A method is described that selectively purifies methionine peptides from enzymic digests of a protein. The peptides, after paper electrophoresis, are treated on paper with iodoacetamide at acid pH. This specifically converts methionine residues into their sulphonium salts. When the paper is submitted to electrophoresis at right angles to the original direction, the carbamoylmethylmethionine peptides emerge from an undifferentiated diagonal. 2. Heating at neutral pH converts carbamoylmethylmethionine into homoserine and thereby specifically cleaves the peptides. 3. The effect of the modifications on amino acid composition and sequence analyses of the peptides was studied. 4. When the method was applied to a tryptic digest of S-aminoethyl-chymotrypsinogen A, two peptides were selectively purified that had the expected amino acid sequence.
