Characterizing the structures and folding of free proteins using 2-D gas-phase separations: observation of multiple unfolded conformers.

Understanding the 3-D structure and dynamics of proteins and other biological macromolecules in various environments is among the central challenges of chemistry. Electrospray ionization can often transfer ions from solution to gas phase with only limited structural distortion, allowing their profiling using mass spectrometry and other gas-phase approaches. Ion mobility spectrometry (IMS) can separate and characterize macroion conformations with high sensitivity and speed. However, IMS separation power is generally insufficient for full resolution of major structural variants of protein ions and elucidation of their interconversion dynamics. Here we report characterization of macromolecular conformations using field asymmetric waveform IMS (FAIMS) coupled to conventional IMS in conjunction with mass spectrometry. The collisional heating of ions in the electrodynamic funnel trap between FAIMS and IMS stages enables investigating the structural evolution of particular isomeric precursors as a function of the intensity and duration of activation that can be varied over large ranges. These new capabilities are demonstrated for ubiquitin and cytochrome c, two common model proteins for structure and folding studies. For nearly all charge states, two-dimensional FAIMS/IMS separations distinguish many more conformations than either FAIMS or IMS alone, including some with very low abundance. For cytochrome c in high charge states, we find several abundant "unfolded" isomer series not distinguishable by IMS, possibly corresponding to different "string of beads" geometries. The unfolding of specific ubiquitin conformers selected by FAIMS has been studied by employing their heating in the FAIMS/IMS interface.