Structural Transitions of Electrosprayed Ubiquitin Ions Stored in an Ion Trap over ∼10 ms to 30 s

Structural transitions of the +6 to +8 charge states of ubiquitin produced by electrospray ionization have been studied in the gas phase by a new ion trap/ion mobility-mass spectrometry technique. The approach allows transitions to be examined in detail over ∼10 ms to 30 s trapping times. This time regime is intermediate between the ∼1 to 5 ms time scales of previous mobility measurements [J. Am. Soc. Mass Spectrom. 1997, 8, 954] and minute to hour time scale measurements associated with trapping experiments done in a Fourier transform mass spectrometer [Int. J. Mass Spectrom. 1999, 185/186/187, 565]. The results show that over the entire time range, the +6 charge state is dominated by compact structures (with cross sections that are near the value expected for folded states in solution). The +7 state shows evidence for at least two types of initial compact structures. One state (∼65% of the population) rapidly unfolds to partially folded and elongated conformers after ∼30 to 40 ms. The remaining 35% of ions also unfolds at a much slower rate. The +8 charge state appears to be formed initially in a range of partially folded states. These states rapidly unfold into elongated structures that persist to the longest trapping times that are employed. These results are compared with the longer time scale measurements, and attempts are made to correlate the features observed in the different experiments.