Structure and fragmentation of b2 ions in peptide mass spectra.

In a number of cases the b2 ion observed in peptide mass spectra fragments directly to the a1 ion. The present study examines the scope of this reaction and provides evidence as to the structure(s) of the b2 ions undergoing fragmentation to the a1 ion. The b2 ion H-Ala-Gly+ fragments, in part, to the a1 ion, whereas the isomeric b2 ion H-Gly-Ala+ does not fragment to the a1 ion. Ab initio calculations of ion energies show that this different behavior can be rationalized in terms of protonated oxazolone structures for the b2 ions provided one assumes a reverse activation energy of approximately 1 eV for the reaction b2-->a2; such a reverse activation energy is consistent with experimental kinetic energy release measurements. Experimentally, the H-Aib-Ala+ b2 ion, which must have a protonated oxazolone structure, fragments extensively to the a1 ion. We conclude that the proposal by Eckart et al. (J. Am. Soc. Mass Spectrom. 1998, 9, 1002) that the b2 ions which undergo fragmentation to a1 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 b2-->a1 reaction occurs extensively when the C-terminus residue in the b2 ion is Gly and with less facility when the C-terminus residue is Ala. When the C-terminus residue is Val or larger, the b2-->a1 reaction cannot compete with the b2-->a2 fragmentation reaction. Some preliminary results on the fragmentation of a2 ions are reported.