Excess enthalpies and apparent molar volumes of some N-methyl substituted amino acids in aqueous solutions

The excess enthalpies and limiting partial molar volumes at 25°C for aqueous solutions of N-methyl glycine, N-methyl alanine, and N-methyl serine are reported and compared with the same properties for the parent amino acids. For each N-methyl derivative the enthalpic contribution to the pairwise interaction is less favorable than that for the parent amino acid. The contribution of the N-methyl substituent to V ₂ ⁰ is similar for each amino acid, and is about 2 cm³-mol^–1 greater than for a methyl substituent on the -carbon. These observations have been rationalized in terms of the likely solute-solvent interactions. In addition a rigid particle model is used with the volume data to examine solute-solvent interactions.