Remarkably Enhanced Enantioselective Hydrolysis of Amino Acid Esters With Tripeptide Catalyst in Micellar Systems

Abstract

Stereoselective hydrolysis of the long-chain substrate (p-nitrophenyl ester of N-dodecanoyl-D and L-phenylalanine: C₁₂-D(L)- Phe-PNP) catalyzed by the active tripeptide (N-benzyloxycarbonyl-Lphenylalanyl-L-histidyl-L-leucine: Z-PheHisLeu) in the micellar systems (cationic hexadecyltrimethylammoniumb romide (CTAB); nonionic α -[4 (1, 1, 3, 3-tetramethylbuthyl) phenyl] - ω hydroxydecakis(oxyethylene)(Triton X-100), polyoxyethylene(20) sorbitan monolaurate (Tween 20), a-tetradecyl-co-hydroxyheptakis (oxyethylene)( C₁₄ (EO)₇ ); anionic sodium dodecylsulfate (SDS))was found to be controlled by changing the concentration and structure of surfactants. The noteworthy aspects are as follows: W ith respect to the rate-enhancement, (a)t he apparent rate constant (k_{a, obsd})is markedly enhanced around critical micelle concentration (CMC) and the head group in micellar surfactants takes an important role for the enhancement of rate (cationic CTAB >> nonionic surfactants (C₁₄(EO)₇, Triton X-100, Tween 20) >> anionic SDS). On the other hand, withrespect tothe stereospecificity, (b) kinetic enantioselectivity is exclusively favored for the L-substrate and the enantioselectivity (reflected in k^L_{a, obsd} / k^D_{a, obsd}) is kept constant above CMC in all of micellar systems in this study. The head group in micelles also take an important role to enhance the enantioselectivity (nonionic surfactants (Triton X-100, Tween 20, C₁₄ (EO)₇ ) > anionic SDS > cationic CTAB). In particular, a remarkably high enantioselectivity ( k^L_{a, obsd} / k^D_{a, obsd}= 77) was attained around CMC in the C₁₄(EO)₇ micellar system.