H D EXCHANGE LEVELS OF SHAPE-RESOLVED CYTOCHROME-C CONFORMERS IN THE GAS-PHASE/

The conformations of cytochrome c ions (+8 through +18) in the gas pha se are examined by simultaneous ion-mobility and hydrogen-deuterium ex change measurements. By varying the voltage used to inject ions into t he drift tube it is possible to study H/D exchange of specific conform ers observed in the ion-mobility spectra: either very diffuse structur es that are favored for all charge states at high injection voltages o r compact structures that can be favored for the +8, +9, and +10 charg e states at low voltages. The number of exchangeable hydrogens for the diffuse conformer is independent of charge state, with an average val ue that is quite low: only 63 +/- 2 of a possible 198. The compact con formers observed for the +8 through +10 charge states exchange fewer h ydrogens (similar to 46), consistent with the idea that compact struct ures protect some hydrogens in the gas phase. Many sites that rapidly exchange in solution appear to be restricted for exchange in the gas p hase, even for very open conformers.