Gas-phase hydrogen-deuterium exchange in phosphorothioate d(GTCAG) and d(TCGAT)

The gas-phase hydrogen-deuterium exchange of phosphorothioate d(GTCAG) and d(TCGAT) has been examined using electrospray Fourier transform ion cyclotron resonance (FTICR) mass spectrometry. The exchange has been monitored for the [M-2H]2−, [M-3H]3−, and [M-4H]4− charge states. The mole fractions of each partially exchanged species have been calculated as a function of time. The rates of initial exchange could not be fit to simple exponential functions. The results suggest that each ion can adopt more than one gas-phase conformation, and that these conformers exchange at different rates. Observation of slowly interconverting populations is consistent with a model where initial ion conformations contain internal hydrogen bonds between the bases and charged residues that are disrupted over time.