Catalysis of ester hydrolysis by cationic micelles of surfactants containing the imidazole ring

Abstract

Micelles of cetyl(imidazol-4-ylmethyl)dimethylammonium chloride, (CDAIM)(IIa), are good catalysts for the hydrolysis of p-nitrophenyl esters in the pH range 7.2–9.2. The large pH dependence of the catalytic activity is indicated by the enhanced contribution of the anionic form of the imidazolyl ring at the polar head of the surfactant. The catalytic effects of CDAIM are compared with those of N^α-myristoylhistidine (MirHis)(Ia) in cationic micelles. The two imidazole-containing surfactants differ markedly in several respects such as the micellar activity in neutral solution, the change in catalytic effect with changing pH, and the rate of regeneration of the micellar active site. A cross examination of the effects of CDAIM and MirHis and those of the corresponding non-surfactant analogues (IIb) and (Ib), allows a dissection of the micellar and structural factors responsible for catalytic activity. Much of the difference observed between CDAIM and MirHis is accounted for by structural factors which make the imidazole ring in compounds (II) a much weaker (by ca.3 pK units) base than that of histidine derivatives (I).