Formation of [b(n−1) + OH + H]+ Ion Structural Analogs by Solution-Phase Chemistry

Derivatization of a variety of peptides by a method known to enhance anhydride formation is demonstrated by mass spectrometry to yield ions that have elemental composition and fragmentation properties identical to [b(n−1) + OH + H]+ ions formed by gas-phase rearrangement and fragmentation. The [b(n−1) + OH + H]+ ions formed by gas-phase rearrangement and fragmentation and the solution-phase [b(n−1) + OH + H]+ ion structural analogs formed by derivatization chemistry show two different forms of dissociation using multiple-collision CAD in a quadrupole ion trap and unimolecular decomposition in a TOF-TOF; one group yields identical product ions as a truncated form of the peptide with a free C-terminal carboxylic acid and fragments at the same activation energy; the other group fragments differently from the truncated peptide, being more resistant to fragmentation than the truncated peptide and yielding primarily the [b(n−2) + OH + H]+ product ion. Nonergodic electron capture dissociation MS/MS suggests that any structural differences between the specific-fragmenting [b(n−1) + OH + H]+ ions and the truncated peptide is at the C-terminus of the peptide. The specific-fragmentation can be readily observed by MSn experiments to occur in an iterative fashion, suggesting that the C-terminal structure of the original [b(n−1) + OH + H]+ ion is maintained after subsequent rearrangement and fragmentation events in peptides which fragment specifically. A mechanism for the formation of specific-fragmenting and nonspecific-fragmenting [b(n−1) + OH + H]+ ions is proposed.