Sequence-specific fragmentation of deprotonated peptides containing H or alkyl side chains

The [M − H] − ions of a variety of di- to pentapeptides containing H or alkyl side chains have been prepared by electrospray ionization and low-energy collision-induced dissociation (CID) of the deprotonated species carried out in the interface region between the atmospheric pressure source and the quadrupole mass analyzer. Using the nomenclature applied to the fragmentation of protonated peptides, deprotonated dipeptides fragment to give a 2 ions (CO 2 loss) and y 1 ions, where the y 1 ion has two fewer hydrogens than the y ″ 1 ions formed from protonated peptides. Deprotonated tri- and tetrapeptides fragment to give primarily y 1, c 1, and ″b 2 ions, where the ″b 2 ion has two fewer hydrogens than the b 2 ion observed for protonated peptides. More minor yields of y 2, c 2, and a 2 ions also are observed. The a ion formed by loss of CO 2 from the [M − H] − ion shows loss of the N-terminal residue for tripeptides and sequential loss of two amino acid residues from the N-terminus for tetrapeptides. The formation of c n ions and the sequential loss of N-terminus residues from the [M − H − CO 2] − ion serves to sequence the peptide from the N-terminus, whereas the formation of y n ions serves to sequence the peptide from the C-terminus. It is concluded that low-energy CID of deprotonated peptides provides as much (or more) sequence information as does CID of protonated peptides, at least for those peptides containing H or alkyl side chains. Mechanistic aspects of the fragmentation reactions observed are discussed.