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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    Nonlinear surface rheology and interfacial microstructure imaging of WPI particles and their constituents
    Yang, Jack - \ 2019
    protein pickering stabilizer - air/water interface - microstructure - surface rheology - Lissajous plots - atomic microscopy
    Langmuir monolayers of non-ionic polymers: Equilibrium of metastability? Case study of PEO and its PPO-PEO diblock copolymers
    Deschenes, L. ; Saint-Germain, F. ; Lyklema, J. - \ 2015
    Journal of Colloid and Interface Science 449 (2015). - ISSN 0021-9797 - p. 494 - 505.
    air-water-interface - poly(ethylene oxide) monolayers - interacting chain molecules - air/water interface - scaling description - statistical-theory - block-copolymers - liquid-films - surface - layers
    Stability and reorganization in Langmuir films of PEO in PEO homopolymers and PPO–PEO block copolymers were investigated using film balance measurements. The apparent fractional losses of EO segments transferred into the subphase resulting from successive compression–expansion cycles have been estimated. The apparent loss is mainly Gmax, Mn and time-dependent. At surface concentrations G ¿ 0.32 mg/m2, PEO films are in equilibrium. For 0.32 ¿ G ¿ 0.7 mg/m2, the losses remain modest. Further compression leads to densification of the monolayer, requiring the interplay of thermodynamics and kinetic factors In the plateau regime, the loss is higher and constant for 1 ¿ Gmax ¿ 2 mg/m2 upon maintaining the achieved surface area for 15 min. Similar losses were obtained for PEO homopolymers of high Mn and PPO353–PEO2295. It suggests that the PEO remains anchored in a metastable state at the air–water interface at surface concentration well above the onset of the plateau. Additional losses are incurred for PEO homopolymers for monolayers kept compressed in the plateau for 2 h. For the interpretation of these phenomena a combination of elements from self-consistent field theory and scaling is desirable with as a trend an increasing contribution of the latter with increasing surface concentration.
    Shear rheology of mixed protein adsorption layers vs their structure studied by surface force measurements
    Danov, K.D. ; Kralchevsky, P.A. ; Radulova, G.M. ; Basheva, E.S. ; Stoyanov, S.D. ; Pelan, E.G. - \ 2015
    Advances in Colloid and Interface Science 222 (2015). - ISSN 0001-8686 - p. 148 - 161.
    nonlinear viscoelastic model - dependent relaxation-times - class-ii hydrophobin - low ionic-strength - beta-casein - interfacial rheology - air/water interface - liquid interfaces - bubble stability - hfbii
    The hydrophobins are proteins that form the most rigid adsorption layers at liquid interfaces in comparison with all other investigated proteins. The mixing of hydrophobin HFBII with other conventional proteins is expected to reduce the surface shear elasticity and viscosity, Esh and ¿sh, proportional to the fraction of the conventional protein. However, the experiments show that the effect of mixing can be rather different depending on the nature of the additive. If the additive is a globular protein, like ß-lactoglobulin and ovalbumin, the surface rigidity is preserved, and even enhanced. The experiments with separate foam films indicate that this is due to the formation of a bilayer structure at the air/water interface. The more hydrophobic HFBII forms the upper layer adjacent to the air phase, whereas the conventional globular protein forms the lower layer that faces the water phase. Thus, the elastic network formed by the adsorbed hydrophobin remains intact, and even reinforced by the adjacent layer of globular protein. In contrast, the addition of the disordered protein ß-casein leads to softening of the HFBII adsorption layer. Similar (an even stronger) effect is produced by the nonionic surfactant Tween 20. This can be explained with the penetration of the hydrophobic tails of ß-casein and Tween 20 between the HFBII molecules at the interface, which breaks the integrity of the hydrophobin interfacial elastic network. The analyzed experimental data for the surface shear rheology of various protein adsorption layers comply with a viscoelastic thixotropic model, which allows one to determine Esh and ¿sh from the measured storage and loss moduli, G' and G¿. The results could contribute for quantitative characterization and deeper understanding of the factors that control the surface rigidity of protein adsorption layers with potential application for the creation of stable foams and emulsions with fine bubbles or droplets.
    Competitive adsorption of the protein hydrophobin and an ionic surfactant: Parallel vs sequential adsorption and dilatational rheology
    Stanimirova, R. ; Marinova, K.G. ; Danov, K.D. ; Kralchevsky, P.A. ; Basheva, E.S. ; Stoyanov, S.D. ; Pelan, E.G. - \ 2014
    Colloids and Surfaces. A: Physicochemical and Engineering Aspects 457 (2014). - ISSN 0927-7757 - p. 307 - 317.
    air-water-interface - sodium dodecyl-sulfate - beta-casein - air/water interface - fluid interfaces - layers - hfbii - elasticity - stability - mixtures
    The competitive adsorption of the protein HFBII hydrophobin and the anionic surfactant sodium dodecyl sulfate (SDS) is investigated in experiments on parallel and sequential adsorption of the two components. The dynamic surface tension and the surface storage and loss dilatational moduli are determined by the oscillating bubble method. A new procedure for data processing is proposed, which allows one to collect data from many different runs on a single master curve and to determine more accurately the dependence of the dilatational elasticity on the surface pressure. Experiments on sequential adsorption are performed by exchanging the HFBII solution around the bubble with an SDS solution. Experiments with separate thin foam films bring additional information on the effect of added SDS. The results indicate that if HFBII has first adsorbed at the air/water interface, it cannot be displaced by SDS at any concentration, both below and above the critical micellization concentration (CMC). In the case of parallel adsorption, there is a considerable difference between the cases below and above the CMC. In the former case, SDS cannot prevent the adsorption of HFBII at the interface, whereas in the latter case adsorption of HFBII is absent, which can be explained with hydrophilization of the hydrophobin aggregates by the SDS in the bulk. The surface dilatational elasticity of the HFBII adsorption layers markedly decreases in the presence of SDS, but it recovers after washing out the SDS. With respect to their dilatational rheology, the investigated HFBII layers exhibit purely elastic behavior, the effect of dilatational viscosity being negligible. As a function of surface tension, the elasticity of the investigated interfacial layers exhibits a high maximum, which could be explained with the occurrence of a phase transition in the protein adsorption layer.
    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%).
    Identifying changes in chemical, interfacial and foam properties of ß-lactoglobulin–sodium dodecyl sulphate mixtures
    Lech, F.J. ; Steltenpool, P. ; Meinders, M.B.J. ; Sforza, S. ; Gruppen, H. ; Wierenga, P.A. - \ 2014
    Colloids and Surfaces. A: Physicochemical and Engineering Aspects 462 (2014). - ISSN 0927-7757 - p. 34 - 44.
    protein-surfactant interactions - air-water interfaces - adsorption layers - competitive adsorption - air/water interface - liquid interfaces - food proteins - mixed layers - rheology - stability
    Techno-functional properties of proteins, such as foam stability, can be affected by the presence of low-molecular-weight surfactants. In order to understand and control the foam properties of such protein–surfactant mixtures, a thorough characterization of foam and interfacial properties needs to be supplemented by a detailed analysis of the structural changes of the protein and possible complexation with the surfactant. In this study, ß-lactoglobulin (BLG) was mixed with sodium dodecyl sulphate (SDS) in different molar ratios (MRs). The foam half-life time of BLG-SDS mixtures decreased from that of pure BLG (315 min at MR 0) to 44 min at MR 20, which is close to the half-life of SDS at the respective concentration. With a further increase in the MR, the foam stability of the mixture increased, similar to the stability of SDS, to 250 min at the highest MR (MR 100). The minimum in the foam stability curve was not reflected in the interfacial properties (¿ and Ed). ¿ decreased and Ed increased continuously with increasing MR from values close to those of protein towards values typically found in pure surfactant solutions. The results show no clear correlation between the interfacial and foaming properties. In addition, it was shown by isothermal titration calorimetry and mass spectrometry that SDS molecules bind to the BLG. This leads to the formation of BLG-SDS complexes. These complexes have large influence on the foam properties in the mixture. The combination of analytical methods that were used give insights about protein complexation and the resulting change of foam properties of the mixture.
    Surface Pressure and Elasticity of Hydrophobin HFBII Layers on the Air-Water Interface: Rheology Versus Structure Detected by AFM Imaging
    Stanimirova, R.D. ; Gurkov, T.D. ; Kralchevsky, P.A. ; Balashev, K.T. ; Stoyanov, S.D. ; Pelan, E.G. - \ 2013
    Langmuir 29 (2013)20. - ISSN 0743-7463 - p. 6053 - 6067.
    class-ii hydrophobins - air/water interface - langmuir monolayers - trichoderma-reesei - proteins - films - adsorption - stability - emulsions - mechanisms
    Here, we combine experiments with Langmuir trough and atomic force microscopy (AFM) to investigate the reasons for the special properties of layers from the protein HFBII hydrophobin spread on the airwater interface. The hydrophobin interfacial layers possess the highest surface dilatational and shear elastic moduli among all investigated proteins. The AFM images show that the spread HFBII layers are rather inhomogeneous, (i.e., they contain voids, monolayer and multilayer domains). A continuous compression of the layer leads to filling the voids and transformation of a part of the monolayer into a trilayer. The trilayer appears in the form of large surface domains, which can be formed by folding and subduction of parts from the initial monolayer. The trilayer appears also in the form of numerous submicrometer spots, which can be obtained by forcing protein molecules out of the monolayer and their self-assembly into adjacent pimples. Such structures are formed because not only the hydrophobic parts, but also the hydrophilic parts of the HFBII molecules can adhere to each other in the water medium. If a hydrophobin layer is subjected to oscillations, its elasticity considerably increases, up to 500 mN/m, which can be explained with compaction. The relaxation of the layers tension after expansion or compression follows the same relatively simple law, which refers to two-dimensional diffusion of protein aggregates within the layer. The characteristic diffusion time after compression is longer than after expansion, which can be explained with the impedence of diffusion in the more compact interfacial layer. The results shed light on the relation between the mesoscopic structure of hydrophobin interfacial layers and their unique mechanical properties that find applications for the production of foams and emulsions of extraordinary stability; for the immobilization of functional molecules at surfaces, and as coating agents for surface modification.
    On the link between surface rheology and foam disproportionation in mixed hydrophobin HFBII and whey protein systems
    Blijdenstein, T.B.J. ; Ganzevles, R.A. ; Groot, P.W.N. de; Stoyanov, S.D. - \ 2013
    Colloids and Surfaces. A: Physicochemical and Engineering Aspects 438 (2013). - ISSN 0927-7757 - p. 13 - 20.
    class-ii hydrophobins - air/water interface - trichoderma-reesei - adsorption layers - shear rheology - films - displacement - stability - casein - sc3
    Here we study the surface dilational properties of spread and adsorbed layers of whey protein isolate (WPI) and hydrophobin HFBII at air/water interface using Langmuir trough and relate them to foam ability and stability. In spread and adsorbed systems, a gradual increase in modulus with the fraction of HFBII on the surface or in the bulk is observed and we can identify distinct regimes of WPI-dominated and HFBII-dominated behaviour. The dominance of HFBII is further substantiated by visual observation of microscopic wrinkles appearing on the surface of the trough. When comparing spread to adsorbed systems, it was found that a higher HFBII fraction is needed to obtain HFBII dominant behaviour in spread layers than in case of adsorbing layers (f(HFBII) = 0.6 and 0.2 respectively). Furthermore, our results indicate that the HFBII-contribution to the interfacial behaviour becomes more pronounced upon sequential large scale compression/expansion cycles. In order to explain this non-trivial behaviour, we propose that there is multi-layer formation at the interface, having a top layer enriched in HFBII and bottom layer enriched in WPI. It is also concluded that coarsening stability in foams corresponds more closely to the surface dilational properties measured in adsorbing layers than those in spread layers. Finally, it was observed that in mixed systems of HFBII and WPI, the coarsening process levels off, which corresponds to the increased dominance of HFBII in mixed WPI:HFBII-layers upon large surface deformation that is occurring at the surface of shrinking bubbles. (C) 2013 Elsevier B.V. All rights reserved.
    (Quasi-) 2D aggregation of polystyrene-b-dextran at the air-water interface
    Bosker, W.T.E. ; Cohen Stuart, M.A. ; Norde, W. - \ 2013
    Langmuir 29 (2013)8. - ISSN 0743-7463 - p. 2667 - 2675.
    diblock copolymer monolayers - surface micelle formation - block polyelectrolytes - air/water interface - molecular-dynamics - solid-state - behavior - brushes - polymers - spectroscopy
    Polystyrene-b-dextran (PS-b-Dextran) copolymers can be used to prepare dextran brushes at solid surfaces, applying Langmuir–Blodgett deposition. When recording the interfacial pressure versus area isotherms of a PS-b-Dextran monolayer, time-dependent hysteresis was observed upon compression and expansion. We argue that this is due to (quasi-) 2D aggregation of the copolymer at the air–water surface, with three contributions. First, at large area per molecule, a zero surface pressure is measured; we ascribe this to self-assembly of block copolymers into surface micelles. At intermediate area we identify a second regime (“desorption regime”) where aggregation into large patches occurs due to van der Waals attraction between PS blocks. At high surface pressure (“brush regime”) we observe hysteretic behavior attributed to H-bonding between dextran chains. When compared to hysteresis of other amphiphilic diblock copolymers (also containing PS, e.g., polystyrene-b-poly(ethylene oxide)) a general criterion can be formulated concerning the extent of hysteresis: when the hydrophobic (PS) block is of equal size as (or bigger than) the hydrophilic block, the hysteresis is maximal. The (quasi-) 2D aggregation of PS-b-Dextran has significant implications for the preparation of dextran brushes at solid surfaces using Langmuir–Blodgett deposition. For each grafting density the monolayer needs to relax, up to several hours, prior to transfer.
    Dynamic surface tension of complex fluid-fluid interfaces: A useful concept, or not?
    Sagis, L.M.C. - \ 2013
    The European Physical Journal. Special Topics 222 (2013)1. - ISSN 1951-6355 - p. 39 - 46.
    scanning angle reflectometry - air-water-interface - neutron reflectivity - 2-dimensional suspensions - air/water interface - bending rigidity - latex-particles - beta-casein - systems - equilibrium
    Dilatational moduli are typically determined by subjecting interfaces to oscillatory area deformations, and are often defined in terms of the difference between the dynamic or transient surface tension of the interface (the surface tension in its deformed state), and the surface tension of the interface in its non-deformed state. Here we will discuss the usefulness of the dynamic surface tension concept in the characterization of dilatational properties of complex fluid-fluid interfaces. Complex fluid-fluid interfaces are interfaces stabilized by components which form mesophases (two-dimensionional gels, glasses, or (liquid) crystalline phases), as a result of in-plane interactions between the components. We will show that for such interfaces dilatational properties are not exclusively determined by the exchange of surface active components between interface and adjoining bulk phases, but also by in-plane viscoelastic stresses. The separation of these contributions remains a challenging problem which remains to be solved.
    Surface rheological properties of liquid-liquid interfaces stabilized by protein fibrillar aggregates and protein-polysaccharide complexes
    Humblet-Hua, K.N.P. ; Linden, E. van der; Sagis, L.M.C. - \ 2013
    Soft Matter 9 (2013)7. - ISSN 1744-683X - p. 2154 - 2165.
    in-water emulsions - egg-white lysozyme - air/water interface - beta-lactoglobulin - amyloid fibrils - oil/water interface - bending rigidity - sodium caseinate - dextran sulfate - food systems
    In this study we have investigated the surface rheological properties of oil-water interfaces stabilized by fibrils from lysozyme (long and semi-flexible and short and rigid ones), fibrils from ovalbumin (short and semi-flexible), lysozyme-pectin complexes, or ovalbumin-pectin complexes. We have compared these properties with those of interfaces stabilized by the native proteins. The surface dilatational and surface shear moduli were determined using an automated drop tensiometer and a stress controlled rheometer with biconical disk geometry. Results show that interfaces stabilized by complexes of these proteins with high-methoxyl pectin have higher surface shear and dilatational moduli than interfaces stabilized by the native proteins only. The interfaces stabilized by ovalbumin and lysozyme complexes have comparable shear and dilatational moduli though ovalbumin-pectin complexes are twice as large in radius as lysozyme-pectin complexes. Under most of the experimental conditions, interfaces stabilized by fibrils have the highest surface rheological moduli. The difference between long semi-flexible lysozyme fibrils or short rigid lysozyme fibrils is not pronounced in interfacial dilation rheology but significant in interfacial shear rheology. The complex surface shear moduli of interfaces stabilized by long semi-flexible fibrils are about 10 times higher than those of interfaces stabilized by short rigid fibrils, over a range of bulk concentrations. Interfaces stabilized by short and more flexible ovalbumin fibrils have a significantly higher surface shear modulus than those stabilized by longer and more rigid lysozyme fibrils. This study has shown that the use of such supra-molecular structural building blocks creates a wider range of microstructural features of the interface, with higher surface shear and dilatational moduli and a more complex dependence on strain.
    Engineering interfacial properties by anionic surfactant-chitosan complexes to improve stability of oil-in-water emulsions
    Zinoviadou, K. ; Scholten, E. ; Moschakis, T. ; Biliaderis, C.G. - \ 2012
    Food & Function 3 (2012)3. - ISSN 2042-6496 - p. 312 - 319.
    freeze-thaw stability - whey-protein isolate - beta-lactoglobulin-pectin - high methoxyl pectin - deposition technique - sodium caseinate - o/w emulsions - electrostatic deposition - environmental-stresses - air/water interface
    Oil-in-water emulsions (10% w/w n-tetradecane) were prepared at pH = 5.7 by using, as surface active agents, electrostatically formed complexes of sodium stearoyl lactylate (SSL) at a concentration of 0.4% (w/w) and chitosan (CH) in a concentration range between 0 and 0.48% w/w. The use of complexes in emulsions with a low concentration of CH (
    New views on foams from protein solutions
    Wierenga, P.A. ; Gruppen, H. - \ 2010
    Current Opinion in Colloid and Interface Science 15 (2010)5. - ISSN 1359-0294 - p. 365 - 373.
    air-water-interface - beta-lactoglobulin - air/water interface - rheological properties - liquid interfaces - adsorption-kinetics - shear rheology - whey-protein - structural conformation - orogenic displacement
    The stabilization of foam by proteins has been mostly studied in relation to the food industry. The main aim of the research is to understand the relation between proteins used and the product properties. The molecular properties of proteins and their foam forming and stabilizing properties are typically linked to the adsorption kinetics and the interfacial properties. Additionally, the properties of thin liquid films formed between neighboring air bubbles are considered. While there are several rules of thumb describing the relations between the different parameters and processes it seems that there is not yet a ‘unifying’ theory on protein stabilized foams. If the different processes could be described by quantitative parameters the applications of traditional proteins and the use of proteins from novel sources could be optimized. However, even for simple protein systems there is a lack of such quantitative rules, and as a result the advancement in the understanding of protein foam seems to progress slowly. This is attributed to the complexity of the system by some authors, but by viewing the literature it also becomes apparent that certain ideas seem to resist change. There are some interesting articles that offer a different point of view. In this article we aim to provide an insight in the different ways in which proteins and their role in foamed systems are described. Based on recent results, it seems that protein adsorption and subsequent changes in interfacial properties could be described in colloidal terms such as the net charge, exposed hydrophobicity and size of the proteins. Such a description can help to understand the behavior of single-component systems, but can also add to the understanding of the more complex systems that seem to attract more and more interest in recent years. An example of the value of using new viewpoints is the exchange of information between fields of food and non-food foams. Examples in this field are the use of particles to stabilize foams, or the production of very stable microbubbles.
    Polymers at the water/air interface, surface pressure isotherms, and molecularly detailed modeling
    Bernardini, C. ; Stoyanov, S.D. ; Cohen Stuart, M.A. ; Arnaudov, L.N. ; Leermakers, F.A.M. - \ 2010
    Langmuir 26 (2010)14. - ISSN 0743-7463 - p. 11850 - 11861.
    interacting chain molecules - air/water interface - light-scattering - langmuir monolayers - methyl-methacrylate - monomolecular films - statistical-theory - air - spread - poly(dimethylsiloxane)
    Surface pressure isotherms at the air/water interface are reproduced for four different polymers, poly-l-lactic acid (PLLA), poly(dimethylsiloxane) (PDMS), poly(methyl methacrylate) (PMMA), and poly(isobutylene) (PiB). The polymers have the common property that they do not dissolve in water. The four isotherms differ strongly. To unravel the underlying details that are causing these differences, we have performed molecularly detailed self-consistent field (SCF) modeling. We describe the polymers on a united atom level, taking the side groups on the monomer level into account. In line with experiments, we find that PiB spreads in a monolayer which smoothly thickens already at a very low surface pressure. PMMA has an autophobic behavior: a PMMA liquid does not spread on top of the monolayer of PMMA at the air/water interface. A thicker PMMA layer only forms after the collapse of the film at a relatively high pressure. The isotherm of PDMS has regions with extreme compressibility which are linked to a layering transition. PLLA wets the water surface and spreads homogeneously at larger areas per monomer. The classical SCF approach features only short-range nearest-neighbor interactions. For the correct positioning of the layering and for the thickening of the polymer films, we account for a power-law van der Waals contribution in the model. Two-gradient SCF computations are performed to model the interface between two coexistent PDMS films at the layering transition, and an estimation of the length of their interfacial contact is obtained, together with the associated line tension value.
    Self-consistent field modeling of non-ionic surfactants at the silica-water interface: Incorporating molecular detail
    Postmus, B.R. ; Leermakers, F.A.M. ; Cohen Stuart, M.A. - \ 2008
    Langmuir 24 (2008)8. - ISSN 0743-7463 - p. 3960 - 3969.
    interacting chain molecules - glycol monododecyl ether - solid-liquid interface - neutron reflection - statistical thermodynamics - hydrophobic surfaces - adsorption-isotherms - air/water interface - fluorescence decay - aqueous-solution
    We have constructed a model to predict the properties of non-ionic (alkyl-ethylene oxide) (C(n)E(m)) surfactants, both in aqueous solutions and near a silica surface, based upon the self-consistent field theory using the Scheutjens-Fleer discretisation scheme. The system has the pH and the ionic strength as additional control parameters. At high ionic strength, the solvent quality for the surfactant head groups is affected, which changes both the bulk and the adsorption behavior of the surfactant. For example, with increasing ionic strength, the CMC drops and the aggregation increases. Surfactants adsorb above the critical surface association concentration (CSAC). The CSAC is a function of the surfactant and the surface properties. Therefore, the CSAC varies with both the ionic strength and the pH. We predict that with increasing ionic strength, the CSAC will first slightly increase but then drop substantially. The charge on the surface is pH dependent, and as the head groups bind through H-bonding to the silanol groups, the CSAC increases with increasing pH. We focus on adsorption/desorption transitions for the surfactants and compare these to the experimental data. Both the equilibrium predictions and the consequences for the kinetics of adsorption follow experimental findings. Our results show that molecularly realistic models can reveal a much richer interfacial behavior than anticipated from more generic models.
    The adsorption and unfolding kinetics determines the folding state of proteins at the air-water interface and thereby the equation of state
    Wierenga, P.A. ; Egmond, M.R. ; Voragen, A.G.J. ; Jongh, H.H.J. de - \ 2006
    Journal of Colloid and Interface Science 299 (2006)2. - ISSN 0021-9797 - p. 850 - 857.
    bovine serum-albumin - neutron reflectivity - air/water interface - beta-casein - liquid interfaces - structural conformation - surface pressure - competitive adsorption - infrared-spectroscopy - circular-dichroism
    Unfolding of proteins has often been mentioned as an important factor during the adsorption process at air-water interfaces and in the increase of surface pressure at later stages of the adsorption process. This work focuses on the question whether the folding state of the adsorbed protein depends on the rate of adsorption to the interface, which can be controlled by bulk concentration. Therefore, the adsorption of proteins with varying structural stabilities at several protein concentrations was studied using ellipsometry and surface tensiometry. For beta-lactoglobulin the adsorbed amount (Gamma) needed to reach a certain surface pressure (Pi) decreased with decreasing bulk concentration. Ovalbumin showed no such dependence. To verify whether this difference in behavior is caused by the difference in structural stability, similar experiments were performed with cytochrome c and a destabilized variant of this protein. Both proteins showed identical Pi-Gamma, and no dependence on bulk concentration. From this work it was concluded that unfolding will only take place if the kinetics of adsorption is similar or slower than the kinetics of unfolding. The latter depends on the activation energy of unfolding (which is in the order of 100-300 kJ/mol), rather than the free energy of unfolding (typically 10-50 kJ/mol).
    Importance of physical vs. chemical interactions in surface shear rheology
    Wierenga, P.A. ; Kosters, H.A. ; Egmond, M.R. ; Voragen, A.G.J. ; Jongh, H.H.J. de - \ 2006
    Advances in Colloid and Interface Science 119 (2006)2-3. - ISSN 0001-8686 - p. 131 - 139.
    air-water-interface - adsorbed protein layers - bovine serum-albumin - air/water interface - beta-lactoglobulin - computer-simulation - hexadecane/water interface - flexible proteins - cross-linking - milk-proteins
    The stability of adsorbed protein layers against deformation has in literature been attributed to the formation of a continuous gel-like network. This hypothesis is mostly based on measurements of the increase of the surface shear elasticity with time. For several proteins this increase has been attributed to the formation of intermolecular disulfide bridges between adsorbed proteins. However, according to an alternative model the shear elasticity results from the low mobility of the densely packed proteins. To contribute to this discussion, the actual role of disulfide bridges in interfacial layers is studied. Ovalbumin was thiolated with S-acetylmercaptosuccinic anhydride (S-AMSA), followed by removal of the acetylblock on the sulphur atom, resulting in respectively blocked (SX) and deblocked (SH) ovalbumin variants. This allows comparison of proteins with identical amino acid sequence and similar globular packing and charge distribution, but different chemical reactivity. The presence and reactivity of the introduced, deblocked sulfhydryl groups were confirmed using the sulfhydryl-disulfide exchange index (SEI). Despite the reactivity of the introduced sulfhydryl groups measured in solution, no increase in the surface shear elasticity could be detected with increasing reactivity. This indicates that physical rather than chemical interactions determine the surface shear behaviour. Further experiments were performed in bulk solution to study the conditions needed to induce covalent aggregate formation. From these studies it was found that mere concentration of proteins (to 200 mg/mL, equivalent to a surface concentration of around 2 mg/m2) is not sufficient to induce significant aggregation to form a continuous network. In view of these results, it was concluded that the adsorbed layer should not be considered a gelled network of aggregated material (in analogy with three-dimensional gels formed from heating protein solutions). Rather, it would appear that the adsorbed proteins form a highly packed system of proteins with net-repulsive interactions.
    Assessing the Extent of Protein Intermolecular Interactions at Air-Water Interfaces Using Spectroscopic Techniques
    Jongh, H.H.J. de; Wierenga, P.A. - \ 2006
    Biopolymers 82 (2006)4. - ISSN 0006-3525 - p. 384 - 389.
    surface shear rheology - air/water interface - ovalbumin - adsorption - layers - dynamics
    There is an ongoing debate about whether a protein surface film at an air-water interface can be regarded as a gelled layer. There is literature reporting that such films show macroscopic fracture behavior and a rheology comparable to three-dimensional protein bulk-networks. Ifthis is the case, a complete description of the formation of adsorbed layers should include a transition from single, freely moving proteins to a gelled layer. This report presents studies using spectroscopic techniques, such as infrared, fluorescence and neutron spectroscopy, or ellipsometry, to derive molecular insight in situ to substantiate the intermolecular networking in surface films of chicken egg ovalbumin. It is concluded that protein films, generated by equilibrium adsorption from the bulk, behave as a densely packed colloidal repulsive particle system, where the proteins still have a significant rotational mobility, have a predominantly retained globular fold, and show distinct (lateral) diffusion. Applied stresses on the surface film (by compressions of the interface) may result in protein denaturation and aggregation. This process renders a surface film from a colloidal particle into that of a gelled system.
    Modulating surface rheology by electrostatic protein/polysaccharide interactions
    Ganzevles, R.A. ; Zinoviadou, K. ; Vliet, T. van; Cohen Stuart, M.A. ; Jongh, H.H.J. de - \ 2006
    Langmuir 22 (2006)24. - ISSN 0743-7463 - p. 10089 - 10096.
    air-water-interface - adsorbed protein layers - beta-lactoglobulin - o/w emulsions - complex coacervation - air/water interface - foam stability - shear rheology - adsorption - pectin
    There is a large interest in mixed protein/polysaccharide layers at air-water and oil-water interfaces because of their ability to stabilize foams and emulsions. Mixed protein/polysaccharide adsorbed layers at air-water interfaces can be prepared either by adsorption of soluble protein/ polysaccharide complexes or by sequential adsorption of complexes or polysaccharides to a previously formed protein layer. Even though the final protein and polysaccharide bulk concentrations are the same, the behavior of the adsorbed layers can be very different, depending on the method of preparation. The surface shear modulus of a sequentially formed ß-lactoglobulin/pectin layer can be up to a factor of 6 higher than that of a layer made by simultaneous adsorption. Furthermore, the surface dilatational modulus and surface shear modulus strongly (up to factors of 2 and 7, respectively) depend on the bulk ß-lactoglobulin/pectin mixing ratio. On the basis of the surface rheological behavior, a mechanistic understanding of how the structure of the adsorbed layers depends on the protein/polysaccharide interaction in bulk solution, mixing ratio, ionic strength, and order of adsorption to the interface (simultaneous or sequential) is derived. Insight into the effect of protein/ polysaccharide interactions on the properties of adsorbed layers provides a solid basis to modulate surface rheological behavior
    Quantitative description of the relation between protein net charge and protein adsorption to air-water interfaces
    Wierenga, P.A. ; Meinders, M.B.J. ; Egmond, M.R. ; Voragen, A.G.J. ; Jongh, H.H.J. de - \ 2005
    The Journal of Physical Chemistry Part B: Condensed Matter, Materials, Surfaces, Interfaces & Biophysical 109 (2005)35. - ISSN 1520-6106 - p. 16946 - 16952.
    der-waals contributions - beta-casein adsorption - surface concentration - electrostatic forces - air/water interface - circular-dichroism - globular-proteins - ovalbumin - kinetics - ph
    In this study a set of chemically engineered variants of ovalbumin was produced to study the effects of electrostatic charge on the adsorption kinetics and resulting surface pressure at the air-water interface. The modification itself was based on the coupling of succinic anhydride to lysine residues on the protein surface. After purification of the modified proteins, five homogeneous batches were obtained with increasing degrees of modification and -potentials ranging from -19 to -26 mV (-17 mV for native ovalbumin). These batches showed no changes in secondary, tertiary, or quaternary structure compared to the native protein. However, the rate of adsorption as measured with ellipsometry was found to decrease with increasing net charge, even at the initial stages of adsorption. This indicates an energy barrier to adsorption. With the use of a model based on the random sequential adsorption model, the energy barrier for adsorption was calculated and found to increase from 4.7kT to 6.1kT when the protein net charge was increased from -12 to -26. A second effect was that the increased electrostatic repulsion resulted in a larger apparent size of the adsorbed proteins, which went from 19 to 31 nm2 (native and highest modification, respectively), corresponding to similar interaction energies at saturation. The interaction energy was found to determine not only the saturation surface load but also the surface pressure as a function of the surface load. This work shows that, in order to describe the functionality of proteins at interfaces, they can be described as hard colloidal particles. Further, it is shown that the build-up of protein surface layers can be described by the coulombic interactions, exposed protein hydrophobicity, and size.
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