Complete protein characterization using top-down mass spectrometry and ultraviolet photodissociation

Metal adduction in mass spectrometric analyses of carbohydrates and glycoconjugates

Glycans, carbohydrates, and glycoconjugates are involved in many crucial biological processes, such as disease development, immune responses, and cell-cell recognition. Glycans and carbohydrates are known for the large number of isomeric features associated with their structures, making analysis challenging compared with other biomolecules. Mass spectrometry has become the primary method of structural characterization for carbohydrates, glycans, and glycoconjugates. Metal adduction is especially important for the mass spectrometric analysis of carbohydrates and glycans. Metal-ion adduction to carbohydrates and glycoconjugates affects ion formation and the three-dimensional, gas-phase structures. Herein, we discuss how metal-ion adduction impacts ionization, ion mobility, ion activation and dissociation, and hydrogen/deuterium exchange for carbohydrates and glycoconjugates. We also compare the use of different metals for these various techniques and highlight the value in using metals as charge carriers for these analyses. Finally, we provide recommendations for selecting a metal for analysis of carbohydrate adducts and describe areas for continued research.

Direct Identification of Intact Proteins Using a Low-Resolution Mass Spectrometer with CIDn/ETnoD

Over the past decades, proteomics has become increasingly important and a heavily discussed topic. The identification of intact proteins remains a major focus in this field. While most intact proteins are analyzed using high-resolution mass spectrometry, identifying them through low-resolution mass spectrometry continues to pose challenges. In our study, we investigated the capability of identifying various intact proteins using collision-induced dissociation (CID) and electron transfer without dissociation (ETnoD). Using myoglobin as our test protein, stable product ions were generated with CID, and the identities of the product ions were identified with ETnoD. ETnoD uses a short activation time (AcT, 5 ms) to create sequential charge-reduced precursor ion (CRI). The charges of the fragments and their sequences were determined with corresponding CRI. The product ions can be selected for subsequent CID (termed CIDn) combined with ETnoD for further sequence identification and validation. We refer to this method as CIDn/ETnoD. The use of a multistage CID activation (CIDn) and ETnoD protocol has been applied to several intact proteins to obtain multiple sequence identifications.

Panda-UV Unlocks Deeper Protein Characterization with Internal Fragments in Ultraviolet Photodissociation Mass Spectrometry

Ultraviolet photodissociation (UVPD) mass spectrometry unlocks insights into the protein structure and sequence through fragmentation patterns. While N- and C-terminal fragments are traditionally relied upon, this work highlights the critical role of internal fragments in achieving near-complete sequencing of protein. Previous limitations of internal fragment utilization, owing to their abundance and potential for random matching, are addressed here with the development of Panda-UV, a novel software tool combining spectral calibration, and Pearson correlation coefficient scoring for confident fragment assignment. Panda-UV showcases its power through comprehensive benchmarks on three model proteins. The inclusion of internal fragments boosts identified fragment numbers by 26% and enhances average protein sequence coverage to a remarkable 93% for intact proteins, unlocking the hidden region of the largest protein carbonic anhydrase II in model proteins. Notably, an average of 65% of internal fragments can be identified in multiple replicates, demonstrating the high confidence of the fragments Panda-UV provided. Finally, the sequence coverages of mAb subunits can be increased up to 86% and the complementary determining regions (CDRs) are nearly completely sequenced in a single experiment. The source codes of Panda-UV are available at https://github.com/PHOENIXcenter/Panda-UV.

Fast peak error correction algorithms for proteoform identification using top-down tandem mass spectra

MOTIVATION

Proteoform identification is an important problem in proteomics. The main task is to find a modified protein that best fits the input spectrum. To overcome the combinatorial explosion of possible proteoforms, the proteoform mass graph and spectrum mass graph are used to represent the protein database and the spectrum, respectively. The problem becomes finding an optimal alignment between the proteoform mass graph and the spectrum mass graph. Peak error correction is an important issue for computing an optimal alignment between the two input mass graphs.

RESULTS

We propose a faster algorithm for the error correction alignment of spectrum mass graph and proteoform mass graph problem and produce a program package TopMGFast. The newly designed algorithms require less space and running time so that we are able to compute global optimal alignments for the two input mass graphs in a reasonable time. For the local alignment version, experiments show that the running time of the new algorithm is reduced by 2.5 times. For the global alignment version, experiments show that the maximum mass errors between any pair of matched nodes in the alignments obtained by our method are within a small range as designed, while the alignments produced by the state-of-the-art method, TopMG, have very large maximum mass errors for many cases. The obtained alignment sizes are roughly the same for both TopMG and TopMGFast. Of course, TopMGFast needs more running time than TopMG. Therefore, our new algorithm can obtain more reliable global alignments within a reasonable time. This is the first time that global optimal error correction alignments can be obtained using real datasets.

AVAILABILITY AND IMPLEMENTATION

The source code of the algorithm is available at https://github.com/Zeirdo/TopMGFast.

Enhancing Top-Down Analysis of Proteins by Combining Ultraviolet Photodissociation (UVPD), Proton-Transfer Charge Reduction (PTCR), and Gas-Phase Fractionation to Alleviate the Impact of Nondissociated Precursor Ions

Recent advances in top-down mass spectrometry strategies continue to improve the analysis of intact proteins. 193 nm ultraviolet photodissociation (UVPD) is one method well-suited for top-down analysis. UVPD is often performed using relatively low photon flux in order to limit multiple-generation dissociation of fragment ions and maximize sequence coverage. Consequently, a large portion of the precursor ion survives the UVPD process, dominates the spectrum, and may impede identification of fragment ions. Here, we explore the isolation of subpopulations of fragment ions lower and higher than the precursor ion after UVPD as a means to eliminate the impact of the surviving precursor ion on the detection of low abundance fragment ions. This gas-phase fractionation method improved sequence coverage harvested from fragment ions found in the m/z regions lower and higher than the precursor by an average factor of 1.3 and 2.3, respectively. Combining this gas-phase fractionation method with proton transfer charge reduction (PTCR) further increased the sequence coverage obtained from these m/z regions by another factor of 1.3 and 1.4, respectively. Implementing a post-UVPD fractionation + PTCR strategy with six fractionation events resulted in a sequence coverage of 75% for enolase, the highest reported for 193 nm UVPD.

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.

Top-Down Protein Analysis by Tandem-Trapped Ion Mobility Spectrometry/Mass Spectrometry (Tandem-TIMS/MS) Coupled with Ultraviolet Photodissociation (UVPD) and Parallel Accumulation/Serial Fragmentation (PASEF) MS/MS Analysis

"Top-down" proteomics analyzes intact proteins and identifies proteoforms by their intact mass as well as the observed fragmentation pattern in tandem mass spectrometry (MS/MS) experiments. Recently, hybrid ion mobility spectrometry-mass spectrometry (IM/MS) methods have gained traction for top-down experiments, either by allowing top-down analysis of individual isomers or alternatively by improving signal/noise and dynamic range for fragment ion assignment. We recently described the construction of a tandem-trapped ion mobility spectrometer/mass spectrometer (tandem-TIMS/MS) coupled with an ultraviolet (UV) laser and demonstrated a proof-of-principle for top-down analysis by UV photodissociation (UVPD) at 2-3 mbar. The present work builds on this with an exploration of a top-down method that couples tandem-TIMS/MS with UVPD and parallel-accumulation serial fragmentation (PASEF) MS/MS analysis. We first survey types and structures of UVPD-specific fragment ions generated in the 2-3 mbar pressure regime of our instrument. Notably, we observe UVPD-induced fragment ions with multiple conformations that differ from those produced in the absence of UV irradiation. Subsequently, we discuss how MS/MS spectra of top-down fragment ions lend themselves ideally for probability-based scoring methods developed in the bottom-up proteomics field and how the ability to record automated PASEF-MS/MS spectra resolves ambiguities in the assignment of top-down fragment ions. Finally, we describe the coupling of tandem-TIMS/MS workflows with UVPD and PASEF-MS/MS analysis for native top-down protein analysis.

Mass Spectrometry-Based Proteomics: Analyses Related to Drug-Resistance and Disease Biomarkers

Mass spectrometry-based proteomics is a key player in research efforts to characterize aberrant epigenetic alterations, including histone post-translational modifications and DNA methylation. Data generated by this approach complements and enrich datasets generated by genomic, epigenetic and transcriptomics approaches. These combined datasets can provide much-needed information on various mechanisms responsible for drug resistance, the discovery and validation of potential biomarkers for different diseases, the identification of signaling pathways, and genes and enzymes to be targeted by future therapies. The increasing use of high-resolution, high-accuracy mass spectrometers combined with more refined protein labeling and enrichment procedures enhanced the role of this approach in the investigation of these epigenetic modifications. In this review, we discuss recent MS-based studies, which are contributing to current research efforts to understand certain mechanisms behind drug resistance to therapy. We also discuss how these MS-based analyses are contributing to biomarkers discovery and validation.

Characterization of Unbranched Ubiquitin Tetramers by Combining Ultraviolet Photodissociation with Proton Transfer Charge Reduction Reactions

Polyubiquitination is an important post-translational modification (PTM) that regulates various biological functions. The linkage sites and topologies of polyubiquitination chains are important factors in determining the fate of polyubiquitinated proteins. Characterization of polyubiquitin chains is the first step in understanding the biological functions of protein ubiquitination, but it is challenging owing to the repeating nature of the ubiquitin chains and the difficulty in deciphering linkage positions. Here, we combine ultraviolet photodissociation (UVPD) mass spectrometry and gas-phase proton transfer charge reduction (PTCR) to facilitate the assignment of product ions generated from Lys6-, Lys11-, Lys29-, Lys33-, Lys48-, and Lys63-linked ubiquitin tetramers. UVPD results in extensive fragmentation of intact proteins in a manner that allows the localization of PTMs. However, UVPD mass spectra of large proteins (>30 kDa) are often congested due to the overlapping isotopic distribution of highly charged fragment ions. UVPD + PTCR improved the identification of PTM-containing fragment ions, allowing the localization of linkage sites in all six tetramers analyzed. UVPD + PTCR also increased the sequence coverage obtained from the PTM-containing fragment ions in each of the four chains of each tetramer by 7 to 44% when compared to UVPD alone.

Studying Membrane Protein-Lipid Specificity through Direct Native Mass Spectrometric Analysis from Tunable Proteoliposomes

Native mass spectrometry (nMS) has emerged as a key analytical tool to study the organizational states of proteins and their complexes with both endogenous and exogenous ligands. Specifically, for membrane proteins, it provides a key analytical dimension to determine the identity of bound lipids and to decipher their effects on the observed structural assembly. We recently developed an approach to study membrane proteins directly from intact and tunable lipid membranes where both the biophysical properties of the membrane and its lipid compositions can be customized. Extending this, we use our liposome-nMS platform to decipher the lipid specificity of membrane proteins through their multiorganelle trafficking pathways. To demonstrate this, we used VAMP2 and reconstituted it in the endoplasmic reticulum (ER), Golgi, synaptic vesicle (SV), and plasma membrane (PM) mimicking liposomes. By directly studying VAMP2 from these customized liposomes, we show how the same transmembrane protein can bind to different sets of lipids in different organellar-mimicking membranes. Considering that the cellular trafficking pathway of most eukaryotic integral membrane proteins involves residence in multiple organellar membranes, this study highlights how the lipid-specificity of the same integral membrane protein may change depending on the membrane context. Further, leveraging the capability of the platform to study membrane proteins from liposomes with curated biophysical properties, we show how we can disentangle chemical versus biophysical properties, of individual lipids in regulating membrane protein assembly.

Characterization of an Omnitrap-Orbitrap Platform Equipped with Infrared Multiphoton Dissociation, Ultraviolet Photodissociation, and Electron Capture Dissociation for the Analysis of Peptides and Proteins

We describe an instrument configuration based on the Orbitrap Exploris 480 mass spectrometer that has been coupled to an Omnitrap platform. The Omnitrap possesses three distinct ion-activation regions that can be used to perform resonant-based collision-induced dissociation, several forms of electron-associated fragmentation, and ultraviolet photodissociation. Each section can also be combined with infrared multiphoton dissociation. In this work, we demonstrate all these modes of operation in a range of peptides and proteins. The results show that this instrument configuration produces similar data to previous implementations of each activation technique and at similar efficiency levels. We demonstrate that this unique instrument configuration is extremely versatile for the investigation of polypeptides.

Differentiation of Aspartic and Isoaspartic Acid Using 193 nm Ultraviolet Photodissociation Mass Spectrometry

Spontaneous conversion of aspartic acid (Asp) to isoaspartic acid (isoAsp) is a ubiquitous modification that influences the structure and function of proteins. This modification of Asp impacts the stability of biotherapeutics and has been linked to the development of neurodegenerative diseases. We explored the use of 193 nm ultraviolet photodissociation (UVPD) to distinguish Asp and isoAsp in the protonated and deprotonated peptides. The differences in the relative abundances of several fragment ions uniquely generated by UVPD were used to differentiate isomeric peptide standards containing Asp or isoAsp. These fragment ions result from the cleavage of bonds N-terminal to Asp/isoAsp residues in addition to the side-chain losses from Asp/isoAsp or the losses of COOH, CO2, CO, or H2O from y-ions. Fragmentation of Asp-containing tryptic peptides using UVPD resulted in more enhanced w/w + 1/y - 1/x ions, while isoAsp-containing peptides yielded more enhanced y - 18/y - 45/y - 46 ions. UVPD was also used to identify an isomerized peptide from a tryptic digest of a monoclonal antibody. Moreover, UVPD of a protonated nontryptic peptide resulted in more enhanced y ions N- and C-terminal to isoAsp and differences in b/y ion ratios that were used to identify the isoAsp peptide.

Structural identification of carbohydrate isomers using ambient infrared-assisted dissociation

Informative dissociation of carbohydrates using an infrared (IR) irradiation system is demonstrated under ambient conditions without the instrumentation of a mass spectrometer. Structural identification of carbohydrates and associated conjugates is essential for understanding their biological functions, but identification remains challenging. Herein, an easy and rugged method is reported for the structural identification of model carbohydrates, including Globo-H, three trisaccharide isomers (nigerotriose/laminaritriose/cellotriose), and two hexasaccharide isomers (laminarihexaose/isomaltohexaose). For Globo-H, the numbers of cross-ring cleavages increased by factors of 4.4 and 3.4 upon ambient IR exposure, compared to an untreated control and a collision-induced dissociation (CID) sample. Moreover, 25-82% enhancement in the numbers of glycosidic bond cleavages upon ambient IR exposure was also obtained compared to untreated and CID samples. Unique features of first-generation fragments produced by ambient IR facilitated the differentiation of three trisaccharide isomers. Semi-quantitative analysis was achieved (coefficient of determination (R²) of 0.982) in a mixture of two hexasaccharide isomers via unique features generated upon ambient IR. Photothermal and radical migration effects induced by ambient IR were postulated as responsible for promoting carbohydrate fragmentation. This easy and rugged method could be a universally applicable protocol and complementary to other techniques for detailed structural characterization of carbohydrates.

Deep Bottom-up Proteomics Enabled by the Integration of Liquid-Phase Ion Trap

In bottom-up proteomics, the complexity of the proteome requires advanced peptide separation and/or fractionation methods to acquire an in-depth understanding of protein profiles. Proposed earlier as a solution-phase ion manipulation device, liquid phase ion traps (LPITs) were used in front of mass spectrometers to accumulate target ions for improved detection sensitivity. In this work, an LPIT-reversed phase liquid chromatography-tandem mass spectrometry (LPIT-RPLC-MS/MS) platform was established for deep bottom-up proteomics. LPIT was used here as a robust and effective method for peptide fractionation, which also shows good reproducibility and sensitivity on both qualitative and quantitative levels. LPIT separates peptides based on their effective charges and hydrodynamic radii, which is orthogonal to that of RPLC. With excellent orthogonality, the integration of LPIT with RPLC-MS/MS could effectively increase the number of peptides and proteins being detected. When HeLa cells were analyzed, peptide and protein coverages were increased by ∼89.2% and 50.3%, respectively. With high efficiency and low cost, this LPIT-based peptide fraction method could potentially be used in routine deep bottom-up proteomics.

Integration of Trapped Ion Mobility Spectrometry and Ultraviolet Photodissociation in a Quadrupolar Ion Trap Mass Spectrometer

There is a growing demand for lower-cost, benchtop analytical instruments with complementary separation capabilities for the screening and characterization of biological samples. In this study, we report on the custom integration of trapped ion mobility spectrometry and ultraviolet photodissociation capabilities in a commercial Paul quadrupolar ion trap multistage mass spectrometer (TIMS-QIT-MSn UVPD platform). A gated TIMS operation allowed for the accumulation of ion mobility separated ion in the QIT, followed by a mass analysis (MS1 scan) or m/z isolation, followed by selected collision induced dissociation (CID) or ultraviolet photodissociation (UVPD) and a mass analysis (MS2 scan). The analytical potential of this platform for the analysis of complex and labile biological samples is illustrated for the case of positional isomers with varying PTM location of the histone H4 tryptic peptide 4-17 singly and doubly acetylated and the histone H3.1 tail (1-50) singly trimethylated. For all cases, a baseline ion mobility precursor molecular ion preseparation was obtained. The tandem CID and UVPD MS2 allowed for effective sequence confirmation as well as the identification of reporter fragment ions associated with the PTM location; a higher sequence coverage was obtained using UVPD when compared to CID. Different from previous IMS-MS implementation, the novel TIMS-QIT-MSn UVPD platform offers a lower-cost alternative for the structural characterization of biological molecules that can be widely disseminated in clinical laboratories.

High sensitivity top-down proteomics captures single muscle cell heterogeneity in large proteoforms

Single-cell proteomics has emerged as a powerful method to characterize cellular phenotypic heterogeneity and the cell-specific functional networks underlying biological processes. However, significant challenges remain in single-cell proteomics for the analysis of proteoforms arising from genetic mutations, alternative splicing, and post-translational modifications. Herein, we have developed a highly sensitive functionally integrated top-down proteomics method for the comprehensive analysis of proteoforms from single cells. We applied this method to single muscle fibers (SMFs) to resolve their heterogeneous functional and proteomic properties at the single-cell level. Notably, we have detected single-cell heterogeneity in large proteoforms (>200 kDa) from the SMFs. Using SMFs obtained from three functionally distinct muscles, we found fiber-to-fiber heterogeneity among the sarcomeric proteoforms which can be related to the functional heterogeneity. Importantly, we detected multiple isoforms of myosin heavy chain (~223 kDa), a motor protein that drives muscle contraction, with high reproducibility to enable the classification of individual fiber types. This study reveals single muscle cell heterogeneity in large proteoforms and establishes a direct relationship between sarcomeric proteoforms and muscle fiber types, highlighting the potential of top-down proteomics for uncovering the molecular underpinnings of cell-to-cell variation in complex systems.

Integrated mass spectrometry strategy for functional protein complex discovery and structural characterization

The discovery of functional protein complex and the interrogation of the complex structure-function relationship (SFR) play crucial roles in the understanding and intervention of biological processes. Affinity purification-mass spectrometry (AP-MS) has been proved as a powerful tool in the discovery of protein complexes. However, validation of these novel protein complexes as well as elucidation of their molecular interaction mechanisms are still challenging. Recently, native top-down MS (nTDMS) is rapidly developed for the structural analysis of protein complexes. In this review, we discuss the integration of AP-MS and nTDMS in the discovery and structural characterization of functional protein complexes. Further, we think the emerging artificial intelligence (AI)-based protein structure prediction is highly complementary to nTDMS and can promote each other. We expect the hybridization of integrated structural MS with AI prediction to be a powerful workflow in the discovery and SFR investigation of functional protein complexes.

Single Molecule Protein Segments Sequencing by a Plasmonic Nanopore

Obtaining sequential and conformational information on proteins is vital to understand their functions. Although the nanopore-based electrical detection can sense single molecule (SM) protein and distinguish among different amino acids, this approach still faces difficulties in slowing down protein translocation and improving ionic current signal-to-noise ratio. Here, we observe the unfolding and multistep sequential translocation of SM cytochrome c (cyt c) through a surface enhanced Raman scattering (SERS) active conical gold nanopore. High bias voltage unfolds SM protein causing more exposure of amino acid residues to the nanopore, which slows down the protein translocation. Specific SERS traces of different SM cyt c segments are then recorded sequentially when they pass through the hotspot inside the gold nanopore. This study shows that the combination of SM SERS with a nanopore can provide a direct insight into protein segments and expedite the development of nanopore toward SM protein sequencing.

Enhanced Characterization of Histones Using 193 nm Ultraviolet Photodissociation and Proton Transfer Charge Reduction

Top-down characterization of histones, proteins that are critical participants in an array of DNA-dependent processes, offers the potential to examine the relationship between histone structure and mechanisms of genetic regulation. Mapping patterns of post-translational modifications (PTMs) of histones requires extensive backbone cleavages to bracket the sites of mass shifts corresponding to specific PTMs. Ultraviolet photodissociation (UVPD) causes substantial fragmentation of proteins, which is well-suited for PTM localization, but the resulting spectra are congested with fragment ions that may have overlapping isotopic distributions that confound deconvolution. Gas-phase proton transfer charge reduction (PTCR) decreases the charge states of highly charged ions, thus alleviating this congestion and facilitating the identification of additional sequence-determining and PTM-localizing fragment ions. By integrating UVPD with PTCR for histone proteoform analyses, sequence coverages up to 91% were achieved for calf thymus histone H4 containing acetylation marks at the N-terminus and Lys12 as well as a dimethylation at Arg3. UVPD-PTCR exhibited large gains in characterization for other histones, such as histone H2A, increasing the sequence coverage from 59 to 77% for monoacetylated H2A.

Ion Mobility Mass Spectrometry (IM-MS) for Structural Biology: Insights Gained by Measuring Mass, Charge, and Collision Cross Section

The investigation of macromolecular biomolecules with ion mobility mass spectrometry (IM-MS) techniques has provided substantial insights into the field of structural biology over the past two decades. An IM-MS workflow applied to a given target analyte provides mass, charge, and conformation, and all three of these can be used to discern structural information. While mass and charge are determined in mass spectrometry (MS), it is the addition of ion mobility that enables the separation of isomeric and isobaric ions and the direct elucidation of conformation, which has reaped huge benefits for structural biology. In this review, where we focus on the analysis of proteins and their complexes, we outline the typical features of an IM-MS experiment from the preparation of samples, the creation of ions, and their separation in different mobility and mass spectrometers. We describe the interpretation of ion mobility data in terms of protein conformation and how the data can be compared with data from other sources with the use of computational tools. The benefit of coupling mobility analysis to activation via collisions with gas or surfaces or photons photoactivation is detailed with reference to recent examples. And finally, we focus on insights afforded by IM-MS experiments when applied to the study of conformationally dynamic and intrinsically disordered proteins.

Recent advances in top-down proteome sample processing ahead of MS analysis

Top-down proteomics is emerging as a preferred approach to investigate biological systems, with objectives ranging from the detailed assessment of a single protein therapeutic, to the complete characterization of every possible protein including their modifications, which define the human proteoform. Given the controlling influence of protein modifications on their biological function, understanding how gene products manifest or respond to disease is most precisely achieved by characterization at the intact protein level. Top-down mass spectrometry (MS) analysis of proteins entails unique challenges associated with processing whole proteins while maintaining their integrity throughout the processes of extraction, enrichment, purification, and fractionation. Recent advances in each of these critical front-end preparation processes, including minimalistic workflows, have greatly expanded the capacity of MS for top-down proteome analysis. Acknowledging the many contributions in MS technology and sample processing, the present review aims to highlight the diverse strategies that have forged a pathway for top-down proteomics. We comprehensively discuss the evolution of front-end workflows that today facilitate optimal characterization of proteoform-driven biology, including a brief description of the clinical applications that have motivated these impactful contributions.

A multiparameter optimization in middle-down analysis of monoclonal antibodies by LC-MS/MS

In antibody-based drug research, a complete characterization of antibody proteoforms covering both the amino acid sequence and all posttranslational modifications remains a major concern. The usual mass spectrometry-based approach to achieve this goal is bottom-up proteomics, which relies on the digestion of antibodies but does not allow the diversity of proteoforms to be assessed. Middle-down and top-down approaches have recently emerged as attractive alternatives but are not yet mastered and thus used in routine by many analytical chemistry laboratories. The work described here aims at providing guidelines to achieve the best sequence coverage for the fragmentation of intact light and heavy chains generated from a simple reduction of intact antibodies using Orbitrap mass spectrometry. Three parameters were found crucial to this aim: the use of an electron-based activation technique, the multiplex selection of precursor ions of different charge states, and the combination of replicates.

Comparison of Top-Down Protein Fragmentation Induced by 213 and 193 nm UVPD

The growing interest in advancing tandem mass spectrometry strategies for top-down proteomics has motivated efforts to optimize ion activation strategies for intact proteins, including the comparison of 193 and 213 nm wavelengths for ultraviolet photodissociation (UVPD). The present study focuses on the performance and outcomes of UVPD for five proteins, ubiquitin, cytochrome C, frataxin, myoglobin, and carbonic anhydrase, with an emphasis on evaluating the similarities and differences in fragmentation promoted by UVPD using 193 nm versus 213 nm photons. Mass spectra were collected as a function of the number of laser pulses, and precursor depletion levels were monitored and controlled to provide consistent energy deposition between 213 and 193 nm UVPD. Fragment ions were confirmed on the basis of their isotopic distributions in m/z space to preserve both charge state and abundance information and were classified on the basis of ion type and frequency. A large portion of the total fragment ion abundance was attributable to preferential cleavages, particularly ones adjacent to proline residues. These cleavages were examined on the basis of the backbone site and abundances, revealing that a and y-2 ions N-terminal to proline residues appeared at disproportionately high abundances in 213 nm UVPD spectra as compared to 193 nm UVPD spectra, highlighting one notable difference in the top-down spectra. We theorize that these fragments are formed more efficiently in 213 nm UVPD than in 193 nm UVPD due to increased absorption of 213 nm photons at the proline amide bond.

Proteoforms expand the world of microproteins and short open reading frame-encoded peptides

Microproteins and short open reading frame-encoded peptides (SEPs) can, like all proteins, carry numerous posttranslational modifications. Together with posttranscriptional processes, this leads to a high number of possible distinct protein molecules, the proteoforms, out of a limited number of genes. The identification, quantification, and molecular characterization of proteoforms possess special challenges to established, mainly bottom-up proteomics (BUP) based analytical approaches. While BUP methods are powerful, proteins have to be inferred rather than directly identified, which hampers the detection of proteoforms. An alternative approach is top-down proteomics (TDP) which allows to identify intact proteoforms. This perspective article provides a brief overview of modified microproteins and SEPs, introduces the proteoform terminology, and compares present BUP and TDP workflows highlighting their major advantages and caveats. Necessary future developments in TDP to fully accentuate its potential for proteoform-centric analytics of microproteins and SEPs will be discussed.

Impact of Internal Fragments on Top-Down Analysis of Intact Proteins by 193 nm UVPD

193 nm ultraviolet photodissociation (UVPD) allows high sequence coverage to be obtained for intact proteins using terminal fragments alone. However, internal fragments, those that contain neither N- nor C- terminus, are typically ignored, neglecting their potential to bolster characterization of intact proteins. Here, we explore internal fragments generated by 193 nm UVPD for proteins ranging in size from 17-47 kDa and using the ClipsMS algorithm to facilitate searches for internal fragments. Internal fragments were only retained if identified in multiple replicates in order to reduce spurious assignments and to explore the reproducibility of internal fragments generated by UVPD. Inclusion of internal fragment improved sequence coverage by an average of 18% and 32% for UVPD and HCD, respectively, across all proteins and charge states studied. However, only an average of 18% of UVPD internal fragments were identified in two out of three replicates relative to the average number identified across all replicates for all proteins studied. Conversely, for HCD, an average of 63% of internal fragments were retained across replicates. These trends reflect an increased risk of false-positive identifications and a need for caution when considering internal fragments for UVPD. Additionally, proton-transfer charge reduction (PTCR) reactions were performed following UVPD or HCD to assess the impact on internal fragment identifications, allowing up to 20% more fragment ions to be retained across multiple replicates. At this time, it is difficult to recommend the inclusion of the internal fragment when searching UVPD spectra without further work to develop strategies for reducing the possibilities of false-positive identifications. All mass spectra are available in the public repository jPOST with the accession number JPST001885.

Deep top-down proteomics revealed significant proteoform-level differences between metastatic and nonmetastatic colorectal cancer cells

Understanding cancer metastasis at the proteoform level is crucial for discovering previously unknown protein biomarkers for cancer diagnosis and drug development. We present the first top-down proteomics (TDP) study of a pair of isogenic human nonmetastatic and metastatic colorectal cancer (CRC) cell lines (SW480 and SW620). We identified 23,622 proteoforms of 2332 proteins from the two cell lines, representing nearly fivefold improvement in the number of proteoform identifications (IDs) compared to previous TDP datasets of human cancer cells. We revealed substantial differences between the SW480 and SW620 cell lines regarding proteoform and single amino acid variant (SAAV) profiles. Quantitative TDP unveiled differentially expressed proteoforms between the two cell lines, and the corresponding genes had diversified functions and were closely related to cancer. Our study represents a pivotal advance in TDP toward the characterization of human proteome in a proteoform-specific manner, which will transform basic and translational biomedical research.

Top-"Double-Down" Mass Spectrometry of Histone H4 Proteoforms: Tandem Ultraviolet-Photon and Mobility/Mass-Selected Electron Capture Dissociations

Post-translational modifications (PTMs) on intact histones play a major role in regulating chromatin dynamics and influence biological processes such as DNA transcription, replication, and repair. The nature and position of each histone PTM is crucial to decipher how this information is translated into biological response. In the present work, the potential of a novel tandem top-"double-down" approach─ultraviolet photodissociation followed by mobility and mass-selected electron capture dissociation and mass spectrometry (UVPD-TIMS-q-ECD-ToF MS/MS)─is illustrated for the characterization of HeLa derived intact histone H4 proteoforms. The comparison between q-ECD-ToF MS/MS spectra and traditional Fourier-transform-ion cyclotron resonance-ECD MS/MS spectra of a H4 standard showed a similar sequence coverage (∼75%) with significant faster data acquisition in the ToF MS/MS platform (∼3 vs ∼15 min). Multiple mass shifts (e.g., 14 and 42 Da) were observed for the HeLa derived H4 proteoforms for which the top-down UVPD and ECD fragmentation analysis were consistent in detecting the presence of acetylated PTMs at the N-terminus and Lys5, Lys8, Lys12, and Lys16 residues, as well as methylated, dimethylated, and trimethylated PTMs at the Lys20 residue with a high sequence coverage (∼90%). The presented top-down results are in good agreement with bottom-up TIMS ToF MS/MS experiments and allowed for additional description of PTMs at the N-terminus. The integration of a 213 nm UV laser in the present platform allowed for UVPD events prior to the ion mobility-mass precursor separation for collision-induced dissociation (CID)/ECD-ToF MS. Selected c305+ UVPD fragments, from different H4 proteoforms (e.g., Ac + Me2, 2Ac + Me2 and 3Ac + Me2), exhibited multiple IMS bands for which similar CID/ECD fragmentation patterns per IMS band pointed toward the presence of conformers, adopting the same PTM distribution, with a clear assignment of the PTM localization for each of the c305+ UVPD fragment H4 proteoforms. These results were consistent with the biological "zip" model, where acetylation proceeds in the Lys16 to Lys5 direction. This novel platform further enhances the structural toolbox with alternative fragmentation mechanisms (UVPD, CID, and ECD) in tandem with fast, high-resolution mobility separations and shows great promise for global proteoform analysis.

The Omnitrap Platform: A Versatile Segmented Linear Ion Trap for Multidimensional Multiple-Stage Tandem Mass Spectrometry

Multidimensional multiple-stage tandem processing of ions is demonstrated successfully in a novel segmented linear ion trap. The enhanced performance is enabled by incorporating the entire range of ion activation methods into a single platform in a highly dynamic fashion. The ion activation network comprises external injection of reagent ions, radical neutral species, photons, electrons, and collisions with neutrals. Axial segmentation of the two-dimensional trapping field provides access to a unique functionality landscape through a system of purpose-designed regions for processing ions with maximum flexibility. Design aspects of the segmented linear ion trap, termed the Omnitrap platform, are highlighted, and motion of ions trapped by rectangular waveforms is investigated experimentally by mapping the stability diagram, tracing secular frequencies, and exploring different isolation techniques. All fragmentation methods incorporated in the Omnitrap platform involving radical chemistry are shown to provide complete sequence coverage for partially unfolded ubiquitin. Three-stage (MS3) tandem mass spectrometry experiments combining collision-induced dissociation of radical ions produced by electron meta-ionization and further involving two intermediate steps of ion isolation and accumulation are performed with high efficiency, producing information rich spectra with signal-to-noise levels comparable to those obtained in a two-stage (MS2) experiment. The advanced capabilities of the Omnitrap platform to provide in-depth top-down MSn characterization of proteins are portrayed. Performance is further enhanced by connecting the Omnitrap platform to an Orbitrap mass analyzer, while successful integration with time-of-flight analyzers has already been demonstrated.

Deciphering combinatorial post-translational modifications by top-down mass spectrometry

Post-translational modifications (PTMs) create vast structural and functional diversity of proteins, ultimately modulating protein function and degradation, influencing cellular signaling, and regulating transcription. The combinatorial patterns of PTMs increase the heterogeneity of proteins and further mediates their interactions. Advances in mass spectrometry-based proteomics have resulted in identification of thousands of proteins and allowed characterization of numerous types and sites of PTMs. Examination of intact proteins, termed the top-down approach, offers the potential to map protein sequences and localize multiple PTMs on each protein, providing the most comprehensive cataloging of proteoforms. This review describes some of the dividends of using mass spectrometry to analyze intact proteins and showcases innovative strategies that have enhanced the promise of top-down proteomics for exploring the impact of combinatorial PTMs in unsurpassed detail.

Nanohydrophobic Interaction Chromatography Coupled to Ultraviolet Photodissociation Mass Spectrometry for the Analysis of Intact Proteins in Low Charge States

The direct correlation between proteoforms and biological phenotype necessitates the exploration of mass spectrometry (MS)-based methods more suitable for proteoform detection and characterization. Here, we couple nano-hydrophobic interaction chromatography (nano-HIC) to ultraviolet photodissociation MS (UVPD-MS) for separation and characterization of intact proteins and proteoforms. High linearity, sensitivity, and sequence coverage are obtained with this method for a variety of proteins. Investigation of collisional cross sections of intact proteins during nano-HIC indicates semifolded conformations in low charge states, enabling a different dimension of separation in comparison to traditional, fully denaturing reversed-phase separations. This method is demonstrated for a mixture of intact proteins from Escherichia coli ribosomes; high sequence coverage is obtained for a variety of modified and unmodified proteoforms.

Exploring the Conformations and Binding Location of HMGA2·DNA Complexes Using Ion Mobility Spectrometry and 193 nm Ultraviolet Photodissociation Mass Spectrometry

Although it is widely accepted that protein function is largely dependent on its structure, intrinsically disordered proteins (IDPs) lack defined structure but are essential in proper cellular processes. Mammalian high mobility group proteins (HMGA) are one such example of IDPs that perform a number of crucial nuclear activities and have been highly studied due to their involvement in the proliferation of a variety of disease and cancers. Traditional structural characterization methods have had limited success in understanding HMGA proteins and their ability to coordinate to DNA. Ion mobility spectrometry and mass spectrometry provide insights into the diversity and heterogeneity of structures adopted by IDPs and are employed here to interrogate HMGA2 in its unbound states and bound to two DNA hairpins. The broad distribution of collision cross sections observed for the apo-protein are restricted when HMGA2 is bound to DNA, suggesting that increased protein organization is promoted in the holo-form. Ultraviolet photodissociation was utilized to probe the changes in structures for the compact and elongated structures of HMGA2 by analyzing backbone cleavage propensities and solvent accessibility based on charge-site analysis, which revealed a spectrum of conformational possibilities. Namely, preferential binding of the DNA hairpins with the second of three AT-hooks of HMGA2 is suggested based on the suppression of backbone fragmentation and distribution of DNA-containing protein fragments.

Trapped Ion Mobility Spectrometry, Ultraviolet Photodissociation, and Time-of-Flight Mass Spectrometry for Gas-Phase Peptide Isobars/Isomers/Conformers Discrimination

Trapped ion mobility spectrometry (TIMS) when coupled with mass spectrometry (MS) offers great advantages for the separation of isobaric, isomeric, and/or conformeric species. In the present work, we report the advantages of coupling TIMS with a low-cost, ultraviolet photodissociation (UVPD) linear ion trap operated at few mbars prior to time-of-flight (ToF) MS analysis for the effective characterization of isobaric, isomeric, and/or conformeric species based on mobility-selected fragmentation patterns. These three traditional challenges to MS-based separations are illustrated for the case of biologically relevant model systems: H3.1 histone tail PTM isobars (K4Me3/K18Ac), lanthipeptide regioisomers (overlapping/nonoverlapping ring patterns), and a model peptide conformer (angiotensin I). The sequential nature of the TIMS operation allows for effective synchronization with the ToF MS scans, in addition to parallel operation between the TIMS and the UVPD trap. Inspection of the mobility-selected UVPD MS spectra showed that for all three cases considered, unique fragmentation patterns (fingerprints) were observed per mobility band. Different from other IMS-UVPD implementations, the higher resolution of the TIMS device allowed for high mobility resolving power (R > 100) and effective mobility separation. The mobility selected UVPD MS provided high sequence coverage (>85%) with a fragmentation efficiency up to ∼40%.

Characterization of the Time-Domain Isotopic Beat Patterns of Monoclonal Antibodies in Fourier Transform Mass Spectrometry

The time-domain transients in the Fourier transform mass spectrometry (FTMS) analysis of monoclonal antibodies (mAbs) are known to exhibit characteristic isotopic beat patterns. These patterns are defined by the isotopic distributions of all gaseous mAb ions present in the FTMS mass analyzer, originating from single or multiple charge states, and from single or multiple proteoforms. For an isolated charge state of a single proteoform, the mAb isotopic beat pattern resembles narrow splashes of signal amplitude (beats), spaced periodically in the time-domain transient, with broad (often exceeding 1 s) "valleys" between them. Here, we reinforce the importance of isotopic beat patterns for the accurate interpretation and presentation of FTMS data in the analysis of mAbs and other large biopolymers. An updated, mAb-grade version of the transient-mediated FTMS data simulation and visualization tool, FTMS Simulator is introduced and benchmarked. We then apply this tool to evaluate the charge-state dependent characteristics of isotopic beats in mAbs analyses with modern models of Orbitrap and ion cyclotron resonance (ICR) FTMS instruments, including detection of higher-order harmonics. We demonstrate the impact of the isotopic beat patterns on the analytical characteristics of the resulting mass spectra of individual and overlapping mAb proteoforms. The results reported here detail highly nonlinear dependences of resolution and signal-to-noise ratio on the time-domain transient period, absorption or magnitude mode spectra representation, and apodization functions. The provided description and the demonstrated ability to routinely conduct accurate simulations of FTMS data for large biopolymers should aid the end-users of Orbitrap and ICR FTMS instruments in the analysis of mAbs and other biopolymers, including viruses.

Multimodal Tandem Mass Spectrometry Techniques for the Analysis of Phosphopeptides

Collisionally activated dissociation (CAD), infrared multiphoton dissociation (IRMPD), ultraviolet photodissociation (UVPD), electron capture dissociation and electron detachment dissociation (EDD) experiments were conducted on a set of phosphopeptides, in a Fourier transform ion cyclotron resonance mass spectrometer. The fragmentation patterns were compared and varied according to the fragmentation mechanisms and the composition of the peptides. CAD and IRMPD produced similar fragmentation profiles of the phosphopeptides, while UVPD produced a large number of complementary fragments. Electron-based dissociation techniques displayed lower fragmentation efficiencies, despite retaining the labile phosphate group, and drastically different fragmentation profiles. EDD produced complex spectra whose interpretation proved challenging.

Investigation of Product Ions Generated by 193 nm Ultraviolet Photodissociation of Peptides and Proteins Containing Disulfide Bonds

Disulfide bridges are unique post-translational modifications (PTM) that contribute to protein architecture and modulate function. This PTM, however, challenges top-down mass spectrometry by cyclizing stretches of the protein sequence. In order to produce and release detectable product ions that contribute to the assignment of proteoforms, regions of a protein encapsulated by disulfide bonds require two fragmentation events: cleavage of the protein backbone and cleavage of the disulfide bond. Traditional collisional activation methods do not cleave disulfide bonds efficiently, often leading to low sequence coverage of proteins that incorporate this feature. To address this challenge, we have evaluated the fragmentation pathways enabled by 193 nm ultraviolet photodissociation (UVPD) and UVPD coupled to electron transfer dissociation for the characterization of protein structures incorporating disulfide bonds. Cleavage of disulfide bonds by either approach results in S-S and C-S dissociation products that result from a combination of homolytic cleavage and hydrogen-transfer processes. Characterization of these product ions elevates interpretation of complex top-down spectra of proteins that incorporate disulfide bonds.

Native top-down mass spectrometry for higher-order structural characterization of proteins and complexes

Progress in structural biology research has led to a high demand for powerful and yet complementary analytical tools for structural characterization of proteins and protein complexes. This demand has significantly increased interest in native mass spectrometry (nMS), particularly native top-down mass spectrometry (nTDMS) in the past decade. This review highlights recent advances in nTDMS for structural research of biological assemblies, with a particular focus on the extra multi-layers of information enabled by TDMS. We include a short introduction of sample preparation and ionization to nMS, tandem fragmentation techniques as well as mass analyzers and software/analysis pipelines used for nTDMS. We highlight unique structural information offered by nTDMS and examples of its broad range of applications in proteins, protein-ligand interactions (metal, cofactor/drug, DNA/RNA, and protein), therapeutic antibodies and antigen-antibody complexes, membrane proteins, macromolecular machineries (ribosome, nucleosome, proteosome, and viruses), to endogenous protein complexes. The challenges, potential, along with perspectives of nTDMS methods for the analysis of proteins and protein assemblies in recombinant and biological samples are discussed.

Tandem Trapped Ion Mobility Spectrometry/Mass Spectrometry (tTIMS/MS) Reveals Sequence-Specific Determinants of Top-Down Protein Fragment Ion Cross Sections

Top-down proteomics provides a straightforward approach to the level of proteoforms but remains technologically challenging. Using ion mobility spectrometry/mass spectrometry (IMS/MS) to separate top-down fragment ions improves signal/noise and dynamic range. Such applications, however, do not yet leverage the primary information obtained from IMS/MS, which is the characterization of the fragment ion structure by the measured momentum transfer cross sections. Here, we perform top-down analysis of intact proteins and assemblies using our tandem trapped ion mobility spectrometer/mass spectrometer (tTIMS/MS) and compile over 1400 cross section values of fragment ions. Our analysis reveals that most fragment ions exhibit multiple, stable conformations similar to those of intact polypeptides and proteins. The data further indicate that the conformational heterogeneity is strongly influenced by the amino acid sequences of the fragment ions. Moreover, time-resolved tTIMS/MS experiments reveal that conformations of top-down fragment ions can be metastable on the timescale of ion mobility measurements. Taken together, our analysis indicates that top-down fragment ions undergo a folding process in the gas phase and that this folding process can lead to kinetic trapping of intermediate states in ion mobility measurements. Hence, because the folding free energy surface of a polypeptide ion is encoded by its amino acid sequence and charge state, our analysis suggests that cross sections can be exploited as sequence-specific determinants of top-down fragment ions.

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.

2-Pyridine Carboxaldehyde for Semi-Automated Soft Spot Identification in Cyclic Peptides

Cyclic peptides are an attractive option as therapeutics due to their ability to disrupt crucial protein-protein interactions and their flexibility in display type screening strategies, but they come with their own bioanalytical challenges in metabolite identification. Initial amide hydrolysis of a cyclic peptide results in a ring opening event in which the sequence is linearized. Unfortunately, the mass of the singly hydrolyzed sequence is the same (M + 18.0106 Da) irrespective of the initial site of hydrolysis, or soft spot. Soft spot identification at this point typically requires time-consuming manual interpretation of the tandem mass spectra, resulting in a substantial bottleneck in the hit to lead process. To overcome this, derivatization using 2-pyridine carboxaldehyde, which shows high selectivity for the alpha amine on the N-terminus, was employed. This strategy results in moderate- to high-efficiency derivatization with a unique mass tag and diagnostic ions that serve to highlight the first amino acid in the newly linearized peptide. The derivatization method and analytical strategy are demonstrated on a whole cell lysate digest, and the soft spot identification strategy is shown with two commercially available cyclic peptides: JB1 and somatostatin. Effective utilization of the automated sample preparation and interpretation of the resulting spectra shown here will serve to reduce the hit-to-lead time for generating promising proteolytically stable peptide candidates.

Characterization of large intact protein ions by mass spectrometry: What directions should we follow?

Theoretically, the gas-phase interrogation of whole proteoforms via mass spectrometry, known as top-down proteomics, bypasses the protein inference problem that afflicts peptide-centric proteomic approaches. Despite this obvious advantage, the application of top-down proteomics remains rare, mainly due to limited throughput and difficulty of analyzing proteins >30 kDa. Here we will discuss some of the problems encountered during the characterization of large proteoforms, and guided by a combination of theoretical background and experimental evidence we will describe some innovative data acquisition strategies and novel mass spectrometry technologies that can at least partially overcome such limitations.

Enhanced Top-Down Protein Characterization with Electron Capture Dissociation and Cyclic Ion Mobility Spectrometry

Tandem mass spectrometry of denatured, multiply charged high mass protein precursor ions yield extremely dense spectra with hundreds of broad and overlapping product ion isotopic distributions of differing charge states that yield an elevated baseline of unresolved "noise" centered about the precursor ion. Development of mass analyzers and signal processing methods to increase mass resolving power and manipulation of precursor and product ion charge through solution additives or ion-ion reactions have been thoroughly explored as solutions to spectral congestion. Here, we demonstrate the utility of electron capture dissociation (ECD) coupled with high-resolution cyclic ion mobility spectrometry (cIMS) to greatly increase top-down protein characterization capabilities. Congestion of protein ECD spectra was reduced using cIMS of the ECD product ions and "mobility fractions", that is, extracted mass spectra for segments of the 2D mobiligram (m/z versus drift time). For small proteins, such as ubiquitin (8.6 kDa), where mass resolving power was not the limiting factor for characterization, pre-IMS ECD and mobility fractions did not significantly increase protein sequence coverage, but an increase in the number of identified product ions was observed. However, a dramatic increase in performance, measured by protein sequence coverage, was observed for larger and more highly charged species, such as the +35 charge state of carbonic anhydrase (29 kDa). Pre-IMS ECD combined with mobility fractions yielded a 135% increase in the number of annotated isotope clusters and a 75% increase in unique product ions compared to processing without using the IMS dimension. These results yielded 89% sequence coverage for carbonic anhydrase.

Improving the Center Section Sequence Coverage of Large Proteins Using Stepped-Fragment Ion Protection Ultraviolet Photodissociation

Ultraviolet photodissociation (UVPD) mass spectrometry has gained attention in recent years for its ability to provide high sequence coverage of intact proteins. However, secondary dissociation of fragment ions, in which fragment ions subjected to multiple laser pulses decompose into small products, is a common phenomenon during UVPD that contributes to limited coverage in the midsection of protein sequences. To counter secondary dissociation, a method involving the application of notched waveforms to modulate the trajectories of fragment ions away from the laser beam, termed fragment ion protection (FIP), was previously developed to reduce the probability of secondary dissociation. This, in turn, increased the number of identified large fragment ions. In the present study, FIP was applied to UVPD of large proteins ranging in size from 29 to 55 kDa, enhancing the abundances of large fragment ions. A stepped-FIP strategy was implemented in which UVPD mass spectra were collected using multiple different amplitudes of the FIP waveforms and then the results from the mass spectra were combined. By using stepped-FIP, the number of fragment ions in the midsections of the sequences increased for all proteins. For example, whereas no fragment ions were identified in the middle section of the sequence for glutamate dehydrogenase (55 kDa, 55+ charge state), 10 sequence ions were identified by using UVPD-FIP.

Fourier-transform ion cyclotron resonance mass spectrometry for characterizing proteoforms

Proteoforms contribute functional diversity to the proteome and aberrant proteoforms levels have been implicated in biological dysfunction and disease. Fourier-transform ion cyclotron resonance mass spectrometry (FT-ICR MS), with its ultrahigh mass-resolving power, mass accuracy, and versatile tandem MS capabilities, has empowered top-down, middle-down, and native MS-based approaches for characterizing proteoforms and their complexes in biological systems. Herein, we review the features which make FT-ICR MS uniquely suited for measuring proteoform mass with ultrahigh resolution and mass accuracy; obtaining in-depth proteoform sequence coverage with expansive tandem MS capabilities; and unambiguously identifying and localizing post-translational and noncovalent modifications. We highlight examples from our body of work in which we have quantified and comprehensively characterized proteoforms from cardiac and skeletal muscle to better understand conditions such as chronic heart failure, acute myocardial infarction, and sarcopenia. Structural characterization of monoclonal antibodies and their proteoforms by FT-ICR MS and emerging applications, such as native top-down FT-ICR MS and high-throughput top-down FT-ICR MS-based proteomics at 21 T, are also covered. Historically, the information gleaned from FT-ICR MS analyses have helped provide biological insights. We predict FT-ICR MS will continue to enable the study of proteoforms of increasing size from increasingly complex endogenous mixtures and facilitate the benchmarking of sensitive and specific assays for clinical diagnostics. © 2020 John Wiley & Sons Ltd. Mass Spec Rev.

Utilization of Fast Photochemical Oxidation of Proteins and Both Bottom-up and Top-down Mass Spectrometry for Structural Characterization of a Transcription Factor-dsDNA Complex

A combination of covalent labeling techniques and mass spectrometry (MS) is currently a progressive approach for deriving insights related to the mapping of protein surfaces or protein-ligand interactions. In this study, we mapped an interaction interface between the DNA binding domain (DBD) of FOXO4 protein and the DNA binding element (DAF16) using fast photochemical oxidation of proteins (FPOP). Residues involved in protein-DNA interaction were identified using the bottom-up approach. To confirm the findings and avoid a misinterpretation of the obtained data, caused by possible multiple radical oxidations leading to the protein surface alteration and oxidation of deeply buried amino acid residues, a top-down approach was employed for the first time in FPOP analysis. An isolation of singly oxidized ions enabled their gas-phase separation from multiply oxidized species followed by CID and ECD fragmentation. Application of both fragmentation techniques allowed generation of complementary fragment sets, out of which the regions shielded in the presence of DNA were deduced. The findings obtained by bottom-up and top-down approaches were highly consistent. Finally, FPOP results were compared with those of the HDX study of the FOXO4-DBD·DAF16 complex. No contradictions were found between the methods. Moreover, their combination provides complementary information related to the structure and dynamics of the protein-DNA complex. Data are available via ProteomeXchange with identifier PXD027624.

Characterization of Structural Hemoglobin Variants by Top-Down Mass Spectrometry and R Programming Tools for Rapid Identification

Hemoglobinopathies are one of the most prevalent genetic disorders, affecting millions throughout the world. These are caused by pathogenic variants in genes that control the production of hemoglobin (Hb) subunits. As the number of known Hb variants has increased, it has become more challenging to obtain unambiguous results from routine chromatographic assays employed in the clinical laboratory. Top-down proteomic analysis of Hb by mass spectrometry is a definitive method to directly characterize the sequences of intact subunits. Here, we apply "chimeric ion loading" to characterize Hb β subunit variants. In this technique, product ions derived from complementary dissociation techniques are accumulated in a multipole storage device before delivery to a 21 T Fourier-transform ion cyclotron resonance mass spectrometer for simultaneous detection. To further improve the efficiency of identification of Hb variants and localization of the mutation site(s), we developed an R programming script, "Variants Identifier", to search top-down data against a database containing accurate intact mass differences and diagnostic ions from investigated Hb variants. A second R script, "PredictDiag", was developed and employed to determine relevant diagnostic ions for additional Hb variants with known sequences. These two R scripts were successfully applied to the identification of a Hb δ-β fusion protein and other Hb variants. The combination of chimeric ion loading and the above R scripts enables rapid and reliable interpretation of top-down mass spectrometry data, regardless of activation type, for Hb variant identification.

Proteoform Identification and Quantification Using Intact Protein Database Search Engine ProteinGoggle

Proteomics studies the proteome of organisms, especially proteins that are differentially expressed under certain physiological or pathological conditions; qualitative identification of protein sequences and posttranslational modifications (PTMs) and their positions can help us systematically understand the structure and function of proteoforms. With the development and relative popularity of soft ionization technology (such as electrospray ionization technology) and high mass measurement accuracy and high-resolution mass spectrometers (such as orbitrap), the mass spectrometry (MS) characterization of complete proteins (the so-called top-down proteomics) has become possible and has gradually become popular. Corresponding database search engines and protein identification bioinformatics tools have also been greatly developed. This chapter provides a brief overview of intact protein database search algorithm "isotopic mass-to-charge ratio and envelope fingerprinting" and search engine ProteinGoggle.

Membrane Ultrafiltration-Based Sample Preparation Method and Sheath-Flow CZE-MS/MS for Top-Down Proteomics

Mass spectrometry (MS)-based denaturing top-down proteomics (dTDP) identify proteoforms without pretreatment of enzyme proteolysis. A universal sample preparation method that can efficiently extract protein, reduce sample loss, maintain protein solubility, and be compatible with following up liquid-phase separation, MS, and tandem MS (MS/MS) is vital for large-scale proteoform characterization. Membrane ultrafiltration (MU) was employed here for buffer exchange to efficiently remove the sodium dodecyl sulfate (SDS) detergent in protein samples used for protein extraction and solubilization, followed by capillary zone electrophoresis (CZE)-MS/MS analysis. The MU method showed good protein recovery, minimum protein bias, and nice compatibility with CZE-MS/MS. Single-shot CZE-MS/MS analysis of an Escherichia coli sample prepared by the MU method identified over 800 proteoforms.

Influence of Primary Structure on Fragmentation of Native-Like Proteins by Ultraviolet Photodissociation

Analysis of native-like protein structures in the gas phase via native mass spectrometry and auxiliary techniques has become a powerful tool for structural biology applications. In combination with ultraviolet photodissociation (UVPD), native top-down mass spectrometry informs backbone flexibility, topology, hydrogen bonding networks, and conformational changes in protein structure. Although it is known that the primary structure affects dissociation of peptides and proteins in the gas phase, its effect on the types and locations of backbone cleavages promoted by UVPD and concomitant influence on structural characterization of native-like proteins is not well understood. Here, trends in the fragmentation of native-like proteins were evaluated by tracking the propensity of 10 fragment types (a, a+1, b, c, x, x+1, y, y-1, Y, and z) in relation to primary structure in a native-top down UVPD data set encompassing >9600 fragment ions. Differing fragmentation trends are reported for the production of distinct fragment types, attributed to a combination of both direct dissociation pathways from excited electronic states and those surmised to involve intramolecular vibrational energy redistribution after internal conversion. The latter pathways were systematically evaluated to evince the role of proton mobility in the generation of "CID-like" fragments through UVPD, providing pertinent insight into the characterization of native-like proteins. Fragmentation trends presented here are envisioned to enhance analysis of the protein higher-order structure or augment scoring algorithms in the high-throughput analysis of intact proteins.