Modication of Sulfur-Containing Amino Acids in Proteins

Methionine and cysteine/cysteine (Figure 5.1) are among the less commonly occurring amino acids in proteins (cysteine, 1.8%; methionine, 2.6%).1 Cysteine can be present as cysteine or as a disulde (occasionally as trisulde);2 the low level of methionine was useful in obtaining polypeptide fragments via cyanogen bromide cleavage.3 The ability of methionine to function as a nucleophile at low pH allows for selective modication with reagents such as iodoacetate under acidic conditions.4