Design and Performance of a Second-Generation Surface-Induced Dissociation Cell for Fourier Transform Ion Cyclotron Resonance Mass Spectrometry of Native Protein Complexes

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.

Surface-induced Dissociation Mass Spectrometry as a Structural Biology Tool

Native mass spectrometry (nMS) is evolving into a workhorse for structural biology. The plethora of online and offline preparation, separation, and purification methods as well as numerous ionization techniques combined with powerful new hybrid ion mobility and mass spectrometry systems has illustrated the great potential of nMS for structural biology. Fundamental to the progression of nMS has been the development of novel activation methods for dissociating proteins and protein complexes to deduce primary, secondary, tertiary, and quaternary structure through the combined use of multiple MS/MS technologies. This review highlights the key features and advantages of surface collisions (surface-induced dissociation, SID) for probing the connectivity of subunits within protein and nucleoprotein complexes and, in particular, for solving protein structure in conjunction with complementary techniques such as cryo-EM and computational modeling. Several case studies highlight the significant role SID, and more generally nMS, will play in structural elucidation of biological assemblies in the future as the technology becomes more widely adopted. Cases are presented where SID agrees with solved crystal or cryoEM structures or provides connectivity maps that are otherwise inaccessible by "gold standard" structural biology techniques.

Surface-Induced Dissociation for Protein Complex Characterization

Native mass spectrometry (nMS) enables intact non-covalent complexes to be studied in the gas phase. nMS can provide information on composition, stoichiometry, topology, and, when coupled with surface-induced dissociation (SID), subunit connectivity. Here we describe the characterization of protein complexes by nMS and SID. Substructural information obtained using this method is consistent with the solved complex structure, when a structure exists. This provides confidence that the method can also be used to obtain substructural information for unknowns, providing insight into subunit connectivity and arrangements. High-energy SID can also provide information on proteoforms present. Previously SID has been limited to a few in-house modified instruments and here we focus on SID implemented within an in-house-modified Q Exactive UHMR. However, SID is currently commercially available within the Waters Select Series Cyclic IMS instrument. Projects are underway that involve the NIH-funded native MS resource (nativems.osu.edu), instrument vendors, and third-party vendors, with the hope of bringing the technology to more platforms and labs in the near future. Currently, nMS resource staff can perform SID experiments for interested research groups.

Implementing Digital-Waveform Technology for Extended m/z Range Operation on a Native Dual-Quadrupole FT-IM-Orbitrap Mass Spectrometer

Here, we describe a digital-waveform dual-quadrupole mass spectrometer that enhances the performance of our drift tube FT-IMS high-resolution Orbitrap mass spectrometer (MS). The dual-quadrupole analyzer enhances the instrument capabilities for studies of large protein and protein complexes. The first quadrupole (q) provides a means for performing low-energy collisional activation of ions to reduce or eliminate noncovalent adducts, viz., salts, buffers, detergents, and/or endogenous ligands. The second quadrupole (Q) is used to mass-select ions of interest for further interrogation by ion mobility spectrometry and/or collision-induced dissociation (CID). Q is operated using digital-waveform technology (DWT) to improve the mass selection compared to that achieved using traditional sinusoidal waveforms at floated DC potentials (>500 V DC). DWT allows for increased precision of the waveform for a fraction of the cost of conventional RF drivers and with readily programmable operation and precision (Hoffman, N. M. . A comparison-based digital-waveform generator for high-resolution duty cycle. Review of Scientific Instruments 2018, 89, 084101).

Surface-induced dissociation of protein complexes on a cyclic ion mobility spectrometer

We describe the implementation of a simple three-electrode surface-induced dissociation (SID) cell on a cyclic ion mobility spectrometer (cIMS) and demonstrate the utility of multipass mobility separations for resolving multiple conformations of protein complexes generated during collision-induced and surface-induced unfolding (CIU & SIU) experiments. In addition to CIU and SIU, SID of protein complexes is readily accomplished within the native instrument software and with no additional external power supplies by entering a single SID collision energy, a simplification in user experience compared to prior implementations. A set of cyclic homomeric protein complexes and a heterohexamer with known CID and SID behavior were analyzed to investigate mass and mobility resolution improvements, the latter of which improved by 20-50% (median: 33%) compared to a linear travelling wave device. Multiple passes of intact complexes, or their SID fragments, increased the mobility resolution by an average of 15% per pass, with the racetrack effect being observed after ∼3 or 4 passes, depending on the drift time spread of the analytes. Even with modest improvements to apparent mobility resolving power, multipass experiments were particularly useful for separating conformations produced from CIU and SIU experiments. We illustrate several examples where either (1) multipass experiments revealed multiple overlapping conformations previously unobserved or obscured due to limited mobility resolution, or (2) CIU or SIU conformations that appeared 'native' in a single pass experiment were actually slightly compacted or expanded, with the change only being measurable through multipass experiments. The work conducted here, the first utilization of multipass cyclic ion mobility for CIU, SIU, and SID of protein assemblies and a demonstration of a wholly integrated SIU/SID workflow, paves the way for widespread adoption of SID technology for native mass spectrometry and also improves our understanding of gas-phase protein complex CIU and SIU conformationomes.

Protein-Small Molecule Interactions in Native Mass Spectrometry

Small molecule drug discovery has been propelled by the continual development of novel scientific methodologies to occasion therapeutic advances. Although established biophysical methods can be used to obtain information regarding the molecular mechanisms underlying drug action, these approaches are often inefficient, low throughput, and ineffective in the analysis of heterogeneous systems including dynamic oligomeric assemblies and proteins that have undergone extensive post-translational modification. Native mass spectrometry can be used to probe protein-small molecule interactions with unprecedented speed and sensitivity, providing unique insights into polydisperse biomolecular systems that are commonly encountered during the drug discovery process. In this review, we describe potential and proven applications of native MS in the study of interactions between small, drug-like molecules and proteins, including large multiprotein complexes and membrane proteins. Approaches to quantify the thermodynamic and kinetic properties of ligand binding are discussed, alongside a summary of gas-phase ion activation techniques that have been used to interrogate the structure of protein-small molecule complexes. We additionally highlight some of the key areas in modern drug design for which native mass spectrometry has elicited significant advances. Future developments and applications of native mass spectrometry in drug discovery workflows are identified, including potential pathways toward studying protein-small molecule interactions on a whole-proteome scale.

High-Resolution Native Mass Spectrometry

Native mass spectrometry (MS) involves the analysis and characterization of macromolecules, predominantly intact proteins and protein complexes, whereby as much as possible the native structural features of the analytes are retained. As such, native MS enables the study of secondary, tertiary, and even quaternary structure of proteins and other biomolecules. Native MS represents a relatively recent addition to the analytical toolbox of mass spectrometry and has over the past decade experienced immense growth, especially in enhancing sensitivity and resolving power but also in ease of use. With the advent of dedicated mass analyzers, sample preparation and separation approaches, targeted fragmentation techniques, and software solutions, the number of practitioners and novel applications has risen in both academia and industry. This review focuses on recent developments, particularly in high-resolution native MS, describing applications in the structural analysis of protein assemblies, proteoform profiling of—among others—biopharmaceuticals and plasma proteins, and quantitative and qualitative analysis of protein–ligand interactions, with the latter covering lipid, drug, and carbohydrate molecules, to name a few.

Standard procedures for native CZE-MS of proteins and protein complexes up to 800 kDa

Native mass spectrometry (nMS) is a rapidly growing method for the characterization of large proteins and protein complexes, preserving "native" non-covalent inter- and intramolecular interactions. Direct infusion of purified analytes into a mass spectrometer represents the standard approach for conducting nMS experiments. Alternatively, CZE can be performed under native conditions, providing high separation performance while consuming trace amounts of sample material. Here, we provide standard operating procedures for acquiring high-quality data using CZE in native mode coupled online to various Orbitrap mass spectrometers via a commercial sheathless interface, covering a wide range of analytes from 30-800 kDa. Using a standard protein mix, the influence of various CZE method parameters were evaluated, such as BGE/conductive liquid composition and separation voltage. Additionally, a universal approach for the optimization of fragmentation settings in the context of protein subunit and metalloenzyme characterization is discussed in detail for model analytes. A short section is dedicated to troubleshooting of the nCZE-MS setup. This study is aimed to help normalize nCZE-MS practices to enhance the CE community and provide a resource for the production of reproducible and high-quality data.

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.

Quantitative Assessment of Six Different Reagent Gases for Charge Transfer Dissociation (CTD) of Biological Ions

Charge transfer dissociation mass spectrometry (CTD-MS) has been shown to induce high energy fragmentation of biological ions in the gas phase and provide fragmentation spectra similar to extreme ultraviolet photodissociation (XUVPD). To date, CTD has typically employed helium cations with kinetic energies between 4-10 keV to initiate radical-directed fragmentation of analytes. However, as a reagent, helium has recently been listed as a critical mineral that is becoming scarcer and more expensive, so this study explored the potential for using cheaper and more readily available reagent gases. A model peptide, bradykinin, and a model oligosaccharide, κ-carrageenan with a degree of polymerization of 4, were fragmented using a variety of CTD reagent gases, which included helium, hydrogen, oxygen, nitrogen, argon and lab air. The CTD results were also contrasted with low-energy collision-induced dissociation (LE-CID), which were collected on the same 3D ion trap. Using constant reagent ion fluxes and kinetic energies, all five alterative reagent gases generated remarkably consistent sequence coverage and fragmentation efficiencies relative to He-CTD, which suggests that the ionization energy of the reagent gas has a negligible effect on the activation of the biological ions. The CTD efficiencies of all the gases ranged from 11-13% for bradykinin and 7-8% for κ-carrageenan. Within these tight ranges, the abundance of the CTnoD peak of bradykinin and the efficiency of CTD fragmentation of bradykinin both correlated with the ionization energy of the CTD reagent gas, which suggests that resonant charge transfer plays a small role in the activation of this peptide. The majority of the excitation energy for bradykinin and for κ-carrageenan comes from an electron stopping mechanism, which is described by long-range interactions between the reagent cations and electrons in the highest occupied molecular orbitals (HOMOs) of the biological ions. The CTD spectra do not provide any evidence for covalently bound products between the biological ions and the more-reactive gases like hydrogen, oxygen and nitrogen, which implies that the high kinetic energies of the reagent ions make them unavailable for covalent reactions. This work demonstrates that any of the substitute reagent gases tested are viable options for future CTD-MS experiments.

Tandem surface-induced dissociation of protein complexes on an ultrahigh resolution platform

We describe instrumentation for conducting tandem surface-induced dissociation (tSID) of native protein complexes on an ultrahigh resolution Fourier transform ion cyclotron resonance (FT-ICR) mass spectrometer. The two stages of SID are accomplished with split lenses replacing the entrance lenses of the quadrupole mass filter (stage 1, referred to herein as SID-Q) and the collision cell (stage 2, Q-SID). After SID-Q, the scattered projectile ions and subcomplexes formed in transit traverse the 20 mm pre-filter prior to the mass-selecting quadrupole, providing preliminary insights into the SID fragmentation kinetics of noncovalent protein complexes. The isolated SID fragments (subcomplexes) are then fragmented by SID in the collision cell entrance lens (Q-SID), generating subcomplexes of subcomplexes. We show that the ultrahigh resolution of the FT-ICR can be used for deconvolving species overlapping in m/z, which are particularly prominent in tandem SID spectra due to the combination of symmetric charge partitioning and narrow product ion charge state distributions. Various protein complex topologies are explored, including homotetramers, homopentamers, a homohexamer, and a heterohexamer.

Simple and Minimally Invasive SID Devices for Native Mass Spectrometry

We describe a set of simple devices for surface-induced dissociation of proteins and protein complexes on three instrument platforms. All of the devices use a novel yet simple split lens geometry that is minimally invasive (requiring a few millimeters along the ion path axis) and is easier to operate than prior generations of devices. The split lens is designed to be small enough to replace the entrance lens of a Bruker FT-ICR collision cell, the dynamic range enhancement (DRE) lens of a Waters Q-IM-TOF, or the exit lens of a transfer multipole of a Thermo Scientific Extended Mass Range (EMR) Orbitrap. Despite the decrease in size and reduction in number of electrodes to 3 (from 10 to 12 in Gen 1 and ∼6 in Gen 2), we show sensitivity improvement in a variety of cases across all platforms while also maintaining SID capabilities across a wide mass and energy range. The coupling of SID, high resolution, and ion mobility is demonstrated for a variety of protein complexes of varying topologies.

A Tilted Surface and Ion Carpet Array for SID

The development of native mass spectrometry (MS) has provided structural biologists an additional tool to probe the structures of large macromolecular systems. Surface-induced dissociation (SID) is one activation method used within tandem MS experiments that has proven useful in interrogating the connectivity and topology of biologically-relevant protein complexes. We present here the use of a tilted surface and ion carpet array within a new SID device design, enabling decreased dimensions along the ion path and fewer lenses to tune. This device works well in fragmenting ions of both low (peptides) and high (protein complexes) m/z. Results show that the ion carpet array, while enabling simplification of the back-end of the device, has deficiencies in product collection and subsequently signal at higher SID energies when fragmenting protein complexes. However, the use of the tilted surface is advantageous as an effective way to shorten the device and reduce the number of independent voltages.