SPECTROSCOPIC AND MICROCALORIMETRIC STUDY OF THE INTERACTION OF N-ALKYL SULFATES WITH INSULIN IN AQUEOUS-SOLUTION

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-degrees-C. The tra nsition induced by a homologous series of C-8-C-12 n-alkyl sulfates, b elow their critical micelle concentrations was studied by difference s pectroscopy and the absorbance changes at 295 nm were analysed to obta in values for the Gibbs energies of the transition in water (DELTAG(w) 0) and in a hydrophobic environment (DELTAG(hc)0) pertaining to satura ted protein-surfactant complexes. The average value of DELTAG(w)0, whi ch was found to be independent of n-alkyl chain length, was 14.6 kJ mo l-1. The values of DELTAG(hc)0 were in the range ca. -40 to -100 kJ mo l-1 for chain lengths from C-8 to C-12. The enthalpies of interaction of the n-alkyl sulfates were measured over the surfactant concentratio n ranges of the transition and were used to estimate an enthalpy chang e for the transition of 97 +/- 10 kJ mol-1.