Rapid Regioselective Oligomerization of l-Glutamic Acid Diethyl Ester Catalyzed by Papain

Abstract

Papain-catalyzed oligomerization of diethyl l-glutamate hydrochloride was conducted in phosphate buffer at 40 °C. Because of rapid oligomerization kinetics, high substrate concentrations were not needed to shift the equilibrium for oligomer synthesis. For example, at 0.03 M diethyl l-glutamate hydrochloride, oligo(γ-ethyl l-glutamate) synthesis and precipitation from solution occurred in 55% yield. MALDI-TOF spectra of precipitated products showed two series of ion peaks separated by 157 m/z units, the mass of oligo(γ-ethyl-l-glutamate) repeat units. The most abundant signals were at DP 8 and 9, in excellent agreement with DPavg values determined by 1H NMR. Lower intensity peaks with m/z less by 28 correspond to hydrolysis of one ester group either at a chain end or a pendant group along chains. Oligo(γ-ethyl-l-glutamate) synthesis at 40 °C in phosphate buffer (0.9 M, pH 7) occurred rapidly so that by 5, 10, and 20 min the yield reached 70 ± 4%, 78 ± 4% and 81 ± 5%, respectively. High product yields were ...