Edman degradation

The N-terminal (amino-terminal) residue of a protein can be recognized through reacting the  protein  with  a compound  which  forms  a stable  covalent  link  with  the  free α-amino group and 6 M HCl prior to hydrolysis. The labeled N-terminal amino acid can then be identified via comparison of its chromatographic properties with standard amino acid derivatives. Generally used reagents for N-terminal analysis are dansyl chloride and fluorodinitrobenzene. If this method was applied to the oligopeptide above the N-terminal residue would be identified as Val but the remainder of the sequence would still be unknown. Additional reaction with dansyl chloride would not reveal the next residue in the series till the peptide is totally degraded in the acid hydrolysis step.

This  problem  was  overcome  through  Pehr  Edman  who  devised  a  technique  for labeling the N-terminal residue and then cleaving it from the rest of the peptide without breaking the peptide bonds among the other amino acids. In supposed Edman degradation, one residue at a period is sequentially erased from the N- terminal end of a peptide or identified and protein.  The uncharged  N-terminal amino  group  of  the  protein  is reacted  with  phenyl  isothiocyanate  to  form  a phenylthiocarbamoyl  derivative  that  is  then  released  from  the  rest  of  the protein as a cyclic PTH (phenylthiohydantoin) amino acid under mildly acidic conditions. This milder cleavage reaction leaves the remainder of the peptide intact available for another round of release and labeling. The released phenylthiohydantoin amino acid is identified through HPLC (high performance   liquid chromatography).  This sequencing methods has been automated and refined so which upwards of 50 residues from the N-terminus of a protein can be sequenced from picomole quantities of material.