WHY ARE B-IONS STABLE SPECIES IN PEPTIDE SPECTRA

Protonated amino acids and derivatives RCH(NH2)C(=O)X . H+ (X = OH, NH 2, OCH3) do not form stable acylium ions on loss of HX, but rather the acylium ion eliminates CO to form the immonium ion RCH = NH2+. By con trast, protonated dipeptide derivatives H2NCH(R)C(=O)NHCH(R')C(=O)X . H+ [X = OH, OCH3, NH2, NHCH(R '')COOH] form stable B-2 ions by elimina tion of HX. These B-2 ions fragment on the metastable ion time scale b y elimination of CO with substantial kinetic energy release (T-1/2 = 0 .3-0.5 eV). Similarly, protonated N-acetyl amino acid derivatives CH3C (=O)NHCH(R')C(=O)X . H+ [X = OH, OCH3, NH2, NHCH(R '')COOH] form stabl e B ions by loss of HX. These B ions also fragment unimolecularly by l oss of CO with T-1/2, values of similar to 0.5 eV. These large kinetic energy releases indicate that a stable configuration of the B ions fr agments by way of activation to a reacting configuration that is highe r in energy than the products, and some of the fragmentation exothermi city of the final step is partitioned into kinetic energy of the separ ating fragments. We conclude that the stable configuration is a proton ated oxazolone, which is formed by interaction of the developing charg e (as HX is lost) with the N-terminus carbonyl group and that the reac ting configuration is the acyclic acylium ion. This conclusion is supp orted by the similar fragmentation behavior of protonated 2-phenyl-5-o xazolone and the B ion derived by loss of H-Gly-OH from protonated C6H 5C(=O)-Gly-Cly-OH. In addition, ab initio calculations on the simplest B ion, nominally HC( =O)NHCH2CO+, show that the lowest energy structu re is the protonated oxazolone. The acyclic acylium isomer is 1.49 eV higher in energy than the protonated oxazolone and 0.88 eV higher in e nergy than the fragmentation products, HC(=O)N+H = CH2 + CO, which is consistent with the kinetic energy releases measured.