Decharging of Globular Proteins and Protein Complexes in Electrospray

Digital Ion Trap Isolation and Mass Analysis of Macromolecular Analytes with Multiply Charged Ion Attachment

Multiply charged ions produced by electrospray ionization (ESI) of heterogeneous mixtures of macromolecular analytes under native conditions are typically confined to relatively narrow ranges of mass-to-charge (m/z) ratio, often with extensive overlap. This scenario makes charge and mass assignments extremely challenging, particularly when individual charge states are unresolved. An ion/ion reaction strategy involving multiply charged ion attachment (MIA) to the mixture components in a narrow range of m/z can facilitate charge and mass assignment. In MIA operation, multiply charged reagent ions are attached to the analyte ions of opposite polarity to provide large m/z displacements resulting from both large changes in mass and charge. However, charge reduction of the high m/z ions initially generated under native ESI conditions requires the ability to isolate high m/z ions and to analyze even higher m/z product ions. Digital ion trap (DIT) operation offers means for both high m/z ion isolation and high m/z mass analysis, in addition to providing conditions for the reaction of oppositely charged ions. The feasibility of conducting MIA experiments in a DIT that takes advantage of high m/z ion operation is demonstrated here using a tandem 2D-3D DIT instrument. Proof-of-concept MIA experiments with cations derived from β-galactosidase using the 20- charge state of human serum immunoglobulin G (IgG, ∼149 kDa) as the reagent anion are described. MIA experiments involving mixtures of ions derived from the E. coli. ribosome are also described. For example, three components in a mixture of 70S particles (>2.2 MDa) were resolved and assigned with masses and charges following an MIA experiment involving the 20- charge state of human serum IgG.

Single-Frequency Ion Parking in a Digital 3D Quadrupole Ion Trap

Single-frequency ion parking, a useful technique in electrospray mass spectrometry (ESI-MS), involves gas-phase charge-reduction ion/ion reactions in an electrodynamic ion trap in conjunction with the application of a supplementary oscillatory voltage to selectively inhibit the reaction rate of an ion of interest. The ion parking process provides a means for limiting the extent of charge reduction in a controlled fashion and allows for ions distributed over a range of charge states to be concentrated into fewer charge states (a single charge state under optimal conditions). As charge reduction inherently leads to an increase in the mass-to-charge (m/z) ratio of the ions, it is important that the means for storing and analyzing ions be able to accommodate ions of high m/z ratios. The so-called 'digital ion trap' (DIT), which uses a digital waveform as the trapping RF, has been demonstrated to be well-suited for the analysis of high m/z ions by taking advantage of its ability to manipulate the waveform frequency. In this study, the feasibility of ion parking in a 3D quadrupole ion trap operated as a DIT using a slow-amplitude single-frequency sine-wave for selective inhibition of an ion/ion reaction is demonstrated. A recently described model that describes ion parking has been adjusted for the DIT case and is used to interpret experimental data for proteins ranging in mass from 8600 Da to 467,000 Da.

Ion parking in native mass spectrometry

A forced, damped harmonic oscillator model for gas-phase ion parking using single-frequency resonance excitation is described and applied to high-mass ions of relevance to native mass spectrometry. Experimental data are provided to illustrate key findings revealed by the modelling. These include: (i) ion secular frequency spacings between adjacent charge states of a given protein are essentially constant and decrease with the mass of the protein (ii) the mechanism for ion parking of high mass ions is the separation of the ion clouds of the oppositely-charged ions with much less influence from an increase in the relative ion velocity due to resonance excitation, (iii) the size of the parked ion cloud ultimately limits ion parking at high m/z ratio, and (iv) the extent of ion parking of off-target ions is highly sensitive to the bath gas pressure in the ion trap. The model is applied to ions of 17 kDa, 467 kDa, and 2 MDa while experimental data are also provided for ions of horse skeletal muscle myoglobin (≈17 kDa) and β-galactosidase (≈467 kDa). The model predicts and data show that it is possible to effect ion parking on a 17 kDa protein to the 1+ charge state under trapping conditions that are readily accessible with commercially available ion traps. It is also possible to park β-galactosidase efficiently to a roughly equivalent m/z ratio (i.e., the 26+ charge state) under the same trapping conditions. However, as charge states decrease, analyte ion cloud sizes become too large to allow for efficient ion trapping. The model allows for a semi-quantitative prediction of ion trapping performance as a function of ion trapping, resonance excitation, and pressure conditions.

Native Mass Spectrometry: Insights and Opportunities for Targeted Protein Degradation

Native mass spectrometry (nMS) is one of the most powerful biophysical methods for the direct observation of noncovalent protein interactions with both small molecules and other proteins. With the advent of targeted protein degradation (TPD), nMS is now emerging as a compelling approach to characterize the multiple fundamental interactions that underpin the TPD mechanism. Specifically, nMS enables the simultaneous observation of the multiple binary and ternary complexes [i.e., all combinations of E3 ligase, target protein of interest, and small molecule proximity-inducing reagents (such as PROteolysis TArgeting Chimeras (PROTACs) and molecular glues)], formed as part of the TPD equilibrium; this is not possible with any other biophysical method. In this paper we overview the proof-of-concept applications of nMS within the field of TPD and demonstrate how it is providing researchers with critical insight into the systems under study. We also provide an outlook on the scope and future opportunities offered by nMS as a core and agnostic biophysical tool for advancing research developments in TPD.

Giving the Green Light to Photochemical Uncaging of Large Biomolecules in High Vacuum

The isolation of biomolecules in a high vacuum enables experiments on fragile species in the absence of a perturbing environment. Since many molecular properties are influenced by local electric fields, here we seek to gain control over the number of charges on a biopolymer by photochemical uncaging. We present the design, modeling, and synthesis of photoactive molecular tags, their labeling to peptides and proteins as well as their photochemical validation in solution and in the gas phase. The tailored tags can be selectively cleaved off at a well-defined time and without the need for any external charge-transferring agents. The energy of a single or two green photons can already trigger the process, and it is soft enough to ensure the integrity of the released biomolecular cargo. We exploit differences in the cleavage pathways in solution and in vacuum and observe a surprising robustness in upscaling the approach from a model system to genuine proteins. The interaction wavelength of 532 nm is compatible with various biomolecular entities, such as oligonucleotides or oligosaccharides.

Onto Grid Purification and 3D Reconstruction of Protein Complexes Using Matrix-Landing Native Mass Spectrometry

Addressing mixtures and heterogeneity in structural biology requires approaches that can differentiate and separate structures based on mass and conformation. Mass spectrometry (MS) provides tools for measuring and isolating gas-phase ions. The development of native MS including electrospray ionization allowed for manipulation and analysis of intact noncovalent biomolecules as ions in the gas phase, leading to detailed measurements of structural heterogeneity. Conversely, transmission electron microscopy (TEM) generates detailed images of biomolecular complexes that show an overall structure. Our matrix-landing approach uses native MS to probe and select biomolecular ions of interest for subsequent TEM imaging, thus unifying information on mass, stoichiometry, heterogeneity, etc., available via native MS with TEM images. Here, we prepare TEM grids of protein complexes purified via quadrupolar isolation and matrix-landing and generate 3D reconstructions of the isolated complexes. Our results show that these complexes maintain their structure through gas-phase isolation.

Surface Activity of Amines Provides Evidence for the Combined ESI Mechanism of Charge Reduction for Protein Complexes

Charge reduction reactions are important for native mass spectrometry (nMS) because lower charge states help retain native-like conformations and preserve noncovalent interactions of protein complexes. While mechanisms of charge reduction reactions are not well understood, they are generally achieved through the addition of small molecules, such as polyamines, to traditional nMS buffers. Here, we present new evidence that surface-active, charge reducing reagents carry away excess charge from the droplet after being emitted due to Coulombic repulsion, thereby reducing the overall charge of the droplet. Furthermore, these processes are directly linked to two mechanisms for electrospray ionization, specifically the charge residue and ion evaporation models (CRM and IEM). Selected protein complexes were analyzed in solutions containing ammonium acetate and selected trialkylamines or diaminoalkanes of increasing alkyl chain lengths. Results show that amines with higher surface activity have increased propensities for promoting charge reduction of the protein ions. The electrospray ionization (ESI) emitter potential was also found to be a major contributing parameter to the prevalence of charge reduction; higher emitter potentials consistently coincided with lower average charge states among all protein complexes analyzed. These results offer experimental evidence for the mechanism of charge reduction in ESI and also provide insight into the final stages of the ESI and their impact on biological ions.

Approaches to Heterogeneity in Native Mass Spectrometry

Native mass spectrometry (MS) is aimed at preserving and determining the native structure, composition, and stoichiometry of biomolecules and their complexes from solution after they are transferred into the gas phase. Major improvements in native MS instrumentation and experimental methods over the past few decades have led to a concomitant increase in the complexity and heterogeneity of samples that can be analyzed, including protein-ligand complexes, protein complexes with multiple coexisting stoichiometries, and membrane protein-lipid assemblies. Heterogeneous features of these biomolecular samples can be important for understanding structure and function. However, sample heterogeneity can make assignment of ion mass, charge, composition, and structure very challenging due to the overlap of tens or even hundreds of peaks in the mass spectrum. In this review, we cover data analysis, experimental, and instrumental advances and strategies aimed at solving this problem, with an in-depth discussion of theoretical and practical aspects of the use of available deconvolution algorithms and tools. We also reflect upon current challenges and provide a view of the future of this exciting field.

Multiply Charged Cation Attachment to Facilitate Mass Measurement in Negative-Mode Native Mass Spectrometry

Native mass spectrometry (MS) is usually conducted in the positive-ion mode; however, in some cases, it is advantageous to use the negative-ion polarity. Challenges associated with native MS using ensemble measurements (i.e., the measurement of many ions at a time as opposed to the measurement of the charge and the mass-to-charge ratio of individual ions) include narrow charge state distributions with the potential for an overlap in neighboring charge states. These issues can either compromise or preclude confident charge state (and hence mass) determination. Charge state determination in challenging instances can be enabled via the attachment of multiply charged ions of opposite polarity. Multiply charged ion attachment facilitates the resolution of charge states and generates mass-to-charge (m/z) information across a broad m/z range. In this work, we demonstrated the attachment of multiply charged cations to anionic complexes generated under native MS conditions. To illustrate the flexibility available in selecting the mass and charge of the reagents, the 15+ and 20+ charge states of horse skeletal muscle apomyoglobin and the 20+ and 30+ charge states of bovine carbonic anhydrase were demonstrated to attach to model complex anions derived from either β-galactosidase or GroEL. The exclusive attachment of reagent ions is observed with no evidence for proton transfer, which is the key for the unambiguous interpretation of the post-ion/ion reaction product ion spectrum. To illustrate the application to mixtures of complex ions, the 10+ charge state of bovine ubiquitin was attached to mixtures of anions generated from the 30S and 50S particles of the Escherichia coli ribosome. Six and five major components were revealed, respectively. In the case of the 50S anion population, it was shown that the attachment of two 30+ cations of carbonic anhydrase revealed the same information as the attachment of six 10+ cations of ubiquitin. In neither case was the intact 50S particle observed. Rather, particles with different combinations of missing components were observed. This work demonstrated the utility of multiply charged cation attachment to facilitate charge state assignments in native MS ensemble measurements of heterogeneous mixtures.

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.

The Charge-State and Structural Stability of Peptides Conferred by Microsolvating Environments in Differential Mobility Spectrometry

The presence of solvent vapor in a differential mobility spectrometry (DMS) cell creates a microsolvating environment that can mitigate complications associated with field-induced heating. In the case of peptides, the microsolvation of protonation sites results in a stabilization of charge density through localized solvent clustering, sheltering the ion from collisional activation. Seeding the DMS carrier gas (N₂) with a solvent vapor prevented nearly all field-induced fragmentation of the protonated peptides GGG, AAA, and the Lys-rich Polybia-MP1 (IDWKKLLDAAKQIL-NH₂). Modeling the microsolvation propensity of protonated n-propylamine [PrNH₃]⁺, a mimic of the Lys side chain and N-terminus, with common gas-phase modifiers (H₂O, MeOH, EtOH, iPrOH, acetone, and MeCN) confirms that all solvent molecules form stable clusters at the site of protonation. Moreover, modeling populations of microsolvated clusters indicates that species containing protonated amine moieties exist as microsolvated species with one to six solvent ligands at all effective ion temperatures (T_eff) accessible during a DMS experiment (ca. 375-600 K). Calculated T_eff of protonated GGG, AAA, and Polybia-MPI using a modified two-temperature theory approach were up to 86 K cooler in DMS environments seeded with solvent vapor compared to pure N₂ environments. Stabilizing effects were largely driven by an increase in the ion's apparent collision cross section and by evaporative cooling processes induced by the dynamic evaporation/condensation cycles incurred in the presence of an oscillating electric separation field. When the microsolvating partner was a protic solvent, abstraction of a proton from [MP1 + 3H]³⁺ to yield [MP1 + 2H]²⁺ was observed. This result was attributed to the proclivity of protic solvents to form hydrogen-bond networks with enhanced gas-phase basicity. Collectively, microsolvation provides analytes with a solvent "air bag," whereby charge reduction and microsolvation-induced stabilization were shown to shelter peptides from the fragmentation induced by field heating and may play a role in preserving native-like ion configurations.

Sample Flow Rate Scan in Electrospray Ionization Mass Spectrometry Reveals Alterations in Protein Charge State Distribution

Sample flow rate is one of the parameters that influence the sensitivity of electrospray ionization (ESI) mass spectrometry. By varying the sample flow rate, initial droplets of different sizes can be generated. Protein molecules in small droplets may form gas-phase ions earlier than the ones in large droplets. Here, we have systematically studied the influence of sample flow rate on the ESI charge state distributions (CSDs) of model proteins. A dedicated programmable sample flow rate scanner was used to infuse protein samples at different flow rates into a mass spectrometer. The synergistic influence of sample flow rate and various electrolytes (ammonium acetate, ammonium bicarbonate, ammonium formate, and piperidine) was studied. Significant alterations to the CSDs with increasing flow rate were observed. For example, in the presence of ammonium acetate, at low flow rates, lower charge states of proteins showed high intensities, while at high flow rates, ions related to higher charge states of proteins dominated the spectra. On the other hand, in the presence of piperidine, a significant reduction in the ion currents of all charge states was observed during the flow rate scan. Our observations suggest that at low flow rates the protein molecules follow a charged residue model of ionization mechanism, and at high flow rates-due to structural changes in protein molecules in large ESI droplets-the charged residue and chain ejection models can possibly coexist. We propose the use of sample flow rate scan as a way to reveal the influence of flow rate on the CSDs of the studied proteins.

Discovery of Potent Charge-Reducing Molecules for Native Ion Mobility Mass Spectrometry Studies

There is growing interest in the characterization of protein complexes and their interactions with ligands using native ion mobility mass spectrometry. A particular challenge, especially for membrane proteins, is preserving noncovalent interactions and maintaining native-like structures. Different approaches have been developed to minimize activation of protein complexes by manipulating charge on protein complexes in solution and the gas-phase. Here, we report the utility of polyamines that have exceptionally high charge-reducing potencies with some molecules requiring 5-fold less than trimethylamine oxide to elicit the same effect. The charge-reducing molecules do not adduct to membrane protein complexes and are also compatible with ion-mobility mass spectrometry, paving the way for improved methods of charge reduction.

Imidazole Derivatives Improve Charge Reduction and Stabilization for Native Mass Spectrometry

Noncovalent interactions between biomolecules are critical to their activity. Native mass spectrometry (MS) has enabled characterization of these interactions by preserving noncovalent assemblies for mass analysis, including protein-ligand and protein-protein complexes for a wide range of soluble and membrane proteins. Recent advances in native MS of lipoprotein nanodiscs have also allowed characterization of antimicrobial peptides and membrane proteins embedded in intact lipid bilayers. However, conventional native electrospray ionization (ESI) can disrupt labile interactions. To stabilize macromolecular complexes for native MS, charge reducing reagents can be added to the solution prior to ESI, such as triethylamine, trimethylamine oxide, and imidazole. Lowering the charge acquired during ESI reduces Coulombic repulsion that leads to dissociation, and charge reduction reagents may also lower the internal energy of the ions through evaporative cooling. Here, we tested a range of imidazole derivatives to discover improved charge reducing reagents and to determine how their chemical properties influence charge reduction efficacy. We measured their effects on a soluble protein complex, a membrane protein complex in detergent, and lipoprotein nanodiscs with and without embedded peptides, and used computational chemistry to understand the observed charge-reduction behavior. Together, our data revealed that hydrophobic substituents at the 2 position on imidazole can significantly improve both charge reduction and gas-phase stability over existing reagents. These new imidazole derivatives will be immediately beneficial for a range of native MS applications and provide chemical principles to guide development of novel charge reducing reagents.

Overcharging Effect in Electrospray Ionization Mass Spectra of Daunomycin-Tuftsin Bioconjugates

Peptide-based small molecule drug conjugates for targeted tumor therapy are currently in the focus of intensive research. Anthracyclines, like daunomycin, are commonly used anticancer drug molecules and are also often applied in peptide-drug conjugates. However, lability of the O-glycosidic bond during electrospray ionization mass spectrometric analysis hinders the analytical characterization of the constructs. “Overprotonation” can occur if daunomycin is linked to positively charged peptide carriers, like tuftsin derivatives. In these molecules, the high number of positive charges enhances the in-source fragmentation significantly, leading to complex mass spectra composed of mainly fragment ions. Therefore, we investigated different novel tuftsin-daunomycin conjugates to find an appropriate condition for mass spectrometric detection. Our results showed that shifting the charge states to lower charges helped to keep ions intact. In this way, a clear spectrum could be obtained containing intact protonated molecules only. Shifting of the protonation states to lower charges could be achieved with the use of appropriate neutral volatile buffers and with tuning the ion source parameters.

Gas-Phase Collisions with Trimethylamine-N-Oxide Enable Activation-Controlled Protein Ion Charge Reduction

Modulating protein ion charge is a useful tool for the study of protein folding and interactions by electrospray ionization mass spectrometry. Here, we investigate activation-dependent charge reduction of protein ions with the chemical chaperone trimethylamine-N-oxide (TMAO). Based on experiments carried out on proteins ranging from 4.5 to 35 kDa, we find that when combined with collisional activation, TMAO removes approximately 60% of the charges acquired under native conditions. Ion mobility measurements furthermore show that TMAO-mediated charge reduction produces the same end charge state and arrival time distributions for native-like and denatured protein ions. Our results suggest that gas-phase collisions between the protein ions and TMAO result in proton transfer, in line with previous findings for dimethyl- and trimethylamine. By adjusting the energy of the collisions experienced by the ions, it is possible to control the degree of charge reduction, making TMAO a highly dynamic charge reducer that opens new avenues for manipulating protein charge states in ESI-MS and for investigating the relationship between protein charge and conformation. ᅟ.

Virus-like particle size and molecular weight/mass determination applying gas-phase electrophoresis (native nES GEMMA)

(Bio-)nanoparticle analysis employing a nano-electrospray gas-phase electrophoretic mobility molecular analyzer (native nES GEMMA) also known as nES differential mobility analyzer (nES DMA) is based on surface-dry analyte separation at ambient pressure. Based on electrophoretic principles, single-charged nanoparticles are separated according to their electrophoretic mobility diameter (EMD) corresponding to the particle size for spherical analytes. Subsequently, it is possible to correlate the (bio-)nanoparticle EMDs to their molecular weight (M_W) yielding a corresponding fitted curve for an investigated analyte class. Based on such a correlation, (bio-)nanoparticle M_W determination via its EMD within one analyte class is possible. Turning our attention to icosahedral, non-enveloped virus-like particles (VLPs), proteinaceous shells, we set up an EMD/M_W correlation. We employed native electrospray ionization mass spectrometry (native ESI MS) to obtain M_W values of investigated analytes, where possible, after extensive purification. We experienced difficulties in native ESI MS with time-of-flight (ToF) detection to determine M_W due to sample inherent characteristics, which was not the case for charge detection (CDMS). nES GEMMA exceeds CDMS in speed of analysis and is likewise less dependent on sample purity and homogeneity. Hence, gas-phase electrophoresis yields calculated M_W values in good approximation even when charge resolution was not obtained in native ESI ToF MS. Therefore, both methods-native nES GEMMA-based M_W determination via an analyte class inherent EMD/M_W correlation and native ESI MS-in the end relate (bio-)nanoparticle M_W values. However, they differ significantly in, e.g., ease of instrument operation, sample and analyte handling, or costs of instrumentation.

Chemical Additives Enable Native Mass Spectrometry Measurement of Membrane Protein Oligomeric State within Intact Nanodiscs

Membrane proteins play critical biochemical roles but remain challenging to study. Recently, native or nondenaturing mass spectrometry (MS) has made great strides in characterizing membrane protein interactions. However, conventional native MS relies on detergent micelles, which may disrupt natural interactions. Lipoprotein nanodiscs provide a platform to present membrane proteins for native MS within a lipid bilayer environment, but previous native MS of membrane proteins in nanodiscs has been limited by the intermediate stability of nanodiscs. It is difficult to eject membrane proteins from nanodiscs for native MS but also difficult to retain intact nanodisc complexes with membrane proteins inside. Here, we employed chemical reagents that modulate the charge acquired during electrospray ionization (ESI). By modulating ESI conditions, we could either eject the membrane protein complex with few bound lipids or capture the intact membrane protein nanodisc complex-allowing measurement of the membrane protein oligomeric state within an intact lipid bilayer environment. The dramatic differences in the stability of nanodiscs under different ESI conditions opens new applications for native MS of nanodiscs.

Strengths and Weaknesses of Molecular Simulations of Electrosprayed Droplets

The origin and the magnitude of the charge in a macroion are critical questions in mass spectrometry analysis coupled to electrospray and other ionization techniques that transfer analytes from the bulk solution into the gaseous phase via droplets. In many circumstances, it is the later stages of the existence of a macroion in the containing solvent drop before the detection that determines the final charge state. Experimental characterization of small (with linear dimensions of several nanometers) and short-lived droplets is quite challenging. Molecular simulations in principle may provide insight exactly in this challenging for experiments regime. We discuss the strengths and weaknesses of the molecular modeling of electrosprayed droplets using molecular dynamics. We illustrate the limitations of the molecular modeling in the analysis of large macroions and specifically proteins away from their native states. Graphical Abstract ᅟ.

Mass Spectrometry Reveals the Direct Action of a Chemical Chaperone

Despite their fundamental biological importance and therapeutic potential, the interactions between chemical chaperones and proteins remain difficult to capture due to their transient and nonspecific nature. Using a simple mass spectrometric assay, we are able to follow the interactions between proteins and the chemical chaperone trimethylamine- N-oxide (TMAO). In this manner, we directly observe that the counteraction of TMAO and the denaturant urea is driven by the exclusion of TMAO from the protein surface, whereas the surfactant lauryl dimethylamine- N-oxide cannot be displaced. Our results clearly demonstrate a direct chaperoning mechanism for TMAO, corroborating extensive computational studies, and pave the way for the use of nondenaturing mass spectrometry and related techniques to study chemical chaperones in molecular detail.

What factors determine the stability of a weak protein-protein interaction in a charged aqueous droplet?

Maintaining the interface of a weak transient protein complex transferred from bulk solution to the gaseous state via evaporating droplets is a critical question in the detection of the complex association (dissociation) constant by using electrospray ionization mass spectrometry (ESI-MS). Here we explore the factors that may affect the stability of a protein-protein interaction (PPI) using atomistic molecular dynamics (MD) modelling of a complex of ubiquitin (Ub) and the ubiquitin-associated domain (UbA) (RCSB PDB code ) and a non-covalent complex of diubiquitin (RCSB PDB code ) in aqueous droplets. A general method is presented to determine the protonation states of the complexes we investigate in particular, and that of a protein in general, under various pH conditions that an evaporating droplet acquires due to its change in size. We find that the combination of high temperature and high charge states of the protein complexes may destabilize the interface by creating new interfaces instead of a direct rupture of the initial stable interface. We provide evidence that highly charged protein complexes are found in droplets that form conical extrusions of the solvent on the surface due to charge-induced instability. This distinct droplet morphology leads to a higher solvent evaporation rate that assists in transferring the complex in the gaseous state without dissociation. The conical solvent protrusions expose on the droplet surface certain amino acids that otherwise would be solvated in a droplet with the protein complex of low charge states. The new vapor-protein interface does not have a direct effect on the stability of the PPI. A common way in experiments to stabilize the protein complexes in droplets is to reduce the protonation state of the proteins. Here we find that weakly bound protein complexes even at high protonation states can be stabilized by the presence of a small number of counterions, without affecting the protonation state of the protein. Our findings may provide guiding principles in ESI-MS experiments to stabilize weak transient PPIs.

Two possible improvements for mass spectrometry analysis of intact biomolecules

The goals of our study were to investigate abilities of two approaches to eliminate possible errors in electrospray mass spectrometry measurements of biomolecules. Passing of a relatively dense supersonic gas jet through ionization region followed by its hitting the spray of the analyzed solution in low vacuum may be effective to keep an initial biomolecule structure in solution. Provided that retention of charge carriers for some sites in the biomolecule cannot be changed noticeably in electrospray ion source, decomposition and separation of charge-state distributions of electrosprayed ions may give additional information about native structure of biomolecules in solution.

Isolating Protein Charge State Reduction in Electrospray Droplets Using Femtosecond Laser Vaporization

Charge state distributions are measured using mass spectrometry for both native and denatured cytochrome c and myoglobin after laser vaporization from the solution state into an electrospray (ES) plume consisting of a series of solution additives differing in gas-phase basicity. The charge distribution depends on both the pH of the protein solution prior to laser vaporization and the gas-phase basicity of the solution additive employed in the ES solvent. Cytochrome c (myoglobin) prepared in solutions with pH of 7.0, 2.6, and 2.3 resulted in the average charge state distribution (Z_avg) of 7.0 ± 0.1 (8.2 ± 0.1), 9.7 ± 0.2 (14.5 ± 0.3), and 11.6 ± 0.3 (16.4 ± 0.1), respectively, in ammonium formate ES solvent. The charge distribution shifted from higher charge states to lower charge states when the ES solvent contained amines additives with higher gas-phase basicity. In the case of triethyl ammonium formate, Z_avg of cytochrome c (myoglobin) prepared in solutions with pH of 7.0, 2.6, and 2.3 decreased to 4.9 (5.7), 7.4 ± 0.2 (9.6 ± 0.3), and 7.9 ± 0.3 (9.8 ± 0.2), respectively. The detection of a charge state distribution corresponding to folded protein after laser vaporized, acid-denatured protein interacts with the ES solvent containing ammonium formate, ammonium acetate, triethyl ammonium formate, and triethyl ammonium acetate suggests that at least a part of protein population folds within the electrospray droplet on a millisecond timescale. Graphical Abstract ᅟ.

Charging of Proteins in Native Mass Spectrometry

Factors that influence the charging of protein ions formed by electrospray ionization from aqueous solutions in which proteins have native structures and function were investigated. Protein ions ranging in molecular weight from 12.3 to 79.7 kDa and pI values from 5.4 to 9.6 were formed from different solutions and reacted with volatile bases of gas-phase basicities higher than that of ammonia in the cell of a Fourier-transform ion cyclotron resonance mass spectrometer. The charge-state distribution of cytochrome c ions formed from aqueous ammonium or potassium acetate is the same. Moreover, ions formed from these two solutions do not undergo proton transfer to 2-fluoropyridine, which is 8 kcal/mol more basic than ammonia. These results provide compelling evidence that proton transfer between ammonia and protein ions does not limit protein ion charge in native electrospray ionization. Both circular dichroism and ion mobility measurements indicate that there are differences in conformations of proteins in pure water and aqueous ammonium acetate, and these differences can account for the difference in the extent of charging and proton-transfer reactivities of protein ions formed from these solutions. The extent of proton transfer of the protein ions with higher gas-phase basicity bases trends with how closely the protein ions are charged to the value predicted by the Rayleigh limit for spherical water droplets approximately the same size as the proteins. These results indicate that droplet charge limits protein ion charge in native mass spectrometry and are consistent with these ions being formed by the charged residue mechanism. Graphical Abstract ᅟ.

Improving sensitivity and linear dynamic range of intact protein analysis using a robust and easy to use microfluidic device

We demonstrate an integrated microfluidic LC device coupled to a QTOF capable of improving sensitivity and linearity for intact protein analysis while also tuning the charge state distributions (CSD) of whole antibodies.

Examining the Heterogeneous Genome Content of Multipartite Viruses BMV and CCMV by Native Mass Spectrometry

Since the concept was first introduced by Brian Chait and co-workers in 1991, mass spectrometry of proteins and protein complexes under non-denaturing conditions (native MS) has strongly developed, through parallel advances in instrumentation, sample preparation, and data analysis tools. However, the success rate of native MS analysis, particularly in heterogeneous mega-Dalton (MDa) protein complexes, still strongly depends on careful instrument modification. Here, we further explore these boundaries in native mass spectrometry, analyzing two related endogenous multipartite viruses: the Brome Mosaic Virus (BMV) and the Cowpea Chlorotic Mottle Virus (CCMV). Both CCMV and BMV are approximately 4.6 megadalton (MDa) in mass, of which approximately 1 MDA originates from the genomic content of the virion. Both viruses are produced as mixtures of three particles carrying different segments of the genome, varying by approximately 0.1 MDA in mass (~2%). This mixture of particles poses a challenging analytical problem for high-resolution native MS analysis, given the large mass scales involved. We attempt to unravel the particle heterogeneity using both Q-TOF and Orbitrap mass spectrometers extensively modified for analysis of very large assemblies. We show that manipulation of the charging behavior can provide assistance in assigning the correct charge states. Despite their challenging size and heterogeneity, we obtained native mass spectra with resolved series of charge states for both BMV and CCMV, demonstrating that native MS of endogenous multipartite virions is feasible. Graphical Abstract ᅟ.

Use of a charge reducing agent to enable intact mass analysis of cysteine-linked antibody-drug-conjugates by native mass spectrometry

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The use of nESI-MS to examine intact Antibody Drug Conjugates (ADC).

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Use of TEAA as charge reducing agent improves cysteine-linked ADC characterization.

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TEAA preserves the intact mAb and facilitate easy drug load determination by native MS.

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This method is particularly beneficial for users of low resolution mass spectrometers.

Antibody-drug-conjugates (ADC) are a growing class of anticancer biopharmaceuticals. Conjugation of cysteine linked ADCs, requires initial reduction of mAb inter-chain disulfide bonds, as the drugs are attached via thiol chemistry. This results in the active mAb moiety being transformed from a covalently linked tetramer to non-covalently linked complexes, which hinders precise determination of drug load with LC–MS. Here, we show how the addition of the charge reducing agent triethylammonium acetate (TEAA) preserves the intact mAb structure, is well suited to the study of cysteine linked conjugates and facilitates easy drug load determination by direct infusion native MS.

Ion Mobility-Mass Spectrometry Reveals Highly-Compact Intermediates in the Collision Induced Dissociation of Charge-Reduced Protein Complexes

Protocols that aim to construct complete models of multiprotein complexes based on ion mobility and mass spectrometry data are becoming an important element of integrative structural biology efforts. However, the usefulness of such data is predicated, in part, on an ability to measure individual subunits removed from the complex while maintaining a compact/folded state. Gas-phase dissociation of intact complexes using collision induced dissociation is a potentially promising pathway for acquiring such protein monomer size information, but most product ions produced are possessed of high charge states and elongated/string-like conformations that are not useful in protein complex modeling. It has previously been demonstrated that the collision induced dissociation of charge-reduced protein complexes can produce compact subunit product ions; however, their formation mechanism is not well understood. Here, we present new experimental evidence for the avidin (64 kDa) and aldolase (157 kDa) tetramers that demonstrates significant complex remodeling during the dissociation of charge-reduced assemblies. Detailed analysis and modeling indicates that highly compact intermediates are accessed during the dissociation process by both complexes. Here, we present putative pathways that describe the formation of such ions, as well as discuss the broader significance of such data for structural biology applications moving forward.

Investigating acid-induced structural transitions of lysozyme in an electrospray ionization source

The effect of acids on the structure of lysozyme (Lyz) during electrospray ionization (ESI) was studied by comparing the solution and gas-phase structures of Lyz. Investigation using circular dichroism spectroscopy and small-angle X-ray scattering demonstrated that the folded conformation of Lyz was maintained in pH 2.2 solutions containing different acids. On the other hand, analysis of the charge state distributions and ion mobility (IM) distributions, combined with molecular dynamics simulations, demonstrated that the gas phase structures of Lyz depend on the pKa of the acid used to acidify the protein solution. Formic acid and acetic acid, which are weak acids (pKa > 3.5), induce unfolding of Lyz during ESI, presumably because the undissociated weak acids provide protons to maintain the acidic groups within Lyz protonated and prevent the formation of salt bridges. However, HCl suppressed the formation of the unfolded conformers because the acid is already dissociated in solution, and chloride anions within the ESI droplet can interact with Lyz to reduce the intramolecular electrostatic repulsion. These trends in the IM distributions are observed for all charge states, demonstrating the significance of the acid effect on the structure of Lyz during ESI.

Extensive Charge Reduction and Dissociation of Intact Protein Complexes Following Electron Transfer on a Quadrupole-Ion Mobility-Time-of-Flight MS

Non-dissociative charge reduction, typically considered to be an unwanted side reaction in electron transfer dissociation (ETD) experiments, can be enhanced significantly in order to reduce the charge state of intact protein complexes to as low as 1+ on a commercially available Q-IM-TOF instrument. This allows for the detection of large complexes beyond 100,000 m/z, while at the same time generating top-down ETD fragments, which provide sequence information from surface-exposed parts of the folded structure. Optimization of the supplemental activation has proven to be crucial in these experiments and the charge-reduced species are most likely the product of both proton transfer (PTR) and non-dissociative electron transfer (ETnoD) reactions that occur prior to the ion mobility cell. Applications of this approach range from deconvolution of complex spectra to the manipulation of charge states of gas-phase ions.

Solvent-induced structural transitions of lysozyme in an electrospray ionization source

The structural characterization of proteins using electrospray ionization mass spectrometry (ESI-MS) has become an important method for understanding protein structural dynamics. The correlation between the structures of proteins in solution and gas phase needs to be understood for the application of ESI-MS to protein structural studies. Hen egg white lysozyme (Lyz) is a small protein with a stable compact structure in solution. Although it was known that denatured Lyz in solution undergoes compaction during transfer into the gas phase via ESI, detailed characterization of the process was not available. In the present study, we show that the organic cosolvent, which denatures Lyz in solution, induces the collapse of the extended Lyz structure into compact structures during ESI. This process is further facilitated by the presence of acids, whose conjugate bases can interact with Lyz to reduce its charge state and the electrostatic repulsion between its charged residues (Analyst, 2015, 140, 661-669). Exposure of ESI droplets to acid and solvent vapors confirms that the overall process most probably occurs in the charged droplets from ESI. This study provides a detailed understanding of the possible influence of the solvent environment on protein structure during transfer into the gas phase.

Charge reduction stabilizes intact membrane protein complexes for mass spectrometry

The study of intact soluble protein assemblies by means of mass spectrometry is providing invaluable contributions to structural biology and biochemistry. A recent breakthrough has enabled similar study of membrane protein complexes, following their release from detergent micelles in the gas phase. Careful optimization of mass spectrometry conditions, particularly with respect to energy regimes, is essential for maintaining compact folded states as detergent is removed. However, many of the saccharide detergents widely employed in structural biology can cause unfolding of membrane proteins in the gas phase. Here, we investigate the potential of charge reduction by introducing three membrane protein complexes from saccharide detergents and show how reducing their overall charge enables generation of compact states, as evidenced by ion mobility mass spectrometry. We find that charge reduction stabilizes the oligomeric state and enhances the stability of lipid-bound complexes. This finding is significant since maintaining native-like membrane proteins enables ligand binding to be assessed from a range of detergents that retain solubility while protecting the overall fold.

Molecular simulation-based structural prediction of protein complexes in mass spectrometry: the human insulin dimer

Protein electrospray ionization (ESI) mass spectrometry (MS)-based techniques are widely used to provide insight into structural proteomics under the assumption that non-covalent protein complexes being transferred into the gas phase preserve basically the same intermolecular interactions as in solution. Here we investigate the applicability of this assumption by extending our previous structural prediction protocol for single proteins in ESI-MS to protein complexes. We apply our protocol to the human insulin dimer (hIns2) as a test case. Our calculations reproduce the main charge and the collision cross section (CCS) measured in ESI-MS experiments. Molecular dynamics simulations for 0.075 ms show that the complex maximizes intermolecular non-bonded interactions relative to the structure in water, without affecting the cross section. The overall gas-phase structure of hIns2 does exhibit differences with the one in aqueous solution, not inferable from a comparison with calculated CCS. Hence, care should be exerted when interpreting ESI-MS proteomics data based solely on NMR and/or X-ray structural information.

Effects of cations on protein and peptide charging in electrospray ionization from aqueous solutions

The effects of eight different cations with ionic radii between 69 and 337 pm on the charging of peptides and proteins with electrospray ionization from aqueous acetate salt solutions are reported. Significant adduction occurs for all cations except NH4(+), and the average protein charge is lower when formed from solutions containing salts compared with solutions without salts added. Circular dichroism and ion mobility results show the protein conformations are different in pure water compared with salt solutions, which likely affects the extent of charging. The average charge of protein and peptide ions formed from solutions with Li(+) and Cs(+), which have Gibbs solvation free energies (GSFEs) that differ by 225 kJ/mol, is similar. Lower charge states are typically formed from solutions with tetramethylammonium and tetraethylammonium that have lower GSFE values. Loss of the larger cations that have the lowest GSFEs is facile when adducted protein ions are collisionally activated, resulting in the formation of lower analyte charge states. This reaction pathway provides a route to produce abundant singly protonated protein ions under native mass spectrometry conditions. The average protein and peptide charge with NH4(+) is nearly the same as that with Rb(+) and K(+), cations with similar GSFE and ionic radii. This indicates that proton transfer from NH4(+) to proteins plays an insignificant role in the extent of protein charging in native mass spectrometry.

Analytical approaches for size and mass analysis of large protein assemblies

Analysis of the size and mass of nanoparticles, whether they are natural biomacromolecular or synthetic supramolecular assemblies, is an important step in the characterization of such molecular species. In recent years, electrospray ionization (ESI) has emerged as a technology through which particles with masses up to 100 MDa can be ionized and transferred into the gas phase, preparing them for accurate mass analysis. Here we review currently used methodologies, with a clear focus on native mass spectrometry (MS). Additional complementary methodologies are also covered, including ion-mobility analysis, nanomechanical mass sensors, and charge-detection MS. The literature discussed clearly demonstrates the great potential of ESI-based methodologies for the size and mass analysis of nanoparticles, including very large naturally occurring protein assemblies. The analytical approaches discussed are powerful tools in not only structural biology, but also nanotechnology.

The role of salt bridges, charge density, and subunit flexibility in determining disassembly routes of protein complexes

Mass spectrometry can be used to characterize multiprotein complexes, defining their subunit stoichiometry and composition following solution disruption and collision-induced dissociation (CID). While CID of protein complexes in the gas phase typically results in the dissociation of unfolded subunits, a second atypical route is possible wherein compact subunits or subcomplexes are ejected without unfolding. Because tertiary structure and subunit interactions may be retained, this is the preferred route for structural investigations. How can we influence which pathway is adopted? By studying properties of a series of homomeric and heteromeric protein complexes and varying their overall charge in solution, we found that low subunit flexibility, higher charge densities, fewer salt bridges, and smaller interfaces are likely to be involved in promoting dissociation routes without unfolding. Manipulating the charge on a protein complex therefore enables us to direct dissociation through structurally informative pathways that mimic those followed in solution.

A computational model for protein ionization by electrospray based on gas-phase basicity

Identifying the key factor(s) governing the overall protein charge is crucial for the interpretation of electrospray-ionization mass spectrometry data. Current hypotheses invoke different principles for folded and unfolded proteins. Here, first we investigate the gas-phase structure and energetics of several proteins of variable size and different folds. The conformer and protomer space of these proteins ions is explored exhaustively by hybrid Monte-Carlo/molecular dynamics calculations, allowing for zwitterionic states. From these calculations, the apparent gas-phase basicity of desolvated protein ions turns out to be the unifying trait dictating protein ionization by electrospray. Next, we develop a simple, general, adjustable-parameter-free model for the potential energy function of proteins. The model is capable to predict with remarkable accuracy the experimental charge of folded proteins and its well-known correlation with the square root of protein mass.

Electrospray ionization-ion mobility spectrometry identified monoclonal antibodies that bind exclusively to either the monomeric or a dimeric form of prostate specific antigen

Macroion mobility spectrometry was used to distinguish between a monoclonal antibody (clone M612165) that bound exclusively to monomeric prostate specific antigen and a different monoclonal antibody (clone M612166) that bound exclusively to a dimeric form of the antigen that only comprised 6.8% of the total protein. In the presence of excess antigen, the mobility spectrum of M612165 was replaced by a composite spectrum that represented a mixture of antibodies that included either one or two equivalents of the protein antigen. In similar circumstances, the mobility spectrum of M612166 was replaced by a composite spectrum that represented a mixture of antibodies that included either two or four equivalents of the protein antigen. When exposed to either of the two antibodies, the mobility spectrum of the prostate specific antigen showed a concomitant decrease in the monomeric antigen in one case and in the dimeric antigen in the other case. While sensitive kinetic exclusion assays demonstrated large differences in the antigen binding behavior of the two antibodies, these functional studies alone were insufficient to reveal the likely structural origins of the observed differences. Macroion mobility measurements were shown to be a useful and informative complement to functional studies in understanding complex macromolecular interactions.

Do charge state signatures guarantee protein conformations?

The extent to which proteins in the gas phase retain their condensed-phase structure is a hotly debated issue. Closely related to this is the degree to which the observed charge state reflects protein conformation. Evidence from electron capture dissociation, hydrogen/deuterium exchange, ion mobility, and molecular dynamics shows clearly that there is often a strong correlation between the degree of folding and charge state, with the most compact conformations observed for the lowest charge states. In this article, we address recent controversies surrounding the relationship between charge states and folding, focussing also on the manipulation of charge in solution and its effect on conformation. 'Supercharging' reagents that have been used to effect change in charge state can promote unfolding in the electrospray droplet. However for several protein complexes, supercharging does not appear to perturb the structure in that unfolding is not detected. Consequently, a higher charge state does not necessarily imply unfolding. Whilst the effect of charge manipulation on conformation remains controversial, there is strong evidence that a folded, compact state of a protein can survive in the gas phase, at least on a millisecond timescale. The exact nature of the side-chain packing and secondary structural elements in these compact states, however, remains elusive and prompts further research.

Supercharging protein complexes from aqueous solution disrupts their native conformations

The effects of aqueous solution supercharging on the solution- and gas-phase structures of two protein complexes were investigated using traveling-wave ion mobility-mass spectrometry (TWIMS-MS). Low initial concentrations of m-nitrobenzyl alcohol (m-NBA) in the electrospray ionization (ESI) solution can effectively increase the charge of concanavalin A dimers and tetramers, but at higher m-NBA concentrations, the increases in charge are accompanied by solution-phase dissociation of the dimers and up to a ~22% increase in the collision cross section (CCS) of the tetramers. With just 0.8% m-NBA added to the ESI solution of a ~630 kDa anthrax toxin octamer complex, the average charge is increased by only ~4% compared with the "native" complex, but it is sufficiently destabilized so that extensive gas-phase fragmentation occurs in the relatively high pressure regions of the TWIMS device. Anthrax toxin complexes exist in either a prechannel or a transmembrane channel state. With m-NBA, the prechannel state of the complex has the same CCS/charge ratio in the gas phase as the transmembrane channel state of the same complex formed without m-NBA, yet undergoes extensive dissociation, indicating that destabilization from supercharging occurs in the ESI droplet prior to ion formation and is not a result of Coulombic destabilization in the gas phase as a result of higher charging. These results demonstrate that the supercharging of large protein complexes is the result of conformational changes induced by the reagents in the ESI droplets, where enrichment of the supercharging reagent during droplet evaporation occurs.

Ion mobility-mass spectrometry reveals conformational changes in charge reduced multiprotein complexes

Characterizing intact multiprotein complexes in terms of both their mass and size by ion mobility-mass spectrometry is becoming an increasingly important tool for structural biology. Furthermore, the charge states of intact protein complexes can dramatically influence the information content of gas-phase measurements performed. Specifically, protein complex charge state has a demonstrated influence upon the conformation, mass resolution, ion mobility resolution, and dissociation properties of protein assemblies upon collisional activation. Here we present the first comparison of charge-reduced multiprotein complexes generated by solution additives and gas-phase ion-neutral reaction chemistry. While the charge reduction mechanism for both methods is undoubtedly similar, significant gas-phase activation of the complex is required to reduce the charge of the assemblies generated using the solution additive strategy employed here. This activation step can act to unfold intact protein complexes, making the data difficult to correlate with solution-phase structures and topologies. We use ion mobility-mass spectrometry to chart such conformational effects for a range of multi-protein complexes, and demonstrate that approaches to reduce charge based on ion-neutral reaction chemistry in the gas-phase consistently produce protein assemblies having compact, 'native-like' geometries while the same molecules added in solution generate significantly unfolded gas-phase complexes having identical charge states.

Mass Spectrometry for Structural Biology: Determining the Composition and Architecture of Protein Complexes

Knowledge of protein structure and protein–protein interactions is vital for appreciating the elaborate biochemical pathways that underlie cellular function. While many techniques exist to probe the structure and complex interplay between functional proteins, none currently offer a complete picture. Mass spectrometry and associated methods provide complementary information to established structural biology tools, and with rapidly evolving technological advances, can in some cases even exceed other techniques by its diversity in application and information content. This is primarily because of the ability of mass spectrometry to precisely identify protein complex stoichiometry, detect individual species present in a mixture, and concomitantly offer conformational information. This review describes the attributes of mass spectrometry for the structural investigation of multiprotein assemblies in the context of recent developments and highlights in the field.