Thermodynamics of transfer of amino acids from water to ethanol-water mixtures

366-370Dissociation constants of -tryptophan, -histidine, -aspartic acid, -glutamic acid and t-serine have been determined in water and ethanol-water mixtures at 298 K potentiometrically by the Calvin-Bjerrum titration technique. These values have been used to calculate the free energies of transfer G(i) involved in the dissociation equilibria. G (RH) and G(R2-) have been calculated (RH±) being the zwitter ion) knowing the solubilities of these acids in ethanol-water mixtures. Contribution due to amino acid side chain to the free energy of transfer from 100% ethanol to water has been estimated in acidic, neutral and basic media. The equivalence of these values confirms the hydrophobicity scale first proposed by Nozaki and Tanford [J bioi Chem, 246 (1971) 2211]