The structure and fragmentation of B-n (n>=3) ions in peptide spectra

The unimolecular and low energy collision-induced fragmentation reactions of the MH(+) ions of N-acetyl-tri-alanine, N-acetyl-tri-alanine methyl ester, N-acetyl-tetra-alanine, tetra-alanine, penta-alanine, hexa-glycine, and Leu-enkephalin have been studied with a particular emphasis on the formation and fragmentation of B-n (n = 3,4,5) ions. In addition, the metastable ion fragmentation reactions of protonated tetra-glycine, penta-glycine, and Leu-enkephalin amide have been studied. B-n ions are prominent stable species in all spectra. The B-n ions fragment, in part, by elimination of CO to form A(n) ions; this reaction occurs on the metastable ion time scale with a substantial release of kinetic energy (T-1/2 = 0.3-0.5 eV) that indicates that a stable configuration of the B-n ion fragments by way of a reacting configuration that is higher in energy than the fragmentation products, A(n) + CO. Ab initio calculations strongly suggest that the stable configuration of the B-3 and B-4 ions is a protonated oxazolone formed by interaction of the developing charge with the next-nearest carbonyl group as HX is lost from the protonated species H-(Yyy)(n)-X . H+. The higher B-n ions also fragment, in part, to form the next-lower B ion, presumably in its stable protonated oxazolone form. This reaction is rationalized in terms of the three-dimensional structure of the B-n ions and it is proposed that the neutral eliminated is an alpha-lactam.