Formation of b2+ ions from protonated peptides: an ab initio study

Systematic ab initio calculations were performed to reveal the mechanism of formation of stable b2+ ions formed during fragmentation of protonated peptides and proteins. Stable oxazolone-type cyclic b2+ ions are formed 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, simultaneously, ring closure takes place in the remaining­-C(O)-N-C-C(O)- fragment leading to the formation of a charged oxazolone-type ring. This reaction takes place through an approximately 10 kcal mol−1 high barrier. The product of this step is a charge-transfer type ion-molecule complex which decomposes in the next step to form the b2+ ion by dropping the amine analogue (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, involving formation of an open-chain acylium ion in the first step and ring closure in the second, is energetically highly unfavorable. Copyright © 1999 John Wiley & Sons, Ltd.

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