Protein-surfactant complexation

It is well established that surfactants can associate with proteins to form molecular complexes involving various kinds of attractive interactions (Coke et ah, 1990 Bos et al., 1997 Bos and van Vliet, 2001 Kelley and McClements, 2003 Semenova et al., 2003, 2006 IE in et al., 2004, 2005 Malhotra and Coupland, 2004 Istarova et al., 2005 Semenova, 2007). The role of protein-surfactant complexes is a more important issue with charged surfactants. As a consequence of complexation, a mixed system of protein + surfactant may have completely different surface activity and rheological properties from those of the pure protein or the pure surfactant (see section 2 in chapter six for more details). 

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Y. Gohon, J.-L. Popot (2003) Membrane protein-surfactant complexes. Curr. Opin. Colloid Interface Sci., in the press... 

A situation that commonly occurs with food foams and emulsions is that there is a mixture of protein and low-molecular-weight surfactant available for adsorption at the interface. The composition and structure of the developing adsorbed layer are therefore strongly influenced by dynamic aspects of the competitive adsorption between protein and surfactant. This competitive adsorption in turn is influenced by the nature of the interfacial protein-protein and protein-surfactant interactions. At the most basic level, what drives this competition is that the surfactant-surface interaction is stronger than the interaction of the surface with the protein (or protein-surfactant complex) (Dickinson, 1998 Goff, 1997 Rodriguez Patino et al., 2007 Miller et al., 2008 Kotsmar et al., 2009). 

The results of surfactant-dependency on protein trahsfer indicate that protein extraction reverse micelles not only provide a hydrophilic droplet in a non-aqueous solvent to facilitate protein partition, but also make proteins sufficiently hydrophobic to solubilize into an organic solvent by coating the protein surface. Consequently, we suggest that proteins in the aqueous phase are extracted through the formation of an interfacial complex, a surfactant-coated protein and that the hydrophobic property dominates the extraction efficiency of the proteins, as seen in Figure 14.4. The unsaturated or branched alkyl chain may contribute to the formation of a soluble protein-surfactant complex into a non-aqueous solvent. 

To obtain the high extraction efficiency of a target protein, we need to design reverse micelles having sufficient hydrophobicity to extract the protein and prepare the conditions for enhancing the protein-surfactant interaction to form a protein-surfactant complex, which is a crucial intermediate for reverse micellar protein extraction. 

The use of fluorous solvents in catalysis has recently moved into the realm of biocatalysis." Protein-surfactant complexes were formed by hydrophobic ion... 

Surfactant-induced unfolding of enzymes is an important issue, and it has been addressed in some more fundamental research studies. In the early 1980s, Jones et al. conducted dialysis experiments to examine binding stoichiometry between surfactants and various protein molecules. Their work showed that anionic surfactants bind to proteins in a cooperative way, with hundreds of surfactant monomers binding to a single protein molecule. There is experimental evidence that the resulting unfolded protein-surfactant complex... 

In that most foodstuffs contain proteins, the formation of protein-surfactant complexes plays a direct role in 2. and an indirect role in 1. and 3., particularly with respect to their adsorption at oil-water interfaces and distribution between the phases. The surfactants used in foodstuffs are necessarily controlled, and permitted food emulsifiers are esters (or partial esters) formed from fatty acids (from animal and vegetable sources) with polyvalent alcohols such as glycerol, propylene glycerol, or sorbitol. 

In the field of research methods probably the most important application of protein surfactant complexation is in the technique of polyacrylamide gel electrophoresis in the presence of SDS, the so-called SDS-PAGE technique, used for the analysis and estimation of molecular masses of protein subunits [22], Protein subunit-SDS complexes are formed from proteins reduced by /3-mercaptoethanol to remove disulphide bonds. The binding of SDS to the polypeptide chains oc-... 

We do not generally find surfactants that only partially denature proteins although the course of denaturation involves the formation of a range of protein-surfactant complexes, the transition from native protein-surfactant complexes to denatured protein-surfactant complexes as a function of surfactant concentration is usually sharp (highly cooperative). 

Sedimentation coefficients of protein-surfactant complexes, subunit dissociation and molecular weights [26,27,43] Hydrodynamic volume and shape factors, protein unfolding [38,39]... 

The determination of binding and conformational changes leaves the question of the detailed structure of complexes unanswered. At present there is no absolute method for structure determination of protein-surfactant complexes apart from x-ray diffraction, which has only been applied to lysozyme with three bound SDS molecules [49]. X-ray diffraction requires a crystal, so in the case of lysozyme cross-linked triclinic crystals of the protein were soaked in 1.1 M SDS and then transferred to water or a lower concentration (0.35 M) of SDS to allow the protein to refold. It was necessary to use cross-linked crystals to prevent them dissolving when exposed to a high SDS concentration. The resulting denatured-renatured crystals were found to have three SDS molecules within a structure that was similar but not identical to that of native lysosyme. Neutron scattering has been applied in a few cases (see Sec. IX), but this is a model-dependent technique. 

The interfacial behavior of protein-surfactant complexes is important in several areas such as the stability of emulsions and foams and the adsorption of proteins and surfactants from their binary solutions onto solid surfaces. Of particular interest is the adsorption of the milk proteins /3-lactoglobulin and /3-casein at the oil-water interface in the presence of nonionic surfactants in relation to food emulsions [56-58] and foam stability [59]. The adsorption of gelatin at the air-water [52,53,60], oil-water [6], and solid-water [62] interfaces in the presence of surfactants has also been studied. Other studies reported include adsorption from aqueous solutions of lysozyme plus ionic surfactants at solid surfaces [63,64], /3-lactoglobulin plus SDS onto... 

Figure 8 Schematic illustration of four different adsorption/displacement models proposed by Wahlgren and Amebrant [63] for protein and surfactant adsorption to solid surfaces. The three diagrams for each model show protein adsorption, surfactant addition, and state after rinsing. Figure A represents the case where surfactant binds to the protein and the protein-surfactant complex desorbs. Figure B represents protein displacement by the surfactant. Figure C represents reversible adsorption of the surfactant by the protein. Figure D represents reversible adsorption by the surfactant resulting in partial desorption of the protein. The figures relate to a hydrophilic surface at a hydrophobic surface the orientation of the surfactant molecules with respect to the surface will be different. (Reproduced from [63] with permission from Academic Press.)...

For a rigorous interpretation of complexation it is important to appreciate that the formation of protein-surfactant complexes is an example of multiple equilibria between monomeric surfactant and complexes having different numbers of bound surfactant molecules. These equilibria, which we assume are reversible (see Sec. VIII), can be represented by a set of equations in which P is the protein and S the surfactant ... 

Table 3 Some Thermodynamic Parameters for the Formation of Protein-Surfactant Complexes... 

VIII. THE REVERSIBILITY OF PROTEIN-SURFACTANT COMPLEX FORMATION... 

The dissociation of protein-surfactant complexes can be induced by the presence of another surfactant with a lower hydrophobic-hydrophilic balance (higher cmc) due to the formation of mixed micelles [105], This effect is illustrated in Fig. 15 for complex formation between insulin and SDS in the presence of a constant concentration of sodium n-decylsulphate (SDeS). In a binary mixture of similar surfactants (SDS + SDeS) that behave as an ideal mixture in the micelles and in the solution, the cmc (C,2 ) is given by [106,107]... 

N(r) can be related to the fractal dimension D of the protein-surfactant complex by the equation N r) = (r/R jP. For a freely diffusing micelle the fractal dimension would be 3, but in a complex the distribution of micelles is dictated by the topology of the polypeptide backbone, so that the fractal dimension is less than 3. At 1% by weight of lithium n-dodecylsulphate, D is 2.3 and decreases to 1.76 at 3%, consistent with a transition from a compact state to a more open random coil in which a string of constant-sized micelles are distributed along the hydrophobic patches of a denatured random coil, although the coil will be restrained by the 17 disulphide bonds in the BSA structure. 

Stationary phase-macromolecule repulsion is yet more evident in SEC of proteins on SW columns in sodium dodecyl sulfate (SDS), in which solvent strongly anionic protein-surfactant complexes are formed. A pronounced ionic strength dependence was observed (76) for the MW calibration curves obtained with a series of globular proteins in 0.1% SDS containing 0.02 M - 0.2 M pH 7 phosphate buffer. The effect of ionic strength on the shapes of these calibration curves is quite similar to observations with strong polyanions. The pronounced influence of I on Ksec °f proteins in SDS eluant was also observed by Takagi et al. (77). 

Equation (3) may be applied to the measurment of the molecular weight for a protein-surfactant complex. The molecular weight, Mx of any protein moiety in a protein-surfactant complex "x" can be calculated from the following equation by comparison with that of standard protein "std" [11]. 

Native membrane proteins are typically stabilized In the presence of surfactants and exist as protein-surfactant complexes in solution. This fact complicates the measurements of the physico-chemical properties of membrane proteins in solution. 

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