Sequencing the peptide from its N-terminal end

Sequencing is a stepwise process of identifying the specific amino acid at each position in the peptide chain, beginning at the N-terminal end.
Phenylisothiocyanate, known as Edman reagent, is used to label the aminoterminal residue under mildly alkaline conditions (Fig. 1). The resulting phenylthiohydantoin (PTH) derivative introduces an instability in the Nterminal peptide bond such that it can be hydrolyzed without cleaving the other peptide bonds. The identity of the amino acid derivative can then be determined. Edman reagent can be applied repeatedly to the shortened peptide obtained in each previous cycle. Automated sequencers are now used.

Figure 1. Determination of the amino (N)-terminal residue of a polypeptide by Edman degradation. PTH = phenylthiohydantoin.