MASS-SPECTROMETRIC AND QUANTUM-MECHANICAL ANALYSIS OF GAS-PHASE FORMATION, STRUCTURE, AND DECOMPOSITION OF VARIOUS B(2) IONS AND THEIR SPECIFICALLY DEUTERATED ANALOGS

B ions represent an important type of fragment ions derived from proto nated peptides by cleavage of an amide bond with N-terminal charge ret ention. Such species have also been discussed as key intermediates dur ing cyclic peptide fragmentation. Detailed structural information on s uch ion types can facilitate the interpretation of multiple step fragm entations such as the formation of inner chain fragments from linear p eptides or the fragmentation of cyclic peptides. The structure of diff erent b(2) ion isomers was investigated with collision-induced dissoci ations (CID) in combination with hydrogen/deuterium (H/D) exchange of the acidic protons. Special care was taken to investigate fragment ion s derived from pure gas-phase processes. Structures deduce from the re sults of the CID analysis were compared with structures predicted on t he basis of quantum chemical density functional theory (DFT) calculati ons to be most stable. The results pointed to different types of struc tures for b(2) ion isomers of complementary amino acid sequences. Eith er the protonated oxazolone structure or the N-terminally protonated i mmonium ion structure were proposed on the basis of the CID results an d the DFT calculations, in addition, the analysis of different selecti vely N-alkylated peptide analogs revealed mechanistic details of the p rocesses generating b ions. (C) 1998 American Society for Mass Spectro metry.