Calorimetric and volumetric studies of the interactions of some amides in water and in 6 mol dm–3 aqueous guanidinium chloride

Abstract

The apparent molar volumes of the amides formamide, N-methylacetamide (NMA) and N,N-diethylacetamide (DEA), in water and in 6 mol dm^–3 guanidinium chloride solutions, have been determined at 25 °C. Results are also given for the enthalpies of dilution of formamide, NMA, acetamide, N-methylformamide (NMF), N,N-dimethylformamide (DMF) and N-acetylglycinamide (GLY), in 6 mol dm^–3 guanidinium chloride solutions, at 25 °C. From the data, partial molar volumes and volumetric and enthalpic interaction coefficients were obtained. The standard state volumes of transfer, from water to 6 mol dm^–3 guanidinium chloride solutions, are small and suggest an interplay between the solvational effects of polar and apolar groups on the amides. The volumetric interaction coefficients become increasingly negative as the apolarity of the solutes increases. The enthalpic coefficients in 6 mol dm^–3 guanidinium chloride solutions have been treated using a group-additivity approach and the indications are that these are dominated by interactions, which are thermochemically repulsive, between solvated amidic and apolar groups. Some comments are made on the possible mode of action of guanidinium chloride and urea as protein denaturants.