Many mechanisms have been proposed for catalysis by -chymotrypsin using the imidazole and serine hydroxyl groups as features of the catalysis. These include: (1) nucleophilic catalysis by imidazole of acylenzyme formation and decomposition with80 or without tetrahedral addition intermediates48•81; these suggestions are not consistent with the deuterium oxide solvent isotope effects; (2) general basic catalysis by imidazole of acyl-enzyme formation and decom-position82•83 ; this suggestion is not consistent with the pH dependence of the process; (3) general basic catalysis by imidazole involving tetrahedral addition in-termediates2•80•83 ; this suggestion, like eq. 6, is not consistent with the pH dependence of the process; (4) general acidic catalysis by an uñ'known acid and general basic catalysis by imidazole of acyl-enzyme formation and decomposition2•83-85; this suggestion is inconsistent with the designation of the acidic pH dependency as the nucleophilic group; (5) general
T h e effect of added nucleophiles such as methanol, ethanol, and hydroxylamine on the kinetics of a-chymotrypsincatalyzed reactions is interpreted in terms of the competitive partitioning of the acyl-enzyme intermediate by water and the added nucleophile according to eq. 2. In agreement with this hypothesis, the rate of deacylation of trans-cinnamoyl-a-chymotrypsiri in methanol-water solutions is dependent on the methanol concentration, and the products of the reaction are both methyl cinnamate and cinnamate ion. Data a t high methanol concentrations do not indicate a saturation phenomenon, thus giving no evidence for a binding of methanol (or water) to the enzyme. The a-chymotrypsin-catalyzed hydrolysis and alcoholysis of both specific and nonspecific substrates of a-chymotrypsin conform to the kinetics predicted by eq. 2. The kinetics of hydrolysis and methanolysis of N-acetyl-L-tyrosineamide and of N-acetyl-L-phenylalanineamide must be explained on the basis of eq. 2. A complete set of,rate constants can be calculated from the alcoholysis experiments for the specific substrates, N-acetyl-L-tryptophan ethyl and methyl esters and N-acetyl-L-phenylalanine methyl ester. The a-chymotrypsin-catalyzed hydrolysis and hydroxylaminolysis of N-acetyl-L-tyrosine ethyl ester and N-acetyl-L-tyrosine hydroxamic acid may be interpreted in terms of an expansion of eq. 2, eq. 16, which takes into account both the enzymatic and nonenzymatic fates of the labile initial product 0-( N-acetyl-Ltyrosy1)hydroxylamine.
The densities of binary solutions of nitromethane in carbon tetrachloride and nitroethane in carbon tetrachloride were measured at 30', 35', and 45'C. The molar volumes and excess molar volumes of mixing were calculated from the observed densities. The data indicated that a single specie of nitroparaffin exists in such binary solutions.Experimental volumetric and spectral data have been used to calculate the thermodynamic properties of 1,1,1,2,2-pentafluoropropane. Properties are presented for the saturated liquid and vapor from -40' F. to the critical temperature, 224.52'F. The data for the superheated vapor range from the saturation temperature to 700' F. The properties listed are volume, enthalpy, entropy, heat capacity at constant pressure, and heat capacity ratio as functions of temperature and pressure.
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