Abstract
Cleavage of chiral p‐nitrophenyl esters derived from the amino acid phenylalanine by histidine‐containing tripeptides has been studied in micellar solutions of four quaternary ammonium surfactants. Enzyme‐like enantioselectivities up to k~I~/k~D~ = 131 (at 0°C) are observed. The enantioselectivity can be rationalized by assuming a hydrophobically driven stabilizing hydrogen bond between the I. enantiomer of the ester and the tripeptide in the transition state of the reaction. This hydrogen bond is absent in the reaction with the D enantiomer of the ester. The transition state has an amphipolar character and is stabilized by the micellar environment. The hydrophilic‐hydrophobic balance of the reactants, which affects the transition state, was optimized by varying the composition of the tripeptide and the length of the N‐protecting groups in the tripeptide and the substrate. The activities and enantioselectivities depend on the structure of the quaternary ammonium surfactant headgroup. Increasing the size of this headgroup leads to an increase in rate of hydrolysis of the L ester and hence to an increase in enantioselectivity. This effect is attributed to a change in the degree of ion‐pair formation with a carboxylate group that is present in the peptides. Compared to previous studies the results indicate that a chiral surfactant is not required for obtaining high enantioselectivities.