THERMODYNAMIC STUDIES ON THE INTERACTION OF N-ALKYL SULFATES WITH INSULIN IN AQUEOUS-SOLUTION

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-degrees-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.