Electron transfer versus proton transfer in gas-phase ion/ion reactions of polyprotonated peptides

Abstract

The ion/ion reactions of several dozen reagent anions with triply protonated cations of the model peptide KGAILKGAILR have been examined to evaluate predictions of a Landau-Zener-based model for the likelihood for electron transfer. Evidence for electron transfer was provided by the appearance of fragment ions unique to electron transfer or electron capture dissociation. Proton transfer and electron transfer are competitive processes for any combination of anionic and cationic reactants. For reagent anions in reactions with protonated peptides, proton transfer is usually significantly more exothermic than electron transfer. If charge transfer occurs at relatively long distances, electron transfer should, therefore, be favored on kinetic grounds because the reactant and product channels cross at greater distances, provided conditions are favorable for electron transfer at the crossing point. The results are consistent with a model based on Landau-Zener theory that indicates both thermodynamic and geometric criteria apply for electron transfer involving polyatomic anions. Both the model and the data suggest that electron affinities associated with the anionic reagents greater than about 60-70 kcal/mol minimize the likelihood that electron transfer will be observed. Provided the electron affinity is not too high, the Franck-Condon factors associated with the anion 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 products also suggest that a significant barrier exists to the isomerization between chemical complexes that, if formed, lead to either proton transfer or electron transfer.

Figure 1

Hypothetical potential energy curves for an ion/ion reaction involving a multiply protonated peptide or protein, MH_nⁿ⁺, and a singly charged anionic reagent, A⁻.

Figure 2

Representations of πr² for the cases of capture into a bound orbit (r_orbit, dashed lines), electron transfer (r_ET), proton transfer (r_PT), and formation of a chemical complex (r_h−s, filled circle in the center) in a reference system in which the scattering center is of infinite mass, and the scattering partner of reduced mass, μ, is that of the ion/ion reactant pair.

Figure 3

Electron transfer probability versus electron affinity as determined using the approximations given in relations 14 and 15.

Figure 4

Post ion/ion reaction spectra of KGAILKGAILR [M + 3H]³⁺ and (a) the azobenzene molecular anion and (b) CS₂^−•.

Figure 5

Post ion/ion reaction spectra of KGAILKGAILR [M + 3H]³⁺ and (a) nitrobenzene (both (M−H)⁻ and M^−• anions were present), (b) SF₆^−•, (c) I⁻, and (d) PDCH [M−F]⁻ obtained after removal of residual multiply charged parent ions.

Figure 6

(a) CID spectrum of SO₂ attachment to guanidinated KGAILKGAILR. (b) CID spectrum of SO₂ loss peak in (a).

Figure 7

(a) CID spectrum of +1 guanidinated KGAILKGAILR from reaction of [M + 3H]³⁺ with SO₂^−•. (b) CID of [M + H]+ guanidinated KGAILKGAILR from proton transfer of [M + 3H]³⁺ with PDCH anions.

Figure 8

Energy diagram representing the barrier to isomerization for the chemical complexes that lead either to electron transfer or proton transfer products.

Scheme 1