Electron transfer with no dissociation ion mobility-mass spectrometry (ETnoD IM-MS). The effect of charge reduction on protein conformation

- Electron transfer to proteins is reported as a novel method to probe changes in protein structure termed ETnoD.
- A single electron is shown to cause dramatic structural rearrangement.
- Smaller conformers preferentially reduce, leaving more extended exposed precursors.
- A given arrival time distribution is preferred for a protein with a given net charge indicating charge dictates structure in the gas phase.
- Improvement in mobility resolution is observed following exposure to the 1,3-dicyanobenzene reagent which is cautiously attributed to anion cation cooling.
- A model is proposed where the transferred electron lessens the contribution of stabilising salt bridges to the gas phase protein structure.

A novel mass spectrometry approach is reported which investigate how ion-molecule charge reduction reactions between radical anions and protein cations modulate protein conformation. An electron transfer reagent (1,3-dicyanobenzene) transfers electrons to positively charged proteins and there are no observable products of dissociation. ETnoD product ions are detected as charged-reduced species with the same molecular weight as the precursor ion, and no significant evidence for proton transfer. We present collision cross section distributions of precursor and product ions before and after exposure to radical ions. Cytochrome c and myoglobin are examined as exemplar systems under both aqueous salt and denaturing conditions before and after exposure to radical anions. We consistently observe depletion of the more compact precursor ion conformers on exposure to the ETD reagent. Remarkably, by examining the collision cross section distributions of the product ions it can be seen that the addition of a single electron can cause a dramatic rearrangement in protein conformation for charge states that are highly populated when sprayed from salty aqueous conditions. Furthermore, a given net charge on an exposed precursor and product ion favours a preferred collision cross section distribution, indicating that the distribution of charge on proteins in the gas phase dictate their conformation. An exception is reported for the low charge state of cytochrome c where compaction was seen in the radical formed post reduction compared to the electrospray generated ion under ETnoD optimised conditions. We propose a model that postulates how electron transfer to conformation stabilising salt bridges may explain our observations.

117