Enthalpies of transfer for amide-water-electrolyte systems

The enthalpies of transfer for model amides, 1,1 -dimethylacetamide, N-methylacetamide, acetamide, formamide 1 N-methylformamide, N-acetylglycine-N-methylamide, N-acetylalanine-N-methylamide, N-acetylleucine-N-methyl-methylamide, and N-acetylglycylglycine-N-methylamide, from water to aqueous salt solution have been measured at 25° using the method of solution calorimetry. Measurements were done at limiting amide concentrations over a wide range of salt concentrations, 0.1 to 7.0 molal in some cases. Salts employed included alkali halides, alkaline earth halides, and guanidine hydrochloride. Trends in the heats of transfer were observed in an attempt to better characterize the forces responsible for isothermal denaturation by electrolytes. From the dependence of the amide transfer upon 1) the total polarization density of the solvent, 2) the length of the peptide, 3) the configuration of the peptide, 4) the substituents on the peptide, and 5) the molecular symmetry of the amide, it is postulated that dispersion forces are ~-t in these systems. Finally, a model is presented for enzyme denaturation baled upon the model compound data and this is tested for ~ case of lysozyme