Transition state stabilization by micelles: the hydrolysis of p-nitrophenyl alkanoates in cetyltrimethylammonium bromide micelles

The cleavage of p-nitrophenyl alkanoates (acetate to octanoate) at high pH is modestly catalyzed by micelles formed from cetyltrimethylammonium bromide (CTAB) in aqueous solution. Rate constants exhibit saturation behaviour with respect to [CTAB], consistent with substrate binding in the micelles. The strength of substrate binding and transition state binding to the micelles increases monotonically with the acyl chain length, and with exactly the same sensitivity. As a result, the extent of acceleration (or catalytic ratio) is independent of the ester chain. These and earlier results are consistent with the reaction centre being located in the Stern layer of the micelle, with the acyl chain of the ester being directed into the hydrophobic micellar interior. The chain length dependence of kinetic parameters found in this work is comparable to that found previously for ester cleavage by cyclodextrins and by various enzymes with hydrophobic binding sites, as well as to that observed for other phenomena involving hydrophobic effects. Keywords: catalysis, ester hydrolysis, micelles, transition state.