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

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Abstract
The 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 α‐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.