Enthalpy of interaction of ribonuclease A and n-alkyl sulphates in aqueous solution

Abstract

The enthalpy changes on interaction of a series of sodium n-alkyl sulphates (C₁₂, C₁₀ and C₈) with ribonuclease A have been measured by microcalorimetry at 25 °C in aqueous solution at pH 3.2, 7.1 and 10.0. Sodium n-dodecyl sulphate interacts exothermally with specific cationic amino-acid sites on ribonuclease and the coincident endothermic unfolding is to a large extent thermally balanced by the ligand binding. In contrast, at pH 3.2 the C₁₀ and C₈ sulphates bind to ribonuclease but the overall enthalpies of interaction are dominated by the endothermic contribution, which is attributed to the unfolding of the polypeptide chain. At pH 7.1 and 10.0 the C₁₀ sulphate unfolds ribonuclease, whereas the C₈ sulphate interacts almost athermally. The combined contribution to the overall enthalpy of interaction of C₁₀ sulphate with ribonuclease at pH 3.2 and 7.0 from unfolding and apolar binding has been evaluated from the present results and is of similar magnitude to that for unfolding reported from thermal denaturation studies.