Electron Transfer versus Proton Transfer in Gas-Phase Ion/Ion Reactions of Polyprotonated Peptides
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- 作者：Harsha P. Gunawardena ; Min He ; Paul A. Chrisman ; Sharon J. Pitteri ; Jason M. Hogan ; Brittany D. M. Hodges ; and Scott A. McLuckey
- 刊名：Journal of the American Chemical Society
- 出版年：2005
- 出版时间：September 14, 2005
- 年：2005
- 卷：127
- 期：36
- 页码：12627 - 12639
- 全文大小：210K
- 年卷期：v.127,no.36(September 14, 2005)
- ISSN：1520-5126
文摘
The ion/ion reactions of several dozen reagent anions with triply protonated cations of the modelpeptide KGAILKGAILR have been examined to evaluate predictions of a Landau-Zener-based model forthe likelihood for electron transfer. Evidence for electron transfer was provided by the appearance of fragmentions unique to electron transfer or electron capture dissociation. Proton transfer and electron transfer arecompetitive processes for any combination of anionic and cationic reactants. For reagent anions in reactionswith protonated peptides, proton transfer is usually significantly more exothermic than electron transfer. Ifcharge transfer occurs at relatively long distances, electron transfer should, therefore, be favored on kineticgrounds because the reactant and product channels cross at greater distances, provided conditions arefavorable for electron transfer at the crossing point. The results are consistent with a model based onLandau-Zener theory that indicates both thermodynamic and geometric criteria apply for electron transferinvolving polyatomic anions. Both the model and the data suggest that electron affinities associated withthe anionic reagents greater than about 60-70 kcal/mol minimize the likelihood that electron transfer willbe observed. Provided the electron affinity is not too high, the Franck-Condon factors associated with theanion and its corresponding neutral must not be too low. When one or the other of these criteria is not met,proton transfer tends to occur essentially exclusively. Experiments involving ion/ion attachment productsalso suggest that a significant barrier exists to the isomerization between chemical complexes that, if formed,lead to either proton transfer or electron transfer.