An amino-terminal tryptophan derivative which is refractory to Edman degradation

A new derivative of tryptophan is proposed to account for the observation that some peptides having amino-terminal tryptophan residues become refractory to Edman degradation. An acid-catalyzed oxidation of the indole nucleus and subsequent cyclization to a unique 3-anilinopyrrolidin-2-one derivative with cleavage of the peptide chain is the most likely chemical explanation. This amino acid is reactive with ninhydrin and contains an aryl amine. However, the amine does not bond to the alpha carbon so while reactive to phenyl isothiocyanate, distances are too great for the residue to be cleaved from the peptide during Edman degradation.