Enantioselective catalysis of histidyl-peptides in surfactant aggregates

Abstract

The enantioselective solvolyses of optically active esters by L-histidine dipeptides in the presence of amphiphile-aggregates composed of cholesteryl derivatives and other surfactants were carried out in 0.02 M Trisbuffer solution. The optically active catalysts employed were N-(benzyloxycarbonyl)-L-phenylalanyl-L-histidine methyl ester (la), N-(benzyloxycarbonyl)-L-phenylalanyl-L-histidine(lb), and N-(benzyloxycarbonyl)-L-leucyl-L-histidine methyl ester(lc), and the substrates solvolyzed were N-carboethoxy-D- and -L-phenylalanine p-nitrophenyl esters (D-NEP AND L-NEP), N-carbobenzoxy-D- and -L-phenylalanine p-nitrophenyl esters (D-NBP and L-NBP), N-dodecanoyl-D- and -L-phenylalanine p-nitrophenyl esters (D-NDP and L-NDP), D- and L-$\alpha$-methoxy phenylacetic acid p-nitrophenyl esters (D-Man-PNP and L-Man-PNP). The characteristic chiral surfactants used in the present system were cationic cholesterol-containing amphiphiles, cholesteryl-oxycarbonyl-methyl-2-(isobutyloyloxy) ethyl dimethyl ammonium bromide (CHODABA) and cholesteryl-oxycarbonyl-decanyl-2-(isobutyloyloxy) ethyl dimethyl ammonium bromide (CHODABU). Also were used 1-methyl-nor- 1,3,5,6-cholestatetraene-3-yl-oxycarbonyl-methyl-2-(isobutyloyloxy) ethyl dimethyl ammonium chloride (ACHODABA) to observe any possible effect of the presence of an aromatic function on the surfactant. In the solvolyses of D-NDP and L-NDP by catalyst la in coaggregate systems composed of steroid surfactants and cationic micelle (CDEAB), the lengthening of the acyl chain of the cholesterol-based surfactant from spacer n=1 (CHODABA) to n=10 (CHODABU) brought about an increase in the deacylation rate of the solvolysis, but the enantioselectivity was low. Particularly noteworthy was the case of the coaggregate system consisted of 58 mole \% CHODABA and 42 mole \& CDEAB. In this system the best enantioselective solvolysis was observed, viz, $k_{cat}$(L)/$k_{cat}$(D) = 98.2). It is also notable that as pH values of the above coagg...