Chemical Transfer Energies of Some Homologous Amino Acids and the -CH_2- Group in Aqueous DMF: Solvent Effect on Hydrophobic Hydration and Three Dimensional Solvent Structure

摘要

Standard transfer Gibbs energies, Δ_(tr)G°, of a series of homologues α-amino acids have been evaluated by determining the solubility of glycine, alanine, amino butyric acid and norvaline gravimetrically at 298.15 K. Standard entropies of transfer, Δ_(tr)S°, of the amino acids have also been evaluated by extending the solubility measurement to five equidistant temperatures ranging from 288.15 to 308.15 K. The chemical contributions Δ_(tr,ch)G° (i) of α-amino acids, as obtained by subtracting theoretically computed contributions to Δ_(tr)G° due to cavity and dipole-dipole interaction effects from the corresponding experimental Δ_(tr)G°, are indicative of the superimposed effect of increased basicity and dispersion and decreased hydrophobic hydration (hbh) in DMF-water solvent mixtures as compared to those in water, while, in addition, TΔ_(tr,ch)S° (i) is guided by structural effects. The computed chemical transfer energies of the -CH_2- group, Δ_(tr,ch)P°(-CH_2-) [P = G or S] as obtained by subtracting the value of lower homologue from that of immediately higher homologue, are found to change with composition indicating involvement of several opposing factors in the calculation of the chemical interactions. The Δ_(tr,ch)G° (-CH2-) values are found to be guided by the decreased hydrophobic effect in DMF-water mixtures, and are indicative of the nature of the three dimensional structure of the aquo-organic solvent system around each solute.