High-resolution Ion Mobility Measurements Research Articles

Collision Cross-Section Calibration Strategy for Lipid Measurements in SLIM-Based High-Resolution Ion Mobility.

Structures for lossless ion manipulation-based high-resolution ion mobility (HRIM) interfaced with mass spectrometry has emerged as a powerful tool for the separation and analysis of many isomeric systems. IM-derived collision cross section (CCS) is increasingly used as a molecular descriptor for structural analysis and feature annotation, but there are few studies on the calibration of CCS from HRIM measurements. Here, we examine the accuracy, reproducibility, and practical applicability of CCS calibration strategies for a broad range of lipid subclasses and develop a straightforward and generalizable framework for obtaining high-resolution CCS values. We explore the utility of using structurally similar custom calibrant sets as well as lipid subclass-specific empirically derived correction factors. While the lipid calibrant sets lowered overall bias of reference CCS values from ∼2–3% to ∼0.5%, application of the subclass-specific correction to values calibrated with a broadly available general calibrant set resulted in biases <0.4%. Using this method, we generated a high-resolution CCS database containing over 90 lipid values with HRIM. To test the applicability of this method to a broader class range typical of lipidomics experiments, a standard lipid mix was analyzed. The results highlight the importance of both class and arrival time range when correcting or scaling CCS values and provide guidance for implementation of the method for more general applications.

Characterizing the resolution and accuracy of a second-generation traveling-wave ion mobility separator for biomolecular ions

High-accuracy, high-resolution ion mobility measurements enable a vast array of important contemporary applications in biological chemistry. With the recent advent of both new, widely available commercial instrumentation and also new calibration datasets tailored for the aforementioned commercial instrumentation, the possibilities for extending such high performance measurements to a diverse set of applications have never been greater. Here, we assess the performance characteristics of a second-generation traveling-wave ion mobility separator, focusing on those figures of merit that lead to making measurements of collision cross-section having both high precision and high accuracy. Through performing a comprehensive survey of instrument parameters and settings, we find instrument conditions for optimized drift time resolution, cross-section resolution, and cross-section accuracy for a range of peptide, protein and multi-protein complex ions. Moreover, the conditions for high accuracy IM results are significantly different from those optimized for separation resolution, indicating that a balance between these two metrics must be attained for traveling wave IM separations of biomolecules. We also assess the effect of ion heating during IM separation on instrument performance.

Disrupting Helix Formation in Unsolvated Peptides

Protonated polyalanine peptides form helices in the gas phase when their most basic protonation site (the N-terminus) is blocked by acetylation: Ac-An+H+ (Ac = acetyl and A = alanine). The glycine analogues, Ac-Gn+H+ (G = glycine), on the other hand, form random globules. The disruption of helix formation in unsolvated Ac-AnGxAm+H+ peptides has been examined as a function of n+m, and x using high resolution ion mobility measurements and molecular dynamics simulations. A surprisingly large block of glycine residues is required to disrupt helix formation in these peptides. For example, Ac-A5G3A5+H+ and Ac-A6G5A6+H+ both remain helical at room temperature. According to molecular dynamics simulations, the glycines do not cause a localized disruption of the helices, as might be expected for a residue considered a helix breaker. This is consistent with helix disruption occurring through a global effect on the relative energies of the helix and globule rather than through a localized entropic effect.

Conformations of Unsolvated Valine-Based Peptides

High-resolution ion mobility measurements and molecular dynamics (MD) simulations have been used to study the conformations of unsolvated valine-based peptides with up to 20 residues. In aqueous solution, valine is known to have a high propensity to form β-sheets and a low propensity to form α-helices. A variety of protonated valine-based peptides were examined in vacuo: Valn+H+, Ac-Valn-Lys+H+, Ac-Lys-Valn+H+, Valn-Gly-Gly-Valm+H+, Valn-LPro-Gly-Valm+H+, Valn-DPro-Gly-Valm+H+, Ac-Valn-Gly-Lys-Valm+H+, Ac-Valn+H+, and Arg-Valn+H+. Peptides designed to be β-hairpins were found to be random globules or helices. The β-hairpin is apparently not favored for valine-based peptides in vacuo, which is in agreement with the predictions of MD simulations. Peptides designed to be α-helices appear to be partial α/partial π-helices. Insertion of Gly-Gly, LPro-Gly, or DPro-Gly into the center of a polyvaline peptide disrupts helix formation. Some of the peptides that were expected to be random globules (because their most basic protonation site is near the N-terminus where protonation destabilizes the helix) were found to be helical with the proton located near the C-terminus. Helix formation appears to be more favorable in unsolvated valine-based peptides than in their alanine analogues. This is the reverse of what is observed in aqueous solution, but appears to parallel the helix propensities determined in polar solvents.

High-resolution ion mobility measurements for silicon cluster anions and cations

High-resolution ion mobility measurements have been performed for silicon cluster anions and cations, Sin− and Sin+, n=6–55. New isomers have been resolved for every cluster size larger than Si20. The results for the anions and the cations have the same global features. However, changing the charge often causes a shift in the isomer distribution, or causes new isomers to emerge. For example, the transition from prolate geometries to more-spherical ones is shifted to larger cluster sizes for the anions than for the cations. The mobilities of the anions are systematically smaller than those of the cations, presumably because of differences in the exterior electron densities.

Ball-and-Chain Dimers from a Hot Fullerene Plasma

The laser desorption of Ck (k = 60 and 70) fullerene is known to produce a broad distribution of cluster sizes strongly peaking around the integer multiples of original fullerene mass. The “exact dimers” (Cn clusters with n = 2k) and species with slightly fewer atoms (n even) have been characterized previously as fully coalesced large single-shell fullerenes and [2 + 2] cycloadducts. Presently, we investigate the species encountered on the high-mass sides of exact dimers, that is, the clusters with n > 2k (n even), using high-resolution ion mobility measurements. Specifically, the drift time distributions for Cn+ and Cn- with n = 122−128, 132−136, and 142−146 have been obtained and compared with the results of trajectory calculations for various trial geometries optimized using the density functional tight binding and semiempirical (AM1) calculations. We find that, besides the normal near-spherical fullerenes and [2 + 2] cycloadducts, these species assume the “ball-and-chain” structures consisting of two fullerene cages (not necessarily those of the original material) connected by chains up to eight atoms long. C122 and C132 cations and anions also reveal a substantial abundance of isomers where the C−C unit is sandwiched between the two fullerenes. Taken in conjunction with earlier findings for smaller fullerene dimers, presently reported results have allowed us to develop a comprehensive model for the chemical reactions occurring in the hot fullerene plasma.

High-resolution ion mobility measurements of indium clusters: electron spill-out in metal cluster anions and cations

High-resolution ion mobility measurements have been used to examine In n − and In n + clusters with n=2–30. The cluster–He scattering cross-sections of the anions are systematically larger than those of the cations. The difference (∼25% for In 2 to ∼2% for In 30) is attributed to the electron density extending further from the surface of the cluster in the anions than in the cations. Geometric cross-sections estimated from electron densities determined from a charged jellium model are in good agreement with the experimental data: the change in cross-section with charge state is reproduced over the entire range.

Helix Formation in Unsolvated Alanine-Based Peptides: Helical Monomers and Helical Dimers

High-resolution ion mobility measurements and molecular dynamics simulations have been used to examine helix formation in protonated alanine-based peptides in a solvent-free environment. Protonated...

Conformations of Gly nH + and Ala nH + Peptides in the Gas Phase

High-resolution ion mobility measurements and molecular dynamics simulations have been used to probe the conformations of protonated polyglycine and polyalanine (Gly nH + and Ala nH +, n = 3–20) in the gas phase. The measured collision integrals for both the polyglycine and the polyalanine peptides are consistent with a self-solvated globule conformation, where the peptide chain wraps around and solvates the charge located on the terminal amine. The conformations of the small peptides are governed entirely by self-solvation, whereas the larger ones have additional backbone hydrogen bonds. Helical conformations, which are stable for neutral Ala n peptides, were not observed in the experiments. Molecular dynamics simulations for Ala nH + peptides suggest that the charge destabilizes the helix, although several of the low energy conformations found in the simulations for the larger Ala nH + peptides have small helical regions.

Structures of the Clusters Produced by Laser Desorption of Fullerenes: [2+2] Cycloadducts of Preshrunk Cages

The laser desorption of C60 or C70 fullerenes yields a distribution of ions with masses corresponding to an even number of carbon atoms around integer multiples of 60 or 70. Clusters with 120 or 140 atoms have been characterized as large coalesced fullerenes and fullerene dimers joined by a [2+2] cycloaddition. Several structures had been proposed for the species with less than an integer multiple of 60 or 70 atoms, but none of them has been validated experimentally. We have examined Cn (n = 112, 114, 116, 118, 136, and 138) cations and anions using high resolution ion mobility measurements and compared the results with trajectory calculations for a number of candidate geometries. We find that these species exist as near-spherical cages and [2+2] cycloadducts of smaller fullerenes derived from C60 or C70. Quadruply bound C116, C118, C136, and C138 isomers that would result from the stepwise elimination of C2 from the preformed [2+2] C60 or C70 dimers were not observed. Hence the fullerene dimers lacking a...

Observation of New Ring Isomers for Carbon Cluster Anions

High-resolution ion-mobility measurements have resolved a number of isomers for carbon cluster anions in the drift-time region previously assigned to monocyclic rings. Tight-binding calculations have been performed to generate feasible geometries, which were compared to the experimental data by calculating their mobilities. Structures composed of a short chain of carbon atoms attached to a carbon ring are found to be the most plausible geometries for these new isomers. A number of different bicyclic ring isomers have also been resolved for the first time.

High resolution ion mobility measurements for gas phase proteins: correlation between solution phase and gas phase conformations

Our high resolution ion mobility apparatus has been modified by attaching an electrospray source to perform measurements for biological molecules. While the greater resolving power permits the resolution of more conformations for BPTI and cytochrome c, the resolved features are generally much broader than expected for a single rigid conformation. A major advantage of the new experimental configuration is the much gentler introduction of ions into the drift tube, so that the observed gas phase conformations appear to more closely reflect those present in solution. For example, it is possible to distinguish between the native state of cytochrome c and the methanol-denatured form on the basis of the ion mobility measurements; the mass spectra alone are not sensitive enough to detect this change. Thus this approach may provide a quick and sensitive tool for probing the solution phase conformations of biological molecules.

Structural Transitions in Sodium Chloride Nanocrystals

High resolution ion mobility measurements have been used to examine structural transitions in $(\mathrm{NaCl}{)}_{n}{\mathrm{Cl}}^{\ensuremath{-}}$ nanocrystals with between 30 and 40 NaCl units. Transformations between bulk fragment geometries with different $j\ifmmode\times\else\texttimes\fi{}k\ifmmode\times\else\texttimes\fi{}l$ dimensions are observed at ambient temperatures. In some of these transformations, close to half the atoms are relocated, yet the Arrhenius activation energies are less than 0.6 eV. A surface diffusion mechanism is proposed to explain the remarkably low activation energies.

High-resolution ion mobility studies of sodium chloride nanocrystals

High-resolution ion mobility measurements have been used to examine the geometries of (NaCl) n Cl − clusters with up to 49 NaCl units. Steps in the relative inverse mobilities as a function of the number of NaCl units are due to completion of the 4 × 3 × 3, 4 × 4 × 3, 4 × 4 × 4, and 5 × 4 × 4 cuboids. For clusters with more than 30 NaCl units several families of isomers, with different cuboid or incomplete cuboid geometries, have been resolved at room temperature. The resolved isomers are assigned to specific geometries by comparing the measured mobilities to mobilities calculated for geometries optimized using an ionic potential.

Structural Elucidation of Fullerene Dimers by High-Resolution Ion Mobility Measurements and Trajectory Calculation Simulations

C120, C130, and C140 cluster anions and cations produced by laser desorption of fullerene films have been examined by high-resolution ion mobility measurements and trajectory calculation simulations. The results for anions and cations are similar. Two peaks are present in the measured drift time distributions. They are attributed to fullerene cage isomers and dimers. The fullerene peak is broad, indicating the presence of a range of fullerene cages with different shapes. The measured mobilities of the dimers are in good agreement with mobilities determined by trajectory calculations for the [2 + 2] cycloadduct geometries of (C60)2, C60·C70, and (C70)2. Comparison of the mobilities calculated for the dimer geometries by the commonly used hard-sphere projection approximation with those determined by trajectory calculations with a realistic potential shows that the projection approximation significantly underestimates the inverse mobilities. The deviations, which are large enough to assign the observed features to the wrong geometries, result mainly from the complete neglect of the scattering process in the hard-sphere projection approximation.

High-resolution ion mobility measurements

Gas phase ion mobility measurements can resolve structural isomers for polyatomic ions and provide information about their geometries. A new experimental apparatus for performing high-resolution ion mobility measurements is described. The apparatus consists of a pulsed laser vaporization/desorption source coupled through an ion gate to a 63-cm-long drift tube. The ion gate is a critical component that prevents the diffusion of neutral species from the source into the drift tube. Ions travel along the drift tube under the influence of a uniform electric field. At the end of the drift tube some of the ions exit through a small aperture. They are focused into a quadrupole mass spectrometer, where they are mass analyzed, and then detected by an off-axis collision dynode and by dual microchannel plates. The apparatus is operated with a drift voltage of up to 14 000 V and a helium buffer gas pressure of around 500 Torr. The resolving power for ion mobility measurements is over an order of magnitude higher than has been achieved using conventional injected-ion drift tube techniques. Examples of the application of the new apparatus in resolving isomers of laser desorbed metallofullerenes, in studying silicon clusters generated by laser vaporization, and in following the isomerization of small nanocrystalline (NaCl)nCl− clusters as a function of temperature, are presented.