Enthalpies of interaction of some N-acetyl amino acid amides in aqueous urea solutions at 298.15 K

Abstract

Enthalpies of dilution of the N-acetyl amino acid amides of glycine, L-alanine, L-valine, L-leucine, L-proline, L-phenylalanine, sarcosine and N-methyl-L-alanine, and the N-acetyl-N′-methyl amino acid amides of glycine, L-alanine, L-leucine, L-proline, sarcosine, and N-methyl-L-alanine dissolved in aqueous 8 mol dm^–3 urea have been measured calorimetrically at 298.15 K. The results were used to calculate enthalpic interaction coefficients employing a concentration expansion of the enthalpies of dilution. The enthalpic pairwise interaction coefficients obtained are more positive in 8 mol dm^–3 urea than in water in most cases. It is proposed that aqueous urea influences the interactions between peptides in two ways. First, urea solvates the amide groups of the peptides and, secondly, urea reduces the hydrophobic interaction of the apolar side chains of the peptides. The consequences for the ability of aqueous urea to denature globular proteins are discussed.