The Edman degradation is a protein-sequencing mechanism whereby the N-terminal amino acid of a peptide reacts with phenylisothiocyanate, followed by reaction with trifluoroacetic acid. This process releases the N-terminal amino acid from the peptide chain as an anilinothiazolinone (ATZ) derivative, shortening the peptide chain by one amino acid residue. In the presence of aqueous acid the ATZ derivative is converted into an isomeric phenylthiohydantoin (PTH) derivative. The PTH derivatives can be identified to yield the primary structure of the peptide chain, the sequence of amino acids. Draw curved arrows to show the movement of electrons in this step of the mechanism of the conversion of the ATZ derivative to the PTH derivative.