The amino acid derivative could be identified by hydrolyzing it to the parent amino acid with acid or alkali. In contrast to N-terminal group determination and sequential analysis of proteins and peptides from the N-terminal end, only a few chemical methods are available for the determination of C-terminal groups and sequential degradation of proteins and peptides from the C terminus. The most commonly used procedure for the determination of protein and peptide sequences from the C-terminal end is the use of exopeptidases, namely, carboxy-peptidases. The peptidyl thiohydantoin was selectively cleaved with ace-tohydroxamic acid, and the C-terminal amino acid was determined by difference amino acid analysis of the peptide before and after the degradation cycle. Side-chain carboxyl groups of aspartic and glutamic acids were also rearranged in the process to the corresponding amines. The sequence of four residues from the C-terminal end of a 22-residue protein fragment was determined.