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The relationship between hydrophobicity and surfactant properties of proteins Keshavarz, Elaheh

Abstract

The composition, structure and functional properties of proteins are thought to be interrelated but the nature of this relationship is not clearly known. In most studies of the structure-function relationships of proteins, more attention has been focused on the polar residues while the importance of the nonpolarity of the protein molecule has been overlooked. It seems necessary to study the relationships, if any, between hydrophobicity and functional properties which are important in food systems. The purpose of this study has been two-fold. First, establishment of methods for determination of hydrophobicity, and second, to correlate the hydrophobicities with the surfactant properties of proteins. The hydrophobicities calculated so far (Bigelow, 1967; Tanford, 1962; Waugh, 1954) from the amino acid compositions do not appear to correlate with the propensity of protein to form hydrophobic interactions. In an attempt to determine the effective hydrophobicity (the capacity to participate in hydrophobic interactions), column chromatography on substituted Sepharose gels was employed. Oleic acid, aliphatic and aromatic amines were coupled to the CNBr activated Sepharose 4B. However, these gels were not suitable for determination of hydrophobicity. This may be due to the presence of undesirable charges or to the exceedingly strong hydrophobic interactions between the ligand and the proteins. Chromatography on Sephadex G-150 was also employed in the presence of Triton X-100 and the amount of the nonionic detergent bound to the protein was determined. However, lack of repeatability in the determination of the detergent bound to the protein prohibited the application of this method for determination of the effective hydrophobicity. Alkylepoxy derivatives of Sepharose 4B (C4, C6 and C8) were synthesized. Hydrophobic chromatography on the butyl and hexyl derivatives was successful in determination of hydrophobicity. However, since octylepoxy-Sepharose, because of its high hydrophobicity, tightly interacted with the proteins it was found to be impractical for the purpose of measuring hydrophobicity. In an aqueous solvent phase, these adsorbents demonstrate some of the properties of an oil/water interface, including the possibility of denaturing some proteins. Therefore, it is assumed that the proteins were denatured on the octylepoxy-Sepharose gel. The hydrophobic partition method was also employed. Two phase polymer systems of polyethylene glycol/dextran and polyethylene glycol palmitate/dextran were used and the extent of the hydrophobic binding of the proteins to the palmitate group was expressed as the "hydrophobic coefficient" of the proteins. The effective hydrophobicities of proteins determined by hydrophobic chromatography with butyl- and hexylepoxy-Sepharoses and by hydrophobic partition significantly correlated with each other. All of these three methods were found suitable for determination of the effective hydrophobicity. No correlation was found between the effective hydrophobicity and the "average hydrophobicity" (Bigelow, 1967) nor with the molecular weights of the proteins. The interfacial tensions of the 0.2% protein solution/ corn oil interfaces were determined as a parameter of the surfactant properties of proteins. A negative correlation was found to exist between the effective hydrophobicities and the interfacial tensions of the proteins. This result suggests that hydrophobicity is involved in the surfactant properties of proteins. The more hydrophobic the protein, the better the surface active properties, thereby facilitating the emulsifying process.

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