Catalytic Efficiency of Synthetic Micellar Catalysts Bearing a Mercapto Group as the Reaction Center

Abstract

In order to obtain a clue to understanding the micro-environmental effect on the reactivity of a mercapto group placed in a reaction center of enzymes, micellar surfactants bearing a mercapto group were synthesized and their catalytic activity in the degradation of p-nitrophenyl carboxylates was studied. N-Hexadecyl-Nα-glutaryl-l-cysteinamide (AM·Cys-1) has an ability to form anionic micelles in aqueous media. The catalytic activity of AM·-Cys-1 was compared with that of another synthetic surfactant, N-hexadecanoyl-l-cysteine (AM·Cys-2). These surfactants below their critical micelle concentrations markedly accelerated the degradation of several p-nitrophenyl carboxylates. On the contrary, the concentration - rate profiles for the degradation of p-nitrophenyl dodecanoate (PNPL) as catalyzed by the surfactants indicate that the reactivity of the mercapto group is reduced upon formation of the anionic micelles. The large rate retardation is primarily due to the decrease in concentration of the active thiolate anion. This was supported by the fact that the pKa values for mercapto groups of the anionic micelles, for which the carboxyl group acts as an anionic head, were increased by 0.8–1.6 pKa unit over those of the corresponding monomeric surfactants in the bulk phase. These surfactants showed profound reactivity even in a neutral pH region when mixed with cationic CTAB micelle. The electrostatic field effect provided by the cationic head of CTAB micelle seems to enhance the nucleophilicity of the mercapto group in the mixed micelles.

Investigation of Physical−Chemical Properties of Agarose Hydrogels with Embedded Emulsions

“…Only a few papers have been published on the studies of the reaction ability of mercapto groups in the reaction with esters. – Thus, Overberger et al have studied the kinetics of thiol groups of 2,6-dimercaptohexane (DMH) and polyvinyl mercaptan (PVM) and copolymers of vinyl mercaptan and vinylimidazole with 4-nitrophenylacetate (4-NPhA), and also with anionic and cationic esters . The reaction of vinyl thiol groups in the copolymers of partially quarternarized poly-4-vinyl pyridine (VPVM) with 4-NPhA and 3-nitro-4-acetobenzoic acid (NABA) was studied in refs and .…”