Formation of b(2)(+) ions from protonated peptides: an ab initio study

Systematic ab initio calculations were performed to reveal the mechanism of formation of stable b(2)(+) ions formed during fragmentation of protonated peptides and proteins. Stable oxazolone-type cyclic b(2)(+) ions are forme d from parent ions containing the -C(O)-N-C-C(O)-unit in a two-step process . In the first step the C-N bond of an N-protonated peptide breaks and, sim ultaneously, ring closure takes place in the remaining -C(O)-N-C-C(O)- frag ment leading to the formation of a charged oxazolone-type ring. This reacti on takes place through an approximately 10 kcal mol-l high barrier. The pro duct of this step is a charge-transfer type ion-molecule complex which deco mposes in the next step to form the b(2)(+) ion by dropping the amine analo gue (C-terminal amino acid or peptide fragment). The dissociation energy of the complex is larger than the height of the barrier through which it was formed so that when the complex decomposes there is not much excess energy to be released as kinetic energy. The alternative multistep mechanism, invo lving formation of an open-chain acylium ion in the first step and ring clo sure in the second, is energetically highly unfavorable. Copyright (C) 1999 John Wiley & Sons, Ltd.