Which statement about the Edman sequencing procedure is not correct?

A The Edman reagent is phenyl isothiocyanate.

B The polypeptide is attached through its C-terminus to an insoluble polymer.

C The Edman reagent adds to the N-terminus of the polypeptide in aqueous acid conditions.

D The modified N-terminal residue is cleaved from the polypeptide using anhydrous acid.

E None of the answers are correct

D. The modified N-terminal residue is cleaved from the polypeptide using anhydrous acid.

Step-by-step explanation

A. The reagent used in the Edman degradation is phenyl isothiocyanate

B. C-terminal of the initial amino acid is covalently attached to an insoluble polymeric support. Thereafter, subsequent N(α)-protected amino acids are introduced to the anchored amino acid/peptide by removal of the N(α)-protecting group of the last target reside, followed by a purification process to remove the soluble by-products.

C. By adding Phenyl isothiocyanate, this creates a phenylthiocarbamoyl derivative with the N-terminal. The N-terminal is then cleaved under less harsh acidic conditions, creating a cyclic compound of phenylthiohydantoin PTH-amino acid. This does not damage the protein and leaves two constituents of the peptide. This method can be repeated for the rest of the residues, separating one residue at a time.

D. Because the Edman degradation proceeds from the N-terminus of the protein, it will not work if the N-terminus has been chemically modified (e.g. by acetylation or formation of pyroglutamic acid). Sequencing will stop if a non-α-amino acid is encountered (e.g. isoaspartic acid), since the favored five-membered ring intermediate is unable to be formed.