Special problems encountered during the cyanogen bromide cleavage of lobster arginine kinase

During structural analysis of Lobster muscle arginine-kinase, we have isolated a CNBr resulting peptide with a blocked N-terminal residue. This peptide was sequenced after unblocking by mild acid treatment (1 N HCl at 100 degrees C for 10 min). The blocked form is not due to the formation of pyroglutamic acid nor is it due to the formation of diketopiperazine. We have applied the experimental conditions used for CNBr cleavage of lobster arginine-kinase to a synthetic peptide the structure of which is similar to the above CNBr peptide. We bring evidence that during CNBr cleavage partial formylation occurs with a possible cyclization of a 7 membered ring of Gly--Asp...