Staff Publications

Staff Publications

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    'Staff publications' is the digital repository of Wageningen University & Research

    'Staff publications' contains references to publications authored by Wageningen University staff from 1976 onward.

    Publications authored by the staff of the Research Institutes are available from 1995 onwards.

    Full text documents are added when available. The database is updated daily and currently holds about 240,000 items, of which 72,000 in open access.

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    Local modification and laser-welding production of functionalized microfluidic chips
    Rijo Carvalho, Rui - \ 2017
    Wageningen University. Promotor(en): H. Zuilhof, co-promotor(en): E.X. Vrouwe; S.P. Pujari. - Wageningen : Wageningen University - ISBN 9789463431781 - 180
    lasers - vloeistoffen (fluids) - microfluidics - glas - kanalen, klein - olefine - organische scheikunde - lasers - fluids - microfluidics - glass - channels - olefin - organic chemistry

    The ability to locally functionalize the surface of glass and polymers allows for myriad biomedical and chemical applications. When we started this research enclosed surfaces such as microfluidic channels were not easily amenable to localized functionalization. Furthermore, pre-functionalization of microfluidic devices was incompatible with the typical high-temperature substrate bonding procedures. The work described in this thesis was performed to tackle these two issues.

    In Chapter 1, an introduction is provided to concepts that are central to microfluidics and surface functionalization. Focus is given to techniques that allow for localized surface functionalization, specifically on silicon oxide and glass surfaces. Although contact methods are often easy and near-ubiquitously used, non-contact methods, such as photolithography, are preferred for scaled-up production and are compatible with enclosed surfaces. In addition, plastics – preferably so-called inert plastics – are emerging as a consistent and cheap substrate for the fabrication of microtechnologies. Therefore, previous methods for the modification of such plastics, e.g. cyclic olefin co-polymers (COC), are described. Most of these methods rely on harsh conditions, e.g. plasma, to oxidize the plastic surface. However, in solution there are reported methods to selectively oxidize C-H bonds on alkanes, under mild conditions. Solution-based procedures are much more compatible with enclosed structures when compared to methods relying on gases or plasmas, as the latter exhibit several diffusion issues. Finally, we address the emergence of laser-based techniques and other low-temperature glass bonding methods which will allow for the pre-functionalization of microfluidic devices prior to the bonding step of their fabrication. An outline of this thesis is provided at the end of the chapter.

    In Chapter 2, we present the first example of a photochemical modification of hydrogen-terminated glass (H-glass) with terminal alkenes. Both flat glass surfaces and the inside of glass microchannels were modified with a well-defined, covalently attached organic monolayer using a range of wavelengths, including 302 nm ultraviolet light. Mechanistic studies of the surface modification showed that it proceeds via an anti-Markovnikov substitution. Reacting H-glass with 10-trifluoro-acetamide-1-decene (TFAAD) followed by basic hydrolysis afforded the corresponding primary amine-terminated monolayer, enabling additional functionalization of the substrate (with fluorescent nanoparticles). A microchannel was photochemically patterned with a functional linker allowing for surface-directed liquid flow, and a passive flow stop-valve. H-glass was shown to be modifiable with a functional tailor-made organic monolayer, has highly tunable wetting properties and presents potential for many applications that are explored in the following chapters.

    In Chapter 3, we studied how to increase the wavelength used in the surface modification approach described in the previous chapter, from 302 to 328 nm. Thus, we show the direct photochemical coupling of a N-hydroxysuccinimide-terminated (NHS) ω-alkene onto hydrogen-terminated silicon oxide, and its subsequent functionalization with a catalytically active DNAzyme. In this chapter, we prepared hydrogen-phenyl-terminated glass (H-Φ-glass) by the reaction of glass with H-SiPhCl2. The presence of a radical-stabilizing substituent on the Si atom enabled the covalent modification of bare glass substrates and of the inside of glass microchannels with a functional organic monolayer that allowed direct reaction with an amine-functionalized bio-molecule. Using these NHS-based active esters on the surface, we performed a direct localized attachment of a horseradish peroxidase (HRP)-mimicking hemin/G-quadruplex (hGQ) DNAzyme complex inside a microfluidic channel. This wall-coated hGQ DNAzyme effectively catalyzed the in-flow oxidation of 2,2'-azino-bis(3-ethylbenzothiazoline-6-sulphonate) [ABTS] in the presence of hydrogen peroxide. This proof-of-concept of mild bio-functionalization will find use in myriad bio-relevant applications.

    In Chapter 4, we explored the modification of plastics, such as cyclic olefin copolymer (COC), which are becoming an increasingly popular material for microfluidics. COC is used, in part, due to its (bio)-chemical resistance. However, its inertness and hydrophobicity can be a major downside for many bio-applications. In this chapter, we showed the first example of a surface-bound selective C-H activation of COC into alcohol C-OH moieties under mild aqueous conditions at room temperature. The nucleophilic COC-OH surface allowed for subsequent covalent attachments, such as of a H-terminated silane. The resulting hybrid material (COC-Si-H) was then modified via a photolithographic hydrosilylation in the presence of ω‑functionalized 1-alkenes to form a new highly-stable, solvent-resistant hybrid surface. The work described in this chapter shows great potential for multiple types of plastics, and further shows that the hydrosilylation chemistry described in the previous chapters can be applied onto a multitude of surfaces that contain nucleophilic groups (such as -OH), and thus be able to react with hydrogen-terminated silanes.

    In Chapter 5, we explored low-temperature bonding methods of glass substrates. These methods are of great interest in the field of microfluidic-based biosensing. We evaluated how laser welding could be used for this, by studying how the compatibility of this technology with the, prior to bonding, modification of glass channels with temperature-sensitive materials. The effects of the welding process were studied by investigating the thermal degradation of a fluorescent monolayer throughout the bonding procedure. Furthermore, these leakage-free devices, as well as preliminary work using picosecond lasers, shows that there is a great potential for improvement and wide applicability.

    In Chapter 6, we summarize the results obtained in this thesis. In addition, we discuss some ideas and topics where the research could either be improved or achieved via alternative approaches, including some leads from unfinished work that was performed within this project.

    Molecular design, synthesis and evaluation of chemical biology tools
    Hoogenboom, Jorin - \ 2017
    Wageningen University. Promotor(en): Han Zuilhof, co-promotor(en): Tom Wennekes. - Wageningen : Wageningen University - ISBN 9789463430388 - 206
    chemical biology - biology - tools - synthesis - organic chemistry - molecules - design - chemische biologie - biologie - gereedschappen - synthese - organische scheikunde - moleculen - ontwerp

    Chapter 1 provides a perspective of synthetic organic chemistry as a discipline involved in the design, synthesis and evaluation of complex molecules. The reader is introduced with a brief history of synthetic organic chemistry, all the while dealing with different aspects of synthetic organic chemistry. These aspects include design, synthesis and evaluation of complex molecules, which are described with representative examples. For instance, chapter 1 described different strategies to design antibiotics like penicillin. Furthermore, efforts at the interface of chemistry and biology has led to the emerging of a new subdisciplines such as chemical biology. Although this discipline is relatively new, the scientific community has witnessed many breakthrough discoveries and some of these discoveries are highlighted in this chapter. After the general introduction presented in chapter 1, the following chapters of this thesis focus on the design, synthesis and evaluation of small molecular probes towards the study of proteins and glycans in plant and mammalian cells. Most of these probes are complex glycomimetic small molecules that in many instances are prepared with a nitrone-olefin [3+2] cycloaddition as a key step.

    In chapter 2, a novel nitrone is presented for the nitrone-olefin [3+2] cycloaddition reaction. This reaction is a powerful tool in synthetic organic chemistry for the synthesis of a wide range of complex molecules. The versatile nature of the reaction is illustrated by the synthesis of several classes of natural product such as vitamins and alkaloids – all complex molecules containing multiple neighboring chiral centers – through the nitrone-olefin [3+2] cycloaddition. However, most nitrones that show good regio- and stereoselectivity are limited in their synthetic versatility, as subsequent synthetic modification of the cycloaddition products are limited. The nitrone described in chapter 2 is a novel masked aldehyde-containing nitrone. This nitrone is prepared by a simple and scalable procedure and can be combined with a diverse set of olefins and other dipolarophiles to afford a broad range of cycloadducts. These cycloadducts can be considered as a masked form of amino-aldehydes, which makes them interesting from a synthetic point of view as illustrated by several postcycloaddition modifications.

    The nitrone-olefin [3+2] cycloaddition is also utilized in chapter 3 for the synthesis of glycomimetic building blocks. Glycomimetics such as iminosugars and pipecolic acids are found in nature and possess a variety of biological activities. The potential of glycomimetics have led to the development of drugs for the treatment of diseases such as type 2 diabetes, Gaucher disease and HIV. Chapter 3 describes how glycomimetic building blocks can be obtained through a nitrone-olefin [3+2] cycloaddition, providing different bicyclic isoxazolidines. These cycloadducts are synthetically versatile, as we report a set of reactions that allow selective modification at each functional position. Accordingly, these versatile bicyclic isoxazolidines enable the synthesis of different glycomimetic building blocks. For example, we were able to make a library of pipecolic acid derivatives – a popular drug-motif – via a one-pot Staudinger/aza-Wittig/Ugi three-component reaction.

    The bicyclic isoxazolidines, discussed in chapter 3, are also reported in chapter 4. This chapter describes the development of a synthetic route towards an activity-based probe (ABP) to study the enzymatic activity of neuraminidases. Neuraminidases are a class of enzymes found in a range of organisms including mammals. The importance of neuraminidases is illustrated by the existence of a neuraminidase-related genetic disease, sialidosis. With no cure for this fatal disease being very limited, there is keen interest in the discovery of novel mechanisms to restore neuraminidase activity. The poor enzyme-activity of a different enzyme-related disease, Gaucher disease, could be improved through the identification of a small molecule that stabilizes and/or promotes the folding of the active enzyme. The validation of this small molecule was aided greatly by the development of a sensitive ABP targeting this disease specific enzyme. Accordingly, the development of a neuramidase-ABP would provide a diagnostic tool to study the activity of neuramidases and ultimately help identify small molecules that could increase the activity of mutant-neuraminidase molecules by stabilizing and/or promoting the folding of this enzyme. The synthesis of a carbocyclic neuraminidase ABP was the goal of chapter 4 and was approached by starting with the nitrone-olefin [3+2] cycloaddition as the key-step. This reaction provided a bicyclic cycloadduct that was used for the development of a synthetic route, which led to a high yielding and practical synthesis of an advance intermediate possessing the majority of the stereochemistry of the required carbocyclic neuraminidase ABP.

    Chapter 5 describes the synthesis and evaluation of chemical tools to study phoshatidyl ethanolamine-binding proteins (PEBPs). This family of proteins is found in a variety of organisms including mammals and plants. This family includes FLOWERING LOCUS T (FT), a signaling protein that acts as a vital flowering hormone in plants. No small molecule inhibitors for FT are known, but an inhibitor called locostatin has been reported to bind in the highly conserved ligand binding site of a structurally related protein. Based on this conservation and overall structurally similarity with FT it was hypothesized that locostatin or derivatives thereof could covalently bind in the ligand binding pocket of FT and hence affect flowering. Chapter 5 reports on the synthesis of novel locostatin-based chemical PEBP probes, followed by evaluation for their ability and selectivity towards FT and a mammalian PEBP.

    Chemical tools were also used to study plants in chapter 6, although different aspects of plants were investigated. This chapter describes the direct molecular imaging of carbohydrates (glycans) in Arabidopsis thaliana. Glycans play a crucial but not fully understood role in plant health and development. The formation of glycans is not genetically encoded, which makes it impossible to image them in vivo using genetically encoded fluorescent tags and related molecular biology approaches. A solution to this problem is the use of tailor-made glycans that are metabolically incorporated in plants via the roots, which may then be visualized with copper-catalyzed click labeling. However, this labelling-technique is toxic to plants and future applications would benefit from bio-orthoganol copper-free labeling techniques. Chapter 6 shows, for the first time in metabolic labeling and imaging of plant glycans, the potential of two copper-free click chemistry methods. These methods are bio-orthogonal and lead to more uniform labeling. Furthermore, this chapter also describes the metabolic incorporation of five novel monosaccharide probes in Arabidopsis thaliana roots and their imaging after (copper-free) fluorescent labeling.

    Finally, chapter 7 contains a general discussion, critically summarizing the body of this thesis along with additional ideas and recommendations for further research.

    Nanostructured imaging surface plasmon resonance biosensing
    Joshi, Sweccha - \ 2017
    Wageningen University. Promotor(en): Michel Nielen; Han Zuilhof, co-promotor(en): Teris van Beek. - Wageningen : Wageningen University - ISBN 9789463430203 - 164
    methodology - techniques - biosensors - resonance - mass spectrometry - organic chemistry - physics - methodologie - technieken - biosensoren - resonantie - massaspectrometrie - organische scheikunde - fysica

    The testing and further development of a prototype nanostructured imaging surface plasmon resonance (iSPR) biosensor, with a focus on surface modification and detailed characterization of the biosensor chip and in-field and at-line applicability in the food industry is described. Furthermore, a simplified coupling of SPR and MS is described that allows identification of the mycotoxins of interest along with any other cross-reacting analytes. Chapter 1 describes general information about SPR, SPR instruments along with their components, development of a multiplex SPR biosensor and coupling of SPR to mass spectrometry.

    In Chapter 2, the surface modification, in-depth characterization and the antifouling performance of the nanostructured iSPR chip is described. Different types of polyethylene glycol (PEG) and zwitterionic polymers were chosen as antifouling chemistries. Various surface characterization techniques such as atomic force microscopy, scanning electron microscopy, water contact angle, X-ray photoelectron spectroscopy and direct analysis in real time high resolution mass spectrometry provided complementary information about the chip before and after the modification. Antifouling chemistry, an essential first step in the development of an SPR biosensor, prevents false positive results arising from non-specific binding of sample components to the SPR chip. Upon comparison of the surface modification and antifouling behavior with conventional flat SPR chips, the latter were only slightly better. Zwitterionic polymers and long chain PEG had the best antifouling performance. A proof-of-principle experiment was done to demonstrate the selective detection of streptavidin binding to a surface partially modified with biotin.

    A 6-plex SPR assay for the detection of mycotoxins in barley was developed in Chapter 3. A benchmark double 3-plex assay was developed for the detection of deoxynivalenol (DON), zearalenone (ZEA), T-2 toxin (T-2), ochratoxin A (OTA), fumonisin B1 (FB1) and aflatoxin B1 (AFB1) using benchtop SPR instrument (Biacore). Preliminary in-house validation of the competitive inhibition assay developed using ovalbumin conjugates of the mycotoxins showed that the method is suitable for detection of DON, ZEA, T-2 and FB1 whereas further improvement is required for OTA and AFB1. The method was then transferred to the nanostructured iSPR, which although less sensitive than the benchtop SPR, was able to detect DON, T-2, ZEA and FB1 at the relevant levels.

    In Chapter 4, the assay developed in Chapter 3 was further optimized and an entire assay along with in-house validation and measurement of naturally contaminated was developed using the nanostructured iSPR. The antifouling chemistry used in Chapter 3, PEG, was replaced by carboxymethylated dextran (CMD) that not only allowed direct immobilization of toxins but also helped to improve the stability of the chip whereby the chip could be used for more than 450 cycles. DON could be detected at the relevant levels in beer with minimal sample preparation whereas for OTA an enrichment step using solid phase extraction was required.

    As demonstrated in Chapter 3 and 4, the nanostructured iSPR instrument can be used for screening of different mycotoxins in beer and related ingredients. However, SPR is not able to provide chemical information of the binding analyte especially in cases where the antibodies have cross-reactivity towards conjugates of the analyte. Therefore, a simplified coupling for SPR with ambient mass spectrometry was developed in Chapter 5. The method allowed identification of DON as well as its cross-reacting conjugates such as deoxynivalenol-3-glucoside and 3-acetyl DON.

    The research presented in this thesis is an important step towards the use of the nanostructured iSPR instrument for label free in-field and at-line detection of various analytes. In Chapter 6, discussion of the main achievements of this thesis, challenges and future perspectives of the technology is described.

    Organocatalysis in continuous flow
    Berg, S.A. van den - \ 2016
    Wageningen University. Promotor(en): Han Zuilhof, co-promotor(en): Tom Wennekes. - Wageningen : Wageningen University - ISBN 9789462576636 - 217
    catalysis - organic chemistry - flow - microfluidics - alkenes - katalyse - organische scheikunde - stroming - microfluidics - alkenen

    Continuous flow chemistry is an enabling technique in organic chemistry. Advantages include extremely fast mixing and heat transfer capabilities as well as rapid screening of reaction conditions. Combining continuous flow chemistry with solid-supported organocatalysis presents challenges that have been investigated in this thesis.

    We have developed a technique to modify the surface of silicon carbide with alkenes using microwaves. Silicon carbide is an extremely tough bio-compatible material and is of interest as a solid support. By using monolayers of bifunctional alkenes we were able to perform follow-up reactions to attach other molecules of interest, thereby further modifying the surface chemistry of silicon carbide. The use of microwaves speeds up the attachment of monolayers by as much as sixteen times.

    We have developed a new method for the surface functionalization of MCM-41, a type of mesoporous silica. Mesoporous silica is an interesting solid support for catalysts because of its large surface area. 1,7-octadiene can be covalently attached under mild conditions (24 h, 100 ºC). Follow-up thiol-ene click reactions can be performed using thiols. The use of 1,2-ethanedithiol leads to a material that can be reacted with the organocatalyst quinine, giving catalytically active mesoporous silica. This material was tested in flow and yielded full conversion of starting materials for the thio-Michael addition of 3-methoxythiophenol and 1-cyclohex-2-enone.

    Sulfonic acid supported on mesoporous silica (SBA-15) made via co-condensation of a thiol-containing silica precursor was used in a packed-bed continuous flow setup. This solid-supported sulfonic acid efficiently catalyzes the protection and deprotection of various alcohols as the tetrahydropyranyl-, trimethylsilyl and dimethylphenylsilyl derivatives under continuous flow conditions. By passing the starting material (primary, secondary, benzylic and phenolic alcohols) together with the appropriate protecting group over the packed bed more than 100 mg of product was produced in flow.

    Finally, the ring-opening polymerization of L-lactide using an organocatalyst was investigated. Using low catalyst loading and short residence times (0.25– 1.2% TBD catalyst, residence times as short as 2 s) well-defined poly(lactic acid) (PLA) is obtained with high (95–100%) monomer conversions and PDIs of typically 1.2. Use of the microreactor allows for a rapid screening of optimal reaction conditions, consistently yielding the optimal values for high conversion and low polydispersity. This quickly revealed that longer residence times will give rise to higher conversions and a concomitantly broader molecular weight distribution due to transesterification of the polymer backbone by TBD.

    The organocatalytic, metal-free continuous flow method described is rapid and mild enough to allow the use of a PEG-5,000 macroinitiator (yielding block copolymers), exo-BCN alcohol and even a base-labile tetrazine-derived alcohol, the latter of which cannot be used under traditional batch conditions. The resulting BCN-PLA and tetrazine-PLA materials were readily functionalized with small molecules and large polymers bearing azides and norbornenes via SPAAC and inverse electron demand Diels-Alder click chemistry.

    Despite the challenges that arise, the combination of continuous flow chemistry and (solid-supported) organocatalysis is a powerful concept that warrants further investigation.

    Kleine deeltjes, grote kansen: nanotechnologie neemt een grote vlucht (Interview met Maarten Jongsma)
    Smit, A. ; Jongsma, M.A. - \ 2012
    WageningenWorld 4 (2012). - ISSN 2210-7908 - p. 34 - 39.
    nanotechnologie - bionanotechnologie - smaak - moleculen - organische scheikunde - toegepast onderzoek - nanotechnology - bionanotechnology - taste - molecules - organic chemistry - applied research
    Een elektronische tong, zeefjes die binnen een uur ziekmakende bacteriën detecteren, of moleculen die helpen bij het vinden en doden van tumoren. Wageningen UR timmert hard aan de weg met nanotechnologie – en onderzoekt meteen de mogelijke risico’s voor mens en natuur.
    'Doorbreek de stilte rond nanotechnologie in voedsel'
    Kampers, F.W.H. - \ 2012
    Kennis Online 9 (2012)december. - p. 4 - 5.
    nanotechnologie - voedseltechnologie - voedselveiligheid - organische scheikunde - toxiciteit - bionanotechnologie - houding van consumenten - nanotechnology - food technology - food safety - organic chemistry - toxicity - bionanotechnology - consumer attitudes
    Levensmiddelenbedrijven vertellen liever niet dat hun eten uit de fabriek komt. Gevaarlijk, zegt Frans Kampers van de Agrotechnolgy & Food Sciences Group. Want als er iets misgaat, zal een ongeïnformeerde consument helemaal in paniek raken.
    Organic monolayers on oxide-free silicon : self-assembly, functionalization, patterning and electronic characterization
    Scheres, L.M.W. - \ 2010
    Wageningen University. Promotor(en): Han Zuilhof. - S.l. : s.n. - ISBN 9789085856641 - 201
    silicium - unimoleculaire films - organische verbindingen - zelf-assemblage - organische scheikunde - silicon - unimolecular films - organic compounds - self assembly - organic chemistry - cum laude
    cum laude graduation (with distinction)
    H-Bond stabilized columnar discotic liquid crystals
    Paraschiv, I. - \ 2007
    Wageningen University. Promotor(en): Ernst Sudhölter, co-promotor(en): Ton Marcelis; Han Zuilhof. - [S.l.] : S.n. - ISBN 9789085045588 - 150
    chemische eigenschappen - synthese - vloeibare kristallen - organische scheikunde - chemical properties - synthesis - liquid crystals - organic chemistry
    Since 1977, more than 2300 publications on discotic (disk-like) liquid crystalline materials have appeared. Discotic liquid crystals, which usually consist of polyaromatic molecules surrounded by long peripheral alkyl tails, can form liquid crystalline mesophases in a wide temperature range. Within these mesophases, the molecules self-assemble in large columnar stacks viap-stacking interactions between the aromatic cores. These assemblies can provide an efficient charge transport pathway, as indicated by their informal designation as " molecular-wires ". As a result, they nowadays attract significant interest for optoelectronic applications. Several possible applications where discotic liquid crystals could be used are field effect transistors (FETs), light emitting diodes (LEDs) and photovoltaic solar cells.

    In all the discotic materials described so far, the molecules inside the columns, can still rotate around the columnar axis, slide out of the columns or oscillate within the columnar stack. This represents a limitation of their applicability as " molecular-wires ". Therefore, in this research, an additional stabilization of columnar discotic mesophases was envisaged in order to increase their organization, without the frequently concomitant loss of processability that would result from extensions of the aromatic core. The research described in this thesis focuses on the stabilization of columnar mesophases by highly tunable (i.e. controlled) H-bonding interactions, without enlarging the aromatic cores and thus maintaining their processability. Functional H-bonding groups, such as urea, amide, thiourea or 1,3,5-benzenetrisamide, have been used in this work, in order to create a H-bonding network parallel to the columnar axis, alongside the existing π-π stacking interactions between the disk-like molecules.

    In Chapter 1 an overview of the different types of liquid crystalline phases is presented, with emphasis on the organization of columnar discotic mesophases. Since this work focuses on the hydrogen bond stabilization of the triphenylene-based discotic liquid crystals, the most important synthetic approaches towards these materials are discussed. Several of these methods were applied to prepare the materials investigated in Chapters 2 to 5. Besides the synthetic aspect, several characterization techniques, which are normally used to investigate the properties of the liquid crystal materials, are shortly discussed.

    Chapter 2deals withthe synthesis and thermotropic properties ofa series of hexaalkoxytriphenylenes that contain one amide, urea or thiourea group in one of their alkoxy tails, as H-bond forming group. The biphenyl route turns out to be the best with respect to yields and versatility, as compared to other methods. The optical polarization microscopy, differential scanning calorimetry and X-ray studies show that the intermolecular hydrogen bonding has a negative influence on the formation and stability of the columnar liquid crystalline phases: The stronger the hydrogen bonding, the more the liquid crystallinity is suppressed. This is probably due to disturbance of thep-pstacking of the triphenylene disks and a lower flexibility of the alkyl tails. So far, the urea and amide containing triphenylene derivatives do not exhibit liquid crystalline properties, probably because an H-bond stabilization of the crystalline state is obtained. However, several thiourea derivatives show columnar hexagonal (Col h ) mesophases, because in these compounds thep-pinteractions are more important than the relatively weak thiourea hydrogen bonding.

    Because in the columnar mesophases the motions of the aromatic core and the alkyl tails are strongly correlated, the length of the alkyl chains, which surround the disk-like aromatic cores, is an important factor that determines the stability of the columns. Since in the previous chapter long substituents carrying an urea, amide or thiourea group have been used, a complementary study, with focus on the synthesis and phase behavior of unsymmetrically substituted hexaalkoxytriphenylenes, is described in Chapter3. Inthis study, one of the hexyloxy chains in hexahexyloxytriphenylene (HAT6) is replaced by either a shorter or longer chain. In the series HAT-(OC 6 H 13 ) 5 -(OCn H 2 n +1 ), with n ranging from 2 to 18, the compounds with n³13 are not liquid crystalline anymore. For all compounds with n£12, Col h phases are found. Furthermore, X-ray investigations show that the intercolumnar distance gradually increases from 20.19 Å to 22.03 Å, with increasing n , while a small odd-even effect on the increase of the intercolumn distance with n is observed. This odd-even effect is also found in the change of ΔH of isotropization with n . The interdisk distance remains constant (3.6 Å).

    Since the stabilization of the columnar mesophase by H-bonding interactions still remains an unsolved issue, Chapter 4 describes a 1,3,5-benzenetrisamide with three pendant hexaalkoxytriphenylene groups, as a new approach for the intermolecular H-bond-stabilization of columnar discotic liquid crystalline materials. This compound forms a columnar hexagonal plastic (Col hp ) discotic phase with a clearing point at 208°C. Surprisingly, the material does not crystallize on cooling from the isotropic phase, even after annealing for a few days at room temperature, but goes into a glassy state due to the enforced H-bonding network formed between the benzenetrisamide units. Modeling studies show that the central 1,3,5-benzenetrisamide cores are rotated 60°with respect to each other, which makes the adjacent triphenylene moieties to stack with a small 15°rotation. This small rotation observed between the adjacent triphenylene units plays a key role, since for such a rotation ~85 % of the maximum charge transfer integral is achieved. This result is correlated to the very high charge carrier mobility of 0.12 cm 2 V -1 s -1 at 180°C, found for this particular compound. This value is the second highest ever reported for liquid crystalline triphenylene systems. This is a first proof for the intermolecular H-bonding stabilization of the columnar organization of triphenylene moieties, without loosing the ease of processing provided by the liquid crystalline phase.

    Since the modeling discussed in the previous chapter suggests that the conformation of the butyl spacer, which is used for that particular compound, dictates thep-poverlap of the triphenylene units within the liquid crystal phase, a new series of 1,3,5-benzenetrisamide derivativeswith threehexaalkoxytriphenylene pendant groups is prepared and their properties discussed in Chapter5. Inthis case, the length of the spacer, as well as the size of the ortho -substituent at the triphenylene core is varied. Taking into account these modifications, all these materials show liquid crystalline behavior with high isotropization points (170-200°C). Different columnar hexagonal phases have been identified going from a columnar hexagonal plastic (Col hp ) phase, when using a butyl spacer, with a shorter butyl ortho -substituent of the triphenylene core, to a columnar hexagonal disordered (Col hd ) phase, when a pentyl spacer is used and a hexyl ortho -substituent is present on the triphenylene groups.Achiral 1,3,5-benzenetrisamide was shown to form columnar stacks with a single helical organization, both in an apolar solvent and in a film, as found by circular dichroism studies. By using pulse-radiolysis time-resolved microwave conductivity, charge carrier mobilities as high as 0.25 cm 2 V -1 s -1 are found for the liquid crystalline phase (Col h ) of this chiral derivative. This mobility is twice the highest value ever reported for triphenylene-based liquid crystalline materials, approaching mobilities found for hexabenzocoronene-based liquid crystals, with a much larger aromatic core. This is a substantial improvement regarding the stabilization of disk-like molecules within their liquid crystal phase, without expanding the aromatic cores, but by tuning the H-bonding and the length of the spacer.
    Uptake of sediment-bound bioavailable polychlorobiphenyls by benthivorous carp (Cyprinus carpio)
    Moermond, C.T.A. ; Roozen, F.C.J.M. ; Zwolsman, J.J.G. ; Koelmans, A.A. - \ 2004
    Environmental Science and Technology 38 (2004)17. - ISSN 0013-936X - p. 4503 - 4509.
    sediment - biologische beschikbaarheid - polychloorbifenylen - benthos - karper - zoetwatervissen - meren - waterinvertebraten - chemicaliën - opname (uptake) - organische scheikunde - ecotoxicologie - waterbodems - sediment - bioavailability - polychlorinated biphenyls - benthos - carp - freshwater fishes - lakes - aquatic invertebrates - chemicals - uptake - organic chemistry - ecotoxicology - water bottoms - hydrophobic organic contaminants - aquatic food webs - contact time - pore-water - ingested sediment - extraction - lake - pollutants - fish
    It is unclear whether accumulation of sediment-bound chemicals in benthivorous fish depends on the degree of sequestration in the sediment like it does for invertebrates. Here, we report on the potential of slow and fast desorbing sediment-bound polychlorobiphenyl (PCB) fractions for accumulation in carp (Cyprinus carpio) in lake enclosures treated with different nutrient doses
    It is unclear whether accumulation of sediment-bound chemicals in benthivorous fish depends on the degree of sequestration in the sediment like it does for invertebrates. Here, we report on the potential of slow and fast desorbing sediment-bound polychlorobiphenyl (PCB) fractions for accumulation in carp (Cyprinus carpio) in lake enclosures treated with different nutrient doses. Routes of PCB uptake were quantitatively evaluated for 15 PCBs (log K-0W range 5.6-7.8) using model analysis. Fast-desorbing PCB fractions in the sediment were defined as the ratio of 6-h Tenax-extractable to (total) Soxhlet-extractable concentrations. These fractions varied between 4 and 22% and did not show a clear trend with log K-0W. However, bioaccumulation of PCBs in carp correlated much better with Tenax-extractable concentrations than with total-extractable concentrations. Nutrient additions in the enclosures had a positive effect on PCB accumulation. Model results show that PCB uptake in carp can be explained from (1) uptake through invertebrate food, (2) uptake from fast-desorbing fractions in ingested sediments, and (3) uptake from water, where PCBs are in partitioning equilibrium with fast-desorbing fractions. The main implication of this research is that fast-desorbing PCB fractions in sediments have great predictive potential for bioaccumulation in benthivorous fish.
    Seed development and carbohydrates
    Wittich, P.E. - \ 1998
    Agricultural University. Promotor(en): M.T.M. Willemse; A.A.M. van Lammeren; C.J. Keijzer. - Wageningen : Wittich - ISBN 9789054858553 - 178
    zaadzetting - zaden - formatie - plantenfysiologie - plantenontwikkeling - vruchten - rijp worden - planten - embryologie - metabolisme - plantenvoeding - assimilatie - koolhydraten - vicia faba - gasteria verrucosa - organische scheikunde - seed set - seeds - formation - plant physiology - plant development - fruits - ripening - plants - embryology - metabolism - plant nutrition - assimilation - carbohydrates - vicia faba - gasteria verrucosa - organic chemistry

    Seeds assure the plant the onset of a next generation and a way of dispersal. They consist of endosperm and an embryo (originating from gametophytic tissue), enveloped by a seed coat (sporophytic tissue). Plants generate different types of seeds. For instance, the endosperm may either be consumed by the embryo during seed development or retained for use by the embryo during germination. Differences in timing of endosperm digestion can be illustrated with broad bean ( Vicia faba ) and Gasteria verrucosa . Broad bean forms seeds in which the endosperm has been consumed by the fully developed embryo, while the embryo of Gasteria is less developed and surrounded by a large amount of endosperm for use during germination.

    An important factor in seed development is the distribution, storage, and utilization of carbohydrates, since carbohydrates are a major source of energy for cell growth. In this thesis the carbohydrate distribution is studied in developing ovules and seeds of maize ( Zea mays ) and Gasteria , by identifying the cells and tissues in which sucrose is degraded. Sucrose is the main carbohydrate supplied by these plants in the developing seeds. The sucrose degrading activity of the enzymes sucrose synthase and invertase indicates the destination of the sucrose transport (Chapters 9 and 10). Immunocytochemical and histochemical techniques are used for the localization of these enzymes in situ .

    The results obtained in this study on maize (Chapter 2 and 3) and Gasteria seed development (Chapters 4 and 5) show a general pattern of carbohydrate transport. First, the greatest amount of carbohydrates is applied for the development of the seed coat and nucellus (sporophytic tissues). An example of such a carbohydrate consuming process is the deposition of phytomelan in the seed coat of Gasteria . Phytomelan is a black cell wall component and chemically very inert. Histochemical and electron microscopy observations (Chapters 6 and 7) show that callose forms a mould for the deposition of phytomelan. The breakdown products of callose (glucose monomers and polymers) seem to be used for the synthesis of the phytomelan. Chemical analysis reveals that phytomelan is a complex polyphenolic polymer, and not a melanin (Chapter 8). Second, carbohydrate transport to the sporophytic tissues is followed by transport of most carbohydrates into the endosperm. These carbohydrates will be used for endosperm growth and for storage. Finally the main carbohydrate flow will go to the embryo. The pattern of carbohydrate usage observed in maize and Gasteria was used to generate a general model for angiosperm seed development (Chapter 10). The model explains differences between seeds by relating carbohydrate distribution during seed development to the timing of seed dispersal.

    Fermentatie van brijvoeders en bijproducten tijdens opslag
    Rijnen, M. ; Scholten, R. - \ 1997
    Praktijkonderzoek varkenshouderij 11 (1997)5. - ISSN 1382-0346 - p. 6 - 6.
    voer - enzymen - fermentatie - voedselindustrie - bijproducten - zetmeelverwerkende industrie - glycolzuur - melkzuur - lactonen - ethanol - voedingswaarde - vloeibare voedering - organische scheikunde - feeds - enzymes - fermentation - food industry - byproducts - starch industry - glycolic acid - lactic acid - lactones - ethanol - nutritive value - liquid feeding - organic chemistry
    Er zijn aanwijzingen dat vloeibare bijproducten en brijvoeders gaan gisten (fermenteren) tijdens opslag, waarbij onder andere organische zuren, melkzuur en alcohol worden gevormd. Dit kan enerzijds de voederwaarde van het product en anderzijds de gezondheid van het dier beonvloeden.
    Sampling and efficient data use for characterizing polluted areas.
    Stein, A. - \ 1997
    In: Pollution assessment and control / Barnett, V., Turkman, K.F., - p. 45 - 69.
    bodem - zware metalen - fysicochemische eigenschappen - bodemeigenschappen - bodemchemie - koolwaterstoffen - organische scheikunde - geostatistiek - soil - heavy metals - physicochemical properties - soil properties - soil chemistry - hydrocarbons - organic chemistry - geostatistics
    Biologische monitoring zoete rijkswateren : microverontreinigingen in driehoeksmosselen 1993
    Pieters, H. ; Verboom, B.L. - \ 1994
    IJmuiden : RIVO-DLO (RIVO rapport C004/94) - 49
    biologische technieken - kanalen - koolwaterstoffen - mossels - mytilidae - nederland - rivieren - waterlopen - oppervlaktewater - water - waterverontreiniging - waterkwaliteit - biologische monitoring - organische scheikunde - biological techniques - canals - hydrocarbons - mussels - mytilidae - netherlands - rivers - streams - surface water - water - water pollution - water quality - biomonitoring - organic chemistry
    Simulation of subsurface biotransformation
    Bosma, T.N.P. - \ 1994
    Agricultural University. Promotor(en): A.J.B. Zehnder; G. Schraa. - S.l. : Bosma - ISBN 9789054852216 - 136
    microbiële afbraak - geologie - biologie - bodembacteriën - organische verbindingen - organische scheikunde - microbial degradation - geology - biology - soil bacteria - organic compounds - organic chemistry

    Hydrophobic organic contaminants like DDT, Polychlorobiphenyls (PCB's) and polyaromatic hydrocarbons (PAH's), have been detected all over the world. They tend to accumulate in the atmosphere and in the soil as a result of their physical and chemical properties. Breakdown mainly proceeds by (photo)chemical reactions in the atmosphere and via microbial transformation in the soil. Microbial transformation can be viewed as part of the ecological process of decomposition, that is, the remineralization of organic material by biota. This Chapter discusses the ecological significance of biotransformation and the dependence of biotransformation rates on enviromnental conditions, and suggests ways to improve the effectiveness of biological soil remediation techniques.

    Contaminant cycling in ecosystems
    Chemicals are released into the environment by human activities. Normally, they enter the abiotic part of the ecosystem which may be viewed as a contaminant pool (Fig. S. 1). Biota take up contaminants directly from the abiotic environment e.g. via leaves or the skin, or ingest them by feeding on a lower trophic level. Organisms have systems at their disposal to excrete or detoxify contaminants. Excretion brings contaminants back to the contaminant pool, while detoxification results in a decontamination as indicated in Fig. S. 1.

    Plants and animals are not always able to detoxify or excrete contaminants after uptake. The inability of organisms to handle xenobiotic compounds may have several causes. One example is the absence of appropriate enzymes to transform the compounds, another the accumulation in (animal) fat tissue before excretion or enzymatic transformation has taken place. Contaminants accumulate in the food chain when organisms are not able to detoxify or excrete them. Accumulation is indicated by the use of different grey shades in Fig. S. 1.

    The population of "decomposers" (Fig. S. 1) is specialized in the uptake and conversion of all kinds of dead organic material, like for instance dead animals and plant debris. Decomposers are crucial for the functioning of ecosystems because they recycle nutrients to the nutrient pool. Contaminants which are accumulated in the tissue of organisms are recycled to the contaminant pool simultaneously. Some bacteria and fungi are able to detoxify and mineralize man-made organic compounds like chlorinated benzenes and polyaromatic hydrocarbons. Thus, they prevent their accumulation in the environment. These microorganisms may therefore be viewed as the "decontaminators" of ecosystems (Fig. S. 1), Many micro-organisms live in soil and ground water where hazardous compounds may accumulate. Microbial transformation is the only mechanism leading to the effective detoxification of such compounds. Therefore, it is of interest to know under which environmental conditions biotransformation is inhibited or stimulated. The potential of micro- organisms to transform contaminants under various environmental conditions is discussed in the following together with the factors governing exposure of micro-organisms to contaminants in soil and ground water.

    Potential of micro-organisms to transform organic contaminants
    The capacity of micro-organisms to detoxify anthropogenic chemicals under similar enviromnental conditions is variable among various habitats. This may be related to previous exposure of the micro-organisms to the compound under consideration. An adapted microflora capable of converting and mineralizing new compounds may evolve after a long exposure time. The microflora in a not pre- exposed environment may not be able to detoxify the same compound. Dichloropropene and 2,4-D (dichlorophenoxy acetic acid) are examples of pesticides that micro-organisms "learned" to transform. Degradation of these compounds in the field can be so rapid nowadays that their effectiveness as pesticide is strongly reduced. As a result, farmers have to apply considerably larger amounts of these pesticides than was necessary in the early times of their use.

    Many non-chlorinated organic compounds can be mineralized by aerobic bacteria. Well documented examples are simple aromatic compounds like benzene, toluene, and xylenes. ,More complicated aromatic structures like PAH's are also susceptible to aerobic degradation. Heavily chlorinated compounds are not readily degraded under aerobic conditions. However, anaerobic bacteria have a great potential to dehalogenate all kinds of such chemicals. Dehalogenation changes the environmental impact of the parent compounds considerably. Partly dechlorinated compounds are often more toxic and more mobile than the original compounds. The carcinogenic compound vinylchloride for example, may arise from the anaerobic dechlorination of tetra- and trichloroethene (PER and TRI). The anoxic transformation products are often biodegradable under aerobic conditions. The increased mobility of the more toxic products allows them to travel to aerobic environments where they
    can be mineralized. Thus, the anaerobic process of dehalogenation may be an important mechanism to initialize the complete mineralization of heavily chlorinated contaminants in the subsurface environment.

    Most of the information regarding the potential of micro-organisms to degrade organic contaminants is obtained from laboratory studies at 20°C. Studies carried out at temperatures down to 4°C, reveal only a slight temperature dependency of aerobic biotransformation rates. Anaerobic dehalogenation rates are reduced and intermediary dehalogenation products accumulate at lower temperatures. It seems that activities of aerobic micro-organisms involved in these processes are less dependent on temperature than those of anaerobes. Therefore, the aerobic removal rates of non- or partly halogenated compounds may be similar in summer and in winter in natural systems, while heavily halogenated compounds will tend to persist more in winter because of the reduced activity of anaerobes.

    A microscopic view of soil pollution and micro-organisms
    A picture of a versatile microbial community that is able to transform and mineralize a variety of hazardous organic compounds arises from the previous section. Nevertheless, biodegradable organic contaminants can persist in soil for decades. The microbial transformation rate of an organic compound is strongly affected by the potential uptake rate which is influenced by the transport rate to individual micro-organisms, The very slow in situ biotransformation is probably caused by the properties of the soil matrix surrounding the micro-organisms which reduces the transport rate. The microscopic spatial distribution of contaminants and micro-organisms will affect biotransformation rates in soil. This paragraph discusses how a spatial separation between micro- organisms and contaminants may develop in case of pollutions from point and non-point sources.

    Soil is polluted from point sources like for instance accidental spills and landfills (local pollution), or from non-point sources like atmospheric deposition and application of pesticides (diffuse pollution). The general characteristic of a local pollution is the presence of high contaminant concentrations in a small volume of soil (Fig. S.2A, upper part). Diffuse pollution is characterized by low contaminant concentrations over a wide area (Fig. S.2B, upper part). The lower part of Fig. S.2 schematically shows the local distribution of micro-organisms and contaminants in both situations. Bacteria are normally present inside soil aggregates. Low concentrations of contaminants flow around these aggregates in the case of diffuse pollution (Fig. S.2B). Local pollution initially contaminates pores around soil aggregates. The easily accessible part in wide pores may be biotransformed rapidly until nutrients become exhausted. This leads to a rapid growth of bacteria in the wide pores. Pollutants which are not biotransformed initially will diffuse into the aggregates. Thus, a situation arises with relatively high numbers of bacteria surrounding contaminated aggregates (Fig. S.2A). Degradation activity is drastically reduced as a result of spatial separation. A similar situation may arise when spots containing pure contaminant exist, where no biological activity is possible anymore. Hence, micro-organisms and contaminants are spatially separated both in the case of local and diffuse pollution. Biotransformation can only take place after diffusion of contaminant through the soil matrix to the micro-organisms.

    Computer calculations based on the concept presented in Fig. S.2 show that intra-aggregate processes of sorption and diffusion are of primary importance in determining the kinetics of biotransformation in soil. Effective diffusion rates in soil aggregates can be up to 1-10 orders of magnitude smaller than in water, depending on the characteristics of the soil matrix. As a consequence, biotransformation rates in different soils are subject to the same variation. When the diffusivity in soil aggregates is small, a steep concentration gradient is needed to maintain a flux of nutrients and contaminants that is sufficient to sustain microbial activity. As soon as the contaminant concentration drops below the value that is needed to maintain the gradient, biotransformation will stop. This threshold concentration is inversely proportional to the effective diffusion rate of contaminant. So, residual concentrations after biotransformation are expected to differ by many orders of magnitude, just like the biotransformation rates do.

    Optimization of bioremediation techniques to relieve limitation of biotransformation
    Limitation of biotransformation is not only the result of slow diffusion rates in soil, but may also be due to physiological or thermodynamic factors, to the presence of undissolved pollutants, or to the coupling of pollutant to soil organic matter via covalent bonds (bound residue formation). All these factors may result in reduced biotransformation rates. A strong association between the pollutants and the soil matrix especially develops at sites which have been polluted for years or decades already, Bound residue formation and extremely slow diffusion into small, highly tortuous pores have both been proposed as causes for this strong association. As a result, bioremediation is particularly difficult for these so-called "aged" pollutants (Fig. S.2A,

    Dissolution rates of undissolved pollutants may be enhanced by dispersing the pure component through the soil, and by the addition of surfactants that increase maximum dissolution rates. These methods have been shown to increase biotransformation rates in practice. However, surfactants do not dissolve bound residues which are covalently bound to organic matter. In addition, they do not increase diffusion rates in small, highly tortuous pores, The existence of bound residues and the extremely slow diffusion rates are causes for high residual concentrations that remain after bioremediation. Therefore, surfactants are not expected to decrease these residual concentrations.

    The possible application of procedures that enhance bound residue formation as means of bioremediation is disputed. It can be argued that pollutants which are present as bound residues are not hazardous anymore, having lost their specific chemical characteristics. Thus, they have also lost their biological activity. However, pollutants not only bind to the humus fraction of the soil, but also to dissolved or colloidal fractions of soil organic matter. This may lead to a mobilization of pollutants instead of the intended immobilization. In addition, dioxinlike products are formed when pollutants with phenolic or carboxylic groups bind to each other. The use of applications involving enhancement of bound residue formation requires that the possible hazards are better understood and that ways are provided to prevent them.

    Considerable residual concentrations will always remain after bioremediation of "aged" pollutions, due to strong sorption and incorporation in organic matter, unless special measures are taken to mobilize the pollutants. In an ex situ scheme, the soil may be pulverized to increase biotransformation rates and decrease residual concentrations. It is imaginable to remove the mobile fraction of pollutant in a relatively short time via a biological treatment during in situ remediation. The residual immobilized fraction which is trapped inside soil aggregates is biologically and chemically inactive. It should be sufficient to monitor and control pollutants that are slowly desorbing from the soil aggregates in an "after-care" phase. An approach may be to monitor the concentration level in the macropores continuously and to stimulate biotransformation by the addition of nutrients as soon as some critical level is reached. An alternative would be to apply a slow pump-and-treat method continuously, The after-care phase can be stopped as soon as total pollution concentrations in the soil are below acceptable limits.

    New pollutions have to be treated biologically as soon as possible to achieve optimal results because long contact times between pollutants and soil have a negative effect on the expected result of bioremediation. A possible strategy is to include biological soil treatment in human activities which almost inherently lead to soil pollution with organic chemicals. Thus, the establishment of a strong association between contaminants and soil can be prevented. This strategy has shown to be effective at tank stations where leaking of benzine or diesel is unavoidable.

    Introduction of specialized bacteria is used as a strategy to enhance the biotransformation of compounds that are not degraded by the indigenous microflora. The success of the addition of micro- organisms depends on their ability to reach the contamination, to survive, and to carry out the desired reaction. A better understanding and control of the transport of bacteria in soil and ground water will help to optimize techniques for bioremediation which employ introduced bacteria. Surface characteristics of bacteria and soil particles together with the ionic strength of the flowing water control the transport of bacteria under saturated flow conditions. The adhesion of bacteria to soil particles is positively correlated with the hydrophobicity of bacteria and the ionic strength of the flowing water. Hence, the ionic strength of the water in which bacteria are introduced can be used to control microbial transport and attachment. If a low ionic strength is used, bacteria may travel long distances and disperse around the point where they are introduced. On the other hand, a high ionic strength will generally stimulate the attachment of bacteria to the solid phase and may prevent bacteria from moving away from the polluted site.

    Concluding remarks
    Microbial transformation is required to achieve detoxification of hydrophobic organic contaminants that accumulate in soil. Micro-organisms can therefore be viewed as a subpopulation of the decomposers with a special function, namely detoxification of the environment. The effectiveness of microbial transformation can severely be reduced by the relative immobility of organic compounds in the soil matrix where micro-organisms live. Limitations resulting from slow diffusion can only be removed effectively during ex situ remediation, e.g. by pulverizing the contaminated soil. During a biological in situ treatment the bulk of contamination can be removed rapidly. The treatment should be followed by an aftercare period in which the possible leaking of the residual amount is monitored to be able to take measures if necessary.

    From an ecological stand-point, it can be argued that production and release rates of toxicants have to be smaller than in situ biotransformation rates to keep environmental pollution within acceptable limits. Treatment as close to the source as possible during the manufacturing and use of chemicals will be an important strategy to reach such a goal. The use of pesticides should be regulated such that the amount applied in a growth. season is completely transformed in situ in the same season.

    ACKNOWLEDGEMENT
    Discussions with Clay L. Montague were very helpful in the conception of the Synopsis.

    Effecten van stoffen op de Nederlandse natuur : een inventarisatie
    Elbers, M.A. ; Douben, P.E.T. - \ 1993
    Wageningen : IBN-DLO (IBN - rapport 005) - 92
    milieu - verontreinigende stoffen - verontreiniging - nadelige gevolgen - koolwaterstoffen - zware metalen - wildbescherming - flora - fauna - wild - bescherming - conservering - nederland - ecotoxicologie - bioaccumulatie - milieuhygiëne - organische scheikunde - environment - pollutants - pollution - adverse effects - hydrocarbons - heavy metals - wildlife conservation - flora - fauna - wildlife - protection - conservation - netherlands - ecotoxicology - bioaccumulation - environmental hygiene - organic chemistry
    Biodegradatie ten dienste van bodemsanering, landbouw en drinkwaterwinning.
    Schraa, G. ; Berg, R. van den - \ 1992
    Bodem 2 (1992)1. - ISSN 0925-1650 - p. 20 - 24.
    luchtverontreiniging - koolwaterstoffen - microbiële afbraak - bodemverontreiniging - biodegradatie - organische scheikunde - air pollution - hydrocarbons - microbial degradation - soil pollution - biodegradation - organic chemistry
    Geordende moleculen in perspectief
    Sudhölter, E.J.R. - \ 1991
    Wageningen : Landbouwuniversiteit - 20
    chemie - organische scheikunde - moleculen - chemistry - organic chemistry - molecules
    Rede Wageningen, 16 mei 1991
    Bepaling van vluchtige vetzuren en ethanol in kuilextracten
    Huisert, H. ; Spoelstra, S.F. - \ 1988
    Lelystad : IVVO (Intern rapport / IVVO no. 247) - 13
    samenstelling - voer - koolwaterstoffen - kuilvoer - organische scheikunde - composition - feeds - hydrocarbons - silage - organic chemistry
    Problematiek bij de analyse van sterolen met betulinol als interne standaard
    Kamp, H.J. van der; Muuse, B.G. - \ 1987
    Wageningen : RIKILT (Rapport / RIKILT 87.50) - 10
    steroïden - alcoholen - analytische methoden - organische scheikunde - steroids - alcohols - analytical methods - organic chemistry
    ISO, IDF en EEG werken aan methoden waarbij de sterolen in % m/m op vetbasis bepaald worden. Hierbij wordt gebruik gemaakt van betulinol als interne standaard. Uit internationale tussenlaboratoriumonderzoeken blijkt dat de sterolsamenstelling m.b.v. gaschromatografie nauwkeurig en reproduceerbaar vastgesteld kan worden. Ook het gehalte aan sterolen in massaprocenten op vetbasis is goed herhaalbaar te bepalen doch de reproduceerbaarheld is zeer slecht. Dit rapport beschrijft stapsgewijs het onderzoek van de methode met interne standaard.
    Oplosbaarheden en aktiviteitscoefficienten van apolaire organische stoffen opgelost in water
    Bosman, H.F.P.M. - \ 1986
    Wageningen : ICW (Nota / Instituut voor Cultuurtechniek en Waterhuishouding no. 1688) - 29
    kanalen - chemische eigenschappen - schade - milieu - koolwaterstoffen - rivieren - oplossingen - waterlopen - oppervlaktewater - water - waterverontreiniging - organische scheikunde - canals - chemical properties - damage - environment - hydrocarbons - rivers - solutions - streams - surface water - water - water pollution - organic chemistry
    Bio-organic heterocycles. Synthesis, mechanisms and bioactivity : proceedings of the 4th FECHEM Conference on Heterocycles in Bio-organic Chemistry, Houthalen, Belgium, 25-28 May 1986
    Plas, H.C. van der; Simonyi, M. ; Alderweireldt, F.C. ; Lepoivre, J.A. - \ 1986
    Amsterdam : Elsevier (Studies in organic chemistry 27) - ISBN 9780444427113 - 325
    heterocyclische verbindingen - organische scheikunde - heterocyclic compounds - organic chemistry
    Handbook of environmental data on organic chemicals
    Verschueren, K. - \ 1983
    New York : Van Nostrand Reinhold - ISBN 9780442288020
    organische verbindingen - toxicologie - chemicaliën - milieu - verontreinigende stoffen - verontreiniging - nadelige gevolgen - handboeken - organische scheikunde - tabellen - organic compounds - toxicology - chemicals - environment - pollutants - pollution - adverse effects - handbooks - organic chemistry - tables
    Organic mutagens and drinking water in The Netherlands : a study on mutagenicity of organic constituents in drinking water in The Netherlands and their possible carcinogenic effects
    Kool, H.J. - \ 1983
    Landbouwhogeschool Wageningen. Promotor(en): J.H. Koeman. - Wageningen : Kool - 116
    carcinoom - chemicaliën - schade - drinkwater - koolwaterstoffen - mutagenese - mutagenen - mutaties - neoplasma's - verontreinigingsbeheersing - waterverontreiniging - waterzuivering - organische scheikunde - carcinoma - chemicals - damage - drinking water - hydrocarbons - mutagenesis - mutagens - mutations - neoplasms - pollution control - water pollution - water treatment - organic chemistry
    Several mutagenic and carcinogenic organic compounds have been detected in Dutch surface waters and in drinking water prepared from these surface waters. Although the levels of these compounds in drinking- and surface water are relatively low, in general below μg per litre, it appeared that organic concentrates tested in the Ames/microsome assay, showed mutagenic activity in 50 ml surface- and 500 ml drinking water.

    Such a result however was not expected based on the concentration of organic mutagens identified in these waters. Therefore the conclusion had been drawn that a number of unknown organic mutagens in combination with the identified mutagens were responsable for the level of mutagenic activity. With this in mind, an extensive investigation was carried out in an attempt to answer the following questions :
    - do drinking waters prepared from surface water, groundwater or a mixture of both show mutagenic activity;
    - do different water treatment processes during the preparation of drinking water influence the mutagenic activity;
    - what kind of physical-chemical properties have the organic compounds in drinking water concentrates showing mutagenic activity in the Ames test;
    - do mutagenic organic concentrates prepared from drinking water show carcinogenic properties?

    Against this background an inventory study was made on the presence of mutagenic activity in drinking water of 18 cities in The Netherlands.

    Besides this study, the influence of different water treatment processes on the mutagenic activity was examined in a number of water works. Furthermore, an attempt was made to characterize the organic compounds which are responsable for the mutagenic activity. Finally, a carcinogenicity study was carried out to see whether mutagenic drinking water concentrates induce carcinogenic effects in rats.

    Chapter 1, describes the aim of this investigation and the factors which have led to the present concern regarding the possible toxic effects of organic (micro)pollutants in drinking water in The Netherlands.

    Furthermore a summary is given of the literature on the identification of individual compounds and some data are given of the mutagenic and (suspect) carcinogenic organic compounds which are detected in surface- and drinking water in The Netherlands.

    In Chapter 2, materials and methods are described which were used for the inventory study on the presence of mutagenic activity in drinking water of 18 cities (Chapter 3), the characterization of organic mutagens (Chapter 4) and for a carcinogenicity study with mutagenic drinking water concentrates (Chapter 5).

    This chapter describes the XAD resins which have been used for concentrating organic mutagens from water, the concentration procedures with the aid of XAD resins and freeze drying, a mutagenicity assay viz. the Ames Salmonella/microsome assay (Ames test). The methods for chemical analysis which are used in the 18 city survey, fractionating techniques like thinlayer chromatography (TLC), high performance liquid chromatography (HPLC) and gelfiltration are described. The treatment, dose levels and experimental design and conduct which are used in a carcinogenicity study are explained. Finally, statistical procedures which have been used for analyzing tumour incidence in the carcinogenicity experiment and relating chemical parameters in drinking water to mutagenic activity in organic concentrates prepared from these drinking waters are described.

    In Chapter 3, the need for concentrating organic mutagens is explained. Mutagenicity results (Ames test) of five surface waters in The Netherlands are presented, in which the organic mutagens are concentrated with the XAD procedures. Depending on the concentration factor applied (103-4.103) it appeared that in all five surface waters mutagenic activity could be detected. The river Rhine showed the highest mutagenic activity viz. a doubling of revertants in 50 ml water.

    To be certain that the XAD procedure is a selective concentration method for organic mutagens, this procedure was compared with a freeze drying technique. The results of this comparative investigation showed that the XAD procedure is a reliable method for concentrating organic mutagens from surface water. on the basis of the surface water results, the XAD procedure was also applied for concentrating organic mutagens from drinking water. It was found that drinking water of four out of six cities showed mutagenic activity in volumes varying from 0.5 to 3 litre.

    To see whether the mutagenic results obtained in the six cities are representive for drinking water in The Netherlands, an extensive inventory study on the presence of mutagenic activity in several types of drinking water was carried out. In this study eighteen cities (twenty drinking waters) were investigated three times for the presence of mutagenic activity and the following chemical parameters : AOCL, EOCl, THM, VOCl, TOC, Tot. N, over a period of 2 years. The chemical parameters were measured to see whether a relationship could be found between one or more of these parameters and mutagenic activity.

    The selection of the drinking water was based on the water source (ground water, surface water) the storage facility (dune filtration, bankfiltration, storage reservoir) and the application of a chlorine treatment during the preparation of drinking water. The results of this study showed that in fourteen of the twenty drinking waters, mutagenic activity could be detected in volumes varying from 0.5 to 3 litre. When the cities were classified according to their water source, storage facility and type of treatment, it appeared that only three of the fifteen cities which prepare their drinking water from surface water or a mixture of surface- and groundwater, did not show mutagenic activity. Two of the five cities which use groundwater as a drinking water source showed mutagenic activity although in one city the activity was marginal.

    Correlating the chemical parameters with the mutagenic activity, it was demonstrated that AOCL showed the highest correlation with the direct mutagenic activity in strain TA 98 and TA 100. It was also shown that a chlorine treatment applied during drinking water preparation correlated well with the direct mutagenic activity in strain TA 98 and TA 100. In addition, chemical parameters which showed a significant difference (p< 0.01) in concentration between mutagenic and non mutagenic samples (Mann-Whitney) and a significant correlation (p< 0.01, Kendall tau) with mutagenic activity were used for straight curve fitting. From these fitted regressions, concentration levels were calculated above which a doubling of revertants may be expected.

    In the second part of Chapter 3 (3.4) the influence of different treatment processes on the mutagenic activity and some chemical parameters were investigated in three waterworks. Application of a chlorine treatment, generally increased the direct and promutagenic activity, but the amount of increase proved to be dependent on the type of water chlorinated. The use of ozone in the preparation of drinking water decreased the mutagenic activity in the water. The amount of reduction was dependent on the type of water ozonated. Dune filtration greatly reduced the mutagenic activity.

    Slow sand filtration could not be evaluated, because of the toxicity of the organic concentrates for the bacterial strains. Filtration over active carbon filters which operated for about 1 year, reduced the mutagenic activity below the detection level. Carbon filters which operated more than 1.5 years in a pilot plant, showed a break-through of mutagenic activity. This result suggests that carbon filters are able to remove organic mutagens only for a short time.

    From the results of the chemical parameters before and after the different treatment processes it appeared that the level of AOCL, behaved very similar with the mutagenic activity in the neutral fraction. These results confirm those obtained in the eighteen city survey and support the idea that A0C1 might be a useful indicator for mutagenic activity in drinking water.

    The physical-chemical characterization of organic mutagens in drinking water concentrates is described in Chapter 4. The first approach was to examine the influence of the pH on the adsorption behaviour of organic mutagens on the XAD resins. Lowering the original pH of drinking water with HCl to pH = 2- 3, revealed the presence of another class of organic mutagens the so-called acid fraction which hardly adsorb on the XAD resins at pH = 7.5.

    Further it was investigated whether different organic solvents are able to eluate the organic mutagens from the XAD column. it was found that diethylether only eluted a minor part of the mutagenic activity from the XAD resins and subsequent elution with acetone eluted the major part of the activity.

    This result showed that it is not likely that the organic mutagens in the acetone fraction are identical with the already identified organics in the ether fraction in this drinking water. Furthermore it appeared that the organic mutagens arenot only less volatile but also resistent to boiling. Another approach to characterize the organic mutagens was to apply fractionation techniques. Using TLC and HPLC, it was found with HPLC analysis that the mutagenic activity was present predominantly in two fractions. Finally, gelfiltration on Sephadex LH20, showed that organic mutagens which demonstrated mutagenic activity with strain TA 98, had a molecular weight in the order of 200.

    Chapter 5, presents the results of a carcinogenicity study, in which Wistar SSP TOX rats were exposed to mutagenic drinking water concentrates of one city in The Netherlands. Drinking water concentrates were prepared every week using the XAD concentration procedure and the organic concentrates (DMSO) were mixed with the non mutagenic drinking water of the Wistar SSP TOX rats.

    Dosage levels were based on multiples of expected human exposure levels. In the calculation the average human exposure was assumed to be approximately 29 ml/kg bw./day. Furthermore it was assumed that a rat of 250 gram would consume about 30 ml water per day. In the experiment rats were divided into four groups (50 males and 50 females per group), a control group and groups which received respectively 10, 30 and 90 times the human exposure level in their drinking water.

    During the experiment (106 weeks) the water consumption of the rats was measured weekly. Body weights were recorded weekly in the first two months of the experiment and once a month thereafter. The mutagenic activity of concentrates which were mixed with non mutagenic drinking water were measured after a week. It was found that the mutagenic activity with strain TA 98 could be recovered for 60-90%, while the TA 100 activity hardly could be recovered. The latter was explained by the observed partial decrease of TA 100 activity after mixing the mutagenic drinking water concentrate with drinking water.

    During the experiment it appeared that the assumed water consumption of 30 ml per day per rat, on which the dose level was based, not was reached.

    Moreover, it appeared that Wistar SSP TOX rats became much heavier than was expected and therefore the actual dose levels in ml/kg bw. of the three exposed groups were not 10, 30 and 90 times the expected human exposure levels, but 4.5, 14 and 40 for male rats respectively and 7, 22 and 68 for female rats. Exposing Wistar SSP TOX rats to these dose levels for 106 weeks, did not result in a significant increase (p <0.05) in tumour induction.

    Furthermore it was shown, that the development and types of tumours were similar in the treated and control groups. The number of animals with tumours and the animals which died of tumours in the exposed groups was not significantly different (p < 0.05) from the control group. The negative results in this carcinogenicity study indicate that mutagenic drinking water concentrates did not contain very potent carcinogens in effective concentrations like for instance N-Nitrosodiethylamine because when these kind of compounds are present, the carcinogenicity should be positive at the dose levels tested.

    On the other hand one cannot exclude the presence of weak carcinogens in the mutagenic concentrates, because one cannot detect a carcinogenic effect at the dosis tested. Based on the carcinogenicity results, an estimation of a risk factor was made for the people who consumed this mutagenic drinking water.

    For the estimation the following three assumptions had been made:
    - when organic carcinogens had been present in drinking water, they were present solely in the mutagenic drinking water concentrates
    - results obtained in the present carcinogenicity study with rats may be extrapolated to man
    - linear extrapolation of high dosis to low dosis give correct results.

    When all these assumption are correct it appeared after statistical analysis that less than 356 of 110.000 people might be at risk. On this basis the contribution of drinking water, if there is a contribution at all, seems relatively small viz. less than 1. 1 % in comparison to the expected tumour incidence of 33.000. The latter value is based on an average tumour incidence of 30 per 100, due to background processes.

    To improve, however the reliability of an estimation of the risk factor for people who consume polluted drinking water, organic concentratres of drinking water which have not been included in the present carcinogenicity study should be examined for complete carcinogenicity in combination with higher doses of concentrates of organic mutagens as used in this study.

    Besides this investigation it is recommended that both organic concentrates will be tested for tumour promotion and initiation activity as well as identification of the organic responsable for these effects should receive priority. Finally Chapter 6 describes a number of conclusions emerging from this investigation. The data presented show that mutagenic activity was detectable in drinking water of many cities in The Netherlands. The number of cities which showed mutagenic activity in their drinking water was the least where groundwater was used as a drinking water source. Therefore it is recommended that groundwater is preferable over other sources of drinking water and the quality of the groundwater should be protected very carefully so that oxidation and disinfection with a chlorine treatment will not be necessary. This recommendation is not only based on the eighteen city survey but also from data obtained in waterworks which prepare their drinking water mainly from surface water, because mutagenic activity is significantly increased by a chlorine treatment which may often result in mutagenic activity in the end product. It is recommended further that the organic mutagens should be identified in order to evalute them with respect to their possible toxic properties. Also higher dosis of mutagenic drinking water concentrates in combination with concentrates prepared from the rest of the organics in drinking water, should be tested for carcinogenicity and chronic toxicity for a more reliable risk estimation. Besides, this investigation it is recommended that both organic concentrates should be investigated for tumour promotion and initiation activity.

    Metabole gevolgen van het gebruik van polyalcoholen en fructose-literatuurstudie. Deel 5, 6 en 7. Maltitol, lactitol en lycasin
    Wiel-Wetzels, W.A.M. van der - \ 1982
    Voeding 43 (1982). - ISSN 0042-7926 - p. 79 - 81.
    alcoholen - fructose - metabolisme - organische scheikunde - alcohols - fructose - metabolism - organic chemistry
    Over de fysische en chemische eigenschappen van genoemde zoetstoffen in relatie tot de stofwisseling
    Studies on carbon metabolism in wild type and mutants of Aspergillus nidulans
    Uitzetter, J.H.A.A. - \ 1982
    Landbouwhogeschool Wageningen. Promotor(en): J.H. van der Veen, co-promotor(en): J. Visser. - Wageningen : Uitzetter - 142
    aspergillus - micro-organismen - biochemie - metabolisme - synthese - organische verbindingen - organische scheikunde - aspergillus - microorganisms - biochemistry - metabolism - synthesis - organic compounds - organic chemistry
    This thesis deals with carbon metabolism in the lower eukaryote Aspergillus nidulans. This fungus is an attractive organism as a model to study genetics in relation to metabolism in lower eukaryotes.

    In chapter I the present state of affairs in this field is summarized for A. nidulans and some of the advantages of this fungus in particular for physiological and biochemical genetic studies are indicated. The picture of the metabolic abilities of Aspergilli is still far from complete notwithstanding the many studies in A. nidulans and A.niger. The latter is a related fungus used for the biosynthesis of industrial enzymes and for the large scale production of metabolites of commercial and economic interest.

    Ever since the early investigations into the physiology of A.nidulans, which started some 40 years ago, the use of mutants to study the effects of mutant genes on metabolism has become increasingly important. Therefore a brief review is given of the various ways in which mutants regarding to carbon metabolism in A. nidulans have been selected and isolated. An up to date list is added of mutations in carbon metabolism and, as far as these are known, of the metabolic defects involved.

    In chapter II a study was made on the metabolic features of mutants in carbon metabolism, since mutations may have effects on the level of a particular enzyme directly as well as on the regulation of other steps in metabolism ( e.g. inhibition, repression). The mutants used for this. purpose were defective in pyruvate kinase, pyruvate dehydrogenase complex, pyruvate carboxylase, transaldolase and in two non-specified lesions in glycerol metabolism. Growth tests with a variety of substrates and combinations of them were performed with the mutants and on the basis of the results obtained criteria were developed to distinguish the mutant phenotypes. Such growth tests can be used as a first step in the elucidation of unknown metabolic pathways and also to find a strategy to isolate particular mutants. From the carbon sources tested for growth, D- galacturonate emerged as a substrate useful to discriminate pyruvate kinase mutants, which showed good growth on this carbon source from mutants defective in the pyruvate dehydrogenase complex or pyruvate carboxylase, which grow badly or not at all. The differences observed between these mutants in their ability to use D-galacturonate as sole carbon source indicated that pyruvate is a metabolite formed during the degradation of D-galacturonate. Moreover, it was predicted that selection for a mutant phenotype unable to use this carbon source would lead, among others, to mutants unable to metabolize pyruvate, for instance pyruvate dehydrogenase complex mutants.

    In chapter III the results are described of an X-ray mutation experiment followed by enrichment on D-galacturonate. This led to a large number of mutants unable to grow on this carbon source. The criteria developed in chapter II were used to assign phenotypes to the mutants isolated. The majority of these were found to be defective in the pyruvate dehydrogenase complex. In addition mutants were isolated defective in pyruvate carboxylase and in glycerol metabolism, whereas a novel class of mutants was found blocked earlier in D-galacturonate metabolism. The mutants isolated were further analyzed by complementation tests with defined tester strains. It was found that all pyruvate dehydrogenase complex mutants studied belonged to one of the three genes already known. The pyruvate carboxylase mutants were novel with respect to their inability to complement in a heterokaryon with two pyruvate carboxylase tester strains which mutually complement. All mutants found in glycerol metabolism belonged to the same gene. Since mutants were obtained which were only defective in D-galacturonate metabolism, a preliminary study was started to analyze the metabolic route which this compound follows in wild type A.nidulans. A pathway for the degradation of D-galacturonate which leads to pyruvate as one of the products is known to occur in Escherichia coli. Although the presence of this pathway was easily demonstrated in the latter microorganism, no positive evidence has been found for a similar pathway operating in A.nidulans.

    In chapter IV the purification and some properties of the pyruvate dehydrogenase complex of A.nidulans are described. A purification scheme was developed using ultracentrifugation of the complex in a crude extract, polyethylene glycol precipitation of the concentrated enzyme complex and affinity chromatography on ethanol-Sepharose 2B. After a final sucrose density gradient centrifugation step, a pure multienzyme complex was obtained with a specific activity of 7.8 U mg -1with a recovery of 16%. The preparation finally obtained still contained pyruvate dehydrogenase kinase, which enzyme was copurified. SDS-polyacrylamide gel electrophoresis showed that the purified pyruvate dehydrogenase complex is composed of 4 proteins which were assigned to the α and βsubunit of pyruvate decarboxylase, lipoate acetyltransferase and of lipoamide dehydrogenase. The molecular weights of the complex enzymes resemble those of the complex of other eukaryotes and of Bacilli. The kinetic constants of the complex were listed and they were compared to those of the complexes of other sources.

    In chapter V a purification scheme for pyruvate kinase is described based mainly on the use of affinity chromatography with reactive dyes. Screening several of these dyes coupled to Sepharose resulted in two dyes which were used in sequence to purify pyruvate kinase: Mikacion Brilliant Yellow 6GS,and Cibacron Blue 3G-A as a dextran conjugate. The addition of ethylene glycol during these purification steps was necessary to warrant an optimal recovery of the enzyme activity. Although some pyruvate kinase activity could be recovered by biospecific elution, in the final purification scheme aspecific elution of the enzyme was preferred by enhancing the ionic strength of the elution buffer. A homogeneous enzyme preparation was obtained with specific activity of 67 U mg -1after a final gel filtration on Sephacryl S-300. The overall yield was low due to instability of the enzyme. The purified pyruvate kinase was used to raise antibodies in a rabbit.

    In chapter VI the levels of pyruvate kinase specific activity were studied in wild type A.nidulans grown under different nutritional conditions. The specific activity of pyruvate kinase was high in mycelium grown on sucrose and low in mycelium grown on polygalacturonate or acetate. The latter two carbon sources allow growth of pyruvate kinase mutants. With antibodies against purified pyruvate kinase raised in a rabbit several pyruvate kinase mutants and wild type grown under pyruvate kinase inducing conditions were analyzed for the presence of a pyruvate kinase gene product in sucrose tolerant and sucrose sensitive pyruvate kinase mutant strains, by the use of immunological methods. Although pyruvate kinase could be detected easily in crude extracts of wild type by Ouchterlony double diffusion and tandem crossed immunoelectrophoresis, these methods failed when applied to extracts of pyruvate kinase mutants. Therefore Western blotting was introduced to screen for a pyruvate kinase gene product. Among 5 mutants, all at the same locus as found by non-complementation, only one was found, totally lacking imunological and enzymatic activity. Since a pyruvate kinase gene product was present in both a sucrose tolerant and a sucrose sensitive strain, the absence of such a gene product is not a prerequisite for sucrose tolerance observed with some pyruvate kinase mutants.

    Nieuwe eiwitbronnen als voedsel voor de mens
    Anonymous, - \ 1979
    Wageningen : Pudoc (Literatuurlijst / Centrum voor landbouwpublikaties en landbouwdocumentatie no. 4210)
    landbouw - voedsel - voedingsmiddelen - eiwit - stikstof - eiwitten - groenten - voedselindustrie - organische verbindingen - bibliografieën - voedingswaarde - voedselsamenstelling - kwaliteit - eigenschappen - organische scheikunde - agriculture - food - foods - protein - nitrogen - proteins - vegetables - food industry - organic compounds - bibliographies - nutritive value - food composition - quality - properties - organic chemistry
    Oxidation of azaheterocycles by free and immobilized xanthine oxidase and xanthine dehydrogenase
    Tramper, J. - \ 1979
    Landbouwhogeschool Wageningen. Promotor(en): H.C. van der Plas. - Wageningen : Tramper - 128
    fermentatie - immobilisatie - oxidoreductasen - heterocyclische stikstofverbindingen - xanthine oxidase - xanthine dehydrogenase - oxidatie - organische scheikunde - fermentation - immobilization - oxidoreductases - heterocyclic nitrogen compounds - xanthine oxidase - xanthine dehydrogenase - oxidation - organic chemistry
    The objective of the study described in this thesis was to show that enzymes, especially immobilized enzymes, can be advantageously used in synthetic organic chemistry.
    In Chapter 1 enzymes are introduced and the advantages discussed when these highly active and specific biocatalysts are immobilized, i.e. , attached to a solid support. Criteria for general acceptation of a specific enzyme as a routine catalyst in the organic-chemistry laboratory are stated. The reasons for the choice of xanthine oxidase as a model enzyme and some relevant properties are given.
    The solid support used for the immobilization determines to a large extent the ultimate characteristics of the immobilized enzyme. In Chapter 2 properties of an ideal support and guidelines for the evaluation of a specific support are
    discussed. Various supports are treated in some detail.
    The mode of immobilization is also very important, especially with respect to the eventual specific activity of the immobilized enzyme. In Chapter 3 the various methods of immobilization are discussed. Just as with the supports (Chapter 2), special attention is given to the ones investigated in this study.
    In Chapter 4 various properties of immobilized milk xantine oxidase are described, as well as the stabilization by coimmobilization with protein, superoxide dismutase and catalase. It is concluded that conversion of substrate on a preparative
    scale can be performed conveniently, but that for efficient application a more stable enzyme preparation is desired.
    Chicken-liver xanthine dehydrogenase is an enzyme very similar to milk xanthine oxidase. In Chapter 5 the reasons are given why it was expected that the liver enzyme would be more stable and practical for synthetic application. The immobilization and the properties of the free and immobilized enzyme are described. Analogous to the milk enzyme, the operational stability of xanthine dehydrogenase is much lower than the storage stability.
    In order to determine the substrate limits of xanthine oxidase more systematical ly, so that more-reliable predictions would be possible, series of substrates were synthesized for this purpose. In Chapter 6 the synthesis of 7-(p-X-phenyl) pteridin-4-ones and the oxidation by free and immobilized xanthine oxidase are presented. As X becomes more electron withdrawing, the rate of oxidation decreases. It is therefore concluded that the electron donation from substrate to enzyme must be the rate-limiting step.
    Convenient oxidation of the above substrates and easy product isolation was possible on a small preparative scale. The limited stability of xanthine oxidase demanded however a relatively large amount of enzyme. In Chapter 7 an easy and mild immobilization procedure, using gelatin as support and glutaraldehyde as cross-linking agent, is presented. Whole milk can be used as starting enzyme solution and no isolation is required. The resulting preparation of immobilized xanthine oxidase is very cheap, highly active, relatively stable and suitable for application in organic synthesis in a continuous manner.
    In Chapter 8 the improvement of the immobilization method with gelatin is described and the extension to the immobilization of whole cells, i.e. , Arthrobacter cells containing xanthine-oxidase activity. These cells were chosen because of their high specific activity, their substrate activation instead of inhibition and their expected higher stability. Immobilized Arthrobacter xanthine
    oxidase is indeed highly active, more stable and has a different substrate specificity than milk xanthine oxidase. Therefore, a wider spectrum of substrates can be conveniently and efficiently oxidized.
    In the final Chapter 9 additional information, mostly gathered at a later stage than the time of publication, is given and discussed.
    The end conclusion is: Xanthine oxidase, either from whole milk or Arthrobacter, when immobilized in glutaraldehyde-crosslinked gelatin, can be conveniently used for the oxidation of a wide range of substrates in a continuous fashion.

    Landschapsoecolochische classificatie
    Anonymous, - \ 1975
    Wageningen : [s.n.] (Literatuurlijst / Centrum voor landbouwpublikaties en landbouwdocumentatie no. 3746)
    bibliografieën - landschap - landschapsbouw - organische verbindingen - ruimtelijke ordening - organische scheikunde - bibliographies - landscape - landscaping - organic compounds - physical planning - organic chemistry
    Samenwerking in de wetenschap
    Hertog, H.J. den - \ 1967
    Wageningen : Veenman & Zonen - 21
    anorganische verbindingen - chemische structuur - ethanol - pyridines - piperidinen - eigenschappen - chemicaliën - samenwerking - anorganische scheikunde - organische scheikunde - invloeden - wetenschap - wetenschappelijke samenwerking - inorganic compounds - chemical structure - ethanol - pyridines - piperidines - properties - chemicals - cooperation - inorganic chemistry - organic chemistry - influences - science - scientific cooperation
    Rede Wageningen
    Modern en klassiek
    Plas, H.C. van der - \ 1966
    Wageningen : Unknown Publisher - 15
    organische verbindingen - analytische methoden - organische scheikunde - fysisch-chemische methoden - organic compounds - analytical methods - organic chemistry - physicochemical methods
    Rede Wageningen
    The electrical double layer on silver iodide in the presence of organic molecules
    Bijsterbosch, B.H. - \ 1965
    Wageningen : Veenman (Mededelingen van de Landbouwhogeschool Wageningen 65-4) - 62
    adsorptie - sorptie - organische verbindingen - organische scheikunde - adsorption - sorption - organic compounds - organic chemistry
    Stadia der experimentele organische chemie
    Hertog, H.J. den - \ 1950
    Groningen [etc.] : Wolters - 16
    geschiedenis - chemische structuur - synthese - organische scheikunde - history - chemical structure - synthesis - organic chemistry
    Rede Wageningen, 16 Mei 1950
    Een eeuw organisch-chemische synthese
    Olivier, S.C.J. - \ 1924
    Wageningen : Veenman - 19
    synthese - organische scheikunde - geschiedenis - vetten - koolhydraten - eiwitten - synthesis - organic chemistry - history - fats - carbohydrates - proteins
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