Protein complexes, interfacial films

Some Properties of Interfacial Films of Colloidal Electrolyte-Protein Complexes... 

Nonsurface-active PS as X strongly increase the surface pressure and the elastic character of the interfacial film. This imexpected behavior would arise from incompatibility with the aqueous bulk phase and possible complexation onto the adsorbed protein. 

Stryer L 1986 Cyclic GMP cascade of vision Anna. Rev. Neurosci. 9 87-119 Cafiso D S and Hubbell W L 1980 Light-induced interfacial potentials in photoreceptor membrane Biophys. J. 30 243-63 Drain C M, Christensen B and Mauzerall D 1989 Photogating of ionic currents across a lipid bilayer Proc. Natl Acad. Sci. USA 86 6959-62 Vsevolodov N N, Druzhko A B and Djukova T V 1989 Actual possibilities of bacteriorhodopsin application Molecular Electronics Biosensors and Biocomputers ed F T Hong (New York Plenum) pp 381-4 Vsevolodov N N and Dyukova T V 1994 Retinal-protein complexes as optoelectronic components Trend. Biotechnol. 12 81-103 Vsevolodov N N, Djukova T V and Druzhko A B 1989 Some methods for irreversible write-once recording in Biochrom films Proc. Annu. Int. Conf IEEE Eng. Med. Biol. Soc. 11 1327... 

Proteins and proteoglycans are important stabilisers of emulsions and foams in many food and non-food applications [3]. The interactions between protein molecules, either when adsorbed at interfaces or when present in the bulk, are very complex and are connected with changes in the conformation of the folded polypeptide chains. These changes have a wide range of characteristic time-scales [4]. When these particular features of proteins are superimposed on the above picture of a deformed interfacial film it is seen that the task of understanding the mechanism of action of proteins as surfactants is a daunting task. 

For SDS, the reaction proceeded to a reproducible end point rapidly —viz., 1 to 2 minutes—when nonionic surface active impurities such as parent dodecyl alcohol, DOH, were removed by ethyl ether extractions. This impurity effect was verified by adding traces of alkyl alcohol—viz., 1 X 10 9 mole per liter—to purified SDS, whereupon the penetration reaction rate was halved. A possible explanation for this behavior is that formation of an SDS-DOH interfacial complex reduced the SDS activity in the interface and consequently its rate of reaction with the protein monolayer. The reasons for the somewhat slower rate of reaction of Cetab with the protein film are more obscure. The reaction rate did not increase after extracting the detergent repeatedly. Two possible reasons for the time dependence in this case may have been that (1) the ether extraction method was not effective in removing surface active impurities, or (2) because of the greater bulk of the Cetab hydrocarbon chain, Ci6 vs. Ci2 for SDS, more time was required for diffusion and appropriate orientation before complex formation. 

These requirements would be fulfilled if SDS were bound to the BSA monolayer in the form of small aggregates or pseudo-micelles. Such aggregates have been demonstrated to be formed as the result of the interaction of SDS and BSA in solution (2). Further, the electrostatic nature of the interaction was demonstrated by the fact that the complex was completely dissociated by adjusting pH to values above 10.0. Therefore, it is suggested that the cause of the marked shift in pK of the ammonium groups of the SDS-BSA surface complex was the presence of aggregates of SDS bound at cationic sites of the protein monolayer. It may be inferred from this hypothesis that the natural result of the interaction of anionic lipids with an interfacial protein film is the formation of a mosaic structure—one of the proposed characteristics of biological membranes. 

Interactions between proteins and polysaccharides give rise to various textures in food. Protein-stabilized emulsions can be made more stable by the addition of a polysaccharide. A complex of whey protein isolate and carboxymethylcellulose was found to possess superior emulsifying properties compared to those of the protein alone (Girard et al., 2002). The structure of emulsion interfaces formed by complexes of proteins and carbohydrates can be manipulated by the conditions of the preparation. The sequence of the addition of the biopolymers can alter the interfacial composition of emulsions. The ability to alter interfacial structure of emulsions is a lever which can be used to tailor the delivery of food components and nutrients (Dickinson, 2008). Polysaccharides can be used to control protein adsorption at an air-water interface (Ganzevles et al., 2006). The interface of simultaneously adsorbed films (from mixtures of proteins and polysaccharides) and sequentially adsorbed films (where the protein layer is adsorbed prior to addition of the polysaccharide) are different. The presence of the polysaccharide at the start of the adsorption process hinders the formation of a dense primary interfacial layer (Ganzelves et al., 2008). These observations demonstrate how the order of addition of components can influence interfacial structure. This has implications for foaming and emulsifying applications. 

Monolayer techniques were used to characterize the interfacial properties of the resultant Fractions. Fraction I contained highly cohesive complexes that did not unfold at the interface and had an average diameter of 9.1 nm. These particles are thought to represent submicelles, previously identified in micelle formation. Fraction II showed interfacial properties that are characteristic of spread casein monomers, and contained mainly a -casein. The results are discussed in relation to casein interactions and micellar formation. Mixed monolayers of sodium caseinate/glyceride monostearate (NaCas/GMS) were also examined at different composition ratios. The results show that for low surface pressures (0-20 mNm ), there is a condensation ascribable to hydrophobic interactions in the mixed film. At high surface pressures, the hydrophobic interaction is modified and the protein is expelled from the monolayer into the subphase. These results are discussed in relation to emulsion stability. 

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