Kinetic Ion Thermometers for Electron Transfer Dissociation

Peptide fragment ions of the z-type were used as kinetic ion thermometers to gauge the internal energy of peptide cation-radicals produced by electron transfer in the gas-phase. Electron transfer dissociation (ETD)-produced z₂ ions containing the leucine residue, z₂(Leu-Lys) and z₂(Leu-Arg), were found to undergo spontaneous dissociation by loss of C₃H₇ that was monitored by time-resolved kinetic measurements on the time scale of the linear ion trap mass spectrometer. Kinetic modeling of the dissociations, including collisional cooling and product loss by neutralization, provided unimolecular rate constants for dissociation that were converted to the z ion internal energies using RRKM theory. The internal energy of z₂(Leu-Lys) and z₂(Leu-Arg) fragment ions was found to decrease with the increasing size of the precursor peptide ion, indicating vibrational energy partitioning between the ion and neutral fragments and ergodic behavior. The experimentally determined excitation in the peptide cation-radicals upon electron transfer (285–327 kJ mol^–1) was found to be lower than that theoretically calculated from the reaction exothermicity. The reasons for this missing energy are discussed.