In this paper, the dissociation enthalpies of three amino acids in ethanol/water mixtures at 298.15K have been determined by LKB-2277 BioActivity Monitor. Combined with the values of free energies, the dissociation entropies have been calculated. Several proton exchange reactions between the given amino acids and the reference acids selected in ethanol/water mixtures have been examined in detail Solute-solvent interactions and the structural effect of the ethanol/water mixtures have been discussed. The results show that substituent effect is most effective in the region with the strongest structural enhancement of the mixed solvents. It is considered that the life of the bulky iceberg-like water clusters is lengthened greatly in the region rich in water.

Key words: WATER, ETHANOL, AMINO ACID, DISSOCIATION, SUBSTITUENT EFFECT, GLYCINE, ALANINE, SERINE