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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    Binding of CML-Modified as Well as Heat-Glycated β-lactoglobulin to Receptors for AGEs Is Determined by Charge and Hydrophobicity
    Zenker, Hannah E. ; Teodorowicz, Malgorzata ; Ewaz, Arifa ; Neerven, R.J.J. van; Savelkoul, Huub F.J. ; Jong, Nicolette W. De; Wichers, Harry J. ; Hettinga, Kasper A. - \ 2020
    International Journal of Molecular Sciences 21 (2020)12. - ISSN 1661-6596
    CD36 - charge - Galectin-3 - glycation - milk protein - Nε-carboxymethyl lysine - sRAGE

    Intake of dietary advanced glycation end products (AGEs) is associated with inflammation-related health problems. Nε-carboxymethyl lysine (CML) is one of the best characterised AGEs in processed food. AGEs have been described as ligands for receptors present on antigen presenting cells. However, changes in protein secondary and tertiary structure also induce binding to AGE receptors. We aimed to discriminate the role of different protein modifications in binding to AGE receptors. Therefore, β-lactoglobulin was chemically modified with glyoxylic acid to produce CML and compared to β-lactoglobulin glycated with lactose. Secondary structure was monitored with circular dichroism, while hydrophobicity and formation of β-sheet structures was measured with ANS-assay and ThT-assay, respectively. Aggregation was monitored using native-PAGE. Binding to sRAGE, CD36, and galectin-3 was measured using inhibition ELISA. Even though no changes in secondary structure were observed in all tested samples, binding to AGE receptors increased with CML concentration of CML-modified β-lactoglobulin. The negative charge of CML was a crucial determinant for the binding of protein bound CML, while binding of glycated BLG was determined by increasing hydrophobicity. This shows that sRAGE, galectin-3, and CD36 bind to protein bound CML and points out the role of negatively charged AGEs in binding to AGE receptors.

    Ion adsorption-induced wetting transition in oil-water-mineral systems
    Mugele, F. ; Bera, B. ; Cavalli, A. ; Siretanu, I. ; Maestro, A. ; Duits, M. ; Cohen Stuart, M.A. ; Ende, D. van den - \ 2015
    Scientific Reports 5 (2015). - ISSN 2045-2322 - 8 p.
    hydration forces - surfaces - recovery - wettability - interfaces - membranes - charge - layer
    The relative wettability of oil and water on solid surfaces is generally governed by a complex competition of molecular interaction forces acting in such three-phase systems. Herein, we experimentally demonstrate how the adsorption of in nature abundant divalent Ca2+ cations to solid-liquid interfaces induces a macroscopic wetting transition from finite contact angles (˜10°) with to near-zero contact angles without divalent cations. We developed a quantitative model based on DLVO theory to demonstrate that this transition, which is observed on model clay surfaces, mica, but not on silica surfaces nor for monovalent K+ and Na+ cations is driven by charge reversal of the solid-liquid interface. Small amounts of a polar hydrocarbon, stearic acid, added to the ambient decane synergistically enhance the effect and lead to water contact angles up to 70° in the presence of Ca2+. Our results imply that it is the removal of divalent cations that makes reservoir rocks more hydrophilic, suggesting a generalizable strategy to control wettability and an explanation for the success of so-called low salinity water flooding, a recent enhanced oil recovery technology.
    Fractioning electrodialysis: a current induced ion exchange process
    Galama, A.H. ; Daubaras, G. ; Burheim, O.S. ; Rijnaarts, H. ; Post, J.W. - \ 2014
    Electrochimica Acta 136 (2014). - ISSN 0013-4686 - p. 257 - 265.
    divalent ions - seawater desalination - membrane - monovalent - diffusion - transport - charge - nanofiltration - permeability - polarization
    In desalination often multi ionic compositions are encountered. A preferential removal of multivalent ions over monovalent ions can be of interest to prevent scaling in the desalination process. Recently, a novel fractionating electrodialysis stack is described by Zhang et al., 2012 (in Sep. purify. Technol. 88). In the present work a small modification to such a stack was made, to create a current induced ion exchange process, in which no longer desalination occurs. This was done by building a membrane stack in which monovalent-selective ion-exchange membranes and standard grade ion-exchange membranes, with similar charge sign (so either anion or cation exchange groups), were placed alternatingly between an anode and a cathode to form a membrane stack. A proof of principle of the fractioning electrodialysis technology is given. Ternary mixtures, with a divalent-monovalent ion ratio similar to seawater, were used as feed water. For a cation and an anion fractioning stack, maximum fractionations of divalent ions were obtained of approximately 90 and 60%. At higher applied current density, ions can be fractionated to a larger extent than at lower applied current density. For both stacks the water recovery was 50%. Coulombic efficiency of both processes decrease rapidly after the start of the experiment. This leads to relatively large volumetric energy consumptions. (C) 2014 Elsevier Ltd. All rights reserved.
    Quantitative description of the parameters affecting the adsorption behaviour of globular proteins
    Delahaije, R.J.B.M. ; Gruppen, H. ; Giuseppin, M.L.F. ; Wierenga, P.A. - \ 2014
    Colloids and Surfaces. B: Biointerfaces 123 (2014). - ISSN 0927-7765 - p. 199 - 206.
    air-water-interface - bovine serum-albumin - beta-lactoglobulin - rheological properties - air/water interface - surface rheology - kinetics - ovalbumin - charge - denaturation
    The adsorption behaviour of proteins depends significantly on their molecular properties and system conditions. To study this relation, the effect of relative exposed hydrophobicity, protein concentration and ionic strength on the adsorption rate and adsorbed amount is studied using ß-lactoglobulin, ovalbumin and lysozyme. The curves of surface elastic modulus versus surface pressure of all three proteins, under different conditions (i.e. concentration and ionic strength) superimposed. This showed that the interactions between the adsorbed proteins are similar and that the adsorbed proteins retain their native state. In addition, the adsorption rate (kadsorb) was shown to scale with the relative hydrophobicity and ionic strength. Moreover, the adsorbed amount was shown to be dependent on the protein charge and the ionic strength. Based on these results, a model is proposed to predict the maximum adsorbed amount (Gmax). The model approximates the adsorbed amount as a close-packed monolayer using a hard-sphere approximation with an effective protein radius which depends on the electrostatic repulsion. The theoretical adsorbed amount was in agreement with experimental Gmax (±10%).
    Foam properties of algae soluble protein isolate: Effect of pH and ionic strength
    Schwenzfeier, A. ; Lech, F.J. ; Wierenga, P.A. ; Eppink, M.H.M. ; Gruppen, H. - \ 2013
    Food Hydrocolloids 33 (2013)1. - ISSN 0268-005X - p. 111 - 117.
    air-water interfaces - tetraselmis sp - adsorption - stability - emulsions - charge
    In this study the foam properties of algae soluble protein isolate (ASPI), a mixture of mainly proteins and polysaccharides, were investigated as function of isolate concentration (0.1–1.0 mg/mL) and pH (3.0–7.0) at 10 mM and 200 mM NaCl. In addition, adsorption kinetics and dilatational elasticity at the air–water interface were studied. Whey protein isolate (WPI) and egg white albumin (EWA) were used as reference proteins. The consistent dilatational behaviour of ASPI at all pH values and ionic strengths tested indicated a similar interfacial composition at all these conditions. Adsorption kinetics, in contrast, were influenced by varying environmental conditions. At increased ionic strength and close to the theoretical isoelectric point calculated based purely on the amino acid composition of ASPI (pH 7) adsorption increased. Since similar adsorption behaviour was also observed for WPI and EWA, the interfacial properties of ASPI are most likely dominated by its protein fraction. This is further confirmed by the fact that ¿-potential measurements suggested an overall isoelectric point of ASPI below pH 3, while adsorption kinetics varied between pH 5 and pH 7 (the theoretical protein-based isoelectric point of ASPI). The overall foam stability of ASPI stabilized foams was superior to those of WPI and EWA in the pH range 5–7. In conclusion, the molecular and interfacial properties of ASPI, a mixture of proteins and polysaccharides, seem to favour the production of very stable foams in this pH range by the selective adsorption of its protein fraction to the air–water interface.
    Emulsifying Property and Antioxidative Activity of Cuttlefish Skin Gelatin Modified with Oxidized Linoleic Acid and Oxidized Tannic Acid
    Aewsiri, T. ; Benjakul, S. ; Visessanguan, W. ; Wierenga, P.A. ; Gruppen, H. - \ 2013
    Food Bioprocess Technology 6 (2013)4. - ISSN 1935-5130 - p. 870 - 881.
    phenolic-compounds - proteins - emulsions - stability - oxidation - charge
    Cuttlefish skin gelatins modified with oxidized linoleic acid (OLA) and oxidized tannic acid (OTA) were characterized and determined for emulsifying properties and antioxidative activity. Modification of gelatin with 5% OTA increased the total phenolic content and 1,1-diphenyl-2-picrylhydrazyl, 2,2'-azinobis(3-ethylbenzothiazoline-6-sulphonic acid) radical scavenging activity and ferric reducing antioxidant power of gelatin–OTA. Incorporation of OLA into gelatin (OLA-to-free amino group molar ratio of 10:1) increased surface hydrophobicity of gelatin from 17.39 to 32.38 and reduce surface tension at air/water interface of gelatin solution from 53 to 32 mN/m. Gelatin–OLA had the increase in emulsion activity index, compared with gelatin without modification and was capable of producing a fine emulsion (d32¿=¿0.79 µm, d43¿=¿0.82 µm). Modification of gelatin–OLA complex with OTA at different concentrations (2.5%, 5%, and 10%) increased antioxidative activity but decrease emulsifying properties. However, gelatin–OLA modified with 5% OTA had higher emulsifying properties than the commercial gelatin (bovine gelatin). The presence of an alkyl group and a hydroxyl group in gelatin after modification with OLA and OTA, respectively, was revealed by Fourier transform infrared study. Coincidental decrease in free amino group was also noticeable in modified gelatin. Menhaden oil-in-water emulsion stabilized by gelatin modified with OLA and 5% OTA was more resistant to lipid oxidation and phase separation as evidenced by the lower thiobarbituric acid reactive substances value and smaller oil droplet size, compared with that stabilized by commercial bovine gelatin. Thus, the modification of gelatin by both OLA and OTA was able to improve antioxidative and emulsifying properties of cuttlefish skin gelatin.
    Improvement of foaming properties of cuttlefish skin gelatin by modification with N-hydroxysuccinimide esters of fatty acid
    Aewsiri, T. ; Benjakul, S. ; Visessanguan, W. ; Wierenga, P.A. ; Gruppen, H. - \ 2011
    Food Hydrocolloids 25 (2011)5. - ISSN 0268-005X - p. 1277 - 1284.
    air-water-interface - proteins - proteolysis - adsorption - surfactant - ovalbumin - charge
    Conformation and foaming properties of cuttlefish skin gelatin modified by N-hydroxysuccinimide esters of different saturated fatty acids including capric acid (C10:0), lauric acid (C12:0) and myristic acid (C14:0) at different molar ratios (0.25, 0.50, 1.00 and 2.00) were investigated. Covalent attachment of fatty acids into gelatin was observed as evidenced by the decrease in amino groups. Fourier transform infrared spectroscopic study indicated the presence of alkyl group of modified gelatin. The higher increase in surface activity with coincidental increase in surface hydrophobicity was observed in gelatin modified with fatty acid ester having a longer chain, especially at the higher molar ratio. The increase in foam expansion was related with the improved surface activity mediated by the modification by N-hydroxysuccinimide esters of fatty acid.
    Investigation of gas/liquid interface of small bubbles formed in solutions of different alkylammonium chlorides
    Sakai, M. ; Murata, T. ; Kamio, K. ; Mukae, K. ; Yamauchi, A. ; Moroi, Y. ; Sugihara, G. ; Norde, W. - \ 2010
    Colloids and Surfaces. A: Physicochemical and Engineering Aspects 359 (2010)1-3. - ISSN 0927-7757 - p. 6 - 12.
    water evaporation - aqueous-solutions - adsorption - microbubbles - surfactants - monolayers - charge
    The electrophoretic mobility of nitrogen gas bubbles was measured in solutions of different nalkylammonium chlorides in a cylindrical cell, 30 mm in diameter and 70 mm in length, rotating in order to keep the bubbles on a line of the rotation axis of the cylinder. An electric charge field was applied to the solutions through two electrodes located at both ends of the cylinder. The mobility was measured as a function of the size of bubbles. The maximum mobility U*(=v/E) and the corresponding diameter of the bubble, d(b)* were used to determine the apparent surface charge density, mat the gas/liquid interface. The surface charge density of bubbles was found (i) to be negative even in solutions of all n-alkylammonium chlorides carbon atoms, the number n of which was selected to be n=1, 2,3, and 10, and (ii) to decrease in magnitude with increasing number of carbon atoms. Decylammonium chloride (DAC: n = 10) is a typical cationic surfactant, however, the surface was revealed to still be negative, which strongly suggests that DAC molecules do not always come out to the gas/liquid interface. The change in surface charge density with the added salt concentration was analyzed, and as a result, it was found that only DAC follows the Langmuir adsorption isotherm. (C) 2010 Published by Elsevier B.V.
    Molecular interpretation of electrokinetic potentials
    Lyklema, J. - \ 2010
    Current Opinion in Colloid and Interface Science 15 (2010)3. - ISSN 1359-0294 - p. 125 - 130.
    silver iodide - double-layer - interface - capacity - charge
    A discussion is given of the electrokinetic, or ¿-potential in terms of the slip process and the composition of the electric double layer. Electrokinetically, only the outer parts of double layers are active. The existence of a stagnant part is generally observed for aqueous solutions adjacent to solid surfaces. It is claimed that this stagnancy is caused by the spontaneous structuring of fluids near solid surfaces. Hence, it is a ubiquitous phenomenon and the thickness of the stagnant layer does not significantly depend on the wettability and the surface charge of the surfaces.
    Poisson analysis of streptococcal bond strengthening on stainless steel with and without salivary conditioning film
    Mei, Li ; Mei, H.C. van der; Ren, Y. ; Norde, W. ; Busscher, H.J. - \ 2009
    Langmuir 25 (2009)11. - ISSN 0743-7463 - p. 6227 - 6231.
    atomic-force microscopy - bacterial adhesion - surface - hydrophobicity - charge
    Poisson analysis of retract force-distance curves in atomic force microscopy (AFM) has yielded a new dimension to the decoupling of individual bond forces into a hydrogen bonding and nonspecific force component. Accordingly, bacterial adhesion forces have been decoupled into a hydrogen bonding and nonspecific Lifshitz-Van der Waals contribution. Due to the forced nature of AFM contact, the nonspecific force contribution has hitherto turned out to be repulsive in the analysis of bacterial adhesion forces on nonconducting surfaces. In this study, we present the results of a Poisson analysis of adhesion forces for streptococci adhering to a conducting surface. Adhesion forces measured between stainless steel, both in the absence and presence of an adsorbed salivary conditioning film, increased with increasing contact time between the streptococcal AFM probe and the surface. Concurrent with the increase in adhesion force, there was an increase in the number of minor force peaks in the retract force-distance curves. Poisson analyses of the adhesion forces indicated repulsive nonspecific Lifshitz-Van der Waals forces for streptococci adhering to saliva-coated stainless steel, but interestingly and for the first time, attractive nonspecific forces were revealed on stainless steel in the absence of a salivary conditioning film. We tentatively attribute this to attraction between the negatively charged streptococci and their positive image charges in the conducting material, which cannot develop in a nonconducting material or in the presence of a nonconductive protein layer on the stainless steel surface.
    Simple Hofmeister series
    Lyklema, J. - \ 2009
    Chemical Physics Letters 467 (2009)4-6. - ISSN 0009-2614 - p. 217 - 222.
    electrical double-layer - silver-iodide - electrolyte interface - agj-soles - adsorption - stability - information - colloids - charge - ions
    Hofmeister, or lyotropic, series date back to 1888, when the founder arranged a large number of electrolytes in sequences with respect to their effectiveness salting out egg white. Since then the name has been applied to various phenomena involving ion specificity. In order to isolate effects attributable to single ionic species an interpretational step has to be taken, because only effects of electroneutral combinations of ions can be measured. The resulting sequence depends on the particular phenomenon studied. As a result, a variety of observations have been reported without a clear organizing principle. Here we describe Hofmeister sequences for well-defined systems which allow clear definition and unambiguous interpretation. Hofmeister sequences are not unique, they depend on the system and are the result of pair interactions.
    Modeling metal-particle interactions with an emphasis on natural organic matter
    Merdy, P. ; Huclier, S. ; Koopal, L.K. - \ 2006
    Environmental Science and Technology 40 (2006)24. - ISSN 0013-936X - p. 7459 - 7466.
    biotic ligand model - nica-donnan model - ion-binding - humic substances - fresh-waters - heavy-metals - adsorption - acid - charge - complexation
    Inner- and outer-sphere complexation of ions at the goethite-solution interface
    Rahnemaie, R. ; Hiemstra, T. ; Riemsdijk, W.H. van - \ 2006
    Journal of Colloid and Interface Science 297 (2006)2. - ISSN 0021-9797 - p. 379 - 388.
    rutile-water interface - surface structural approach - electrical double-layer - sulfate adsorption - salt-solutions - organic-acids - calcium - oxide - sorption - charge
    Formation of inner- and outer-sphere complexes of environmentally important divalent ions on the goethite surface was examined by applying the charge distribution CD model for inner- and outer-sphere complexation. The model assumes spatial charge distribution between the surface (0-plane) and the next electrostatic plane (1-plane) for innersphere complexation and between the 1-plane and the head end of the diffuse double layer (2-plane) for the outersphere complexation. The latter approach has been used because the distance of closest approach to a charged surface may differ for different ions. The surface structural approach implies the use of a Three-Plane model for the compact part (Stern layer) of solid-solution interface, which is divided into two layers. The thickness of each layer depends on the capacitance and the local dielectric constant. The new approach has been applied to describe the adsorption of magnesium, calcium, strontium, and sulfate ions. It is shown that the concept can successfully describe the development of surface charge in the presence of Ca+2, Mg+2, Sr+2, and SO(-2)4 as a function of loading, pH, and salt level, and also the shift in the isoelectric point (IEP) of goethite. The CD modeling revealed that, for the conditions studied, magnesium is mainly adsorbed as a bidentate innersphere complex, calcium can be a combination of bidentate innersphere and a monodentate inner- or outer-sphere complexes, and strontium is probably adsorbed as an outersphere complex. Sulfate is present as a mixture of inner- and outer-sphere monodentate complexes. Outersphere complexation is less pH dependent than innersphere complexation. The CD model predicts that the outersphere complexation of divalent cations and anions is relatively favorable at respectively low and high pH. Increase of ion loading favors the formation of innersphere complexes.
    Bulk conductivity of soft surface layers : experimental measurement and electrokinetic implications
    Yezek, L.P. - \ 2005
    Langmuir 21 (2005)22. - ISSN 0743-7463 - p. 10054 - 10060.
    poly(n-isopropylacrylamide) microgel particles - grafted polyelectrolyte layers - electrophoretic mobility - streaming current - swelling behavior - donnan potentials - gel layers - interfaces - charge - diffusion
    Conductivity measurements were carried out on a family of polyacrylamide-co-sodium acrylate gels cross-linked with N,N¿ -methylenebisacrylamide in a homemade electrokinetic cell. The conductivity data allowed the equilibrium Donnan potential difference between the bulk gel and the bulk electrolyte solution to be estimated at various ionic strengths. The data were fit to a simple model assuming full dissociation of functional groups as well as to a more complete model (Dukhin, S. S.; Zimmerman, R.; Werner, C. J. Colloid Interface Sci. 2004, 274, 309) that accounts for the weak electrolyte nature of the acrylate groups fixed within the gel structure. The conductivity of the gel layers was observed to be significantly larger than the conductivity of the bulk electrolyte solutions at low ionic strengths. The increased conductivity reflects the enhanced concentration of ions within the gel structure due to Donnan equilibrium and the mobility of ions within the high water content gel layers. The electrokinetic implications of the bulk conductivity of gel-like soft surface layers are discussed in terms of the influence of the gel conductance on the resulting streaming potential.
    Electrokinetic characterization of poly(acrylic acid) and poly(ethylene oxide) brushes in aqueous electrolyte solutions
    Zimmermann, R. ; Norde, W. ; Cohen Stuart, M.A. ; Werner, C. - \ 2005
    Langmuir 21 (2005)11. - ISSN 0743-7463 - p. 5108 - 5114.
    self-assembled monolayers - surface conductivity - polyacrylamide-gels - hindered diffusion - donnan potentials - polymers - charge - flow - proteins - model
    Surfaces carrying hydrophilic polymer brushes were prepared from poly(styrene)-poly(acrylic acid) and poly(styrene)-poly(ethylene oxide) diblock copolymers, respectively, using a Langmuir-Blodgett technique and employing poly(styrene)-coated planar glass as substrates. The electrical properties of these surfaces in aqueous electrolyte were analyzed as a function of pH and KCl concentration using streaming potential/streaming current measurements. From these data, both the zeta potential and the surface conductivity could be obtained. The poly(acrylic acid) brushes are charged due to the dissociation of carboxylic acid groups and give theoretical surface potentials of -160 mV at full dissociation in 10-3 M solutions. The surface conductivity of these brushes is enormous under these conditions, accounting for more than 93% of the total measured surface conductivity. However, the mobility of the ions within the brush was estimated from the density of the carboxylic acid groups and the surface conductivity data to be only about 14% of that of free ions. The poly(ethylene oxide) (PEO) brushes effectively screen the charge of the underlying substrate, giving a very low zeta potential except when the ionic strength is very low. From the data, a hydrodynamic layer thickness of the PEO brushes could be estimated which is in good agreement with independent experiments (neutron reflectivity) and theoretical estimates. The surface conductivity in this system was slightly lower than that of the polystyren substrate. This also indicates that no significant amount of preferentially, i.e., nonelectrostatically attracted, ions taken up in the brush.
    Cylindrical cell model for the electrostatic free energy of polyelectrolyte complexes
    Biesheuvel, P.M. ; Cohen Stuart, M.A. - \ 2004
    Langmuir 20 (2004)1. - ISSN 0743-7463 - p. 4764 - 4770.
    electrical double-layer - exponential-growth - multilayers - equilibria - binding - charge - force - films - salt
    Associative phase separation (complex coacervation) in a mixture of oppositely charged polyelectrolytes can lead to different types of (inter-)polyelectrolyte complexes (soluble micelles, macroscopic precipitation). In a previous report [Langmuir 2004, 20, 2785-2791], we presented a model for the electrostatic free energy change when (weakly charged) polyelectrolyte forms a homogeneous complex phase. The influence of ionization of the polymer on the electrostatic free energy of the complex was incorporated but the influence of complex density neglected. In the present effort, cylindrical cells are assumed around each polyelectrolyte chain in the complex, and on the basis of the Poisson-Boltzmann equation, the electrostatic free energy is calculated as a function of the complex density. After combination with Flory-Huggins mixing free energy terms and minimization of the total free energy, the equilibrium complex density is obtained, for a given ratio of polycations to polyanions in the complex. The analysis is used in an example calculation of polyelectrolyte film formation by alternatingly applying a polycation and a polyanion solution. The calculation suggests that the often observed exponential growth of a polyelectrolyte film when the polymer is weakly charged has a thermodynamic origin: the polyelectrolyte complex shifts repeatedly between two equilibrium states of different densities and compositions. However, when the polyelectrolytes are strongly charged the difference in the compositions between the two equilibrium states is very small, and exponential growth by an absorption mechanism is no longer possible.
    Modeling the binding of fulvic acid by goethite: The speciation of adsorbed FA molecules
    Filius, J.D. ; Meeussen, J.C.L. ; Hiemstra, T. ; Riemsdijk, W.H. van - \ 2003
    Geochimica et Cosmochimica Acta 67 (2003)8. - ISSN 0016-7037 - p. 1463 - 1474.
    natural organic-matter - nica-donnan model - humic substances - surface complexation - competitive adsorption - proton binding - hydrous oxides - ion-binding - iron-oxide - charge
    Under natural conditions, the adsorption of ions at the solid-water interface may be strongly influenced by the adsorption of organic matter. In this paper, we describe the adsorption of fulvic acid (FA) by metal (hydr)oxide surfaces with a heterogeneous surface complexation model, the ligand and charge distribution (LCD) model. The model is a self-consistent combination of the nonideal competitive adsorption (NICA) equation and the CD-MUSIC model. The LCD model can describe simultaneously the concentration, pH, and salt dependency of the adsorption with a minimum of only three adjustable parameters. Furthermore, the model predicts the coadsorption of protons accurately for an extended range of conditions. Surface speciation calculations show that almost all hydroxyl groups of the adsorbed FA molecules are involved in outer sphere complexation reactions. The carboxylic groups of the adsorbed FA molecule form inner and outer sphere complexes. Furthermore, part of the carboxylate groups remain noncoordinated and deprotonated. Copyright (C) 2003 Elsevier Science Ltd.
    The role of the abundant phenylalanines in the mode of action of the antimicrobial peptide clavanin
    Kan, E.J.M. van; Demel, R.A. ; Bent, A. van der; Kruijff, B. de - \ 2003
    Biochimica et Biophysica Acta. Biomembranes 1615 (2003)1-2. - ISSN 0005-2736 - p. 84 - 92.
    protein secondary structure - antibacterial peptides - hydrophobic moment - circular-dichroism - tunicate hemocytes - membrane-activity - charge - modulate - diastereomers - biomembranes
    Clavanin A is a special antimicrobial peptide that acts at the level of the membrane via a pH-dependent mechanism. At neutral pH, clavanin disrupts biological and model membranes in a nonspecific manner, causing efflux of large molecules. At mildly acidic conditions, however, the peptide efficiently kills bacteria by permeabilizing their membrane most likely by interacting with proteins involved in proton translocation [Biochemistry 41 (2002) 7529]. Clavanin A is unusually rich in phenylalanines with 5 out of 23 residues, which suggests that these residues are functionally important. A set of mutants, in which all Phe residues are replaced by either Ile, Leu, Trp, or Tyr was used to investigate the role of these amino acids. The antimicrobial activities of the different peptides both at neutral and low pH show that the presence of phenylalanine is not essential nor optimal, as the Trp, Leu, and Ile mutant are equally or more active than the wild-type component. In general, at neutral pH, the biological activities correlate well with the peptides' ability to interact with membrane lipids. Correspondingly, the permeabilization efficiencies of biological and model membranes of the various derivatives were found to be closely related to their ability to adopt -helical structures, and follows the order 5L>5W>5I>5Y>wild type. The results suggest an important role for the Phe residues, in providing the peptide in a balanced manner with sufficient hydrophobicity, and therewith membrane affinity, as well as conformational flexibility
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