Rapid regioselective oligomerization of L-glutamic acid diethyl ester catalyzed by papain

Papain-catalyzed oligomerization of diethyl L-glutamate hydrochloride was conducted in phosphate buffer at 40 degrees 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(gamma-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(gamma-ethyl-L-glutamate) repeat units. The most abundant signals were at DP 8 and 9, in excellent agreement with DPavg values determined by H-1 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(gamma-ethyl-L-glutamate) synthesis at 40 degrees 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 observed over a broad range of pH values. As long as the pH was maintained from 5.5 to 8.5, the product yield was >= 60%. Ionic strength had no significant effect on oligopeptide yield. The dominant role of phosphate buffer in reactions was its control of pH. Other influences of phosphate ions on papain, such as nonspecific salt interactions or a salting out of product, appear to be of little or no importance. Loss in protein concentration and activity in the supernatant was observed after one reaction. A second reaction cycle performed using recovered supernatants resulted in a decrease in oligo(gamma-ethyl-L-glutamate) yield from about 75% to 20%.