Mass Spectrometry of N-Methylated Peptide Derivatives

AN important method of peptide sequence determination, namely mass spectrometry of N-methylated acyl peptides, has been described recently1,2. We devised the same method during studies of gastrin3, and have found that methylation by methyl iodide and silver oxide in dimethylformamide is not straightforward for residues of methionine, glutamic acid and aspartic acid. These more complex reactions, which involve the peptide backbone, do not prevent the method being used, but understanding of them is a prerequisite for its general application.