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

Abstract

In order to clarify micro-environmental effects on the reactivity of mercapto groups in enzymes, cationic surfactants bearing a mercapto group, N-hexadecyl-Nα-(3-trimethylammoniopropionyl)-l-cysteinamide bromide (CM·Cys-1) and N-dodecyl-Nα-(6-trimethylammoniohexanoyl)-l-cysteinamide bromide (CM·Cys-2), were synthesized and their kinetic behavior investigated. The surfactants above their critical micelle concentrations markedly catalyzed the decomposition p-nitrophenyl hexanoate (PNPH) and acetate (PNPA), the concentrationrate profiles being found to be those for typical micelle-catalyzed reactions. The rate constants for the degradation of PNPH as catalyzed by CM·Cys-1 and -2 in the micellar phase are 0.482 and 0.197 s−1, respectively, in 9.8% (v/v)ethanol–1.0%(v/v)dioxane–1.0%(v/v)methanol-water at 30.0±0.1 °C, pH 8.65, and μ0.10 (KCl). The difference in catalytic activity can be attributed partly to the micro-environmental effect on the pKa value of the mercapto group lying at the reaction center. The rate constants for the thiolate anions (true reactive species) of CM·Cys-1 and -2 to react with PNPH were identical with each other irrespective of the nature of the surfactants. The electrostatic effect provided by the cationic charge in the Stern layer, which acts to reduce the pKa value of the reactive mercapto group, seems to play a more important role than the desolvation effect on the thiolate anion by the hydrophobic field.

Artificial Enzymes

“…While micelles provide hydrophobic reaction sites effective for acceleration of various enzyme-mimetic reactions, those aggregates are generally soft and difficult to achieve conformational fixation of substrates for regio- and stereospecific reactions. Peptide surfactants bearing amino acid residue(s), such as 7 , developed by Murakami et al may improve such a weak aspect of micellar aggregates, since the amphiphiles tend to form tight aggregates relative to those formed with simple surfactants, originating from the intramicellar hydrogen-bonding interaction. − …”

“…Peptide surfactants bearing amino acid residue(s), such as 7, developed by Murakami et al may improve such a weak aspect of micellar aggregates, since the amphiphiles tend to form tight aggregates relative to those formed with simple surfactants, originating from the intramicellar hydrogen-bonding interaction. [34][35][36][37] It is well recognized that incorporation of substrate species into aqueous micelles proceeds through hydrophobic, electrostatic, and charge-transfer interactions. On the other hand, the hydrogen-bonding interaction between a substrate and a catalyst seems to be also effective for stereoselective catalysis in the hydrophobic micellar phases.…”

Enantioselective catalysis of deacylation of p-nitrophenyl N-acylphenyl-alanates by mixed micelles composed of Nα-hexadecanoyl-L-histidine and a cationic surfactant

“…Based on the data obtained at 20-35 "C, pH 7.6 (0.083 mol dm-3 Tris buffer) in CH3CN-H20 (3 : 97 vlv) containing 0.083 mol dm-j KC1 with concentrations: PalHis,(1.5-7.5) x mol dmF3; OTAC,(3)(4)(5)(6)(7)(8)(9)(10)(11)(12)(13)(14)(15) x 10-4 mol dm-3 at [OTAC]/[PalHis] 20; substrate,…”