Elucidating Protein Structures in the Gas Phase: Traversing Configuration Space with Biasing Methods.

Achieving accurate characterization of protein structures in the gas phase continues to be a formidable challenge. To tackle this issue, the present study employs Molecular Dynamics (MD) simulations in tandem with enhanced sampling techniques (methods designed to efficiently explore protein conformations). The objective is to identify suitable structures of proteins by contrasting their calculated Collision Cross-Section (CCS) with those observed experimentally.

Ion mobility calculations of flexible all-atom systems at arbitrary fields using two-temperature theory.

Ion mobility spectrometry (IMS) separates and analyzes ions based on their mobility in a gas under an electric field. When the field is increased, the mobility varies in a complex way that depends on the relative velocity between gas and ion, their electrostatic potential interactions, and the effects from direct impingement. Recently, the two-temperature theory, primarily developed for monoatomic ions in monoatomic gases, has been extended to study mobilities at arbitrary fields using polyatomic ions in polyatomic gases, with some success.

Investigation of Zero-/High-Field Ion Mobility Orthogonal Separation Using a Hyphenated DMA-FAIMS System and Validation of the Two-Temperature Theory at Arbitrary Field for Tetraalkylammonium Salts in Nitrogen.

Toward greater separation techniques for ions, a differential mobility analyzer (DMA) has been coupled with field asymmetric waveform ion mobility spectrometry (FAIMS) to take advantage of two mobility-related but different methods of separation. The filtering effect of the DMA allows ions to be selected individually based on low-field mobility and studied in FAIMS at variable electric field, yielding mobility separations in two dimensions. Because spectra fully describe ion mobility at variable field strength, results are then compared with a two-temperature theory-predicted mobility up to the fourth-order approximation.

Enhancing Separation and Constriction of Ion Mobility Distributions in Drift Tubes at Atmospheric Pressure Using Varying Fields.

A linearly decreasing electric field has been previously proven to be effective for diffusional correction of ions in a varying field drift tube (VFDT) system, leading to higher resolving powers compared to a conventional drift tube due to its capacity to narrow distributions midflight. However, the theoretical predictions in resolving power of the VFDT were much higher than what was observed experimentally. The reason behind this discrepancy has been identified as the difference between the theoretically calculated resolving power (spatial) and the experimental one (time).

Field-Switching Repeller Flowing Atmospheric-Pressure Afterglow Drift Tube Ion Mobility Spectrometry.

In this work, a field-switching (FS) technique is employed with a flowing atmospheric pressure afterglow (FAPA) source in drift tube ion mobility spectrometry (DTIMS). The premise is to incorporate a tip-repeller electrode as a substitute for the Bradbury-Nielsen gate (BNG) so as to overcome corresponding disadvantages of the BNG, including the gate depletion effect (GDE). The DTIMS spectra were optimized in terms of peak shape and full width by inserting an aperture at the DTIMS inlet that was used to control the neutral molecules' penetration into the separation region, thus preventing neutral-ion reactions inside.

Measurement and Theory of Gas-Phase Ion Mobility Shifts Resulting from Isotopomer Mass Distribution Changes.

The unanticipated discovery of recent ultra-high-resolution ion mobility spectrometry (IMS) measurements revealing that isotopomers─compounds that differ only in the isotopic substitution sites─can be separated has raised questions as to the physical basis for their separation. A study comparing IMS separations for two isotopomer sets in conjunction with theory and simulations accounting for ion rotational effects provides the first-ever prediction of rotation-mediated shifts. The simulations produce observable mobility shifts due to differences in gas-ion collision frequency and translational-to-rotational energy transfer.

Predicting ion mobility as a function of the electric field for small ions in light gases.

High resolution mobility devices such as Field Asymmetric Waveform Ion Mobility Spectrometry (FAIMS) and Differential Mobility spectrometers (DMS) use strong electric fields to gas concentration ratios, E/N, to separate ions in the gas phase. While extremely successful, their empirical results show a non-linear, ion-dependent relation between mobility K and E/N that is difficult to characterize. The one-temperature theory Mason-Schamp equation, which is the most widely used ion mobility equation, unfortunately, cannot capture this behavior.