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Dissolution enthalpies of L-alpha-aminobutyric acid, L-alpha-isoleucine, L-alpha-phenylalanine, L-alpha-methionine, L-alpha-serine, L-alpha-threonine, L-alpha-cysteine, L-alpha-asparagine and L-alpha-glutamine in aqueous solutions of urea have been measured by calorimetry at 298.15 K. The obtained results were used to calculate the enthalpic interaction coefficients between the zwitterions of the L-alpha-amino acids and a molecule of urea in water. These values were interpreted in terms of the hydrophobic or hydrophilic effects of the side chains of amino acids on their interactions with a polar molecule of urea in water.

Abstract: The enthalpies of solution of N-acetyl-N′-methylglycinamide, N-acetyl-N′-methyl-l-α-alaninamide, N-acetyl-N′-methyl-l-α-leucinamide and N-acetyl-N′-methyl-l-α-serinamide have been measured in water and in aqueous urea solutions with molalities from 0.25 to 3.0molkg−1 at 298.15K. From these data the standard dissolution enthalpies of amides in aqueous urea solutions have been determined. The results have been treated according to McMillan–Mayer''s theory in order to obtain the enthalpic coefficients of the interactions between amino acid derivatives and urea molecules. The obtained parameters were compared with the hydrophobic scale for the amino acid side chains. [Copyright &y& Elsevier]