Combining HD exchange and ESI-FAIMS-MS for detecting gas-phase conformers of equine cytochrome c

Conformers of equine cytochrome c were investigated in the gas phase using a combination of high-field asymmetric waveform ion mobility spectrometry (FAIMS) and hydrogen–deuterium (H–D) exchange. Electrospray generated ions of equine cytochrome c were exposed to a low concentration of D 2 O vapour while being transported by a flow of nitrogen through a FAIMS device. During this transport period of about 250 ms in the FAIMS analyzer, the various conformers of multiply charged ions of cytochrome c were simultaneously undergoing H–D exchange and being separated from each other. The extent of H–D exchange was calculated from the observed m/z of conformers after exposure to D 2 O vapour in FAIMS. The complementary nature of these two methods resulted in observations supporting a greater number of conformers (e.g., at least 11 conformers were identified for the +16 charge state) than would be expected by analyzing the FAIMS data and the H–D exchange data independently.