Catalyses by Polymer Complexes. V. The Heterotropic (Allosteric) Interaction of Histamine- and Hydroxamate-Containing Polymer Catalysts with Hydrophobic Ammonium Salts in the Hydrolysis of Phenyl Esters

Three nucleophilic copolymers, (N-methacryloylhistamine—methacrylic acid (MHistm—MAc), N-methacryloylhistamine—Methacrylamide (MHistm—MAm), and methacrylohydroxamic acid—Methacrylic acid (MHAc—MAc)) were synthesized, and their hydrolytic reactivities toward p-nitrophenyl acetate (PNPA) and p-nitrophenyl hexanoate (PNPH) were studied in the absence and the presence of hydrophobic ammonium salts. The nucleophilic reactivity of MHistm—MAc toward PNPA was hardly affected by the addition of hydrophobic ammonium ions, while a marked increase of rates was found in the reaction with PNPH. Addition of these ammonium ions to the MHAc—MAc system enhanced the rate of the reaction with PNPH. This was produced by a PK_a lowering (0.3—0.4PK unit) and by the increase in the second-order rate constant (ca. 3 fold), as inferred from the pH—rate profile. The rate-enhancing effect of the hydrophobic ammonium salts was analyzed by using the Hill equation which has been employed for analyzing the allosteric behavior in the enzyme system. The observed coefficient (n=3∼4) suggests that polymer-bound ammonium ions facilitate the subsequent binding considerably.