Electron transfer to peptide ions from anions such as fluoranthene is a highly exothermic process. Recent investigations indicate that backbone fragments can be produced from excited electronic states of the charge-reduced peptide ion. This raised the question of the nature of such states and the internal energy distribution in the charge reduced ion. ETD is too fast to allow experimental kinetic analysis to be carried out on the time scale of ion trap measurements. We now report a new method that relies on the kinetics of consecutive dissociations of primary ETD fragments to gauge the internal energy of the charge-reduced peptide ion.Kinetic ion thermometers (KIT) were z-type fragment ions LR, LAR, LK, and LAK that were produced by ETD of doubly and triply charged Xn-KIT ion precursors. The Xn part of the peptide was selected to vary the amino acid residues (His, Ala2-5, Tyr, 2-aminododecanoyl) and internal degrees of freedom in the complementary c fragments. In the first series of measurements, the dissociation kinetics of KIT by loss of C3H7 radicals were calibrated by comparing experimental energy-resolved breakdown diagrams with those based on RRKM calculations. ETD kinetics was modeled by using rate constants for C3H7 loss from KIT ions (k1), collisional cooling (kc), and ion depletion by neutralization for the reactant (k2) and product (k3). The last two rate constants were obtained experimentally, kc was estimated from the ion collisional cross sections, and k1 was modeled by ab initio/RRKM calculations. Kinetic equations describing these processes provided time-resolved breakdown diagrams that showed very good fit with experimental measurements. Examples of single-mode and multimodal energy deposition upon ETD will be discussed.

Kinetic ion thermometers (KIT) for the determination of internal energy in transient peptide cation-radicals formed by electron transfer

248th National Meeting (2014)

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