Poly(L‐histidyl‐L‐alanyl‐α‐L‐glutamic acid). II. Catalysis of p‐nitrophenyl acetate hydrolysis

Abstract

SynopsisThe hydrolysis of p-nitrophenyl acetate is catalyzed by imidazole, free in solution or as the side chain in poly(His-Ala-Glu). This is based on the observations that the reaction is first order in ester and first order in nonprotonated imidazole. Catalysis of p-nitrophenyl acetate hydrolysis is dependent on solvent conditions. The effect of low concentrations of ethanol, dioxane, and trifluoroethanol were investigated. As the concentration of organic solvent is increased, the second-order rate constant for imidazole catalysis decreases. The decrease, however, is greater for imidazole than for poly(His-Ala-Glu). In 2% trifluoroethanol/water solution, free imidazole has twice the catalytic activity of polymeric imidazole, while in 40% trifluoroethanol/water they have equal activity. Since under the latter solvent conditions poly(His-Ala-Glu) is partially a-helical, the relative improvement in polymeric-imidazole catalysis may be attributed to imidazole hydrogen-bonded to a carboxylate ion. With this assumption the carboxylate-imidazole hydrogen-bonded system has been calculated to have three times the base catalytic activity of imidazole.