Structure and fragmentation of b2 ions in peptide mass spectra

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 structure(s) 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 similar to 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. Sec. 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. (C) 2000 American Society for Mass Spectrometry.