Electron transfer dissociation: Effects of cation charge state on product partitioning in ion/ion electron transfer to multiply protonated polypeptides

Abstract

The effect of cation charge state on product partitioning in the gas-phase ion/ion electron transfer reactions of multiply protonated tryptic peptides, model peptides, and relatively large peptides with singly charged radical anions has been examined. In particular, partitioning into various competing channels, such as proton transfer (PT) versus electron transfer (ET), electron transfer with subsequent dissociation (ETD) versus electron transfer with no dissociation (ET,noD), and fragmentation of backbone bonds versus fragmentation of side chains, was measured quantitatively as a function of peptide charge state to allow insights to be drawn about the fundamental aspects of ion/ion reactions that lead to ETD. The ET channel increases relative to the PT channel, ETD increases relative to ET,noD, and fragmentation at backbone bonds increases relative to side-chain cleavages as cation charge state increases. The increase in ET versus PT with charge state is consistent with a Landau-Zener based curve-crossing model. An optimum charge state for ET is predicted by the model for the ground state-to-ground state reaction. However, when the population of excited product ion states is considered, it is possible that a decrease in ET efficiency as charge state increases will not be observed due to the possibility of the population of excited electronic states of the products. Several factors can contribute to the increase in ETD versus ET,noD and backbone cleavage versus side-chain losses. These factors include an increase in reaction exothermicity and charge state dependent differences in precursor and product ion structures, stabilities, and sites of protonation.

Extensive Charge Reduction and Dissociation of Intact Protein Complexes Following Electron Transfer on a Quadrupole-Ion Mobility-Time-of-Flight MS

“…Whereas the charge-reduced tetramers are annotated as Q n+ for the sake of simplicity, it should be noted that these are not the same [Q+nH] n+ charge states produced by electrospray ionization, but rather the products of a combination of proton transfer and nondissociative electron transfer (ETnoD), which are both known to occur under these conditions [24,[37][38][39][40]. It would, therefore, be more accurate to consider them as [Q+xH] n+ , with n<= x<= N, where N is the charge state of the precursor.…”

Detailed characterization of the O-linked glycosylation of the neuropilin-1 c/MAM-domain

“…Precursor charge state emerged as a most relevant criterion for ETD of other compounds [21,22], it made little difference for Nrp1 glycopeptides. In contrast, co-eluting peptides severely affected ionization and hence fragmentation.…”

Charge Transfer Dissociation (CTD) Mass Spectrometry of Peptide Cations: Study of Charge State Effects and Side-Chain Losses

“…Similar to electron-based fragmentation methods [14,25], CTD of substance P also shows certain charge statedependence on fragmentation. Product ion spectra of He-CTD of 2+ and 3+ substance P produced more than twice the number of fragment ions than the 1+ precursor, mainly because of the addition of c and z ions.…”

“…In addition, ExD retains PTMs to a much greater extent than CID, which facilitates the elucidation of PTM site information [12]. However, the fact that ExD relies on charge reduction makes it incompatible with 1+ precursor ions, and its performance is compromised for 2+ precursor ions [14]. The inefficiency with peptide dications can be problematic for implementing ExD with enzymatic digestion workflows because many tryptically digested peptides are doubly charged [15].…”

“…Contrary to the common ion/ion dissociation methods, CTD utilizes the interaction between homo-polarity ions such as peptide cations and helium cations, which, in the case of 1+ substance P, results in a dominant series of a ions. It has been widely reported that the fragmentation pattern of MS/MS techniques shows certain dependence on the charge state of precursor ions and the type of mass analyzer [14,19,25]. To investigate this dependence, CTD fragmentation of substance P and bradykinin at different charge states (1+, 2+, and 3+) was carried out in a 3D ion trap mass spectrometer.…”