High‐performance liquid chromatographic mass spectrometric examination of C ‐methylation artifacts from the permethylation reaction of peptides

Combined high performance liquid chromatography mass spectrometry using a moving belt interface was used to investigate C ‐methylation artifacts arising from the permethylation reaction of peptides with methyl sulfinyl carbanion and methyl iodide. C ‐Methylation was not limited to glycine but, depending on the reaction conditions, other amino acids were also susceptible to it. In the absence of a glycine residue, preference for C ‐methylation of the C‐terminus amino acid was observed. The use of on‐line high performance liquid chromatography mass spectrometry allowed the separation of N, O, C ‐methylated from the N, O, S ‐permethylated derivatives, thus facilitating interpretation of the mass spectra.