THERMODYNAMICS OF THE HYDROLYSIS OF N-ACETYL-L-PHENYLALANINE ETHYL-ESTER IN WATER AND IN ORGANIC-SOLVENTS

Equilibrium measurements have been performed on the alpha-chymotrypsin -catalyzed hydrolysis reaction of N-acetyl-L-phenylalanine ethyl ester to (N-acetyl-L-phenylalanine + ethanol) with carbon tetrachloride, di chloromethane, toluene, and aqueous phosphate buffer as solvents for t he reactants and products. Apparent equilibrium constants were measure d as a function of temperature for this reaction in all four solvents. Calorimetric measurements were also performed for this reaction in aq ueous phosphate buffer. The principal reaction occurring in the aqueou s phosphate buffer at pH 6-7 is N-acetyl-L-phenylalanine ethyl ester(a q) + H2O(aq) N-acetyl-L-phenylalanine(-)(aq) + ethanol(aq) + H+(aq). T herefore, to compare the results for the reaction in water with those for the reaction in the organic solvents where it is assumed only neut ral species are present, it was necessary to adjust the experimental r esults to the reaction involving neutral species: N-acetyl-L-phenylala nine ethyl ester(sln) + H2O(sln) = N-acetyl-L-phenylalanine(sln) + eth anol-(sln), where sln denotes either aqueous media, carbon tetrachlori de, dichloromethane, or toluene. The values of the equilibrium constan t for this latter reaction, with the concentration of water included i n the expression for the equilibrium constant, ranged from 0.057 to 0. 20 at T = 298.15 K for the four solvents. This rather limited range of values for the equilibrium constants is significant. The very limited amount of information available from the literature is also suggestiv e of the rule that equilibrium constants for hydrolysis reactions in d ifferent solvents are comparable if the reaction refers to neutral spe cies and the concentration of water is included in the formulation of the equilibrium constant. Also, the standard molar enthalpy of reactio n was found to be a linear function (slope = 313 K) of the standard mo lar entropy of reaction. This is indicative of an enthalpy-entropy com pensation effect.