Unlocking the N-Terminus: Understanding Edman Degradation
Edman degradation is a classical biochemical technique used to identify the sequence of amino acids in a peptide, specifically targeting the N-terminal residue. It relies on the selective chemical reaction between the N-terminal amino group and phenyl isothiocyanate (PITC) under mildly alkaline conditions.
In the first step, the N-terminal amino acid reacts with PITC to form a phenylthiocarbamoyl derivative. Under acidic conditions, this intermediate undergoes cyclization and cleavage, yielding a thiazolinone derivative and a new peptide chain shortened by one amino acid. The thiazolinone is then converted to a stable product known as phenylthiohydantoin (PTH), which can be identified through chromatography or spectroscopy. Each PTH corresponds uniquely to a known amino acid.
Because the remaining peptide is left intact (minus the first residue), the process can be repeated iteratively, allowing the sequencing of peptides residue by residue from the N-terminus. However, Edman degradation is most effective for peptides up to about 30–50 residues long.
This method played a foundational role in protein sequencing before the rise of mass spectrometry and still offers educational insight into peptide chemistry. The key strength of Edman degradation lies in its precision in identifying the N-terminal residue, making it a cornerstone technique in early protein chemistry.