Impact of charge state on gas-phase behaviors of noncovalent protein complexes in collision induced dissociation and surface induced dissociation.

Charge states of noncovalent protein complexes in the gas phase are known to affect their propensity for unfolding and dissociation. In this work, C-reactive protein (CRP) pentamer and Concanavalin A (ConA) tetramer at different charge states were subjected to collision induced dissociation (CID) and surface induced dissociation (SID) in a modified quadrupole/ion mobility/time-of-flight mass spectrometer. Charge manipulation was achieved through solution addition of charge reducing (triethylammonium acetate) or supercharging (3-nitrobenzylalcohol) reagents. The results show that charge reduction increases the stability of the proteins to dissociation and suppresses unfolding of the precursors. While CID becomes less effective at dissociation of charge reduced CRP and ConA, SID showed better preserved subunit contacts that are useful for quaternary structure elucidation. In contrast, supercharging of CRP and ConA leads to facile dissociation into subunits even for CID. The extent of precursor unfolding also increases with greater charge. Another interesting finding is that low-charge multimer products (dimers, trimers, etc.) seem to be collapsed after being released from the complexes. Further investigation is necessary to fully understand this behavior. The data presented here suggest that charge manipulation can be used to "tune" the dissociation behavior of noncovalent protein complexes in order to obtain the most useful information desired for structural analysis.