Which of the following reagents would be most useful in determining the N-terminal amino acid of a polypeptide?

Phenylisothiocyanate is used in the Edman degradation for N-terminal analysis. Trypsin is an endopeptidase and cleaves internal peptide bonds and therefore is not generally useful in determining the N-terminal amino acid residue. Carboxypeptidase is an exopeptidase that sequentially removes one amino acid at a time from the carboxyl end of the polypeptide chain and is used to determine the C-terminal amino acid. Cyanogen bromide cleaves at internal methionine residues in a polypeptide chain.