Performic Acid Oxidation
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Where it is necessary to carry out quantitative amino acid analysis of cysteine or cystine residues, carboxymethylation ( Chapter 59) or pyridethylation ( Chapter 62) is not the method of choice as it may be difficult to assess the completeness of the reactions. It is often preferable to oxidize the thiol or disulfide groups with performic acid to give cysteic acid. This also has the effect of converting methionine residues to methionine sulfone. Tyrosine residues may be protected from reaction by the use of phenol in the reaction mixture.