Enthalpy of interaction of some amides with ammonium methanoate in water at 25°C Interactions between ionic groups and peptide and hydrophobic groups

Abstract

The enthalpies of dilution at 25°C of aqueous solutions containing ammonium methanoate and a series of amides: formamide, acetamide, N-methylformamide, N-methylacetamide, N-ethylformamide, N-methylpropionamide, N,N-dimethylformamide, N,N-dimethylacetamide and N,N-diethylformamide have been measured. The dilution experiments were performed on solutions equimolal in the salt and the amides. The enthalpies of dilution of solutions containing only ammonium methanoate were also obtained. The results on the dilutions of the binary solute solutions were analysed using an excess function approach and enthalpic heterotactic interaction coefficients were obtained representing interactions between the ammonium methanoate and the amides. Consideration of these interaction coefficients using a group additivity approach shows that ammonium methanoate interacts more favourably, in a thermochemical sense, in water with tertiary amide (secondary peptide) groups than with secondary amide (primary peptide) groups, and that the interactions of ammonium methanoate with methylene groups in water are thermochemically repulsive. These results are qualitatively similar to those found for a range of salts interacting with groups present on peptides. Examination of the information currently available on salt–amide interactions in aqueous solutions, gives further support to the idea that these are fairly quantitative correlations between the propensity solutes in water have to interact with each other and their hydration energetics.