Chemical Transfer Energetics of a Series of Homologous α-Amino Acids in Quasi-Aprotic 2-Methoxyethanol-Water Mixtures

The zwitterionic solvation behavior of a series of the homologous alpha-amino acids glycine (Gly), DL-alanine (Ala), DL-alpha-amino butyric acids (Aba) and DL-nor-valine (Val), in water and in aqueous mixtures of 2-methoxyethanol, have been studied at equidistant temperature intervals in the range of 288.15-308.15 K, using the solubility method. The solubility data were analyzed according to the 'formol titrimetric' process. Standard Gibbs energies and entropies of transfer were calculated and are discussed. The observed and versus composition profiles are complicated because of the various interaction effects. The chemical effects of the transfer Gibbs energies and entropies of transfer have been obtained after elimination of the cavity effect, estimated by the scaled particle theory and dipole-dipole interaction effects, calculated by use of the Keesom orientation expression. The chemical contributions to transfer energetics of homologous alpha-amino acids are guided by the combined effects of increased dispersion interaction, basicity and decreased acidity, hydrogen bonding effects and hydrophilic hydration of aqueous 2-methoxy ethanol as compared to that of reference solvent, water. In this context the solvating characters of protic ethylene glycol are also referred to for comparison.