Reduction and selective S-methylation

Conversion of half-cystine residues in proteins and peptides to the S-methyl derivatives is advantageous in subsequent studies of amino acid sequence. Under the usual conditions of acid hydrolysis ( 2.1), S-methylcysteine is recovered in a 90% yield (Heinrikson 1971). The phenylthiohydantoin of S-methylcysteine is readily identified by routine thin layer chromatography procedures (Rochat et al. 1970). With the increasing use of the sequenator, PTH-S-methylcysteine offers a marked advantage over derivatives such as PTH-cysteic acid, or PTFl-carboxymethylcysteine, which have to be identified by special techniques (Edman 1960, 1970). S-methylcysteinyl residues provide a new point of cleavage for cyanogen bromide (5). 

The procedure described below is taken from a detailed publication (jf Heinrikson (1971). 

Under the above conditions, the modification is totally restricted to sulfhydryl groups (Heinrikson 1971).S-methylcysteine is quantitated by amino acid analysis after acid hydrolysis ( 2.5.5). 

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