Online Protein Unfolding Characterized by Ion Mobility Electron Capture Dissociation Mass Spectrometry: Cytochrome C from Neutral and Acidic Solutions

Electrospray ionization mass spectrometry (ESI-MS) experiments, including ion mobility-mass spectrometry (ESI-IM-MS), and electron capture dissociation (ECD) of proteins ionized from aqueous solutions, have been used for the study of solution-like structures of intact proteins. By mixing aqueous proteins with denaturants online before ESI, the amount of protein unfolding can be precisely controlled and rapidly analyzed, permitting the characterization of protein folding intermediates in protein folding pathways. Herein, we mixed various pH solutions online with aqueous cytochrome C for unfolding and characterizing its unfolding intermediates with ESI-MS charge state distribution measurements, ion mobility, and ECD. The presence of folding intermediates and unfolded cytochrome c structures were detected from changes in charge states, arrival time distributions (ATDs), and ECD fragmentation. We also compared structures from nondenaturing and denaturing solution mixtures measured under “gentle” (i.e., low energy) ion transmission conditions with structures measured under “harsh” (higher energy) transmission. This work confirms that when using “gentle” instrument conditions, the gas-phase cytochrome c ions reflect attributes of the various solution-phase structures. However, “harsh” conditions that maximize ion transmission produce annealed, extended structures that no longer correlate with changes in solution structure.