Optical resolution of unusual amino acids using microbial proteases

Abstract

Abstract We have developed novel enzymatic methods for the optical resolution of unusual amino acids. In this work, we tried two microbial proteases, available inexpensively in a crude state, from Aspergillus oryzae and from Bacillus subtilis . The enantioselective hydrolysis of the methyl esters of the N ‐benzyloxycarbonyl (Z) derivatives of a number of amino acids, both aliphatic and aromatic, was examined using these microbial proteases. The enantiomeric purities of the resolved Z‐amino acids were determined accurately by methods based on the reversed‐phase HPLC separation of diastereomeric derivatives or the HPLC separation of enantiomeric derivatives on chiral stationary phases. In general, B. subtilis protease yielded better results than A. oryzae protease. Using the former protease, the amino acids bearing aliphatic side chains were resolved with good to excellent enantioselectivities and reasonable hydrolysis rates. The speed of hydrolysis was reduced significantly when the length of the side chain was longer than 5 carbon atoms. Phenylalanine, halogenated phenylalanines, and phenylalanine homologs were also resolved, generally with high enantiomeric purities, though the hydrolysis rates were not always reasonably fast. In all the cases examined, the L ‐enantiomers were preferentially hydrolyzed as in the lipase‐catalyzed enantioselective hydrolysis reported previously. © 1992 Wiley‐Liss, Inc.