MECHANISM OF ENANTIOSELECTIVE ESTER CLEAVAGE BY HISTIDINE-CONTAINING DIPEPTIDES AT A MICELLAR INTERFACE

Chiral p-nitrophenyl esters derived from the amino acid phenylalanine are cleaved by histidine-containing dipeptides at a micellar interface. High enantioselectivities (up to k(L)/k(D) = 30.4 at 0-degrees-C) are observed. Both the substrates and the catalysts contain an alternating sequence of hydrophobic and hydrophilic groups. Due to the need for hydration of the hydrophilic groups, the hydrophobic groups cannot dissolve completely into the micellar hydrocarbon phase. The kinetic data suggest that the micellar interface is capable of discriminating between transition states that have different hydrophilic and hydrophobic properties. One of the diastereomeric transition states is characterized by a hydrogen bond between the amide CO group of the ester and an NH group of the histidine-containing dipeptide. Upon formation of this hydrogen bond these polar CO and NH groups lose their hydrophilicity which allows the transfer of the adjacent apolar groups to the micellar hydrocarbon phase. The other diastereomeric transition state cannot form this hydrogen bond and the hydrophobic groups remain hydrated. Consequently, the latter transition state is of higher energy. The kinetic data reveal that it is important to prevent steric hinderance between the reactants in order to allow the unhindered formation of the hydrogen bond.