Protein-surfactant complexes, solubilizing

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. 

The present chapter demonstrates Chat the macromolecular properties of membrane proteins composed of subunits can be determined by the SEC/LALLS technique. The molecular weight of the protein moiety in protein-surfactant complex cannot be determined without measurement of the value of the relevant refractive index Increment. Furthermore, the determination of the molecular weight of the protein moiety in a protein-surfactant complex is complicated because of the necessity of a rigorous measurement of the protein concentration. Weight measurement is Inadequate for this purpose because solubilized membrane proteins bind surfactant molecules. The microburet method, which has been employed for this purpose in our receptor protein study, is reasonable nonspecific for this type of protein [34]. 

Homogeneous biocatalysis in both fluorous biphasic and supercritical CO systems has been demonstrated [6]. By forming protein-surfactant complexes by hydrophobic ion pairing with a highly fluorinated anionic surfactant cytochrome c can be solubilized in perfluoromethylcyclohexane (PMFC) and in scCO. The secondary structure of the proteins within these ion-paired complexes has been shown to remain intact, and particle size analysis indicated that small aggregates of protein molecules surrounded by surfactant molecules are formed. The presence of the KDP (perfluoropolyether carboxylate surfactants) ion paired with a-chymotrypsin appears to enhance its catal)4ic activity as compared to that of the native enzyme in a fluorous biphasic system. The facile recycling of the a-chymotrypsin-KDP complex in a fluorous biphasic system has been demonstrated with retention of enzyme artivity over four reaction cycles. 

A recent and exciting area of research is the solubilization of enzymes in nonaqueous solvents. One way solubilization is achieved is through noncovalent complexes of lipid (surfactant) and protein, to be referred to here as enzyme-lipid aggregates, or ELAs. Such complexes are reported to be highly active and stable. Moreover, the activity of ELAs can be significantly higher than free, suspended enzyme (in the absence or presence of surfactant), enzymes solubilized in aqueous-organic biphasic systems, or reverse micellar solutions, and can approach catalytic rates in... 

One of the major goals of these many investigations of lipids is, of course, a better understanding of the in - vivo behavior of membranes. Beyond studies of binary lipid mixtures, as mentioned above, a further step which is necessary is the incorporation of proteins into the layers. In many respects, this increase in the complexity of the bilayer systems resembles that encountered in the use of synthetic surfactants in "real - world" situations, where blends, rather than single, surfactants are used. Surfactant blends in aqueous solutions are often further modified in use by the solubilization of oily organic compounds, as in the cases of detergency or cosmetic formulation. 

Detergents are used to solubilize and study biological membranes and proteins (63-65). Detergents or surfactants disrupt membranes by intercalating into phospholipid bilayers and solubilizing lipids and proteins. Nonionic surfactants are deemed to be milder detergents and do not usually result in protein denaturation. Nonionic surfactants form complexes with membrane proteins that are more or less fixed in the cell membrane (61). 

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