Structure and fragmentation of b 2 ions in peptide mass spectra

Abstract

In a number of cases the b 2 ion observed in peptide mass spectra fragments directly to the a 1 ion. The present study examines the scope of this reaction and provides evidence as to the of the b 2 ions undergoing fragmentation to the a 1 ion. The b 2 ion H–Ala–Gly + fragments, in part, to the a 1 ion, whereas the isomeric b 2 ion H–Gly–Ala + does not fragment to the a 1 ion. Ab initio calculations of ion energies show that this different behavior can be rationalized in terms of protonated oxazolone structures for the b 2 ions provided one assumes a reverse activation energy of ∼1 eV for the reaction b 2 → a 2; such a reverse activation energy is consistent with experimental kinetic energy release measurements. Experimentally, the H–Aib–Ala + b 2 ion, which must have a protonated oxazolone structure, fragments extensively to the a 1 ion. We conclude that the proposal by Eckart et al. (J. Am. Soc. Mass Spectrom. 1998, 9, 1002) that the b 2 ions which undergo fragmentation to a 1 ions have an immonium ion structure is not necessary to rationalize the results, but that the fragmentation does occur from a protonated oxazolone structure. It is shown that the b 2 → a 1 reaction occurs extensively when the C-terminus residue in the b 2 ion is Gly and with less facility when the C-terminus residue is Ala. When the C-terminus residue is Val or larger, the b 2 → a 1 reaction cannot compete with the b 2 → a 2 fragmentation reaction. Some preliminary results on the fragmentation of a 2 ions are reported.