Spectroscopic and microcalorimetric study of the interaction of n-alkyl sulfates with insulin in aqueous solution

Abstract

A surfactant-induced conformational transition of bovine insulin that leads to difference spectra assigned to changes in the environment of the tyrosine residues has been studied at pH 10, 25 °C. The transition induced by a homologous series of C₈–C₁₂n-alkyl sulfates, below their critical micelle concentrations was studied by difference spectroscopy and the absorbance changes at 295 nm were analysed to obtain values for the Gibbs energies of the transition in water (ΔG⁰_w) and in a hydrophobic environment (ΔG⁰_hc) pertaining to saturated protein–surfactant complexes. The average value of ΔG⁰_w, which was found to be independent of n-alkyl chain length, was 14.6 kJ mol^–1. The values of ΔG⁰_hc were in the range ca.–40 to –100 kJ mol^–1 for chain lengths from C₈ to C₁₂. The enthalpies of interaction of the n-alkyl sulfates were measured over the surfactant concentration ranges of the transition and were used to estimate an enthalpy change for the transition of 97 ± 10 kJ mol^–1.