Practical Protein Chemistry - A. Darbre 1989

Determination of the composition of protein oligomers. Preparation of monomers and polypeptide chains
Preparation of monomers for sequencing
Reduction of sulfoxides with N-methylmercaptoacetamide

Based on the results of Amino acid analysis, it is difficult to assess the oxidation state of Methionine residues during the preparation of protein samples, if only because the reverse reaction of partial reduction may occur during Hydrolysis [156]. However, methionine losses can be inferred from the low yield upon Cyanogen bromide Cleavage [66]. A low cystine yield may also result from The formation of Cysteine sulfoxide [85] or alkylcysteine [86]. Partial oxidation significantly complicates the identification of the corresponding PTH-Amino Acids during deep sequencing. Among numerous Other Reagents, N-methylmercaptoacetamide is the most effective reducing agent for methionine S-oxide [80]. The reaction of methionine S-oxide (1 mg/mL) with 0.7–2.8 M N-methylmercaptoacetamide is carried out at pH 2.6–8.5 and 37 °C for 12–14 h under a nitrogen atmosphere.

The reduction of S-carboxymethylcysteine sulfoxide in Peptides and Proteins is performed under the same conditions, except that the reaction time is increased 5-fold.

Upon protein oxidation with performic acid, methionine is converted into a stable S,S-dioxide. Prior to the oxidation step [87], the Tryptophan residue is converted into oxindolyl-3-Alanine [155] or kynurenine [11], which are more stable under Edman Degradation conditions. The method proposed in [155] is more specific (see Section 2.5.3). During the Conversion of Tryptophan, a side reaction is the formation of methionine S-oxide, which can be selectively reduced back to methionine [156].