Catalysis by Triton X-100 micelles at neutral pH

Abstract

Hydrolysis of nitrophenyl esters in micelles has served as an excellent model for studies on micellar catalysis. Non-ionic micelles do not support ester hydrolysis due to the lack of charged species in the micelles. p-Nitrophenyloleate dissolved in Triton X-100 micelles does not undergo hydrolysis at pH 7.2; however, the hydrolysis increases with increasing concentration of guanidinium chloride (GdnHCl) up to 6 M. Such hydrolysis was not observed in the presence of urea (8 M). The chaotropes, GdnHCl and urea, destabilize the micelles by increasing the CMC. The decrease in micelle radius, measured by dynamic light scatter, and increase in polarity inside the palisade layer, measured by emission of 2-anthroyloxy fatty acids, was significant in GdnHCl but not in urea. Since hydrolysis was observed exclusively in the presence of GdnHCl, we have investigated the interaction of guanidinium ion with Triton X-100 micelle by using the chemical shift analysis of protons in ethylene oxide (PEO) using 1H NMR. A significant shift in the resonance frequency of the protons of the polyethylene oxide of Triton X-100 in GdnHCl, and not in urea, shows a hydrogen bonding between guanidinium ion and the PEO chains. The altered partitioning of the counter ion, hydroxyl, inside the micelles in the presence of GdnHCl was suggested to be responsible for enhanced hydrolysis of PNPO in Triton X-100. Such behavior of a neutral micelle, Triton X-100, as functional micelles was not demonstrated at neutral pHs earlier and this phenomenon is discussed with respect to the binding ability of PEO chains with cations. © 2008 Elsevier B.V. All rights reserved.