Calorimetric studies of the interactions of guanidinium hydrochloride and potassium iodide with model amides in aqueous solution.

Abstract

AbstractThe enthalpies of transfer, ΔHtr, of a series of amides from water to aqueous solutions of either guanidinium hydrochloride (GuHCl) or potassium iodide were obtained from calorimetric measurements at 25°C. The amides were studied at molalities around 10−2 m while salt molalities ranged from 0–10 m. The amides investigated were Ac‐Gly‐NHMe, Ac‐Gly‐Gly‐NHMe, Ac‐Ala‐NHMe, and Ac‐Leu‐NHMe. Use of an additivity assumption allowed the calculation of group contributions to ΔHtr in these two salt systems for the methyl group, leucyl side chain, and the peptide backbone unit. Values of the entropy of transfer were also obtained. The great ability of GuHCl to randomize protein structures appears to arise from effects on polar and nonpolar groups, which are characterized by enthalpies and entropies of transfer not substantially different from those with KI, a salt comprised of ions of comparable size and polarizability. The difference in the sign of the free energies of transfer of nonpolar groups from water to MX solutions, negative for GuHCl and positive for KI, is the result of these small differences in enthalpies and entropies of transfer. Variations in water structure produced by differences in ionic properties rather than a mode of action for GuHCl very different from that of other salts characterizes its superior denaturing ability.