Aqueous solutions containing amino acids and peptides. Part 27.—Partial molar heat capacities and partial molar volumes of some N-acetyl amino acid amides, some N-acetyl peptide amides and two peptides at 25 °C

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Abstract
Partial molar heat capacities, in water at 25 °C, of the compounds N-acetylglycinamide, N-acetyl-L-alaninamide, N-acetylglycylglycinamide, N-acetylglycyl-L-alaninamide, N-acetyl-L-alanylglycinamide, N-acetyl-L-alanyl-L-alaninamide, glycyl-DL-alanine and DL-alanylglycine have been obtained. The information obtained, for the amides, is discussed in terms of group-additivity approaches, and group contributions are evaluated. The values for these differ in some instances, and particularly that for the heat capacity of the peptide group, from those obtained in an earlier study. Comparison of the parent amino acids and peptides with the amides shows that there is a marked contribution to the heat capacities of the zwitterionic compounds from electrostatic sources. Some results are presented of the partial molar volumes, also at 25 °C, of N-acetylglycyl-L-alaninamide, N-acetyl-L-alanylglycinamide and DL-alanylglycine. A correlation is evident between the heat capacity and volumetric interaction coefficients, for amino acids, peptides and N-acetylamides, obtained from the molality dependences of the apparent molar properties, and this has been rationalised using a simple electrostatic approach.

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