Tunable charge tags for electron-based methods of peptide sequencing: design and applications.

Charge tags using basic auxiliary functional groups 6-aminoquinolinylcarboxamido, 4-aminopyrimidyl-1-methylcarboxamido, 2-aminobenzoimidazolyl-1-methylcarboxamido, and the fixed-charge 4-(dimethylamino)pyridyl-1-carboxamido moiety are evaluated as to their properties in electron transfer dissociation mass spectra of arginine C-terminated peptides. The neutral tags have proton affinities that are competitive with those of amino acid residues in peptides. Charge reduction by electron transfer from fluoranthene anion-radicals results in peptide backbone dissociations that improve sequence coverage by providing extensive series of N-terminal c-type fragments without impeding the formation of C-terminal z fragments. Comparison of ETD mass spectra of free and tagged peptides allows one to resolve ambiguities in fragment ion assignment through mass shifts of c ions. Simple chemical procedures are reported for N-terminal tagging of Arg-containing tryptic peptides.