Dissociation pathways of alkali-cationized peptides: opportunities for C-terminal peptide sequencing

Dissociation pathways of alkali-cationized peptides have been studied using multiple stages of mass spectrometry (MSx) with a quadrupole ion trap mass spectrometer. Over 100 peptide ions ranging from 2 to 10 residues in length and containing each of the 20 common amino acids have been examined. The formation of the [b(n-1) + Na + OH]+ product ion is the predominant dissociation pathway for the majority of the common amino acids. This product corresponds to a sodium-cationized peptide one residue shorter in length than the original peptide. In a few cases, product ions such as [b(n-1) + Na - H]+ and those formed by loss, or partial loss, of a sidechain are observed. Both [b(n-1) + Na + OH]+ and [b(n-1) + Na - H]+ product ions can be selected as parent ions for a subsequent stage of tandem mass spectrometry (MS/MS). It is shown that these dissociation patterns provide opportunities for peptide sequencing by successive dissociation from the C-terminus of alkali-cationized peptides. Up to seven stages of MS/MS have been performed on a series of [b + Na + OH]+ ions to provide sequence information from the C-terminus. This method is analogous to Edman degradation except that the cleavage occurs from the C-terminus instead of the N-terminus, making it more attractive for N-terminal blocked peptides. The isomers leucine and isoleucine cannot be differentiated by this method but the isobars lysine and glutamine can.