Ion Mobility Spectrometry of Macromolecules with Dipole Alignment Switchable by Varying the Gas Pressure

Since inception in the 1980s, differential or field asymmetric waveform ion mobility spectrometry (FAIMS) was implemented at or near the ambient gas pressure (AP). Recently, we developed FAIMS at 15-30 Torr within a mass spectrometer and demonstrated it for small and medium sized ions, including peptides. The overall separation properties mirrored those at AP, reflecting the shared underlying physics. Here we extend these analyses to macromolecules, namely, multiply charged proteins generated by electrospray ionization. The spectra for smaller proteins (ubiquitin, cytochrome c, myoglobin) again resemble those at AP, producing features for one or a few adjacent well-defined conformers with type C behavior. Large proteins (single aldolase domain and albumin) now follow, with no broad bands for type A or B species that dominated at 1 atm. Those unique behaviors were ascribed to pendular ions with electric dipoles reversibly locked by the strong field in FAIMS. Disappearance of those bands shows loss of alignment predicted by first-principles theory, further supporting dipole locking at AP. The capability to modulate dipole alignment by varying gas pressure at constant normalized field provides the basis for determining the ion dipole moment and direction within the molecular frame from the pressure of onset and characteristics of spectral drift. This new approach to alter FAIMS separations of proteins could make a powerful tool for structural biology and be useful for proteomics and imaging.