Protein-surfactant complex

Figure 6.9 Effect of CITREM concentration on the molecular and thermodynamic parameters of complex protein-surfactant nanoparticles in aqueous medium (phosphate buffer, pH = 7.2, ionic strength = 0.05 M 20 °C) (a) extent of protein association, k = Mwcomplex/Mwprotem (b) structure-sensitive parameter, p (c) second virial coefficient, A2 (rnolal scale) (d) effective charge, ZE (net number n of moles of negative charges per mole of original sodium caseinate nanoparticles existing at pH = 7.2 (Mw = 4xl06 Da)). The indicated cmc value refers to the pure CITREM solution. Reproduced from Semenova et al. (2007) with permission.

The interaction between macromolecules and surfactants in aqueous solution has been extensively investigated for a few decades. First, the protein-surfactant systems were stressed because of their biological importance. With the appearance of well-defined synthetic polymer m< els, the research was extended to the neutral polymer-surfactant systems, too, with the promise of a deeper insight into the more complex protein-surfactant interaction. 

A protein-surfactant-water complex forms in the solid phase. Upon adsorption of a certain number of surfactant molecules, the complex s hydrophobicity increases, driving its transport into the liquid phase. 

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

Self-organization systems under thermodynamic control (spontaneous processes with a negative free-energy change), such as supramolecular complexes, crystallization, surfactant aggregation, certain nano-structures, protein folding, protein assembly, DNA duplex. 

Analysis of recent experimental data has shown that there are various issues that have to be considered when interpreting the complex character of the interactions in mixed protein + surfactant systems (Kelley and McClements, 2003 II in et al., 2004, 2005 Malhotra and Coupland, 2004 Istarova et al., 2005). Below we identify these general issues as factors A, B, C and D. 

In general, surface activity behaviour in food colloids is dominated by the proteins and the low-molecular-weight surfactants. The competition between proteins and surfactants determines the composition and properties of adsorbed layers at oil-water and air-water interfaces. In the case of mixtures of proteins with non-surface-active polysaccharides, the resulting surface-activity is usually attributed to the adsorption of protein-polysaccharide complexes. By understanding relationships between the protein-protein, protein-surfactant and protein-polysaccharide interactions and the properties of the resulting adsorbed layers, we can aim to... 

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). 

Gurov, A.N., Nuss, P.V. (1986). Protein-polysaccharide complexes as surfactants. Nah-rung, 30, 349-353. 

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. 

On the protein extraction behaviour using reverse micelles, the protein-surfactant interaction between charged protein surfaces and surfactant headgroups is a dominant factor in distinguishing the target proteins. In particular, some researchers have suggested that a protein can be extracted as a hydrophobic ion complex between a protein and surfactant molecules [6,7,12-15]. Therefore, an intrinsic factor of proteins also gives considerable modulation in the extraction behaviour, in which the environmental factors were maintained. 

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... 

Another feature of adsorption from solution is the variety and complexity of molecules that may be involved in the processes. Indeed one can be interested either by a simple organic molecule, like benzene and its derivatives, or by much larger molecules like proteins, surfactants, or polymers, which bear many different chemical functions and may adopt a large number of conformations at the interface. For such molecules, a good knowledge of both the surface chemistry and the accessibility of porous materials are crucial to understand the adsorption phenomenon. 

In order to assess the effect of compression (expansion) on more complex mixed layers (protein + protein or protein + surfactant), we have simulated four different binary systems. The mixtures are composed of two species of the same spherical size in a 1 1 molar ratio. In all cases, one of the species (Type 1) interacts solely through the repulsive core potential both with particles of its same type and with particles of Type 2. The Type 2 particles, however, are able to form bonds with particles of their ovm type. The four different cases correspond to different classes of bonding between the particles of Type 2 (a) no bonds, (b) very-easy-to-break bonds (fcmax = 0-3)i (c) breakable bonds (fcmax = 0-5), and (d) permanent bonds (fcmax = °°)-The structures of fhe four differenf sysfems after 6 X 10 equilibration time steps are shown in Figure 23.3. Case (a) represents a perfect mixture since... 

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]... 

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