Vapor pressures of aqueous solutions of l-amino acids (glycine, l-alanine, l-valine, and l-serine) were measured at various concentrations at 298 K by a differential pressure static method. Activities and activity coefficients of water were determined. The activity coefficients of the amino acids in water were obtained from the activity coefficients of water using the UNIQUAC equation. The obtained activity coefficients of the amino acids were evaluated by comparison of the values obtained by a conventional virial expansion method and the literature data.

ABSTRACT

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The enantioselectivity of cross-linked crystals of γ-chymotrypsin in the transesterification of the medicinally important compound methyl 3-hydroxy-2-phenylpropionate (1) with propanol has been examined in a variety of organic solvents. The (k cat/K M) S /k cat/K M) R ratio in this enzymatic process can be forced to span a 20-fold range simply by switching from one solvent to another; in fact, while the enzyme strongly prefers the S-enantiomer of 1 in some solvents, the R-antipode is more reactive in others. These striking observations are quantitatively rationalized by accounting for the energetics of desolvation of S-1 and R-1 in the enzyme-bound transition states. In order to accomplish this, explicit rules have been established for the modeling and thermodynamic quantification of the partially desolvated substrate's transition state moieties.

The solubility of an amino acid in an aqueous solution containing another amino acid was measured at 298 K. The measured amino acid pairs were DL-alanine/DL-serine and DL-alanine/DL-valine. The invariant eutectic compositions of these pairs of amino acids were also determined. In addition, a prediction model of solubility for such a system was developed on the basis of solid-liquid equilibria. To represent more accurately the activity coefficient of amino acids and homopeptides by Larsen's UNIFAC model, new assignments of UNIFAC groups were proposed and their interaction parameters were determined from experimental osmotic coefficient data. Finally, the prediction of solubilities of mixed solid amino acids was made and the results were compared with experimental data.