Understanding reaction pathways in top-down ETD by dissecting isotope distributions: A mammoth task

Abstract

Electron transfer dissociation (ETD) is becoming increasingly more important in mass spectrometry-based structural proteomics due to its ability to cleave the protein backbone without annihilating the higher-order structure. Concomitant with ETD, non-dissociative charge reduction occurs primarily by proton transfer from the protein precursor to the ETD reagent, and by non-dissociative electron transfer from the reagent to the protein. While it has been known for some time that the preference for either proton or electron transfer is largely a function of the properties of the reagent, the extent of dissociative versus non-dissociative electron transfer is also dependent on instrument choice, precursor charge state and conformation, as well as ion activation before, during, and after cation/anion interaction. In this work, we use a novel software named MASSTODON to quantify the different reaction pathways in top-down spectra of ubiquitin, acquired on two different instruments: a Synapt G2 (Waters) and an LTQ Orbitrap Velos (Thermo). We compare the behavior of different precursor charge states, representing more folded or more extended conformations, and also evaluate the effect of increasing supplemental activation (on the Synapt) and reaction time (on the Orbitrap). In agreement with previous work, increasing supplemental activation promotes fragment release and hydrogen radical migration from (N-terminal) c fragments to (C-terminal) z fragments. A longer reaction time leads to increasingly more extensive charge reduction and a decrease in the relative occurrence of non-dissociative electron transfer compared to proton transfer, possibly indicating some degree of time- and charge-state dependent loss of electrostatic interactions on a timescale of a few tens of milliseconds.