T. Yalcin et al., THE STRUCTURE AND FRAGMENTATION OF B-N (N-GREATER-THAN-OR-EQUAL-TO-3)IONS IN PEPTIDE SPECTRA, Journal of the American Society for Mass Spectrometry, 7(3), 1996, pp. 233-242
The unimolecular and low energy collision-induced fragmentation reacti
ons of the MH(+) ions of N-acetyl-tri-alanine, N-acetyl-tri-alanine me
thyl ester, N-acetyl-tetra-alanine, tetra-alanine, penta-alanine, hexa
-glycine, and Leu-enkephalin have been studied with a particular empha
sis on the formation and fragmentation of B-n (n = 3,4,5) ions. In add
ition, 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 fragm
ent, in part, by elimination of CO to form A(n) ions; this reaction oc
curs on the metastable ion time scale with a substantial release of ki
netic energy (T-1/2 = 0.3-0.5 eV) that indicates that a stable configu
ration of the B-n ion fragments by way of a reacting configuration tha
t is higher in energy than the fragmentation products, A(n) + CO. Ab i
nitio calculations strongly suggest that the stable configuration of t
he 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 l
ost 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 i
ts stable protonated oxazolone form. This reaction is rationalized in
terms of the three-dimensional structure of the B-n ions and it is pro
posed that the neutral eliminated is an alpha-lactam.