Enzymes as reagents in peptide synthesis: enzymatic removal of amine protecting groups

A model system is described for the enzymatic deprotection of suitably masked amino groups during stepwise peptide synthesis. Nitrophenyl esters of amino acids, N-protected with trypsin-labile benzyloxycarbonylarginyl groups, were prepared as crystalline, analytically pure picrate salts in a standardized procedure. These intermediates were shown to react with amino compounds to form the expected peptide linkages. A pair of diasteriomeric peptides prepared in this way and featuring benzyloxycarbonylarginyl-L-, AND -D-glutaminyl sequences, respectively, were subjected to tryptic digestion. In both cases, a specific cleavage of the arginyl bond was achieved; however, the peptide containing the L-glutaminyl residue was deprotected much more rapidly than its diasteriomer containing the D-glutaminyl residue. The hydrolysis of the former isomer was not noticeably impeded by the presence of the latter. The results of these studies suggest that C-activated amino-acid derivatives, N-protected with trypsin-labile groups, are readily prepared in convenient form and that the peptide derivatives prepared from these intermediates are readily freed of their amino-protecting groups under mild, aqueous conditions with a potentially useful degree of stereospecificity. Theoretical implications of this first enzyme-catalyzed step in the repetitive cycle of peptide elaboration are discussed along with the procedural advantages implicit in the alternation of strongly and weakly basic groups in the protected and unprotected peptide intermediates, respectively.