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

Top/Middle-Down Characterization of α-Synuclein Glycoforms

α-Synuclein is an intrinsically disordered protein that plays a critical role in the pathogenesis of neurodegenerative disorders, such as Parkinson's disease. Proteomics studies of human brain samples have associated the modification of the O-linked N-acetyl-glucosamine (O-GlcNAc) to several synucleinopathies; in particular, the position of the O-GlcNAc can regulate protein aggregation and subsequent cell toxicity. There is a need for site specific O-GlcNAc α-synuclein screening tools to direct better therapeutic strategies. In the present work, for the first time, the potential of fast, high-resolution trapped ion mobility spectrometry (TIMS) preseparation in tandem with mass spectrometry assisted by an electromagnetostatic (EMS) cell, capable of electron capture dissociation (ECD), and ultraviolet photodissociation (213 nm UVPD) is illustrated for the characterization of α-synuclein positional glycoforms: T72, T75, T81, and S87 modified with a single O-GlcNAc. Top-down 213 nm UVPD and ECD MS/MS experiments of the intact proteoforms showed specific product ions for each α-synuclein glycoforms associated with the O-GlcNAc position with a sequence coverage of ∼68 and ∼82%, respectively. TIMS-MS profiles of α-synuclein and the four glycoforms exhibited large structural heterogeneity and signature patterns across the 8+-15+ charge state distribution; however, while the α-synuclein positional glycoforms showed signature mobility profiles, they were only partially separated in the mobility domain. Moreover, a middle-down approach based on the Val40-Phe94 (55 residues) chymotrypsin proteolytic product using tandem TIMS-q-ECD-TOF MS/MS permitted the separation of the parent positional isomeric glycoforms. The ECD fragmentation of the ion mobility and m/z separated isomeric Val40-Phe94 proteolytic peptides with single O-GlcNAc in the T72, T75, T81, and S87 positions provided the O-GlcNAc confirmation and positional assignment with a sequence coverage of ∼80%. This method enables the high-throughput screening of positional glycoforms and further enhances the structural mass spectrometry toolbox with fast, high-resolution mobility separations and 213 nm UVPD and ECD fragmentation capabilities.

Resolving D-Amino Acid Containing Peptides Using Ion Mobility-Mass Spectrometry: Challenges and Recent Developments

Peptides and proteins having D-amino acids in their sequences are now believed to be widespread among different living organisms. Their significance is attributed to the diverse functions of these molecules, such as having a certain pathological implication or enhancing biological activity. Indeed, some peptide molecules with D-amino acids in their structure have already found their way to clinical use such as the antibacterial gramicidin and the antidiabetic nateglinide. Ion mobility mass spectrometry (IM-MS) added an additional dimension of separation as it depends on ions mobility in the space, which is dependent on their shapes, and the shape depends on the orientation of atoms. Thus, D-amino acids containing peptides (DAACPs) will have different mobility and collision cross-section values than those with L-amino acids. Eventually, this will lead to baseline separation of the two peptides. Additionally, ion mobility can precisely locate the position of D-amino acids by analyzing the difference in the arrival times of the fragment ions. The importance of DAACPs, as well as the difficulties in discovering them, were addressed in this review. Similarly, we emphasized how recent developments in IM-MS have improved their detection and analysis. Consequently, the LC-IM-MS/MS platform appears to be promising in isomeric mixture analysis.

Implementation of charged microdroplet-based derivatization of bile acids on a cyclic ion mobility spectrometry-mass spectrometry platform

Herein, we report the first implementation of charged microdroplet-based derivatization on a commercially-available cyclic ion mobility spectrometry-mass spectrometry platform. We have demonstrated the potential of our approach to improve separability of challenging isomers, but more importantly to rapidly screen derivatization reactions through droplet chemistry. Additionally, the use of cyclic ion mobility separations and tandem mass spectrometry reveals insights into product formation that would be lost with single stage mass spectrometry. Overall, we anticipate broad utility of our methodology owing to the simple design and setup for performing these droplet-based reactions and future work coupling these reactions online with liquid chromatography.

Enhancing the Depth of Analyses with Next-Generation Ion Mobility Experiments

Recent developments in ion mobility (IM) technology have expanded the capability to separate and characterize gas-phase ions of biomolecules, especially when paired with mass spectrometry. This next generation of IM technology has been ushered in by creative innovation focused on both instrument architectures and how electric fields are applied. In this review, we focus on the application of high-resolution and multidimensional IM to biomolecular analyses, encompassing the fields of glycomics, lipidomics, peptidomics, and proteomics. We highlight selected research that demonstrates the application of the new IM toolkit to challenging biomolecular systems. Through our review of recently published literature, we outline the current strengths of respective technologies and perspectives for future applications.

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.

General Method to Obtain Collision Cross-Section Values in Multipass High-Resolution Cyclic Ion Mobility Separations

In recent years, ion mobility spectrometry-mass spectrometry (IMS-MS) has advanced the field of omics-based research, especially with the development of high-resolution platforms; however, these separations have generally been qualitative in nature. The rotationally averaged ion neutral collision cross section (CCS) is one of the only quantitative metrics available for aiding in characterizing biomolecules in IMS-MS. However, determining the CCS of an ion for multipass IMS systems, such as in cyclic ion mobility-mass spectrometry (cIMS-MS) and structures for lossless ion manipulations, has been challenging due to the lack of methods available for calculating CCS when more than a single pass is required for separation as well as the laborious nature of requiring calibrants and unknown compounds to be subjected to identical number of passes, which may not be possible in certain instances because of peak splitting, high levels of diffusion, etc. Herein, we present a general method that uses average ion velocities for calculating CCS values in cIMS-MS-based separations. Initially, we developed calibration curves using common CCS calibrants [i.e., tetra-alkylammonium salts, polyalanine, and hexakis(fluoroalkoxy)phosphazines] at different traveling wave (TW) conditions and the calculated cIMS CCS values were within ∼1% error or less compared to previously established drift tube IMS CCS measurements. Since it has been established that glycans can split into their α/β anomers, we utilized this method for two glycan species, 2α-mannobiose and melibiose. Both glycans were analyzed at the same TW conditions as the calibrants, and we observed anomer splitting at pathlengths of 20 m for 2α-mannobiose and 40 m for melibiose and thus assigned two unique CCS values for each glycan, which is the first time this has ever been done. We have demonstrated that the use of average ion velocities is a robust approach for obtaining CCS values with good agreement to CCS measurements from the previous literature and anticipate that this methodology can be applied to any IMS-MS platform that utilizes multipass separations. Our future work aims to incorporate this methodology for the development of a high-resolution CCS database to aid in the characterization of human milk oligosaccharides.

Hydrogen-Deuterium-Exchange-Based Mass Distribution Shifts in High-Resolution Cyclic Ion Mobility Separations

The mass distribution of ions influences separations in ion mobility spectrometry-mass spectrometry (IMS-MS). Herein, we introduce a method to induce mass distribution shifts for various analytes using hydrogen-deuterium exchange (HDX) immediately prior to ionization using a dual syringe approach. By replacing labile hydrogens on analytes with deuteriums, we were able to differentiate isomers using separations of isotopologues. For each analyte studied, every possible level of deuteration (from undeuterated to fully deuterated) was generated and then separated using cyclic ion mobility spectrometry-mass spectrometry (cIMS-MS). The information gained from such separations (relative arrival times; t_Rel. values) was found to be orthogonal to conventional IMS-MS separations. Additionally, the observed shifts were linearly additive with increasing deuteration, suggesting that this methodology could be extended to analytes with a larger number of labile hydrogens. For one isomer pair, as few as two deuteriums were able to produce a large enough mass distribution shift to differentiate isomers. In another experiment, we found that the mass distribution shift was large enough to overcome the reduced mass contribution, resulting in a "flipped" arrival time where the heavier deuterated isotopologue arrived before the lighter one. In this work, we present a proof-of-concept demonstration that mass-distribution-based shifts, t_Rel. values, could potentially act as an added dimension to characterize molecules in IMS-MS. We anticipate, along with future work in this area, that mass-distribution-based shifts could enable the identification of unknown molecules through a database-driven approach in an analogous fashion to collision cross section (CCS) measurements.

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.

Developments in Trapped Ion Mobility Mass Spectrometry to Probe the Early Stages of Peptide Aggregation

Ion mobility mass spectrometry (IM-MS) has proven to be an excellent method to characterize the structure of amyloidogenic protein and peptide aggregates, which are formed in coincidence with the development of neurodegenerative diseases. However, it remains a challenge to obtain detailed structural information on all conformational intermediates, originating from the early onset of those pathologies, due to their complex and heterogeneous environment. One way to enhance the insights and the identification of these early stage oligomers is by employing high resolution ion mobility mass spectrometry experiments. This would allow us to enhance the mobility separation and MS characterization. Trapped ion mobility spectrometry (TIMS) is an ion mobility technique known for its inherently high resolution and has successfully been applied to the analysis of protein conformations among others. To obtain conformational information on fragile peptide aggregates, the instrumental parameters of the TIMS-Quadrupole-Time-of-Flight mass spectrometer (TIMS-qToF-MS) have to be optimized to allow the study of intact aggregates and ensure their transmission toward the detector. Here, we investigate the suitability and application of TIMS to probe the aggregation process, targeting the well-characterized M307-N319 peptide segment of the TDP-43 protein, which is involved in the development of amyotrophic lateral sclerosis. By studying the influence of key parameters over the full mass spectrometer, such as source temperature, applied voltages or RFs among others, we demonstrate that by using an optimized instrumental method TIMS can be used to probe peptide aggregation.

Assessing the Use of Host-Guest Chemistry in Conjunction with Cyclic Ion Mobility Separations for the Linkage-Specific Characterization of Human Milk Oligosaccharides

Human milk oligosaccharides (HMOs) are a class of glycans that are highly abundant in human milk and contribute to the healthy growth of an infant's immune system. While new advancements in analytical methodologies have been made in glycomics, the high degree of isomeric heterogeneity and lack of authentic standards have made the high-resolution separation and accurate characterization of linkage positioning of all HMO species very challenging. Herein, we present an evaluation of the use of host-guest chemistry in conjunction with cyclic ion mobility spectrometry-mass spectrometry (cIMS-MS)-based separations for the identification of linkage positioning in three pairs of di-, tetra-, and hexasaccharide HMO isomers that only differ in the positioning of one glycosidic linkage (β1,3 versus β1,4). Suitable hosts, such as α/β cyclodextrins, cucurbit[n]urils (n = 5, 7), crown ethers, cyclic peptides, and an ionophore, were used to assess host-guest inclusion complex formation as well as linkage-specific cIMS-MS trends. Our results indicated a linkage-specific trend for the [M + 2α + 2H]2+ cyclodextrin-based host-guest inclusion complexes where the β1,3 linkage-containing isomers were always higher mobility than the β1,4 linkage-containing ones as well one for the [M + α + β + 2H]2+ complexes where the β1,4 linkage-containing isomers were always higher mobility than the β1,3 linkage-containing ones. We also observed diagnostic mobility fingerprints for the cucurbituril-based complexes. We anticipate that linkage-specific and mobility fingerprint trends can potentially aid in identifying linkage positioning for other HMO isomers as well as in complex human milk samples.

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.