Gas-phase bovine ubiquitin cation conformations resolved by gas-phase hydrogen/deuterium exchange rate and extent

The gas-phase hydrogen/deuterium exchange of [M + nH](n+) (n = 5-13) ions o f bovine ubiquitin with the H/D exchange reagent D2O are examined by electr ospray ionization Fourier transform ion cyclotron resonance (FT-ICR) mass s pectrometry. All of the odd or all of the even charge states were isolated by stored waveform inverse Fourier transform excitation and simultaneously reacted with D2O leaked steadily into the ICR cell for reaction periods ran ging from 1 s to 1 h. Different gas-phase protein conformations could be re solved according to difference in extent of H/D exchange. The 5+ and 6+ cha rge states display broad distributions of conformations ranging from 0-80% deuterium incorporation. In contrast, each of the higher charge states, 7-1 1+ and 13+, displays a single major isotopic distribution, whereas the 12charge state separates into two isotopic distributions of comparable abunda nce. In general, H/D exchange rates decrease with increasing charge state. External electrospray ionization source conditions (capillary current and e xternal accumulation period) were varied while observing the conformational distribution of the 7+ charge state: increased heating in either region re duced the number of slow-exchanging conformations. At 9.4 T, it is possible to trap a large number of ions for a long reaction period (up to 1 h) at r elatively high pressure (2 x 10(-7) Torr). These results demonstrate the ca pability of FT-ICR mass analysis following gaseous H/D exchange of electros prayed proteins to disperse different gas-phase protein conformations for s ubsequent isolation acid characterization. (C) 1999 Elsevier Science B.V.