Thermodynamic studies on the interaction of n-alkyl sulfates with insulin in aqueous solution

Abstract

Binding isotherms for the interaction of a homologous series of sodium n-alkyl sulfates (chain lengths 8–10 and 12) on interaction with bovine insulin at 25 °C, at pH 3.2 and 10.0, have been determined. The isotherms show two unusual features, first the binding ranges up to 60–70 surfactant molecules per insulin molecule and secondly it is only slightly dependent on the n-alkyl chain length. The enthalpy changes on surfactant–insulin interaction have been measured by microcalorimetry and have been discussed in terms of exothermic contributions from sulfate head-group interactions with cation amino acid residues in the protein and an endothermic unfolding contribution. The exothermicities of the head-group interactions increase with decreasing pH but are in excess of those predicted from model studies on surfactant–polypeptide interactions. The Gibbs energies of binding per surfactant molecule decrease with increasing binding to a limiting value of ca.– 15 kJ (mol surfactant)–1 independent of pH. The high binding levels coupled with the slight chain length dependence suggest a micellar-based structure for the insulin–surfactant complexes.