Identifying nonspecific ligand binding in electrospray ionization mass spectrometry using the reporter molecule method

Characterization of Electrospray Drop Size Distributions by Mobility-Classified Mass Spectrometry: Implications for Ion Clustering in Solution and Ion Formation Pathways

One of the outcomes of electrospray ionization is the size distribution of the droplets, which determines, together with the solvent composition and the source gas temperature, the minimum distance from the sprayer tip to the mass spectrometer inlet and therefore the ion transfer efficiency. Even more importantly, the average number of analyte molecules and, if present, contaminant species per droplet depend on the drop size. Consequently, the drop size distribution is a key parameter in nonspecific ion clustering in solution and ion suppression. The finding that small droplet sizes improve the mass spectral quality led to the development of nanoelectrospray sources, which dispense liquid flow rates below 0.1 μL/min and can generate drops with diameters smaller than 100 nm. However, current discussions on the effect of drop size on ion formation pathways and efficiencies remain qualitative because the exact drop size distributions are unknown. Here, we show that ion mobility-classified mass spectrometry of raffinose cluster ions allows us to determine very precisely the drop size distribution generated by the electrospray source in positive- and negative-ion modes. Based on the derived drop size distributions, we can quantitatively predict nonspecific ion clustering and can extract accurate probabilities for emission of species from parent drops upon Coulomb fission.

Mobility-Classified Mass Spectrometry Reveals a Complete Picture of the Electrospray Outcome

Electrospray ionization mass spectrometry supported by mobility classification (ESI-DMA-MS) is a strong technique for a comprehensive analysis of organic and inorganic clusters and small nanoparticles. In-depth understanding and optimization of the electrospray process are key for unlocking new fields of application and for extension of the accessible range of data. We show that the combination of mobility-classified mass spectrometry (DMA-MS) with an electrospray operated in a well-defined cone-jet mode is capable of providing a full picture of the outcome of the electrospray process including the evaporation residues of the parent droplets. Based on ESI-DMA-MS measurements of lysozyme, we find that clusters up to almost 1 MDa (around 60 lysozyme molecules per cluster) formed as evaporation residues of the parent droplets can be detected. The escape probability of lysozyme molecules from the parent droplets is polarity-dependent. A quantitative analysis of clusters is possible at analyte concentrations where the likeliness of finding more than a single molecule in the generated droplet is low. At higher concentrations, one can, however, quantify the analyte concentration from the detected size of the droplet evaporation residues. The approach is widely applicable for organic and inorganic nanostructures. The results allow clear statements on the conditions under which mixtures of larger molecules, clusters, and nanoparticles are accessible to quantitative analysis by mass spectrometry.

Sliding Window Adduct Removal Method (SWARM) for Enhanced Electrospray Ionization Mass Spectrometry Binding Data

Electrospray ionization mass spectrometry (ESI-MS) screening of compound libraries against target proteins enables the rapid identification of ligands and measurement of the stoichiometry and affinity of the interactions. However, non-specific association of buffer or salts (added or present as impurities) to the protein ions during gas-phase ion formation can complicate the analysis of ESI-MS data acquired for mixtures of compounds with similar molecular weights. Spectral overlap of ions corresponding to free protein and protein-ligand complexes and their corresponding adducts can hinder the identification of ligands and introduce errors in the measured affinities. Here, we present a straightforward approach, called the sliding window adduct removal method (SWARM), to quantitatively correct ESI mass spectra of low-to-moderate resolution for signal overlap associated with adducts. The method relies on the statistical nature of adduct formation in ESI and the assumption that the distributions of adducts associated with a given protein (free protein and ligand-bound forms) are identical at a given charge state. Analysis of ESI mass spectra measured for protein-oligosaccharide interactions using solutions that produced either low- or high-abundance adducts provides support for this assumption. Implementation of SWARM involves the stepwise subtraction of the adduct signal associated with the detected protein-ligand complexes from the mass spectrum. This is accomplished using the adduct distribution measured for an appropriate reference species (usually free protein). To demonstrate the utility of the method, we applied SWARM to ESI-MS screening data acquired for libraries of oligosaccharides and bifunctional ligands consisting of a sulfonamide moiety linked to human glycan structures. Graphical Abstract.

Ligand-protein target screening from cell matrices using reactive desorption electrospray ionization-mass spectrometry via a native-denatured exchange approach

Native mass spectrometry has been recognized as a powerful tool for probing interactions between small molecules, such as drugs and natural products, and target proteins. However, the presence of heterogeneous proteins and metabolites in real biological systems can alter the conformations of target proteins or compete with candidate ligands, thus necessitating a method for measuring binding stoichiometries in matrices aside from the extensively used pure/recombinant protein systems. Furthermore, some small molecule-protein interactions have a transient and low-affinity nature and thus can be mis-assigned as nonspecific binding complexes that are often formed during the native ESI process. A native-denatured exchange (NDX) approach was recently developed using a reactive desorption electrospray ionization-mass spectrometer (DESI-MS) setup to screen specific interacting partners. The method works by gradually increasing the composition of denaturing solvents contained in the DESI spray and thus conferring a switch from a native to denatured ionization environment. This change impairs three-dimensional structures of target proteins and disrupts specific ligand-protein interactions, leading to decreased holo/apo ratios. In contrast, ligand-protein complexes exhibiting different trends are assigned as nonspecific interactions. Herein, we applied the NDX approach to probe specific ligand-protein interactions in biological matrices. We first used mixtures of model ligands and proteins to examine the use of reactive DESI-MS in recognizing ligand-target binding in mixtures. Subsequently, we used the NDX approach to analyze binding affinity curves of ligands to target proteins spiked in cell lysates with the aid of size exclusion chromatography and demonstrated its use in probing specific ligand-protein interactions from cell matrices.

Probing specific ligand-protein interactions by native-denatured exchange mass spectrometry

Probing ligand-target protein interactions provides essential information for deep understanding of biochemical machinery and design of drug screening assays. Native electrospray ionization-mass spectrometry (ESI-MS) is promising for direct analysis of ligand-protein complexes. However, it lacks the ability to distinguish between specific and non-specific ligand-protein interactions, and to further recognize the specifically bound proteins as drug target candidates, which remains as a major challenge in the field of drug developments by far. Herein we report a native-denatured exchange (NDX) mass spectrometry (MS) acquisition approach using a liquid sample-desorption electrospray ionization (LS-DESI) setup, and demonstrate its capability in enabling a change from native detection of noncovalent ligand-protein complexes to denatured analysis using three model ligand-protein complexes including myoglobin, CDP-ribonuclease and N,N',N″-triacetylchitotriose (NAG3)-lysozyme. Notably, we found the NDX-MS approach can readily discriminate specific ligand-protein interactions from nonspecific ones, as revealed by their distinct dynamic profiles of K_d as a function of the DESI spraying flow rate. Consequently, this NDX-MS approach holds promise for future applications to discovering specific protein targets for ligands of interest, and to screening compounds with high specificity to drug targets and thus eliminates off-target effects.

How can native mass spectrometry contribute to characterization of biomacromolecular higher-order structure and interactions?

Native mass spectrometry (MS) is an emerging approach for characterizing biomacromolecular structure and interactions under physiologically relevant conditions. In native MS measurement, intact macromolecules or macromolecular complexes are directly ionized from a non-denaturing solvent, and key noncovalent interactions that hold the complexes together can be preserved for MS analysis in the gas phase. This technique provides unique multi-level structural information such as conformational changes, stoichiometry, topology and dynamics, complementing conventional biophysical techniques. Despite the maturation of native MS and greatly expanded range of applications in recent decades, further dissemination is needed to make the community aware of such a technique. In this review, we attempt to provide an overview of the current body of knowledge regarding major aspects of native MS and explain how such technique contributes to the characterization of biomacromolecular higher-order structure and interactions.

Sizing Up Protein-Ligand Complexes: The Rise of Structural Mass Spectrometry Approaches in the Pharmaceutical Sciences

Capturing the dynamic interplay between proteins and their myriad interaction partners is critically important for advancing our understanding of almost every biochemical process and human disease. The importance of this general area has spawned many measurement methods capable of assaying such protein complexes, and the mass spectrometry-based structural biology methods described in this review form an important part of that analytical arsenal. Here, we survey the basic principles of such measurements, cover recent applications of the technology that have focused on protein-small-molecule complexes, and discuss the bright future awaiting this group of technologies.

Biophysical and structural characterization of mono/di-arylated lactosamine derivatives interaction with human galectin-3

Combination of biophysical and structural techniques allowed characterizing and uncovering the mechanisms underlying increased binding affinity of lactosamine derivatives for galectin 3. In particular, complementing information gathered from X-ray crystallography, native mass spectrometry and isothermal microcalorimetry showed favorable enthalpic contribution of cation-π interaction between lactosamine aryl substitutions and arginine residues from the carbohydrate recognition domain, which resulted in two log increase in compound binding affinity. This incrementing strategy allowed individual contribution of galectin inhibitor moieties to be dissected. Altogether, our results suggest that core and substituents of these saccharide-based inhibitors can be optimized separately, providing valuable tools to study the role of galectins in diseases.

Preliminary mass spectrometry characterization studies of galectin-3 samples, prior to carbohydrate-binding studies using Affinity mass spectrometry

RATIONALE

Investigation of non-covalent complexes of proteins using Affinity Mass Spectrometry (AMS) represents a major challenge in modern biomedical research. However, many experimental obstacles can make AMS data analysis complex. Additionally, sample purity and size of the protein may still pose significant challenges.

METHODS

Matrix-assisted laser desorption/ionization-time-of-flight (MALDI-TOF) mass spectrometry (MS) was used for initial mapping of protein samples. nanoESI (electrospray ionization) quadrupole-time-of-flight (QTOF) MS was used for mapping of protein samples under native conditions and subsequent AMS studies. The human galectin-3 protein sample was expressed in E. coli.

RESULTS

Full length galectin-3 was difficult to work with, due to several truncated forms observed after the purification procedures. On the other hand, galectin-3C produced excellent quality nanoESI-MS spectra. A covalent adduct of lactose was found to be located on residue Lys 176. Functional AMS control studies indicated that galectin-3 interactions with oligosaccharides may be dependent on its charge.

CONCLUSIONS

Mass spectrometry represents a valuable tool that can be efficiently used for structural characterization of protein samples prior to functional analyses. By means of accurate mass measurements, many protein truncations can be identified based on mass alone. Analysis of covalent adducts is more challenging. Finally, for AMS studies, careful use of controls may reveal charge-dependence of protein-oligosaccharide interactions. Copyright © 2016 John Wiley & Sons, Ltd.

Optimization of Electrospray Ionization by Statistical Design of Experiments and Response Surface Methodology: Protein-Ligand Equilibrium Dissociation Constant Determinations

Electrospray ionization mass spectrometry (ESI-MS) binding studies between proteins and ligands under native conditions require that instrumental ESI source conditions are optimized if relative solution-phase equilibrium concentrations between the protein-ligand complex and free protein are to be retained. Instrumental ESI source conditions that simultaneously maximize the relative ionization efficiency of the protein-ligand complex over free protein and minimize the protein-ligand complex dissociation during the ESI process and the transfer from atmospheric pressure to vacuum are generally specific for each protein-ligand system and should be established when an accurate equilibrium dissociation constant (KD) is to be determined via titration. In this paper, a straightforward and systematic approach for ESI source optimization is presented. The method uses statistical design of experiments (DOE) in conjunction with response surface methodology (RSM) and is demonstrated for the complexes between Plasmodium vivax guanylate kinase (PvGK) and two ligands: 5'-guanosine monophosphate (GMP) and 5'-guanosine diphosphate (GDP). It was verified that even though the ligands are structurally similar, the most appropriate ESI conditions for KD determination by titration are different for each. Graphical Abstract ᅟ.

Catenation and encapsulation induce distinct reconstitutions within a dynamic library of mixed-ligand Zn4L6 cages

Two new Zn4L6 cages composed of diamine subcomponents containing either naphthalene diimide (NDI) or porphyrin moieties are described. Their structural differences allow these cages to exhibit distinct interactions with different chemical stimuli, yielding different supramolecular products. The electron-poor NDI subunits of the first cage were observed to thread through electron-rich aromatic crown-ether macrocycles, forming mechanically-interlocked species up to a [3]catenane, whereas the porphyrin ligands of the second cage interacted favourably with C70, causing it to be bound as a guest. When mixed, the two cages were observed to form a dynamic combinatorial library (DCL) of seven constitutionally distinct mixed-ligand Zn4L6 cages. The DCL was observed to reconstitute in opposing ways when treated with either the crown ether or C70: the electron-rich macrocycle templated the formation of heteroleptic catenanes, whereas C70 caused the DCL to self-sort into homoleptic structures.

Deconvolution method for specific and nonspecific binding of ligand to multiprotein complex by native mass spectrometry

In native mass spectrometry, it has been difficult to discriminate between specific bindings of a ligand to a multiprotein complex target from the nonspecific interactions. Here, we present a deconvolution model that consists of two levels of data reduction. At the first level, the apparent association binding constants are extracted from the measured intensities of the target/ligand complexes by varying ligand concentration. At the second level, two functional forms representing the specific and nonspecific binding events are fit to the apparent binding constants obtained from the first level of modeling. Using this approach, we found that a power-law distribution described nonspecific binding of α-amanitin to yeast RNA polymerase II. Moreover, treating the concentration of the multiprotein complex as a fitting parameter reduced the impact of inaccuracies in this experimental measurement on the apparent association constants. This model improves upon current methods for separating specific and nonspecific binding to large, multiprotein complexes in native mass spectrometry, by modeling nonspecific binding with a power-law function.

Screening and classifying small-molecule inhibitors of amyloid formation using ion mobility spectrometry-mass spectrometry

The search for therapeutic agents which bind specifically to precursor protein conformations and inhibit amyloid assembly is an important challenge. Identifying such inhibitors is difficult since many protein precursors of aggregation are partially folded or intrinsically disordered, ruling out structure-based design. Furthermore, inhibitors can act by a variety of mechanisms, including specific or non-specific binding, as well as colloidal inhibition. Here we report a high throughput method based on ion mobility spectrometry-mass spectrometry (IMS-MS) that is capable of rapidly detecting small molecules that bind to amyloid precursors, identifying the interacting protein species, and defining the mode of inhibition. Using this method we have classified a variety of small molecules that are potential inhibitors of human islet amyloid polypeptide (hIAPP) aggregation or amyloid-beta 1-40 (Aβ40) aggregation as either specific, non-specific, colloidal or non-interacting. We also demonstrate the ability of IMS-MS to screen for inhibitory small molecules in a 96-well plate format and use this to discover a new inhibitor of hIAPP amyloid assembly.

Application of the Exactive Plus EMR for automated protein-ligand screening by non-covalent mass spectrometry

RATIONALE

Non-covalent mass spectrometry (MS) offers considerable potential for protein-ligand screening in drug discovery programmes. However, there are some limitations with the time-of-flight (TOF) instrumentation typically employed that restrict the application of non-covalent MS in industrial laboratories.

METHODS

An Exactive Plus EMR mass spectrometer was investigated for its ability to characterise non-covalent protein-small molecule interactions. Nano-electrospray ionisation (nanoESI) infusion was achieved with a TriVersa NanoMate. The transport multipole and ion lens voltages, dissociation energies and pressure in the Orbitrap™ were optimised. Native MS was performed, with ligand titrations to judge retention of protein-ligand interactions, serial dilutions of native proteins as an indication of sensitivity, and a heterogeneous protein analysed for spectral resolution.

RESULTS

Interactions between native proteins and ligands are preserved during analysis on the Exactive Plus EMR, with the binding affinities determined in good agreement with expected values. High spectral resolution allows baseline separation of adduct ions, which should improve the accuracy and limit of detection for measuring ligand interactions. Data are also presented showing baseline resolution of glycoforms of a highly glycosylated protein, allowing binding of a fragment molecule to be detected.

CONCLUSIONS

The high sensitivity and spectral resolution achievable with the Orbitrap technology confer significant advantages over TOF mass spectrometers, and offer a solution to current limitations regarding throughput, data analysis and sample requirements. A further benefit of improved spectral resolution is the possibility of using heterogeneous protein samples such as glycoproteins for fragment screening. This would significantly expand the scope of applicability of non-covalent MS in the pharmaceutical and other industries.

Advances in ion mobility spectrometry-mass spectrometry reveal key insights into amyloid assembly

Interfacing ion mobility spectrometry to mass spectrometry (IMS-MS) has enabled mass spectrometric analyses to extend into an extra dimension, providing unrivalled separation and structural characterization of lowly populated species in heterogeneous mixtures. One biological system that has benefitted significantly from such advances is that of amyloid formation. Using IMS-MS, progress has been made into identifying transiently populated monomeric and oligomeric species for a number of different amyloid systems and has led to an enhanced understanding of the mechanism by which small molecules modulate amyloid formation. This review highlights recent advances in this field, which have been accelerated by the commercial availability of IMS-MS instruments. This article is part of a Special Issue entitled: Mass spectrometry in structural biology.

Reliable determinations of protein-ligand interactions by direct ESI-MS measurements. Are we there yet?

The association-dissociation of noncovalent interactions between protein and ligands, such as other proteins, carbohydrates, lipids, DNA, or small molecules, are critical events in many biological processes. The discovery and characterization of these interactions is essential to a complete understanding of biochemical reactions and pathways and to the design of novel therapeutic agents that may be used to treat a variety of diseases and infections. Over the last 20 y, electrospray ionization mass spectrometry (ESI-MS) has emerged as a versatile tool for the identification and quantification of protein-ligand interactions in vitro. Here, we describe the implementation of the direct ESI-MS assay for the determination of protein-ligand binding stoichiometry and affinity. Additionally, we outline common sources of error encountered with these measurements and various strategies to overcome them. Finally, we comment on some of the outstanding challenges associated with the implementation of the assay and highlight new areas where direct ESI-MS measurements are expected to make significant contributions in the future.

Studying protein-protein affinity and immobilized ligand-protein affinity interactions using MS-based methods

This review discusses the most important current methods employing mass spectrometry (MS) analysis for the study of protein affinity interactions. The methods are discussed in depth with particular reference to MS-based approaches for analyzing protein-protein and protein-immobilized ligand interactions, analyzed either directly or indirectly. First, we introduce MS methods for the study of intact protein complexes in the gas phase. Next, pull-down methods for affinity-based analysis of protein-protein and protein-immobilized ligand interactions are discussed. Presently, this field of research is often called interactomics or interaction proteomics. A slightly different approach that will be discussed, chemical proteomics, allows one to analyze selectivity profiles of ligands for multiple drug targets and off-targets. Additionally, of particular interest is the use of surface plasmon resonance technologies coupled with MS for the study of protein interactions. The review addresses the principle of each of the methods with a focus on recent developments and the applicability to lead compound generation in drug discovery as well as the elucidation of protein interactions involved in cellular processes. The review focuses on the analysis of bioaffinity interactions of proteins with other proteins and with ligands, where the proteins are considered as the bioactives analyzed by MS.

Native mass spectrometry as a tool in structural biology

Native mass spectrometry (native MS) gives information about the composition, topological arrangements, dynamics, and structural properties of protein complexes. The mass range is principally unlimited and highly dynamic, allowing the detection of small subunits and large complexes within the same measurement. The amount of protein needed for an analysis is, compared to most other structural biology methods, very low. This unit provides an introduction to native MS. It starts with an explanation of the basic method and details on how to measure intact proteins and protein complexes, and continues with the study of dynamics and complex stability in the gas phase. The final section discusses the most recent extension to the native MS field, ion mobility, which allows the direct assessment of the structural properties of the complexes of interest.

Differential interactions of isomeric amino sugars with insulin studied under electrospray ionisation mass spectrometry

Protein-ligand interactions were studied for bovine insulin-amino sugar systems under electrospray ionisation mass spectrometry conditions. The isomeric amino sugars showed differences in the relative abundance of 1:1 protein-ligand complex formation. The electrospray ionisation and tandem mass spectrometry results of the complex clearly demonstrated that the differences in the interaction of isomeric sugars with insulin are mainly due to the differences in their gas-phase basicity. The same phenomenon is replicated in the formation of complexes between insulin and other ligands, such as amino acids, as well as in the binding of the amino sugars with amyloid β 1-40 peptide.

Nonspecific interactions between proteins and charged biomolecules in electrospray ionization mass spectrometry

An investigation of the nonspecific association of small charged biomolecules and proteins in electrospray ionization mass spectrometry (ES-MS) is described. Aqueous solutions containing pairs of proteins and a small acidic or basic biomolecule that does not interact specifically with either of the proteins were analyzed by ES-MS and the distributions of the biomolecules bound nonspecifically to each pair of proteins compared. For the basic amino acid arginine and the peptide RGVFRR, nonequivalent distributions were measured in positive ion mode, but equivalent distributions were measured in negative ion mode. In the case of uridine 5'-diphosphate, nonequivalent distributions were measured in negative ion mode, but equivalent distributions observed in positive ion mode. The results of dissociation experiments performed on the gaseous ions of the nonspecific complexes suggest that the nonequivalent distributions result from differences in the extent to which the nonspecific complexes undergo in-source dissociation. To test this hypothesis, the distributions of nonspecifically bound basic molecules measured in the presence of imidazole, which protects complexes from in-source dissociation, were compared. In all cases, equivalent distributions were obtained. The results indicate that nonspecific binding of charged molecules to proteins during ES is a statistical process, independent of protein structure and size. However, the kinetic stabilities of the nonspecific interactions are sensitive to the nature of the protein ions. It is concluded that the reference protein method for correcting ES mass spectra for nonspecific ligand-protein binding can be applied to the analysis of ionic ligands, provided that in-source dissociation of the nonspecific interactions is minimized.

Native MS: an ’ESI‚ way to support structure- and fragment-based drug discovery

The success of early drug-discovery programs depends on the adequate combination of complementary and orthogonal technologies allowing hit/lead compounds to be optimized and improve therapeutic activity. Among the available biophysical methods, native MS recently emerged as an efficient method for compound-binding screening. Native MS is a highly sensitive and accurate screening technique. This review provides a description of the general approach and an overview of the possible characterization of ligand-binding properties. How native MS supports structure- and fragment-based drug research will also be discussed, with examples from the literature and internal developments. Native MS shows strong potential for in-depth characterization of ligand-binding properties. It is also a reliable screening technique in drug-discovery processes.