Proteomes exhibit a 2 net charge at pH 2.7 and can be separated from other peptides according to single charge differences using SCX chromatography (33). Because trypsin cuts poorly at Meth-R, the missed cleavages produce peptides with charge 3 at pH 2.7. Thus, most Meth-R peptides should be present in the 3 peptide-containing fractions after SCX chromatography. SCX fractions enriched for tryptic