High-resolution mass spectrometry for the analysis of interfacial kinetics of organic surface reactions
Sen, Rickdeb - \ 2017
Wageningen University. Promotor(en): H. Zuilhof. - Wageningen : Wageningen University - ISBN 9789463436243 - 308
surface chemistry - unimolecular films - chemical reactions - analytical methods - mass spectrometry - oppervlaktechemie - unimoleculaire films - chemische reacties - analytische methoden - massaspectrometrie
In this thesis, XPS and DART–HRMS have been used in close conjugation to supplement each other, since the latter is a relatively new addition to surface chemist’s repertoire that – after development – needed a firm comparison to build up a reputation of its own. The strength of our approach has been underlined by the high correlation between these two independent analytical techniques. Central to our approach has been the formation of mixed monolayers in case of aluminum oxide substrates. As presented in Chapters 2, 3 and 4, we have succeeded in the rapid formation of range stable, covalently bound mixed monolayers. The subsequent development of a general and fast analytical technique to determine the interfacial reaction kinetics, including the activation parameters DH‡ and DS‡, provided unparalleled insights. We have developed a “MS–ionizable tag” technique, which has been applied for the analysis of surface–bound organic reactions, to the best of our knowledge, for the first time.
The Strain–Promoted Alkyne–Azide Cycloaddition (SPAAC) reaction was chosen as a model reaction given the fact that its kinetics had been well–studied in solution. As shown in Chapter 2, the microenvironment around the reactive surface group was carefully controlled by the length of the inert alkyl chains surrounding it. We observed a few interesting trends which could be of great interest to future surface chemists. First, the SPAAC reaction – which is a click reaction in solution – does not retain this nature on the surface (It does not proceed to full conversion and converges sluggishly to around 37% yield after significant temporal passage). A partially accessible microenvironment, where the motion of reactive groups is slightly restricted, was found to provide a high rate with the highest surface yield. In contrast, a freely accessible reactive moiety afforded a lower surface yield albeit with the highest overall rate. Finally, a buried microenvironment led to the highest overall rate albeit with a lower surface yield. As a corollary, for the surface–bound SPAAC reaction we can compare the partially accessible microenvironment to a marathon runner who is able to run further but at a pace slower than a sprinter (free microenvironment). This provides the surface chemist with a handle for tuning the monolayer as per her/his reaction goals.
Harnessing the valuable insights gained from the SPAAC reaction, our concept of ionizable MS tag coupled with DART–HRMS was further extended to a more novel and yet unstudied interfacial reaction in Chapter 3. The Strain–Promoted Oxidation–Controlled cycloalkyne–1,2–Quinone (SPOCQ) cycloaddition was applied for the first time on a surface and afforded a quantitative yield for a free microenvironment in under 4 h. It is to be noted here, that for the first time a 100% (quantitative) metal–free click reaction was observed at a surface. This proved that our approach of engineering the microenvironment around the reactive site provides a distinct edge needed to attain quantitative yields. Quinones are hard to synthesize/store/use in solution given their high propensity to polymerize. However, we demonstrated that on the surface, quinones can be easily generated and stored over–extended period of time by a facile periodate oxidation. Auto–polymerization of surface–bound quinones is precluded by their tether and enforced distal separation by surrounding inert alkyl chains (3:1 ratio). The wider application of this interesting mixture has been further rigorously demonstrated in later chapters too. The bioorthogonality of the SPOCQ reaction coupled with its higher speed and its quantitative yields on the surface are definitely its most salient features.
After studying strain–promoted click reactions on the surface (culminating for SPOCQ in quantitative conversion within 4 h), the question arose if DART–HRMS could also be used to reproducibly and precisely determine a different class of cycloadditions, for which we selected the interfacial inverse electron demand Diels–Alder (IEDDA) reaction as this reaction was reported to be really fast –at least for click reactions– in solution. This was studied in Chapter 4 extensively and we surpassed our previous kinetic record (SPOCQ) by obtaining a quantitative yield in a mere 15 min. The other interesting observation of this study was that reversing the reaction counterparts on the surface produced a discernible reaction rate difference. We found that one of the reactants when tethered in a particular stereochemistry (exo– form) gave the highest surface coverage (100%) within the shortest amount of time. This was also the first time that the effect of diastereomerism on interfacial reaction rates was studied.
In Chapter 5, covalent modification of native non–activated mica has been carried out utilizing catechol linkers. Previous studies for mica modification produced poorly defined polymeric structures on the surface or required extensive and tedious organic synthesis. We have addressed both these issues head–on in this thesis. Well–defined and characterized ultrathin layers were constructed on mica using a catechol–based molecule involving a two–step synthesis. Mica being atomically flat provides an ideal surface upon which to study various phenomena by AFM and other forms of microscopy. However, most research until now was restricted to simply drop–casting the pre–fabricated moieties followed by studying their final structures. Our method now allows for the step–wise formation and characterization of these very interesting structures. Along with it, we also performed several click attachment chemistries on these ultrathin layers which can be harnessed by surface chemists to put various functional and structurally complex moieties on the surface. This opens the pathway for the attachment of more complex architectures on the surface with higher functionality along with the ability to study their formation in a step–wise controlled fashion.
Overall, this thesis wishes to understand organic surface chemistry and several of its intricate mysteries. It clearly outlines several modification techniques and unravels interfacial kinetics of several interesting “metal–free click reactions”. It strives to rationalize the activation parameters in conjunction with classical organic chemistry and gives details on how surrounding “inert” alkyl chains can play a profound role in reaction rates. Lastly, we have striven to and achieved rapid and quantitative reactions on the surface by virtue of optimization of this microenvironment. Personally I believe, we have treaded on a road seldom traveled and unraveled a new understanding about molecular interactions on the ever–interesting and an infinitely–complex surface.
Chemie - Van suiker naar mobiel : Kennisclip Bogo-project e-learning
Baltissen, A.H.M.C. - \ 2016
biopolymeren - suiker - biobased economy - biochemie - chemische reacties - polymelkzuur - melkzuur - bioplastics - tuinbouw - lesmaterialen - biopolymers - sugar - biobased economy - biochemistry - chemical reactions - polylactic acid - lactic acid - bioplastics - horticulture - teaching materials
Deze kennisclip maakt onderdeel uit van de lesmodule Biobased Economy van het CIV T&U.
New insight into enzymatic cross-linking of globular proteins: from nanostructure to functionality
Sariçay, Y. - \ 2014
Wageningen University. Promotor(en): Martien Cohen Stuart, co-promotor(en): Renko de Vries; Peter Wierenga. - Wageningen : Wageningen University - ISBN 9789462571211 - 217
enzymatische cross-linking - eiwitten - chemische reacties - alfa-lactalbumine - lactalbumine - nanotechnologie - enzymatic cross-linking - proteins - chemical reactions - alpha-lactalbumin - lactalbumin - nanotechnology
In last two decades, enzymatic cross-linking of proteins has a growing interest in food technology for better tailoring protein functionality. However, the relation between physical and functional properties of enzymatically cross-linked proteins has been hardly addressed so far. The aim of this thesis was to elucidate the effect of enzymatic protein cross- linking on the physical and functional properties of protein nanoparticles at multiple lengthscale. In the first part of this thesis, as a model system, the enzymatic cross-linking of globular whey protein apo-α-lactalbumin (α-LA) by horseradish peroxidase (HRP) was discussed in details. In comparison with HRP, in the second part of the thesis, we also addressed to what extent both laccase (LC) (from trametes versicolor) and tyrosinase (TYR) (from agaricus bisporus) differ in catalyzing oxidative cross-linking of α-LA. Both HRP and LC were capable of creating self-similar large α-LA nanoparticles that have an open architecture at similar lengthscales whereas TYR led to the formation of α-LA oligomers only. All HRP-, LC-nanoparticles and TYR-oligomers exhibited a high extent of secondary structure content preserved whereas their almost all tertiary structure was lost upon enzymatic cross-linking. HRP-catalyzed cross-linking of α-LA resulted in more hydrophilic nanoparticles than LC-cross-linked α-LA nanoparticles. Whereas both HRP- and LC-nanoparticles exhibited very high colloidal and thermal stability against protein aggregation at pH 5.8 and 7.0, HRP- nanoparticles were more stable than LC-nanoparticles upon heating and in the presence of dithiothreitol (DTT). This suggests that, unlike HRP- nanoparticles, not only the dityrosine bonds but also disulfide cross-linking stabilizes LC-nanoparticles. Dilute dispersions of HRP-nanoparticles exhibited a high viscosity and a hydrophilic nature. As these dispersions were concentrated, they jammed above their critical overlapping concentration and thus created physical transparent protein hydrogels at relatively low protein concentration (4% w/v). These properties of HRP- nanoparticle dispersions offer high thickening properties that are comparable with polysaccharide in food applications as protein-based thickeners.
Multi-response modeling of acrylamide formation in biscuits
Capuano, E. ; Fels, H.J. van der; Atac-Mogol, B. ; Kocadagli, T. ; Göncüoglu, N. ; Hamzalioglu, B.A. ; Gokmen, V. - \ 2013
biscuits - acrylamiden - bakken (in de oven) - kinetica - modellen - chemische reacties - biscuits - acrylamides - baking - kinetics - models - chemical reactions
The aim of this study was to model acrylamide formation during baking of biscuits, using multi-response kinetic modelling.
Reaction and separation opportunities with microfluidic devices
Kolfschoten, R.C. - \ 2011
Wageningen University. Promotor(en): Remko Boom, co-promotor(en): Anja Janssen. - [S.l.] : S.n. - ISBN 9789085858645 - 153
microfluidics - chemische reacties - scheiding - kunstmatige membranen - enzymen - diffusie - microfluidics - chemical reactions - separation - artificial membranes - enzymes - diffusion
Microfluidic devices make precisely controlled processing of substances possible on a microliter level. The advantage is that, due to the small sizes, the driving forces for mass and heat transfer are high. The surface to volume ratios are also high, which can benefit many surface oriented processes. In addition, because of their small volumes, microfluidic devices reduce reagent consumption and risk of failure compared to larger counterparts. Furthermore, the parallelization of such devices can increase productivity while maintaining their characteristics. Overall, these advantageous properties give many opportunities for reaction and separation processes.
Although researchers have intensively studied microfluidics for analytical and sensory applications, microfluidics for preparative processes is still in its infancy. This thesis research involved exploring these processes for biocatalysis and bio-separations with microfluidic devices. The purpose of this thesis was to yield a better understanding of microfluidics for the preparative processes and larger-scale production. We therefore addressed subjects including microfluidic parallelization, membrane separation, biocatalysis, and design. The presented research is useful for further developing innovative process intensification by means of microfluidic devices.
Parallelization of microfluidic devices can facilitate the generation of more data or product in less time. In Chapter 2, we present a proof of concept of such a parallelization for obtaining information on reaction and separation kinetics. We assembled different microfluidic contactors into a single device in order to perform distinct experiments simultaneously. The concept of the parallelization was based on the decoupling of pressure drop from residence time. We demonstrated this by microfluidic membrane separations and determination of membrane properties. The reported device enabled a three times higher throughput compared to devices with a single separation region.
Processes such as chromatographic separation and nanofiltration can remove low molecular weight sugars from liquid mixtures of oligosaccharides. In Chapter 3, we present a novel separation process based on the concept of mass diffusion. Differences between diffusivities of the components drive such a separation, while membranes, in particular nanofiltration membranes, can enhance the separation. We demonstrate this by the use of a membrane microfluidic device for the separation of small molecular weight components. Our results show that mass diffusion separation in liquids is a feasible concept. With optimized microchannel and membrane dimensions, the presented separation process might compete with currently available separation technologies.
For diffusion-based processes, such as mass diffusion separation shown in Chapter 3, small diffusion distances – and thus thinner membranes – can reduce diffusion times significantly. In Chapter 4, we used a microfluidic contactor to contact liquid streams via such extremely thin membranes. We show that the presented concept can be useful for diffusion-based pre-concentration or downstream processes such as fractionation and enrichment. Our results indicate that also this method can yield a feasible process. Moreover, the technology is generally applicable to any diffusing component – regardless of its absolute diffusivity or concentration.
Fast mass transfer and low reagent consumption have made enzyme microreactors popular research tools. In Chapter 5, we used such a microreactor to study the effect of diffusion on enzyme activity. We found that the Michaelis-Menten kinetic parameters were similar at the microscale and bench scale. Our results show that with residence times below a few seconds, diffusion effects limited the reaction rate and therefore reduced the conversion per volume of enzyme microreactor. The critical residence time where this limitation occurred increased quadratically with channel width, increased with enzyme concentration, and decreased with substrate concentration. We concluded that in order to use an enzyme microreactor efficiently, such effects should be taken into account.
Many parameters such as the enzyme properties, operating conditions, and dimensions of the microreactor determine to what extent mass transfer restrictions affect the reaction rate and the productivity. The use of microchannels can indeed shorten the characteristic mass transfer time, as shown in Chapter 5, but may also affect the productivity of the microreactor. Chapter 6 provides the correlations between these parameters for coflow enzyme microreactors obeying Michaelis-Menten kinetics. These correlations outline the design space based on reduced mass transfer restrictions and maximum productivity respectively. The methodology that yields the design space provides a generic hands-on approach to optimally design coflow enzyme microreactors.
Microfluidics involves the exploitation of the phenomena that manifest themselves on microscale. This thesis shows that microscale applications can indeed offer unprecedented benefits. The discussion in Chapter 7 summarizes and reflects on the previous parts of this thesis. We conclude that it is important to explore and exploit other characteristics of continuous production in microfluidic devices beyond mass transfer effects in order to develop novel processes. In addition, we stress the importance of adoption of microfluidics, and show which determinants are involved in this. Knowledge of these determinants is of utmost importance to reduce skepticism towards and stimulate the adoption of microfluidics by industry.
New approach to analyse spin probe and spin trap ESR
Makarova, K. - \ 2011
Wageningen University. Promotor(en): Herbert van Amerongen, co-promotor(en): Henk van As. - [S.l. : S.n. - ISBN 9789085858409 - 145
paramagnetische elektronenresonantiespectroscopie - chemische reacties - vrije radicalen - biofysica - electron paramagnetic resonance spectroscopy - chemical reactions - free radicals - biophysics
The goal of this thesis is to develop new comprehensive methods for the analysis of ESR spectra and interpretation of magnetic parameters. A new approach for the analysis of fast isotropic spectra is proposed. It is based on a combination of an experimental approach (multifrequency ESR) and accurate spectra simulation using an improved model, that will be further introduced below. The determined magnetic parameters of the spin probe are directly interpreted in terms of structural information about the spin probe surroundings (lipid bilayer). The obtained magnetic parameters of various spin traps are interpreted by artificial neural networks (ANN) in order to obtain information about the identities of trapped radicals. Then, Density Functional Theory (DFT) calculations are applied to study the mechanism of reactions involving free radicals detected by spin trapping ESR and to calculate magnetic parameters of the radical adducts.
Kinetic modeling of reactions in Foods
Boekel, M.A.J.S. van - \ 2008
Boca Raton : CRC Press - ISBN 9781574446142 - 400
wiskundige modellen - kinetica - chemische reacties - voedselkwaliteit - voedingsmiddelen - houdbaarheid (kwaliteit) - opslagkwaliteit - fysische eigenschappen - fysicochemische eigenschappen - modelleren - mathematical models - kinetics - chemical reactions - food quality - foods - keeping quality - storage quality - physical properties - physicochemical properties - modeling
The level of quality that food maintains as it travels down the production-to-consumption path is largely determined by the chemical, biochemical, physical, and microbiological changes that take place during its processing and storage. Kinetic Modeling of Reactions in Foods demonstrates how to effectively capture these changes in an integrative fashion using mathematical models. Thus, kinetic modeling of food changes creates the possibility to control and predict food quality from a technological point of view.
High-Rate Sulfate Reduction at High Salinity (up to 90 mS.cm-1) in Mesophilic UASB Reactors
Vallero, M.V.G. ; Sipma, J. ; Lettinga, G. ; Lens, P.N.L. - \ 2004
Biotechnology and Bioengineering 86 (2004)2. - ISSN 0006-3592 - p. 226 - 235.
sulfaat - reductie - chemische reacties - zoutgehalte - geactiveerd slib - biodegradatie - acetaten - propionaten - ethanol - waterzuivering - sulfate - reduction - chemical reactions - salinity - activated sludge - biodegradation - acetates - propionates - ethanol - water treatment - desulfitobacterium-frappieri pcp-1 - anaerobic granular sludge - long-term competition - waste-water - methanogenic bacteria - biological treatment - reducing reactors - bed reactor - wastewaters - ammonia
Sulfate reduction in salt-rich wastewaters using unadapted granular sludge was investigated in 0.9 L UASB reactors (pH 7.0 ± 0.2; hydraulic retention time from 8-14 h) fed with acetate, propionate, or ethanol at organic loading rates up to 10 gCOD.L-1.day-1 and in excess sulfate (COD/SO of 0.5). High-rate sulfate reduction rates (up to 3.7 gSO42-.L-1.day-1) were achieved at salinities exceeding 50 gNaCl.L-1 and 1 gMgCl2.L-1. Sulfate reduction proceeded at a salinity of up to 70 gNaCl.L-1 and 1 gMgCl2.L-1 (corresponding to a conductivity of about 85-90 mS.cm-1), although at lower rates compared to a conductivity of 60-70 mS.cm-1. Ethanol as well as propionate were suitable substrates for sulfate reduction, with acetate and sulfide as the end products. The successful high-rate treatment was due to the proliferation of a halotolerant incomplete oxidizing SRB population present in the unadapted inoculum sludge. Bioaugmentation of this sludge with the acetate oxidizing halotolerant SRB Desulfobacter halotolerans was unsuccessful, as the strain washed out from the UASB reactor without colonizing the UASB granules. © 2004 Wiley Periodicals, Inc.
|Vetoxidatie en het voorspellen van houdbaarheid
Dekker, M. - \ 2002
Voedingsmiddelentechnologie 35 (2002)1-2. - ISSN 0042-7934 - p. 37 - 39.
vetten - lipiden - geoxideerde vetten - oxidatie - chemische reacties - houdbaarheid (kwaliteit) - kwaliteit - bewaartijd - opslagkwaliteit - voedingsmiddelen - conferenties - fats - lipids - oxidized fats - oxidation - chemical reactions - keeping quality - quality - storage life - storage quality - foods - conferences
Verslag van een symposium als afsluiting van een onderzoeksproject
Wheat bran glucuronoarabinoxylans : biochemical and physical aspects
Schooneveld - Bergmans, M.E.F. - \ 1997
Agricultural University. Promotor(en): A.G.J. Voragen; G. Beldman. - S.l. : Schooneveld-Bergmans - ISBN 9789054857167 - 125
graansoorten - maling - Triticum aestivum - tarwe - hexaploïdie - voedsel - voedingsmiddelen - koolhydraten - zetmeel - vezel - polysacchariden - structuur - chemische reacties - cereals - milling - Triticum aestivum - wheat - hexaploidy - food - foods - carbohydrates - starch - fibre - polysaccharides - structure - chemical reactions
Arabinoxylans are present in cereal cell walls and in vitro they have interesting physicochemical properties, such as viscosity and gelation. Although many studies on these properties were reported for wheat flour arabinoxylan, not much research has been directed towards exploitation of these polysaccharides as food gum. For that purpose glucuronoarabinoxylans of wheat bran, a cheap by-product of the cereal industry, were studied with regard to their extractability, their structural and physicochemical properties.
Approximately 50% of the glucuronoarabinoxylans of wheat bran cell wall material were recovered in high purity by barium hydroxide extraction at 70 to 95°C. Delignification or other treatments to open up the cell wall structure were not effective in increasing the yield. The extracted glucuronoarabinoxylans were very diverse in chemical structure and physicochemical properties. About 30% of them had a low degree of substitution, were easily degradable by xylanolytic enzymes and hardly influenced the viscosity of the solvent as a result of extensive aggregation. Over 50% of them had a high degree of substitution, were supposed to contain dimeric branches of arabinose and xylose, were scarcely degradable by xylanolytic enzymes, gave moderate viscosity to solutions and were very effective in stabilizing emulsions. The structure of these glucuronoarabinoxylans could only be speculated upon and it could not be enzymatically modified as a consequence of its complexity and the lack of appropriate enzymes. The remaining glucuronoarabinoxylans either had an intermediate or very high degree of substitution, of which the latter was presumed to be connected to lignin-fragments.
Gel-forming glucuronoarabinoxylans were recovered only in low yield by dilute alkali extraction and subsequent purification was necessary. These feruloylated glucuronoarabinoxylans gelled upon addition of oxidative agents, of which peroxide - peroxidase, glucose - glucoseoxidase - peroxidase and ammonium persulphate were investigated. In comparison with wheat flour arabinoxylans, those of wheat bran appeared to give less flexible networks at high concentration, which was ascribed to their high degree of substitution and high ferulic acid content. Of the dimers formed upon cross-linking, the generally known diferulic acid, being a 5-5 coupled dimer, was only present in relatively low amounts. Dimers, in which the 8-position of the ferulic acid residue is involved were preponderant. The distribution of the dimers was not affected by the type of cross- linking agent or the type of arabinoxylan. However, the presence of lignin fragments in the bran extract was presumed to cause a low ferulic acid recovery upon cross-linking.
Growth and death of animal cells in bioreactors
Martens, D.E. - \ 1996
Agricultural University. Promotor(en): J. Tramper; C.D. de Gooijer. - S.l. : Martens - ISBN 9789054855934 - 222
weefselkweek - celkweek - meristemen - zoölogie - chemische reacties - uitrusting - biotechnologie - chemische industrie - biochemie - tissue culture - cell culture - meristems - zoology - chemical reactions - equipment - biotechnology - chemical industry - biochemistry - cum laude
Animal-cell cultivation is becoming increasingly important especially for the area of hunian- health products. The products range from vaccines to therapeutic proteins and the cells themselves. The therapeutic application of proteins puts high demands upon their quality with respect to purity and structure. For example, a correct folding and glycosylation is of importance for the activity, the in vivo clearance rate and the possible immunogenicity of the protein, and can often only be obtained by production in animal cells. An important class of proteins produced by animal cells is formed by monoclonal antibodies. Monoclonal antibodies are produced by hybridoma cells and have the capacity to bind very specifically to a particular molecular structure (epitope), a quality that makes them suitable for application in in vivo and in vitro diagnostics, in separation technology and for the in vivo targeting of drugs.
The occurrence of substantial cell death and the presence of cell debris is a major problem in animal-cell cultivation. It interferes with the attainment of high volumetric productivities and with a proper functioning of the process. In addition, it may affect the quality of the product and cause problems in down-stream processing. Cell death may follow two different pathways, being apoptosis and necrosis, which have very distinct physiological and morphological features. Necrosis is a passive process generally caused by sudden high levels of environmental stress, whereas apoptosis is an active, genetically controlled process induced by mild stress conditions or specific signals from the environment.
After the introduction in Chapter 1, the application of a general framework for the construction of segregated models is discussed in Chapter 2 with respect to the behaviour of animal-cell populations. For the construction of segregated models, the physiological state of an animal cell must be specified, which is discussed in this chapter with special attention for the experimental verification of the models. Finally, a number of age-structured, segregated models, which are of importance for animal-cell cultivation are reviewed in this chapter.
The required amounts of animal-cell products are expected to be in the order of kilograms or even tonnes on a yearly basis. In order to produce these amounts, scale-up is necessary, which is most easily done in conventional reactor systems like the stirred-tank, bubble-column, and air-lift reactor. A main problem in the scale-up of these reactors is the supply of sufficient oxygen to the culture, which often requires sparging. Hydrodynamic forces associated with sparging cause cell death. In Chapters 3, 4, and 5 the specific death rate of hybridoma cells in bubble-column and air-lift reactors is studied with the hypothetical-killing-volume theory as a central theme. The hypothetical killing volume is a hypothetical volume associated with an air bubble during its lifetime in the reactor in which all cells are killed. The first-order death-rate constant in bubble- column and air-lift reactors can then be derived to be the product of this hypothetical killing volume and the number of bubbles introduced into the reactor per unit time and per unit reactor volume. The specific death rate of the hybridoma cells in the bubble-column and air-lift reactors is shown to be proportional to the gas flow rate and the reciprocal reactor height. Furthermore, in bubble columns the specific death rate is shown to be proportional to the square of the reciprocal reactor diameter. These results are in accordance with the hypothetical-killing-volume theory. The main cause of cell death is found to be bubble breakup at the surface, although detrimental effects at the sparger cannot be excluded. In Chapter 6 the specific death rate of Vero cells immobilized on microcarriers is shown to be proportional to the gas flow rate. Since the height of the reactor is not varied, it cannot be excluded that in this case also the rising of the bubbles or the associated liquid flow cause cell damage.
A common method to reduce the detrimental effects of air bubbles is the use of protective additives. In this thesis it is shown that the addition of two such protectants, Pluronic F68 (Chapter 3) and serum (Chapter 4), respectively, reduces the amount of cell death as a consequence of sparging. Furthermore, as demonstrated in Chapter 4, the protective effect of serum has a fast-acting, physical, and a slow-acting, physiological component. In Chapter 5 the effect of the specific growth rate on the specific death rate of cells due to sparging is studied in air-lift loop reactors. Cells with varying specific growth rates are obtained from steady-state continuous cultures run at different dilution rates. Remarkably, the specific death rate of the cells due to sparging decreased as their specific growth rate decreased. Furthermore, in Chapter 6 it is shown that the specific death rate of Vero cells is reduced by immobilisation of the cells inside porous carriers.
Below a critical dilution rate in continuous culture as well as towards the end of batch cultures, the specific death rate of hybridoma cells increases rapidly. In this case, the cells mainly die through apoptosis as a consequence of substrate depletion and the accumulation of toxic products. In Chapter 7 an age-structured model is developed to describe the rate of apoptosis as a function of the dilution rate in continuous culture. In this model a critical specific growth rate is introduced below which the cells start becoming apoptotic. In addition to the specific deathand growth rate, the average cell volume of the viable cells and the specific consumption and production rates for glucose, glutamine, lactate and ammonia are calculated. The model can reasonably well describe a set of literature data, with respect to the specific growth- and death rate and the concentrations of viable cells, dead cells, glucose, glutamine, lactate, and ammonia. In Chapter 8 the model is extended with equations concerning two hypotheses for the production of monoclonal antibody being:
Cell death associated with sparging may be minimised by:
Glucuronoarabinoxylans from sorghum grain
Verbruggen, M.A. - \ 1996
Agricultural University. Promotor(en): A.G.J. Voragen; G. Beldman. - S.l. : Verbruggen - ISBN 9789054855026 - 131
sorghum bicolor - polysacchariden - structuur - chemische reacties - sorghum - sorghum bicolor - polysaccharides - structure - chemical reactions - sorghum
Water-unextractable cell wall materials (WUS) were prepared from raw, polished, and malted sorghum ( Sorghum vulgare cv. Fara Fara). Except for the amounts, hardly any difference could be observed between the WUS of these three raw materials. This means that cell wall materials of the endosperm cell walls are basically the same as those of the outer endosperm and pericarp layers, and that the cell walls largely persist, during malting. These preparations were further fractionated by a sequential extraction procedure using aqueous solutions of saturated Ba(OH) 2 , 1M KOH and 4M KOH. The WUS preparations were composed of glucuronoarabinoxylans (GAX), (1→3),(1→4)-β-D-glucans, cellulose, and some protein. GAX was primarily extracted by Ba(OH) 2 solutions. All GAX fractions were composed of a highly substituted (1→4)-β-D-xylan backbone, substituted by arabinose and uronic acid. It was concluded that sorghum GAX populations were characterized by a reasonable homogeneity, since they could not be separated further by several chromatographic and precipitation techniques.
Degradation studies using purified xylanases, arabinofuranosidases and a glucuronidase alone or in combination, showed that the GAX populations were hardly broken down. Some oligomers were formed by digesting Ba(OH) 2 extracted GAX with a combination of endoxylanase I and (1→4)-β-D-arabinoxylan arabinofuranohydrolase, both purified from Aspergillus awamori . These oligomers were found to have a main chain of three or four xylose units, and to contain α-glucuronic acid linked to O -2 of the non-reducing terminal xylose unit. Two oligomers were found to have a dimeric (1→2)-linked arabinose side, chain linked at O -3 of an internal xylose unit. Also single arabinose substitution occured at O -3 of an internal xylose unit. There are strong indications that these side groups can also be linked at O -2 of an internal xylose residue. The reducing xylose units were unsubstituted. A model for the GAX populations from sorghum was proposed combining the results of the degradation studies, the identification of the oligomers, and knowledge about the mode of action of the enzymes used.
Finally, the developed techniques to investigate GAX in particular, were used to study the behaviour of GAX in the brewing process. Worts and spent grains of mashes, supplemented with commercial enzyme preparations containing xylanases among others were studied. Except for the amount of solubilized GAX, the GAX hardly changed with respect to the sugar composition and molecular weight distribution. A direct relationship between GAX, xylanases, and filtration behaviour of worts prepared from malted sorghum, could therefore not be established.
Modelling nutrient removal in a sequencing batch reactor with respirometry
Bernardes, R.S. - \ 1996
Agricultural University. Promotor(en): W.H. Rulkens; A. Klapwijk. - S.l. : Bernardes - ISBN 9789054854715 - 173
waterzuivering - verwijdering - chemische reacties - uitrusting - afvalwaterbehandeling - rioolafvalwaterverwijdering - zuiveringsinstallaties - water treatment - removal - chemical reactions - equipment - waste water treatment - sewage effluent disposal - purification plants
The main objectives of the present thesis can be summarized as: i) the development and validation of simplified mathematical models for activated sludge processes in an SBR treating real domestic wastewater; ii) the application of these simplified models for analysing the respirometric response and for obtaining information about the oxygen uptake for the different processes; iii) the application of the monitored respirometric values for model calibration and determination of parameter values, which are used to predict the processes in the next cycle; iv) the use of models as theoretical background for the development of control strategies for plug-flow systems and for SBR; v) relating the basic time scale for the models to the short term.
The starting points for the model development and simplification were: i) the Activated Sludge Model No. 1, for carbon oxidation, nitrification and denitrification; and ii) the Activated Sludge Model No. 2, for biological phosphorus removal .
In this study an SBR pilot plant was used and seen as a model for a plug-flow system. During the two and a half years of operation, the plant underwent three different technological phases. The first phase began with the removal of organics and nitrification. Denitrification was incorporated in the second phase. The last phase included biological phosphorus removal.
In the first phase, two simplified mathematical activated sludge models are presented. The first model gives the response of the respiration rate in an SBR with nitrification, the oxidation of readily biodegradable matter, and endogenous respiration during one cycle. This model is used to predict the respiration rate during a complete SBR cycle. For this, it uses parameter values calibrated during the previous cycle, some default values and information about the ammonia concentration in the influent. The endogenous respiration rate is described with an exponential equation. The second model is used to predict the changes in nitrification capacity after a change in the loading rate and/or sludge wastage rate. For model calibration and validation, an SBR pilot plant receiving domestic wastewater was operated for nine weeks.
In the second phase, a mathematical model is presented for the behaviour of the respiration rate and nitrate removal in an SBR with nitrification, denitrification and carbon oxidation. This model is based on the response of the respiration rate measured during nitrification and carbon oxidation and the nitrate removal rate during the post-denitrification period. For model calibration and validation, an SBR pilot plant receiving domestic wastewater was operated for three months. The respiration rate was used to calibrate several parameters of the model.
In the third phase, a mathematical model for an activated sludge SBR with nitrification, denitrification, carbon oxidation and phosphate removal is presented. This model is based on the response of the respiration rate measured during nitrification, carbon oxidation and phosphate removal, together with the behaviour of phosphate and acetate as proposed in the Activated Sludge Model No. 2. For model calibration and validation, an SBR pilot plant receiving settled domestic wastewater plus acetic acid solution was operated for five months.
In all the three phases the model for the respiration rate ( r ) in an SBR during one cycle, including nitrification, oxidation of readily biodegradable matter, endogenous respiration and a fraction for the respiration rate for phosphorus uptake, gives a good simulation of the measured respiration rate. A good prediction of the total oxygen consumption and distribution during one cycle is found from a simulation, using parameters calculated from the previous cycle together with the variables from the influent. Therefor this model can be used in control strategies as long as it is used for a short time- scale. During long-term operation, parameter variation is significant and too complex to be predicted. In the particular case of nitrification capacity variation in an SBR during long-term operation, the model can explain the variation trend but cannot explain the abrupt changes.
Simplified mathematical models for the activated sludge process on the bases of the respiration rate are validated. On short-time scale, the models give a good response prediction of the activated sludge process feed with wastewater. The models are good tools for control strategies, however periodical parameter calibration is needed.
Transannular cyclisation reactions and the germacrane system mediated by enzymes from Cichorium intybus
Piet, D.P. - \ 1996
Agricultural University. Promotor(en): Æ. de Groot; M.C.R. Franssen. - S.l. : Piet - ISBN 9789054855880 - 155
enzymen - biofysica - cichorium intybus - chemische reacties - cyclische verbindingen - enzymes - biophysics - cichorium intybus - chemical reactions - cyclic compounds
Chicory ( Cichorium intybus L.), one of the many species of the Compositae family, has been cultivated for the production of the leaves since 300 BC as a food supplement and since the 16th century as a substitute for coffee. The sprouts of the chicory are appreciated for their bitter taste. This bitter taste is associated with the presence of sesquiterpene lactones. The majority of these sesquiterpene lactones possess a guaiane framework, a small number possesses a eudesmane- or a germacrane framework. The abundance of these sesquiterpene lactones is not limited to the leaves of the plant. Considerable amounts of are also present in the root, currently an agricultural waste product. Not only is the root a rich source of sesquiterpene lactones, it also contains a large amount of inulin ( 1 ), a storage carbohydrate which is based on fructose instead of glucose. Fructose, an interesting sweetener, is a versatile building block in the synthesis of several polymers and natural products. The bitter principles in the chicory may find their application as a bitter tasting additive in consumer goods.
The biosynthesis of the sesquiterpene lactones in the chicory is believed to start from a head to tail cyclisation of farnesyl pyrophosphate ( 23 ) into a germacrane, followed by cyclisation into eudesmanes and guaianes. This thesis deals with the cyclisation of germacrane synthons and natural germacranes, induced by a root homogenate of fresh chicory. The goal is to determine the substrate specificity of the germacrane cyclase of chicory and to obtain more insight in the biosynthesis of the sesquiterpene bitter principles in C.intybus.
A general introduction on the history, use and contents of the chicory is given in chapter 1. In chapter 2, an overview of the literature on the (bio)synthesis of germacrane sesquiterpenes and their possible biotransformation into a variety of cyclised products, is presented.
In chapter 3, the synthesis of two (E,E)-cyclodeca-1,5-dienols possessing the germacrane framework ( 100 and 101 ), is described. The cyclisation behaviour of these compounds and the natural germacrane (+)-hedycaryol ( 39 ) towards a chicory root homogenate is discussed. The cyclising enzymes in this homogenate transform the 10-membered ring compounds into products with a eudesmane skeleton by protonation of the C l -C 10 double bond followed by transannular cyclisation and subsequent stereoselective incorporation of a water molecule at C 4 . The flexibility of the 10-membered ring system was demonstrated by the formation of the epimeric diols 117-120 from 100 and 101 . The relatively small hydroxyl function at C 7 permitted inversion of the germacrane framework, enabling cyclisation through two different syn- conformations. The large isopropanol group of 39 prohibits this inversion to such an extent that cyclisation takes place only through one conformation to give cryptomeridiol ( 125 ).
In chapter 4, the synthesis of the three (E,E)-cyclodeca-1,6-dienols 131-133 and their cyclisation by a chicory root homogenate is described. Two kinds of hydroazulene alcohols were obtained in these reactions arising from 1,5- and 1,7-cyclisation. The 1,5-cyclisation products ( 134-136 ) are formed through an internal nucleophilic displacement of the allylic alcohol moiety by the Cj-CjO double bond, while in the formation of the 1,7-cyclisation products ( 137-139 ), an allylic isomerisation reaction of the (E,E)-cyclodeca-1,6-dienol skeleton into an allylic (E,E)-cyclodeca-1,5- dienol skeleton preceded the internal nucleophilic displacement reaction. Hydroazulenes possessing a C 6 -C 7 double bond like 134 resemble natural products like alismol ( 43) .
Recently, the structure of a trinor-guaiane, dictamnol ( 140 ), similar to alismol, was published. The ring fusion of dictamnol ( 140 ) was postulated as cis. Since 140 was already synthesised at our laboratory and major discrepancies were found between our NMR spectral data of 140 and those reported in the literature, serious doubt about the stereochemistry and the ring junction arose. Therefore, natural dictamnol was isolated, its stereochemistry was reinvestigated and a structural revision into a transfused hydroazulene ( 152 ) is proposed.
In chapter 5, the biotransformation of derivatives of germacrone, a readily available sesquiterpene germacrane, is described. In a number of cases, enzyme mediated cyclisation of the chemically epoxidised germacrone derivatives, had to compete with spontaneous cyclisation reactions. However, some selectivity was observed, especially in the biotransformation of germacrone-4,5-epoxide ( 48 ) into neoprocurcumenol ( 161 ). Compound 161 is the only product obtained through an enzyme-mediated cyclisation of 48 . The C l -C 10 double bond in 161 is characteristic for guaiane bitter principles in the chicory. In boiled root samples, the only conversion that was observed was a homofragmentation reaction of 48 into curcumenone ( 160 ).
The synthesis of isogermacrone ( 166 ) paved the way for studying the influence of the position and the stereochemistry of the double bond on the ring fusion of the cyclisation products. The 4,5-epoxides of isogermacrone ( 167 ) and isogermacrene B ( 176 ) were transformed by a chicory root homogenate into a cis-fused eudesmane and two tricyclo[22.214.171.124]sesquiterpenes.
Chapter 6 deals with the synthesis of (E,Z)-cyclodeca-1,5-dienone 184 and the biotransformation of 184 and structurally related compounds. Transannular cyclisation reactions of (E,Z)-cyclodeca- 1,5-dienes appear to proceed in a different way as compared to the (E,E)-cyclodeca-1,5-dienes in chapter 3-5. Instead of a carbon-carbon bond formation between both double bonds of the germacrane skeleton, ring substituents are involved in the cyclisation process to relieve ring strain. If no additional ring substituents are present, e.g. in E-epoxide 201 , a cis-fused hydroazulene diol ( 202 ) is obtained. The Z-epoxides 211 and 214 were not or not unambiguously transformed by a chicory root homogenate.
In chapter 7, the biotransformation of farnesyl pyrophosphate ( 23 ) by a partially purified chicory root homogenate is described. Radio-GC and GC-MS analysis of the incubation products obtained from [1- 3H]-farnesyl pyrophosphate revealed that 23 was initially transformed into germacrene A ( 36 ). However, the Cope rearrangement product, (β-elemene, 222) and two cyclisation products of 36 , α- and β-selinene ( 223 and 224 ) were the only products that were detected in the assay, since 36 is sensitive towards acid and elevated temperatures.
In conclusion, the substrate specificity of the germacrane cyclase is discussed and an active site model for the germacrane cyclase in proposed together with two tentative biosyntheses of the sesquiterpene lactones in chicory. The most likely biosynthesis starts with cyclisation of farnesyl pyrophosphate ( 23 ) in germacrene A ( 36 ) followed by several oxidation steps to give intermediate 227 . Enzyme mediated cyclisation of 227 would start with the protonation and subsequent dehydration of the C 3 -hydroxyl group giving allylic cation 228 . This cation then would give 229 after a 1,5-cyclisation, followed by a selective deprotonation towards the bridgehead carbon atom. Further oxidation of 229 would give the guaianolides 12-17 .
Glucosidation of the C 3 -hydroxyl function of 227 gives sonchuside A ( 20 ) and cichorioside C ( 21 ) which may be cyclised by germacrane cyclasing enzymes into the corresponding eudesmanolides, e.g. 18 . Presumably, glucosidation of the C 3 -hydroxyl group prevents the 1,5-cyclisation process towards the guaianolides.
Two-stage baculovirus production in insect-cell bioreactors
Lier, F. van - \ 1995
Agricultural University. Promotor(en): J. Tramper; J.M. Vlak. - S.l. : Van Lier - ISBN 9789054853978 - 184
baculovirus - kernpolyedervirussen - experimenten - weefselkweek - celkweek - chemische reacties - uitrusting - biologische bestrijding - virussen - organismen ingezet bij biologische bestrijding - baculovirus - nuclear polyhedrosis viruses - experiments - tissue culture - cell culture - chemical reactions - equipment - biological control - viruses - biological control agents
Baculoviruses are insect-pathogenic viruses with a narrow host range. The viruses can be an alternative to chemical insecticides. From research aimed at improving the efficacy of the viruses in insect control another application evolved: the use of the baculovirus to express foreign proteins in insect cells. To produce large amounts of baculovirus or baculovirus- expressed proteins, large-scale culture and subsequent infection of insect cells is necessary. Chapter 1 of this thesis reviews the research on factors influencing insect-cell culture and infection and the status of production with insect-cell cultures.
One of the production strategies reviewed in chapter 1 is a continuous two-stage bioreactor configuration. In the first reactor cells are cultured and the effluent is led to second reactor where the cells are infected with the baculovirus. The first results reported with this system showed two disadvantages. The number of cells infected was relatively small and production in the infection reactor was limited to about one month.
To increase the number of infected cells the residence time distribution in the infection reactor was altered (chapter 2). The infection reactor was replaced by two infection reactors in series, each containing half the volume of the original infection reactor. Therefore, the mixing characteristics of the infection part of the production system became more plug-flow like. This resulted in a higher number of cells which produced polyhedra (encapsulated virus). However, the time during which the cells in the infection reactors produced polyhedra was diminished to about two weeks.
To gain more insight in the decrease of production a recombinant virus was used in a study described in chapter 3. In this recombinant virus the polyhedrin gene (coding for the major protein in the virus matrix) was replaced by the lacZ gene of Escherichia coli. Production of β- galactosidase in a continuous two-reactor configuration gave analogous results to production with the wild-type virus. Production was maintained for about four weeks then it rapidly decreased. Upon restriction enzyme analysis it was shown that the decrease of β-galactosidase resulted from the disappearance of the gene from the virus population rather than inactivation of the gene.
Research at the department of Virology of the Agricultural University of Wageningen revealed that the reduction of production resulted from the occurrence of defective mutants of the baculovirus. These defective virus lack the polyhedrin-promoter driven gene and became predominant in the bioreactor due to interference with the replication of non-mutated virus.
On the defective viruses another highly expressed gene, the p10 gene, was found to be still present. A recombinant virus containing the lacZ from E. coli under control of the p10 promoter was used in the study described in chapter 4. The recombinant virus produced both polyhedra and β-galactosidase in a continuous bioreactor system. Again production lasted for about four weeks both for polyhedra and β-galactosidase This indicates that the presence of the p10-promoter driven gene per se is not enough for recombinant protein production.
In chapter 5 a model is presented which describes the kinetics of virus infection. Besides the infectious virus particles and the defective mutants, a third virus type is taken into account. This so-called abortive virus is capable of blocking an entry site of a cell but is not leading to production of new virus particles. By assuming a limited amount of entry sites on an insect cell it was possible to calculate when production of infectious virus decreased. The model indicates that the build-up of defective viruses can be postponed by keeping the number of infectious viruses per cell low. A way of accomplishing this is the use of repeated batch infections.
In chapter 6 experiments with repeated (fed-)batch infections are presented. These experiments confirmed calculations with the model. A production involving a series of batch infections resulted in prolongation of production time to 60 days.
The application of the baculovirus expression vector in the light of the findings described in the thesis is discussed in chapter 7.
Design, characterization and application of the multiple air-lift loop bioreactor
Bakker, W.A.M. - \ 1995
Agricultural University. Promotor(en): J. Tramper; C.D. de Gooijer; H.H. Beeftink. - S.l. : Bakker - ISBN 9789054854791 - 172
chemische reacties - uitrusting - chemische eigenschappen - automatische regeling - instrumentatie - systemen - zuurstof - monoclonale antilichamen - hybridoma's - chemical reactions - equipment - chemical properties - automatic control - instrumentation - systems - oxygen - monoclonal antibodies - hybridomas
A new bioreactor is introduced: the Multiple Air-lift Loop reactor (MAL). The MAL consists of a series of air-lift loop reactors within one vessel. With the MAL, a new type of geometry for air-lift reactors with an internal loop is introduced. This new geometry was characterized with respect to hydrodynamics, mixing and oxygen transfer. The hydrodynamics were described by an existing model. Hydrodynamics, mixing and oxygen transfer in the new reactor configuration were comparable to that in conventional air-lifts with an internal loop.
The design and use of the MAL as a reactor cascade, to approximate plug-flow behaviour, were studied. Biological model systems were used to compare the reactor series to a single vessel. These model systems included immobilized invertase and nitrifying bacteria. With the immobilized invertase it was shown that a threecompartment MAL gives an improved substrate conversion when compared to a single vessel of the same overall volume. This could be described with a previously developed model. Also for the immobilized nitrifying bacteria improved substrate conversion was shown in the comparison between a series and a single vessel. Free suspended hybridomas were used for monoclonal antibody (MAb) production. It was shown that reactor series can be useful research tools for kinetic studies. In the second vessel in the series conditions were obtained that can hardly be reached in a single vessel. Not only growth, but also death could be studied under stable conditions. A model was derived that describes hybridoma growth and their MAb production.
Vessels in a series can be of equal volume, but very often unequal volumes can be more advantageous. Therefore, choosing the appropriate reactor volumes is an important design step, which is discussed for different applications. Finally, a general procedure for choosing the optimal bioreactor cascade configuration for any application is given.
Chemical consequences of long-range orbital interactions in Perhydronaphtalene-1,4-diol monosulfonate esters
Orru, R.V.A. - \ 1994
Agricultural University. Promotor(en): Æ. de Groot; J.B.P.A. Wijnberg. - S.l. : Orru - ISBN 9789054852995 - 145
diterpenoïden - sesquiterpenoïden - terpenen - etherische oliën - sesquiterpenen - chemische structuur - chemische reacties - sulfonaten (esters) - diterpenoids - sesquiterpenoids - terpenoids - essential oils - sesquiterpenes - chemical structure - chemical reactions - sulfonates (esters)
In this thesis the base-induced reactions of perhydronaphthalene-1,4-diol monosulfonate esters are described. These compounds undergo smoothly, typical carbocationic processes upon treatment with sodium tert -amylate in refluxing benzene. The product outcome, product ratio, and (relative) rate of these reactions is satisfactorily explained when through-bond orbital interactions (TBI) over four abonds are invoked. In order to gather more detailed information about the basic stereochemical and stereoelectronic principles underlying these processes, synthetical organic, computational, and kinetic investigations were undertaken.
Most experimental studies on TBI have focussed on its spectroscopic manifestations and are reviewed repeatedly. On the other hand, there are relatively few reviews on the chemical consequences of TBI over three or more σ-bonds. A number of illustrative examples of chemical reactions in which long-range orbital interactions are believed to play an essential role are discussed in chapter 1. In the same chapter also attention is drawn to the synthetic utility of some of these reactions.
In chapter 2 the syntheses of the mesylates 39 , 40 , and 58-67 are described. The compounds 39 , 40 , 58 , and 59 are prepared in order to investigate how the orientation of the sulfonate ester group in combination with the orientation of the tertiary hydroxyl group determines the outcome and rate of their reactions with sodium tert -amylate. The results of these investigations are described in chapter 3. It was found that an equatorial sulfonate ester group favors homofragmentation leading to the cyclopropane derivative 105 . In case of an axial sulfonate ester group β-elimination, which strongly depends on the stereochemistry of the tertiary deprotonated hydroxyl group, is the main reaction path. In the chapter 3 the synthesis of the O-silylated mesylates 106 and 107 is also described. These compounds show no reaction at all upon treatment with strong base. On the other hand, fast reactions are observed when 106 and 107 are treated with TBAF. Generation of an alcoholate is crucial for the observed reactions. Homofragmentation and an internal return reaction with inversion of configuration of the mesylate group in the axial mesylates 39 and 58 is explained by assuming a 1,3-bridged intermediate carbocation.
The mesylates 60-65 are prepared (Chapter 2) to determine the influence of the geometry of the relaying σ-bonds on the reactions with sodium tert -amylate. In chapter 4 the results of these studies are described in detail. An alcoholate function intramolecularly induces heterolysis of the sulfonate ester group in an apolar solvent via orbital interactions through three intervening C-C single bonds. It is shown that the reactivity of the compounds 60-65 is only affected by the relative position of the hydroxyl function to the sulfonate ester group and not by the orientation of the hydroxyl group. The two chief pathways by which these compounds react are rearrangement ( 60 , 62 , and 63 ) and homofragmentation ( 64 and 65 ). Stereoelectronic effects play a dominant role here, except in compound 61 where steric factors primarily determine the reactivity and product outcome (ether formation). Homofragmentation is much faster than rearrangement and is only possible when a 1,3-bridged through-space interaction accompanies TBI. The extent of TBI as well as the product composition is strongly determined by the σ-relay of the four σ-bonds between the electron donor (alcoholate) and the electron acceptor (sulfonate ester bond). These results are consistent with the " trans rule", which is in line with predictions from theoretical models regarding TBI.
The product outcome, product ratio, and relative rate of the base-induced reactions of perhydronaphthalene-1,4-diol monosulfonate esters described in chapter 4 are satisfactorily explained with the concept of TBI. However, the conclusions are all based on empirical results. In chapter 5 the results of semi-empirical calculations, using the MNDO method, performed on model systems are presented. In this way a more detailed understanding of the stereoelectronic features underlying the homofragmentation and rearrangement reaction is obtained. The trends in the results of the MNDO simulations are the same as those found in the reactions of the compounds 60-65 . Whether rearrangement or homofragmentation takes place depends on the geometry of the σ-relay and the inductivity of the system. Cyclopropanoid bridged structures seem to be involved in the rearrangement process as well as in the homofragmentation process.
In order to explore the effects of the order of substitution of the carbon atom that borders the carbon atom to which the mesylate group is attached the compounds 40 , 66 , and 67 were synthesized. This subject is discussed in chapter 6. The product formation is strongly dependent on the steric consequences of alkyl substituents at βcarbon atoms. Homofragmentation is highly favored when the repulsive steric interactions do not prevent a homohyperconjugatively stabilized transition state. This is only possible in an ideal "W" arrangement of theσ-relay ( 40 ). Due to the repulsive 1,3- peri -effect in 66 , and a combination of the 1,3- peri -effect and the 1,3- diaxial -effect in 67 the σ-relay diverges from the ideal "all trans " geometry as a result of which other reaction pathways (elimination, 1,3-H, and 1,2-Me shifts) are favored over homofragmentation. Introduction of inductively electron-donating substituents leads to an increase in reaction rate, despite the (slight) deviation of the "W" arrangement. It is concluded that although bridged ions are important intermediates in the observed reaction paths, they are not decisive for the reactivity of these compounds.
The O-silylated mesylates 106 and 107 react fast upon treatment with TBAF in refluxing benzene (Chapter 3). At room temperature only desilylation takes place. To investigate the influence of a remote nucleofugal mesylate group on the rate of desilylation, apart from 106 and 107 , also the O-silylated compounds 127-131 are synthesized and treated with TBAF as is described in chapter 7. The rates of desilylation are determined conveniently by HPLC monitoring of the disappearance of the starting material. The desilylation rate of compounds with a mesylate group is much higher than the desilylation rate of corresponding compounds with a hydroxyl group instead ( 130 and 131 ). Furthermore, compounds having a "W" arrangement ( 107 and 129 ) of the relaying σ-bonds react considerably faster than their "sickle relay" analogs ( 106 , 127 , and 128 ). The results presented in this chapter show nicely that longrange electronic effects of distant substituents can exert a substantial influence on the reactivity of certain functional groups in general.
In conclusion, the concept of TBI offers a good explanation for the reactivity of the compounds studied throughout this thesis. The stereochemical and stereoelectronic requirements for the base-induced reactions of perhydronaphthalene-1,4-diol monosulfonate esters are now well established. The general utilitly of the concept of σ-delocalization and TBI in everyday chemistry is demonstrated
Gasvormige stikstofverliezen bij de teelt van sla op een zandbed onder glas.
Postma, R. ; Oenema, O. ; Bussink, D.W. ; Heinen, M. - \ 1994
Meststoffen : Dutch/English annual on fertilizers and fertilization (1994). - ISSN 0169-2267 - p. 28 - 34.
ammoniak - watervrije ammoniak - lactuca sativa - slasoorten - hydrocultuur - chemische reacties - mineralen - boekhouding - stikstofdioxide - glastuinbouw - ammonia - anhydrous ammonia - lactuca sativa - lettuces - hydroponics - chemical reactions - minerals - accounting - nitrogen dioxide - greenhouse horticulture
Uit mineralenbalansen van teelten op kunstmatige substraten in de glastuinbouw is gebleken dat de aanvoer van stikstof (N) via recirculerende voedingsoplossingen wat hoger is dan de afvoer van N via oogstprodukten en gewasresten. In deze studie zijn de ammoniak-vervluchtiging, denitrificatie en lachgas-emissie bij de teelt van sla gekwantificeerd
|Oligosacchariden als bifidogene factoren.
Hartemink, R. ; Nout, M.J.R. ; Rombouts, F.M. - \ 1994
Voedingsmiddelentechnologie 27 (1994)20. - ISSN 0042-7934 - p. 27 - 29.
bifidobacterium - koolhydraten - chemische reacties - microbiële afbraak - polysacchariden - structuur - carbohydrates - chemical reactions - microbial degradation - polysaccharides - structure
Levende bacterien die een gunstige invloed hebben op de darmflora. Ze worden probiotica genoemd en ze worden toegepast in zowel de humane voeding als de veevoeding
|Tailor-made produktie van oligosacchariden.
Laere, K.M.J. van; Schols, H.A. ; Voragen, A.G.J. - \ 1994
Voedingsmiddelentechnologie 27 (1994)20. - ISSN 0042-7934 - p. 33 - 35.
koolhydraten - chemische reacties - chemische structuur - chemicaliën - voedselindustrie - voedseltechnologie - polysacchariden - eigenschappen - structuur - invloeden - carbohydrates - chemical reactions - chemical structure - chemicals - food industry - food technology - polysaccharides - properties - structure - influences
De gewenste oligosacchariden kunnen met behulp van specifieke enzymen en chromatografische technieken 'op maat' worden gemaakt