Monitoring Structural Changes of Proteins in an Ion Trap over ~10-200 ms: Unfolding Transitions in Cytochrome c Ions

A new technique for studying the time dependence of conformational changes of gas-phase protein ions is described. In this approach, a short pulse of electrosprayed protein ions is introduced into an ion trap and stored. After a defined time period, the distribution of ions is ejected from the trap into an ion mobility/time-of-flight mass spectrometer. Combined measurements of mobilities and flight times in the mass spectrometer provide information about the abundances of different conformer types and charge-state distributions. By varying the storage time in the trap, it is possible to monitor changes in ion conformation that occur over extended time periods (~10-200 ms). The method is demonstrated by examining changes in cytochrome c ion conformations for the +7 to +10 charge states.