Towards the Analysis of High Molecular Weight Proteins and Protein complexes using TIMS-MS

Revisiting Dissolved Organic Matter Analysis Using High-Resolution Trapped Ion Mobility and FT-ICR Mass Spectrometry

The molecular level characterization of complex mixtures remains an analytical challenge. We have shown that the integration of complementary, high-resolution, gas-phase separations allows for chemical formula level isomeric content description. In the current work, we revisited the current challenges associated with the analysis of dissolved organic matter using high-resolution trapped ion mobility separation (TIMS) and Fourier transform ion cyclotron resonance mass spectrometry (FT-ICR MS). In particular, we evaluated the separation capabilities provided by TIMS-MS compared to MS alone, the use of ICR complementary data acquisition (DAQ) systems and transient processing strategies, ICR cell geometries (e.g., Infinity cell vs harmonized cell), and magnetic field strengths (7 T vs 9.4 T vs 21 T) for the case of a Harney River DOM sample. Results showed that the external high-performance DAQ enables direct representation of mass spectra in absorption mode FT (aFT), doubling the MS resolution compared to the default magnitude mode FT (mFT). Changes between half- vs full-apodization result in greater MS signal/noise vs superior MS resolving power (RP); in the case of DOM analysis, a 45% increase in assigned formulas is observed when employing the DAQ half (Kaiser-type)-apodization window and aFT when compared to the default instrument mFT. Results showed the advantages of reprocessing 2D-TIMS-FT-ICR MS data with higher RP and magnetic field chemical formulas generated list acquired (e.g., 21 T led to a 24% increase in isomers reported) or the implementation of alternative strategies.

Evaluation of a Commercial TIMS-Q-TOF Platform for Native Mass Spectrometry

Mass-spectrometry based assays in structural biology studies measure either intact or digested proteins. Typically, different mass spectrometers are dedicated for such measurements: those optimized for rapid analysis of peptides or those designed for high molecular weight analysis. A commercial trapped ion mobility-quadrupole-time-of-flight (TIMS-Q-TOF) platform is widely utilized for proteomics and metabolomics, with ion mobility providing a separation dimension in addition to liquid chromatography. The ability to perform high-quality native mass spectrometry of protein complexes, however, remains largely uninvestigated. Here, we evaluate a commercial TIMS-Q-TOF platform for analyzing noncovalent protein complexes by utilizing the instrument's full range of ion mobility, MS, and MS/MS (both in-source activation and collision cell CID) capabilities. The TIMS analyzer is able to be tuned gently to yield collision cross sections of native-like complexes comparable to those previously reported on various instrument platforms. In-source activation and collision cell CID were robust for both small and large complexes. TIMS-CID was performed on protein complexes streptavidin (53 kDa), avidin (68 kDa), and cholera toxin B (CTB, 58 kDa). Complexes pyruvate kinase (237 kDa) and GroEL (801 kDa) were beyond the trapping capabilities of the commercial TIMS analyzer, but TOF mass spectra could be acquired. The presented results indicate that the commercial TIMS-Q-TOF platform can be used for both omics and native mass spectrometry applications; however, modifications to the commercial RF drivers for both the TIMS analyzer and quadrupole (currently limited to m/z 3000) are necessary to mobility analyze protein complexes greater than about 60 kDa.

Separation and Collision Cross Section Measurements of Protein Complexes Afforded by a Modular Drift Tube Coupled to an Orbitrap Mass Spectrometer

New developments in analytical technologies and biophysical methods have advanced the characterization of increasingly complex biomolecular assemblies using native mass spectrometry (MS). Ion mobility methods, in particular, have enabled a new dimension of structural information and analysis of proteins, allowing separation of conformations and providing size and shape insights based on collision cross sections (CCSs). Based on the concepts of absorption-mode Fourier transform (aFT) multiplexing ion mobility spectrometry (IMS), here, a modular drift tube design proves capable of separating native-like proteins up to 148 kDa with resolution up to 45. Coupled with high-resolution Orbitrap MS, binding of small ligands and cofactors can be resolved in the mass domain and correlated to changes in structural heterogeneity observed in the ion-neutral CCS distributions. We also demonstrate the ability to rapidly determine accurate CCSs for proteins with 1-min aFT-IMS-MS sweeps without the need for calibrants or correction factors.

Collision-Induced Unfolding of Native-like Protein Ions Within a Trapped Ion Mobility Spectrometry Device

Native mass spectrometry and collision-induced unfolding (CIU) workflows continue to grow in utilization due to their ability to rapidly characterize protein conformation and stability. To perform these experiments, the instrument must be capable of collisionally activating ions prior to ion mobility spectrometry (IMS) analyses. Trapped ion mobility spectrometry (TIMS) is an ion mobility implementation that has been increasingly adopted due to its inherently high resolution and reduced instrumental footprint. In currently deployed commercial instruments, however, typical modes of collisional activation do not precede IMS analysis, and thus, the instruments are incapable of performing CIU. In this work, we expand on a recently developed method of activating protein ions within the TIMS device and explore its analytical utility toward the unfolding of native-like protein ions. We demonstrate the unfolding of native-like ions of ubiquitin, cytochrome C, β-lactoglobulin, and carbonic anhydrase. These ions undergo extensive unfolding upon collisional activation. Additionally, the improved resolution provided by the TIMS separation uncovers previously obscured unfolding complexity.

Structural O-Glycoform Heterogeneity of the SARS-CoV-2 Spike Protein Receptor-Binding Domain Revealed by Top-Down Mass Spectrometry

Severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) utilizes an extensively glycosylated surface spike (S) protein to mediate host cell entry, and the S protein glycosylation plays key roles in altering the viral binding/function and infectivity. However, the molecular structures and glycan heterogeneity of the new O-glycans found on the S protein regional-binding domain (S-RBD) remain cryptic because of the challenges in intact glycoform analysis by conventional bottom-up glycoproteomic approaches. Here, we report the complete structural elucidation of intact O-glycan proteoforms through a hybrid native and denaturing top-down mass spectrometry (MS) approach employing both trapped ion mobility spectrometry (TIMS) quadrupole time-of-flight and ultrahigh-resolution Fourier transform ion cyclotron resonance (FTICR)-MS. Native top-down TIMS-MS/MS separates the protein conformers of the S-RBD to reveal their gas-phase structural heterogeneity, and top-down FTICR-MS/MS provides in-depth glycoform analysis for unambiguous identification of the glycan structures and their glycosites. A total of eight O-glycoforms and their relative molecular abundance are structurally elucidated for the first time. These findings demonstrate that this hybrid top-down MS approach can provide a high-resolution proteoform-resolved mapping of diverse O-glycoforms of the S glycoprotein, which lays a strong molecular foundation to uncover the functional roles of their O-glycans. This proteoform-resolved approach can be applied to reveal the structural O-glycoform heterogeneity of emergent SARS-CoV-2 S-RBD variants as well as other O-glycoproteins in general.

Surface-Induced Dissociation of Protein Complexes Selected by Trapped Ion Mobility Spectrometry

Native mass spectrometry (nMS), particularly in conjunction with gas-phase ion mobility spectrometry measurements, has proven useful as a structural biology tool for evaluating the stoichiometry, conformation, and topology of protein complexes. Here, we demonstrate the combination of trapped ion mobility spectrometry (TIMS) and surface-induced dissociation (SID) on a Bruker SolariX XR 15 T FT-ICR mass spectrometer for the structural analysis of protein complexes. We successfully performed SID on mobility-selected protein complexes, including the streptavidin tetramer and cholera toxin B with bound ligands. Additionally, TIMS-SID was employed on a mixture of the peptides desArg1 and desArg9 bradykinin to mobility-separate and identify the individual peptides. Importantly, results show that native-like conformations can be maintained throughout the TIMS analysis. The TIMS-SID spectra are analogous to SID spectra acquired using quadrupole mass selection, indicating little measurable, if any, structural rearrangement during mobility selection. Mobility parking was used on the ion or mobility of interest and 50-200 SID mass spectra were averaged. High-quality TIMS-SID spectra were acquired over a period of 2-10 min, comparable to or slightly longer than SID coupled with ion mobility on various instrument platforms in our laboratory. The ultrahigh resolving power of the 15 T FT-ICR allowed for the identification and relative quantification of overlapping SID fragments with the same nominal m/z based on isotope patterns, and it shows promise as a platform to probe small mass differences, such as protein/ligand binding or post-translational modifications. These results represent the potential of TIMS-SID-MS for the analysis of both protein complexes and peptides.

Utilizing Ion Mobility-Mass Spectrometry to Investigate the Unfolding Pathway of Cu/Zn Superoxide Dismutase

Native mass spectrometry has emerged as a powerful tool for structural biology as it enables the evaluation of molecules as they occur in their physiological conditions. Ion mobility spectrometry-mass spectrometry (IMS-MS) has shown essential in these analyses as it allows the measurement of the shape of a molecule, denoted as its collision cross section (CCS), and mass. The structural information garnered from native IMS-MS provides insight into the tertiary and quaternary structure of proteins and can be used to validate NMR or crystallographic X-ray structures. Additionally, due to the rapid nature (millisecond measurements) and ability of IMS-MS to analyze heterogeneous solutions, it can be used to address structural questions not possible with traditional structural approaches. Herein, we applied multiple solution conditions to systematically denature bovine Cu/Zn-superoxide dismutase (SOD1) and assess its unfolding pathway from the holo-dimer to the holo-monomer, single-metal monomer, and apo-monomer. Additionally, we compared and noted 1–2% agreement between CCS values from both drift tube IMS and trapped IMS for the SOD1 holo-monomer and holo-dimer. The observed CCS values were in excellent agreement with computational CCS values predicted from the homo-dimer crystal structure, showcasing the ability to use both IMS-MS platforms to provide valuable structural information for molecular modeling of protein interactions and structural assessments.

Trapped Ion Mobility Spectrometry of Native Macromolecular Assemblies

The structural elucidation of native macromolecular assemblies has been a subject of considerable interest in native mass spectrometry (MS), and more recently in tandem with ion mobility spectrometry (IMS-MS), for a better understanding of their biochemical and biophysical functions. In the present work, we describe a new generation trapped ion mobility spectrometer (TIMS), with extended mobility range (K0 = 0.185-1.84 cm2·V-1·s-1), capable of trapping high-molecular-weight (MW) macromolecular assemblies. This compact 4 cm long TIMS analyzer utilizes a convex electrode, quadrupolar geometry with increased pseudopotential penetration in the radial dimension, extending the mobility trapping to high-MW species under native state (i.e., lower charge states). The TIMS capabilities to perform variable scan rate (Sr) mobility measurements over short time (100-500 ms), high-mobility resolution, and ion-neutral collision cross-section (CCSN2) measurements are presented. The trapping capabilities of the convex electrode TIMS geometry and ease of operation over a wide gas flow, rf range, and electric field trapping range are illustrated for the first time using a comprehensive list of standards varying from CsI clusters (n = 6-73), Tuning Mix oligomers (n = 1-5), common proteins (e.g., ubiquitin, cytochrome C, lysozyme, concanavalin (n = 1-4), carbonic anhydrase, β clamp (n = 1-4), topoisomerase IB, bovine serum albumin (n = 1-3), topoisomerase IA, alcohol dehydrogenase), IgG antibody (e.g., avastin), protein-DNA complexes, and macromolecular assemblies (e.g., GroEL and RNA polymerase (n = 1-2)) covering a wide mass (up to m/z 19 000) and CCS range (up to 22 000 Å2 with <0.6% relative standard deviation (RSD)).

Reduction of Taylor-Aris dispersion by lateral mixing for chromatographic applications

Chromatographic columns are suffering from Taylor-Aris dispersion, especially for slowly diffusing molecules such as proteins. Since downscaling the channel size to reduce Taylor-Aris dispersion meets fundamental pressure limitations, new strategies are needed to further improve chromatography beyond its current limits. In this work we demonstrate a method to reduce Taylor-Aris dispersion by lateral mixing in a newly designed silicon AC-electroosmotic flow mixer. We obtained a reduction in κ_aris by a factor of three in a 40 μm × 20 μm microchannel, corresponding to a plate height gain of 2 to 3 under unretained conditions at low to high Pe values. We also demonstrate an improvement of a reverse-phase chromatographic separation of coumarins.

Advances in MS Based Strategies for Probing Ligand-Target Interactions: Focus on Soft Ionization Mass Spectrometric Techniques

The non-covalent interactions between small drug molecules and disease-related proteins (ligand-target interactions) mediate various pharmacological processes in the treatment of different diseases. The development of the analytical methods to assess those interactions, including binding sites, binding energies, stoichiometry and association-dissociation constants, could assist in clarifying the mechanisms of action, precise treatment of targeted diseases as well as the targeted drug discovery. For the last decades, mass spectrometry (MS) has been recognized as a powerful tool to study the non-covalent interactions of the ligand-target complexes with the characteristics of high sensitivity, high-resolution, and high-throughput. Soft ionization mass spectrometry, especially the electrospray mass spectrometry (ESI-MS) and matrix assisted laser desorption ionization mass spectrometry (MALDI-MS), could achieve the complete transformation of the target analytes into the gas phase, and subsequent detection of the small drug molecules and disease-related protein complexes, and has exerted great advantages for studying the drug ligands-protein targets interactions, even in case of identifying active components as drug ligands from crude extracts of medicinal plants. Despite of other analytical techniques for this purpose, such as the NMR and X-ray crystallography, this review highlights the principles, research hotspots and recent applications of the soft ionization mass spectrometry and its hyphenated techniques, including hydrogen-deuterium exchange mass spectrometry (HDX-MS), chemical cross-linking mass spectrometry (CX-MS), and ion mobility spectrometry mass spectrometry (IMS-MS), in the study of the non-covalent interactions between small drug molecules and disease-related proteins.

Importance of collision cross section measurements by ion mobility mass spectrometry in structural biology

The field of ion mobility mass spectrometry (IM-MS) has developed rapidly in recent decades, with new fundamental advances underpinning innovative applications. This has been particularly noticeable in the field of biomacromolecular structure determination and structural biology, with pioneering studies revealing new structural insight for complex protein assemblies which control biological function. This perspective offers a review of recent developments in IM-MS which have enabled expanding applications in protein structural biology, principally focusing on the quantitative measurement of collision cross sections and their interpretation to describe higher order protein structures.

Evidence of Cis/Trans-Isomerization at Pro7/Pro16 in the Lasso Peptide Microcin J25

Microcin J25 is a ribosomal synthesized and post-translationally modified peptide (RiPP) characterized by a mechanically interlocked topology called the lasso fold. This structure provides microcin J25 a potent antimicrobial activity resulting from internalization via the siderophore receptor FhuA and further inhibition of the RNA polymerase. In the present work, nuclear magnetic resonance (NMR) and trapped ion mobility spectrometry-mass spectrometry (TIMS-MS) were used to investigate the lasso structure of microcin J25. NMR experiments showed that the lasso peptide microcin J25 can adopt conformational states where Pro16 can be found in the cis- and trans-orientations. The high-resolution mobility analysis, aided by site-directed mutagenesis ([P7A], [P16A], and [P7A/P16A] variants), demonstrated that microcin J25 can adopt cis/cis-, cis/trans-, trans/cis-, and trans/trans-conformations at the Pro7 and Pro16 peptide bonds. It was also shown that interconversion between the conformers can occur as a function of the starting solvent conditions and ion heating (collision-induced activation, CIA) despite the lasso topology. Complementary to NMR findings, the cis-conformations at Pro7 were assigned using TIMS-MS. This study highlights the analytical power of TIMS-MS and site-directed mutagenesis for the study of biological systems with large micro-heterogeneity as a way to further increase our understanding of the receptor-binding dynamics and biological activity.

Ultra-high-resolution ion mobility spectrometry-current instrumentation, limitations, and future developments

With recent advances in ionization sources and instrumentation, ion mobility spectrometers (IMS) have transformed from a detector for chemical warfare agents and explosives to a widely used tool in analytical and bioanalytical applications. This increasing measurement task complexity requires higher and higher analytical performance and especially ultra-high resolution. In this review, we will discuss the currently used ion mobility spectrometers able to reach such ultra-high resolution, defined here as a resolving power greater than 200. These instruments are drift tube IMS, traveling wave IMS, trapped IMS, and field asymmetric or differential IMS. The basic operating principles and the resulting effects of experimental parameters on resolving power are explained and compared between the different instruments. This allows understanding the current limitations of resolving power and how ion mobility spectrometers may progress in the future. Graphical abstract.

Peptide Sequence Influence on the Conformational Dynamics and DNA binding of the Intrinsically Disordered AT-Hook 3 Peptide

The intrinsically disordered ATHP3 was studied at native conditions and in complex with DNA using single amino acid substitutions and high-resolution ion mobility spectrometry coupled to mass spectrometry (trapped IMS-MS). Results showed that ATHP3 can exist in multiple conformations at native conditions (at least 10 conformers were separated), with a variety of proline cis/trans orientations, side chain orientations and protonation sites. When in complex with AT rich DNA hairpins, the -RGRP- core is essential for stabilizing the ATHP3: DNA complex. In particular, the arginine in the sixth position plays an important role during binding to AT-rich regions of hairpin DNA, in good agreement with previous NMR and X-ray data. Mobility based correlation matrices are proposed as a way to reveal differences in structural motifs across the peptide mutants based on the conformational space and relative conformer abundance.

Towards Discovery and Targeted Peptide Biomarker Detection Using nanoESI-TIMS-TOF MS

In the present work, the potential of trapped ion mobility spectrometry coupled to TOF mass spectrometry (TIMS-TOF MS) for discovery and targeted monitoring of peptide biomarkers from human-in-mouse xenograft tumor tissue was evaluated. In particular, a TIMS-MS workflow was developed for the detection and quantification of peptide biomarkers using internal heavy analogs, taking advantage of the high mobility resolution (R = 150-250) prior to mass analysis. Five peptide biomarkers were separated, identified, and quantified using offline nanoESI-TIMS-CID-TOF MS; the results were in good agreement with measurements using a traditional LC-ESI-MS/MS proteomics workflow. The TIMS-TOF MS analysis permitted peptide biomarker detection based on accurate mobility, mass measurements, and high sequence coverage for concentrations in the 10-200 nM range, while simultaneously achieving discovery measurements of not initially targeted peptides as markers from the same proteins and, eventually, other proteins. Graphical Abstract ᅟ.

Conformational Landscapes of Ubiquitin, Cytochrome c, and Myoglobin: Uniform Field Ion Mobility Measurements in Helium and Nitrogen Drift Gas

In this study, a commercial uniform field drift tube ion mobility-mass spectrometer (IM-MS) was utilized to measure the gas-phase conformational populations of three well-studied proteins: ubiquitin (8566 Da), cytochrome c (12,359 Da), and myoglobin in both apo and holo forms (16,951 and 17,567 Da, respectively) in order to evaluate the use of this technology for broadscale structural proteomics applications. Proteins were electrosprayed from either acidic organic (pH ~3) or aqueous buffered (pH ~6.6) solution phase conditions, which generated a wide range of cation charge states corresponding to both extended (unfolded) and compact (folded) gas-phase conformational populations. Corresponding collision cross section (CCS) measurements were compiled for significant ion mobility peak features observed at each charge state in order to map the conformational landscapes of these proteins in both helium and nitrogen drift gases. It was observed that the conformational landscapes were similar in both drift gases, with differences being attributed primarily to ion heating during helium operation due to the necessity of operating the instrument with higher pressure differentials. Higher resolving powers were observed in nitrogen, which allowed for slightly better structural resolution of closely-spaced conformer populations. The instrumentation was found to be particularly adept at measuring low abundance conformers which are only present under gentle conditions which minimize ion heating. This work represents the single largest ion mobility CCS survey published to date for these three proteins with 266 CCS values and 117 ion mobility spectra, many of which have not been previously reported.

Discovery and targeted monitoring of polychlorinated biphenyl metabolites in blood plasma using LC-TIMS-TOF MS

In the present work, the potential for rapid, targeted analysis of hydroxylated metabolites of polychlorinated biphenyls (OH-PCBs) in diluted human blood plasma using liquid chromatography coupled with trapped ion mobility spectrometry and TOF high resolution mass spectrometry (LC-TIMS-TOF MS) was evaluated. Experimental OH-PCB collisional cross section (CCS_N2) and gas-phase candidate structures (<3% error) are reported for the first time and used, in addition to the LC retention time and accurate m/z, as OH-PCB identification features in order to increase the detection selectivity. The proposed LC-TIMS-TOF MS workflow combines a "dilute-and-shoot" sample preparation strategy, a robust liquid chromatography step, a high-resolving power mobility separation (R ~ 150-250) and high-resolution mass spectrometry (R ~ 30-40k) for the separation, identification and quantification of common OH-PCB isomers with limits of detection comparable to traditional workflows (e.g., LOD and LOQ of ~10 pg/mL and ~50 pg/mL, respectively). The higher selectivity and low detection limits provides multiple advantages compared to current methodologies that typically require long, labor-intensive preparation and/or derivatization steps prior to gas or liquid chromatography-mass spectrometry.

Metal ions induced secondary structure rearrangements: mechanically interlocked lassovs.unthreaded branched-cyclic topoisomers

Metal ions can play a significant role in a variety of important functions in protein systems including cofactor for catalysis, protein folding, assembly, structural stability and conformational change.

Adding a new separation dimension to MS and LC-MS: What is the utility of ion mobility spectrometry?

Ion mobility spectrometry is an analytical technique known for more than 100 years, which entails separating ions in the gas phase based on their size, shape, and charge. While ion mobility spectrometry alone can be useful for some applications (mostly security analysis for detecting certain classes of narcotics and explosives), it becomes even more powerful in combination with mass spectrometry and high-performance liquid chromatography. Indeed, the limited resolving power of ion mobility spectrometry alone can be tackled when combining this analytical strategy with mass spectrometry or liquid chromatography with mass spectrometry. Over the last few years, the hyphenation of ion mobility spectrometry to mass spectrometry or liquid chromatography with mass spectrometry has attracted more and more interest, with significant progresses in both technical advances and pioneering applications. This review describes the theoretical background, available technologies, and future capabilities of these techniques. It also highlights a wide range of applications, from small molecules (natural products, metabolites, glycans, lipids) to large biomolecules (proteins, protein complexes, biopharmaceuticals, oligonucleotides).

Fast and Effective Ion Mobility-Mass Spectrometry Separation of d-Amino-Acid-Containing Peptides

Despite often minute concentrations in vivo, d-amino acid containing peptides (DAACPs) are crucial to many life processes. Standard proteomics protocols fail to detect them as d/l substitutions do not affect the peptide parent and fragment masses. The differences in fragment yields are often limited, obstructing the investigations of important but low abundance epimers in isomeric mixtures. Separation of d/l-peptides using ion mobility spectrometry (IMS) was impeded by small collision cross section differences (commonly ∼1%). Here, broad baseline separation of DAACPs with up to ∼30 residues employing trapped IMS with resolving power up to ∼340, followed by time-of-flight mass spectrometry is demonstrated. The d/l-pairs coeluting in one charge state were resolved in another, and epimers merged as protonated species were resolved upon metalation, effectively turning the charge state and cationization mode into extra separation dimensions. Linear quantification down to 0.25% proved the utility of high resolution IMS-MS for real samples with large interisomeric dynamic range. Very close relative mobilities found for DAACP pairs using traveling-wave IMS (TWIMS) with different ion sources and faster IMS separations showed the transferability of results across IMS platforms. Fragmentation of epimers can enhance their identification and further improve detection and quantification limits, and we demonstrate the advantages of online mobility separated collision-induced dissociation (CID) followed by high resolution mass spectrometry (TIMS-CID-MS) for epimer analysis.

Coming to Grips with Ambiguity: Ion Mobility-Mass Spectrometry for Protein Quaternary Structure Assignment

Multiprotein complexes are central to our understanding of cellular biology, as they play critical roles in nearly every biological process. Despite many impressive advances associated with structural characterization techniques, large and highly-dynamic protein complexes are too often refractory to analysis by conventional, high-resolution approaches. To fill this gap, ion mobility-mass spectrometry (IM-MS) methods have emerged as a promising approach for characterizing the structures of challenging assemblies due in large part to the ability of these methods to characterize the composition, connectivity, and topology of large, labile complexes. In this Critical Insight, we present a series of bioinformatics studies aimed at assessing the information content of IM-MS datasets for building models of multiprotein structure. Our computational data highlights the limits of current coarse-graining approaches, and compelled us to develop an improved workflow for multiprotein topology modeling, which we benchmark against a subset of the multiprotein complexes within the PDB. This improved workflow has allowed us to ascertain both the minimal experimental restraint sets required for generation of high-confidence multiprotein topologies, and quantify the ambiguity in models where insufficient IM-MS information is available. We conclude by projecting the future of IM-MS in the context of protein quaternary structure assignment, where we predict that a more complete knowledge of the ultimate information content and ambiguity within such models will undoubtedly lead to applications for a broader array of challenging biomolecular assemblies. Graphical Abstract ᅟ.

The Solution Assembly of Biological Molecules Using Ion Mobility Methods: From Amino Acids to Amyloid β-Protein

Ion mobility spectrometry-mass spectrometry (IMS-MS) methods are increasingly used to study noncovalent assemblies of peptides and proteins. This review focuses on the noncovalent self-assembly of amino acids and peptides, systems at the heart of the amyloid process that play a central role in a number of devastating diseases. Three different systems are discussed in detail: the 42-residue peptide amyloid-β42 implicated in the etiology of Alzheimer's disease, several amyloid-forming peptides with 6-11 residues, and the assembly of individual amino acids. We also discuss from a more fundamental perspective the processes that determine how quickly proteins and their assemblies denature when the analyte ion has been stripped of its solvent in an IMS-MS measurement and how to soften the measurement so that biologically meaningful data can be recorded.

Analysis of Photoirradiated Water Accommodated Fractions of Crude Oils Using Tandem TIMS and FT-ICR MS

For the first time, trapped ion mobility spectrometry (TIMS) in tandem with Fourier transform ion cyclotron resonance mass spectrometry (FT-ICR MS) is applied to the analysis of the low energy water accommodated fraction (WAF) of a crude oil as a function of the exposure to light. The TIMS-FT-ICR MS analysis provided, in addition to the heteroatom series identification, new insights into the WAF isomeric complexity (e.g., [m/z; chemical formula; collision cross section] data sets) for a better evaluation of the degree of chemical and structural photoinduced transformations. Inspection of the [m/z; chemical formula; collision cross section] data sets shows that the WAF composition changes as a function of the exposure to light in the first 115 h by initial photosolubilization of HC components and their photo-oxidation up to O_4-5 of mainly high double bond equivalence species (DBE > 9). The addition of high resolution TIMS (resolving power of 90-220) to ultrahigh resolution FT-ICR MS (resolving power over 400k) permitted the identification of a larger number of molecular components in a single analysis (e.g., over 47k using TIMS-MS compared to 12k by MS alone), with instances of over 6-fold increase in the number of molecular features per nominal mass due to the WAF isomeric complexity. This work represents a stepping stone toward a better understanding of the WAF components and highlights the need for better experimental and theoretical approaches to characterize the WAF structural diversity.

Structures of the kinetically trapped i-motif DNA intermediates

In the present work, the conformational dynamics and folding pathways of i-motif DNA were studied in solution and in the gas-phase as a function of the solution pH conditions using circular dichroism (CD), photoacoustic calorimetry analysis (PAC), trapped ion mobility spectrometry-mass spectrometry (TIMS-MS), and molecular dynamics (MD). Solution studies showed at thermodynamic equilibrium the existence of a two-state folding mechanism, whereas during the pH = 7.0 → 4.5 transition a fast and slow phase (ΔHfast + ΔHslow = 43 ± 7 kcal mol-1) with a volume change associated with the formation of hemiprotonated cytosine base pairs and concomitant collapse of the i-motif oligonucleotide into a compact conformation were observed. TIMS-MS experiments showed that gas-phase, kinetically trapped i-motif DNA intermediates produced by nanoESI are preserved, with relative abundances depending on the solution pH conditions. In particular, a folded i-motif DNA structure was observed in nanoESI-TIMS-MS for low charge states in both positive and negative ion mode (e.g., z = ±3 to ±5) at low pH conditions. As solution pH increases, the cytosine neutralization leads to the loss of cytosine-cytosine+ (C·CH+) base pairing in the CCC strands and in those conditions we observe partially unfolded i-motif DNA conformations in nanoESI-TIMS-MS for higher charge states (e.g., z = -6 to -9). Collisional induced activation prior to TIMS-MS showed the existence of multiple local free energy minima, associated with the i-motif DNA unfolding at z = -6 charge state. For the first time, candidate gas-phase structures are proposed based on mobility measurements of the i-motif DNA unfolding pathway. Moreover, the inspection of partially unfolded i-motif DNA structures (z = -7 and z = -8 charge states) showed that the presence of inner cations may or may not induce conformational changes in the gas-phase. For example, incorporation of ammonium adducts does not lead to major conformational changes while sodium adducts may lead to the formation of sodium mediated bonds between two negatively charged sides inducing the stabilization towards more compact structures in new local, free energy minima in the gas-phase.