Internal chain cleavage and product heterogeneity during Edman degradation of isosteric peptide analogs lacking the alpha-carbonyl function.

A synthetic peptide analog, with one peptide carbonyl group replaced by a methylene bridge, was submitted to structural analysis by Edman degradation. Multiple cleavages were obtained in the first cycle, due to phenylthiocarbamylation of the internal secondary amine as well as spontaneous alkaline cyclization and subsequent recoupling with the Edman reagent. Three fragments from cleavage of the peptide analog after a single Edman cycle were purified by reverse-phase high-performance liquid chromatography. The results support previous observations in a novel combination. The reactions may also be important with native polypeptides since non-quantitative alkaline cyclization now encountered can mimic apparent N-terminal heterogeneity in agreement with earlier data, while quantitative cyclization can mimic loss of N-terminal residues.