Negative Polarity Helium Charge Transfer Dissociation Tandem Mass Spectrometry: Radical-Initiated Fragmentation of Complex Polysulfated Anions

Radiation-Induced Molecular Processes in DNA: A Perspective on Gas-Phase Interaction Studies

Studying the direct effects of DNA irradiation is essential for understanding the impact of radiation on biological systems. Gas-phase interactions are especially well suited to uncover the molecular mechanisms underlying these direct effects. Only relatively recently, isolated DNA oligonucleotides were irradiated by ionizing particles such as VUV or X-ray photons or ion beams, and ionic products were analyzed by mass spectrometry. This article provides a comprehensive review of primarily experimental investigations in this field over the past decade, emphasizing the description of processes such as ionization, fragmentation, charge and hydrogen transfer triggered by photoabsorption or ion collision, and the recent progress made thanks to specific atomic photoabsorption. Then, we outline ongoing experimental developments notably involving ion-mobility spectrometry, crossed beams or time-resolved measurements. The discussion extends to potential research directions for the future.

Installation protocol for charge transfer dissociation mass spectrometry on ion trapping mass spectrometers

RATIONALE

Charge transfer dissociation (CTD) is a novel fragmentation technique that demonstrates enhanced structural characterization for a wide variety of molecules compared to standard fragmentation techniques like collision-induced dissociation (CID). Alternative fragmentation techniques, such as electron transfer dissociation, electron capture dissociation, and ultraviolet photodissociation, also overcome many of the shortfalls of CID, but none of them are a silver bullet that can adequately characterize a wide variety of structures and charge states of target compounds. Given the diversity of structural classes and their occasional obstinance towards certain activation techniques, alternative fragmentation techniques are required that rely on novel or alternative modes of activation.

METHODS

Herein, we present a step-by-step protocol for the installation of CTD on a quadrupole ion trap mass spectrometer and best practices for optimizing the signal-to-noise ratio and acquisition times for CTD mass spectra.

RESULTS

In addition to two CTD installations in the Jackson laboratory, CTD has also been installed, and is currently in operation, on two 3D ion trap mass spectrometers in France: one in the laboratory of Dr. David Ropartz and Dr. Hélène Rogneaux at INRAE in Nantes, and the other in the laboratory of Dr. Jean-Yves Salpin at Université d'Évry Val-d'Essonne, part of the Paris-Saclay University system.

CONCLUSIONS

Here, we provide a visual protocol to help others accomplish the instrument modification.

Radiation-Induced Transfer of Charge, Atoms, and Energy within Isolated Biomolecular Systems

In biological tissues, ionizing radiation interacts with a variety of molecules and the consequences include cell killing and the modification of mechanical properties. Applications of biological radiation action are for instance radiotherapy, sterilization, or the tailoring of biomaterial properties. During the first femtoseconds to milliseconds after the initial radiation action, biomolecular systems typically respond by transfer of charge, atoms, or energy. In the condensed phase, it is usually very difficult to distinguish direct effects from indirect effects. A straightforward solution for this problem is the use of gas-phase techniques, for instance from the field of mass spectrometry. In this review, we survey mainly experimental but also theoretical work, focusing on radiation-induced intra- and inter-molecular transfer of charge, atoms, and energy within biomolecular systems in the gas phase. Building blocks of DNA, proteins, and saccharides, but also antibiotics are considered. The emergence of general processes as well as their timescales and mechanisms are highlighted.

Structural characterization of human milk oligosaccharides using ultrahigh performance liquid chromatography-helium charge transfer dissociation mass spectrometry

The combination of helium charge transfer dissociation mass spectrometry (He-CTD-MS) with ultrahigh performance liquid chromatography (UHPLC) is presented for the analysis of a complex mixture of acidic and neutral human milk oligosaccharides (HMOs). The research focuses on the identification of the monosaccharide sequence, the branching patterns, the sialylation/fucosylation arrangements, and the differentiation of isomeric oligosaccharides in the mixture. Initial studies first optimized the conditions for the UHPLC separation and the He-CTD-MS conditions. Results demonstrate that He-CTD is compatible with UHPLC timescales and provides unambiguous glycosidic and cross-ring cleavages from both the reducing and the nonreducing ends, which is not typically possible using collision-induced dissociation. He-CTD produces informative fragments, including 0,3An and 0,4An ions, which have been observed with electron transfer dissociation, electron detachment dissociation, and ultraviolet photodissociation (UVPD) and are crucial for differentiating the α-2,3- versus α-2,6-linked sialic acid (Neu5Ac) residues present among sialyllacto-N-tetraose HMOs. In addition to the linkage positions, He-CTD is able to differentiate structural isomers for both sialyllacto-N-tetraoses and lacto-N-fucopentaoses structures by providing unique, unambiguous cross-ring cleavages of types 0,2An, 0,2Xn, and 1,5An while preserving most of the labile Neu5Ac and fucose groups.

Structural Characterization of Natural and Synthetic Macrocycles Using Charge-Transfer Dissociation Mass Spectrometry

Research in natural products (NPs) has gained interest as drug developers turn to nature to combat problems with drug resistance, drug delivery, and emerging diseases. Whereas NPs offer a tantalizing source of new pharmacologically active compounds, their structural complexity presents a challenge for analytical characterization and organic synthesis. Of particular concern is the characterization of cyclic-, polycyclic-, or macrocyclic compounds. One example of endogenous compounds as inspiration for NP development are cobalamins, like vitamin B₁₂. An example of exogenous NPs is the class of macrolides that includes erythromycin. Both classes of macrocycles feature analogues with a range of modifications on their macrocyclic cores, but because of their cyclic nature, they are generally resistant to fragmentation by collision-induced dissociation (CID). In the present work, charge-transfer dissociation (CTD) was employed, with or without supplemental collisional activation, to produce radical-driven, high-energy fragmentation products of different macrocyclic precursors. With the assistance of collisional activation of CTnoD products, CTD frequently cleaved two covalent bonds within the macrocycle cores to reveal rich, informative spectra that helped identify sites of modification and resolve structural analogues. In a third example of macrocycle fragmentation, CTD enabled an impurity in a biological sample to be characterized as a cyclic polymer of nylon-6,6. In each example, CTD spectra are starkly different from CID and are highly reminiscent of other high-energy fragmentation techniques like extreme ultraviolet dissociative photoionization (XUV-DPI) and electron ionization-induced dissociation (EID). The results indicate that CTD-MS is a useful tool for the characterization of natural and synthetic macrocycles.

In-depth structural characterization of oligosaccharides released by GH107 endofucanase MfFcnA reveals enzyme subsite specificity and sulfated fucan substructural features

The extracellular matrix of brown algae represents an abundant source of fucose-containing sulfated polysaccharides (FCSPs). FCSPs include sulfated fucans, essentially composed of fucose, and highly heterogeneous fucoidans, comprising various monosaccharides. Despite a range of potentially valuable biological activities, the structures of FCSPs are only partially characterized and enzymatic tools leading to their deconstruction are rare. Previously, the enzyme MfFcnA was isolated from the marine bacterium Mariniflexile fucanivorans and biochemically characterized as an endo-α-1 → 4-l-fucanase, the first member of glycoside hydrolase family 107. Here, MfFcnA was used as an enzymatic tool to deconstruct the structure of the sulfated fucans from Pelvetia canaliculata (Fucales brown alga). Oligofucans released by MfFcnA at different time points were characterized using mass spectrometry coupled with liquid chromatography and tandem mass spectrometry through Charge Transfer Dissociation. This approach highlights a large diversity in the structures released. In particular, the analyses show the presence of species with less than three sulfates per two fucose residues. They also reveal species with monosaccharides other than fucose and the occurrence of laterally branched residues. Precisely, the lateral branching is either in the form of a hexose accompanied by a trisulfated fucose nearby, or of a side chain of fucoses with a pentose as the branching point on the polymer. Overall, the results indicate that the structure of sulfated fucans from P. canaliculata is more complex than expected. They also reveal the interesting capacity of MfFcnA to accommodate different substrates, leading to structurally diverse oligofucan products that potentially could be screened for bioactivities.

Combination of High-Resolution Multistage Ion Mobility and Tandem MS with High Energy of Activation to Resolve the Structure of Complex Chemoenzymatically Synthesized Glycans

Carbohydrates, in particular microbial glycans, are highly structurally diverse biomolecules, the recognition of which governs numerous biological processes. Of special interest, glycans of known monosaccharide composition feature multiple possible isomers, differentiated by the anomerism and position of their glycosidic linkages. Robust analytical tools able to circumvent this extreme structural complexity are increasing in demand to ensure not only the correct determination of naturally occurring glycans but also to support the rapid development of enzymatic and chemoenzymatic glycan synthesis. In support to the later, we report the use of complementary strategies based on mass spectrometry (MS) to evaluate the ability of 14 engineered mutants of sucrose-utilizing α-transglucosylases to produce type/group-specific Shigella flexneri pentasaccharide bricks from a single lightly protected non-natural tetrasaccharide acceptor substrate. A first analysis of the reaction media by UHPLC coupled to high-accuracy MS led to detect six reaction products of enzymatic glucosylation out of the eight possible ones. A seventh structure was evidenced by an additional step of ion mobility at a resolving power (R_p) of approximately 100. Finally, a R_p of about 250 in ion mobility made it possible to detect the eighth and last of the expected structures. Complementary to these measurements, tandem MS with high activation energy charge transfer dissociation (CTD) allowed us to unambiguously characterize seven regioisomers out of the eight possible products of enzymatic glucosylation. This work illustrates the potential of the recently described powerful IMS and CTD-MS methods for the precise structural characterization of complex glycans.

Charge transfer dissociation of a branched glycan with alkali and alkaline earth metal adducts

Alkali and alkaline earth metal adducts of a branched glycan, XXXG, were analyzed with helium charge transfer dissociation (He-CTD) and low-energy collision-induced dissociation (LE-CID) to investigate if metalation would impact the type of fragments generated and the structural characterization of the analyte. The studied adducts included 1+ and 2+ precursors involving one or more of the cations: H⁺ , Na⁺ , K⁺ , Ca²⁺ , and Mg²⁺ . Regardless of the metal adduct, He-CTD generated abundant and numerous glycosidic and cross-ring cleavages that were structurally informative and able to identify the 1,4-linkage and 1,6-branching patterns. In contrast, the LE-CID spectra mainly contained glycosidic cleavages, consecutive fragments, and numerous neutral losses, which complicated spectral interpretation. LE-CID of [M + K + H]²⁺ and [M + Na]⁺ precursors generated a few cross-ring cleavages, but they were not sufficient to identify the 1,4-linkage and 1,6-branching pattern of the XXXG xyloglucan. He-CTD predominantly generated 1+ fragments from 1+ precursors and 2+ product ions from 2+ precursors, although both LE-CID and He-CTD were able to generate 1+ product ions from 2+ adducts of magnesium and calcium. The singly charged fragments derive from the loss of H⁺ from the metalated product ions and the formation of a protonated complementary product ion; such observations are similar to previous reports for magnesium and calcium salts undergoing electron capture dissociation (ECD) activation. However, during He-CTD, the [M + Mg]²⁺ precursor generated more singly charged product ions than [M + Ca]²⁺ , either because Mg has a higher second ionization potential than Ca or because of conformational differences and the locations of the charging adducts during fragmentation. He-CTD of the [M + 2Na]²⁺ and the [M + 2 K]²⁺ precursors generated singly charged product ions from the loss of a sodium ion and potassium ion, respectively. In summary, although the metal ions influence the mass and charge state of the observed product ions, the metal ions had a negligible effect on the types of cross-ring cleavages observed.

Ultra-high-performance liquid chromatography charge transfer dissociation mass spectrometry (UHPLC-CTD-MS) as a tool for analyzing the structural heterogeneity in carrageenan oligosaccharides

Ultra-high-performance liquid chromatography (UHPLC) with charge transfer dissociation mass spectrometry (CTD-MS) is presented for the analysis of a mixture of complex sulfated oligosaccharides. The mixture contained kappa (κ), iota (ι), and lambda (λ) carrageenans that contain anhydro bridges, different degrees of sulfation ranging from one to three per dimer, different positioning of the sulfate groups along the backbone, and varying degrees of polymerization (DP) between 4 and 12. Optimization studies using standard mixtures of carrageenans helped establish the optimal conditions for online UHPLC-CTD-MS/MS analysis. Optimization included (1) UHPLC conditions; (2) ion source conditions, such as the capillary voltage, drying gas and nebulizing gas temperature, and flow rate; and (3) CTD-MS conditions, including data-dependent CTD-MS. The UHPLC-CTD results were contrasted with UHPLC-CID results of the same mixture on the same instrument. Whereas CID tends to produce B/Y and C/Z ions with many neutral losses, CTD produced more abundant A/X ions and less abundant neutral losses, which enabled more confident structural detail. The results demonstrate that He-CTD is compatible with the timescale of UHPLC and provides more structural information about carrageenans compared to state-of-the-art methods like UHPLC-CID analysis.

Anomeric Retention of Carbohydrates in Multistage Cyclic Ion Mobility (IMSn): De Novo Structural Elucidation of Enzymatically Produced Mannosides

Carbohydrates are complex structures that still challenge analysts today because of their different levels of isomerism, notably the anomerism of the glycosidic bond. It has been shown recently that anomerism is preserved upon gas-phase fragmentation and that high-resolution ion mobility (IMS) can distinguish anomers. However, these concepts have yet to be applied to complex biological products. We have used high-resolution IMS on a cyclic device to characterize the reaction products of Uhgb_MS, a novel mannoside synthase of the GH130 family. We designed a so-called IMSⁿ sequence consisting of (i) separating and isolating specific IMS peaks, (ii) ejecting ions to a pre-array store cell depending on their arrival time, (iii) inducing collisional activation upon reinjection, and (iv) performing multistage IMS analysis of the fragments. First, we applied IMS² sequences to purely linked α1,2- and β1,2-mannooligosaccharides, which provided us with reference drift times for fragments of known conformation. Then, we performed IMSⁿ analyses of enzymatically produced mannosides and, by comparison with the references, we succeeded in determining the intrachain anomerism of a α1,2-mannotriose and a mix-linked β/α1,2-mannotetraose-a first for a crude biological medium. Our results show that the anomerism of glycosides is maintained through multiple stages of collisional fragmentation, and that standalone high-resolution IMS and IMSⁿ can be used to characterize the intrachain anomerism in tri- and tetrasaccharides in a biological medium. This is also the first evidence that a single carbohydrate-active enzyme can synthesize both α- and β-glycosidic linkages.

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

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

Approaches to Investigate Selective Dietary Polysaccharide Utilization by Human Gut Microbiota at a Functional Level

The human diet is temporally and spatially dynamic, and influenced by culture, regional food systems, socioeconomics, and consumer preference. Such factors result in enormous structural diversity of ingested glycans that are refractory to digestion by human enzymes. To convert these glycans into metabolizable nutrients and energy, humans rely upon the catalytic potential encoded within the gut microbiome, a rich collective of microorganisms residing in the gastrointestinal tract. The development of high-throughput sequencing methods has enabled microbial communities to be studied with more coverage and depth, and as a result, cataloging the taxonomic structure of the gut microbiome has become routine. Efforts to unravel the microbial processes governing glycan digestion by the gut microbiome, however, are still in their infancy and will benefit by retooling our approaches to study glycan structure at high resolution and adopting next-generation functional methods. Also, new bioinformatic tools specialized for annotating carbohydrate-active enzymes and predicting their functions with high accuracy will be required for deciphering the catalytic potential of sequence datasets. Furthermore, physiological approaches to enable genotype-phenotype assignments within the gut microbiome, such as fluorescent polysaccharides, has enabled rapid identification of carbohydrate interactions at the single cell level. In this review, we summarize the current state-of-knowledge of these methods and discuss how their continued development will advance our understanding of gut microbiome function.

Structural Characterization of Isomeric Oligogalacturonan Mixtures Using Ultrahigh-Performance Liquid Chromatography-Charge Transfer Dissociation Mass Spectrometry

Pectins are natural polysaccharides made from galacturonic acid residues, and they are widely used as an excipient in food and pharmaceutical industries. The degree of methyl-esterification, the monomeric composition, and the linkage pattern are all important factors that influence the physical and chemical properties of pectins, such as the solubility. This work focuses on the successful online coupling of charge transfer dissociation-mass spectrometry (CTD-MS) with ultrahigh-performance liquid chromatography (UHPLC) to differentiate isomers of oligogalacturonans derived from citrus pectins. This work employed CTD fragmentation of the pectin mixtures in data-dependent acquisition mode. Compared to the UHPLC with collision-induced dissociation mass spectrometry (UHPLC-CID-MS), UHPLC-CTD-MS yielded fewer ambiguous ions and more structurally informative results. The developed UHPLC-CTD-MS method resulted in abundant cross-ring cleavages-and especially ^1,4X_n, ^1,5X_n, and ^2,4X_n ions-which helped to identify most of the isomers. The Gal A isomers differed only in the methyl group position along the galacturonic acid backbone. The combination of CTD in real time with UHPLC provides a new tool for the structural characterization of complex mixtures of oligogalacturonans and potentially other classes of oligosaccharides.

Structural Characterization of Sulfated Glycosaminoglycans Using Charge-Transfer Dissociation

Glycosaminoglycans (GAGs) participate in a broad range of physiological processes, and their structures are of interest to researchers in structural biology and medicine. Although they are abundant in tissues and extracellular matrices, their structural heterogeneity makes them challenging analytes. Mass spectrometry, and more specifically, tandem mass spectrometry, is particularly well suited for their analysis. Many tandem mass spectrometry techniques have been examined for their suitability toward the structural characterization of GAGs. Threshold activation methods such as collision-induced dissociation (CID) produce mainly glycosidic cleavages and do not yield a broad range of structurally informative cross-ring fragments. Considerable research efforts have been directed at finding other means of dissociating gas-phase GAG ions to produce more comprehensive structural information. Here, we compare the structural information on GAGs obtained by charge-transfer dissociation (CTD) and electron detachment dissociation (EDD). EDD has previously been applied to GAGs and is known to produce both glycosidic and cross-ring cleavages in similar abundance. CTD has not previously been used to analyze GAGs but has been shown to produce abundant cross-ring cleavages and no sulfate loss when applied to another class of sulfated carbohydrates like algal polysaccharides. In contrast to EDD, which is restricted to FTICR mass spectrometers, CTD can be implemented on other platforms, such as ion trap mass spectrometers (ITMS). Here, we show the capability of CTD-ITMS to produce structurally significant details of the sites of modification in both heparan sulfate (HS) and chondroitin sulfate (CS) standards ranging in length from degree of polymerization (dp) 4 to dp6. EDD and CTD both yield more structural information than CID and yield similar fractional abundances to one another for glycosidic fragments, cross-ring fragments, and neutral losses.

Discrimination of β-1,4- and β-1,3-Linkages in Native Oligosaccharides via Charge Transfer Dissociation Mass Spectrometry

The connection between monosaccharides influences the structure, solubility, and biological function of carbohydrates. Although tandem mass spectrometry (MS/MS) often enables the compositional identification of carbohydrates, traditional MS/MS fragmentation methods fail to generate abundant cross-ring fragments of intrachain monosaccharides that could reveal carbohydrate connectivity. We examined the potential of helium-charge transfer dissociation (He-CTD) as a method of MS/MS to decipher the connectivity of β-1,4- and β-1,3-linked oligosaccharides. In contrast to collision-induced dissociation (CID), He-CTD of isolated oligosaccharide precursors produced both glycosidic and cross-ring cleavages of each monosaccharide. The radical-driven dissociation in He-CTD induced single cleavage events, without consecutive fragmentations, which facilitated structural interpretation. He-CTD of various standards up to a degree of polymerization of 7 showed that β-1,4- and β-1,3-linked carbohydrates can be distinguished based on diagnostic ^3,5A fragment ions that are characteristic for β-1,4-linkages. Overall, fragment ion spectra from He-CTD contained sufficient information to infer the connectivity specifically for each glycosidic bond. When testing He-CTD to resolve the order of β-1,4- and β-1,3-linkages in mixed-linked oligosaccharide standards, He-CTD spectra sometimes provided less confident assignment of connectivity. Ion mobility spectrometry-mass spectrometry (IMS-MS) of the standards indicated that ambiguity in the He-CTD spectra was caused by isobaric impurities in the mixed-linked oligosaccharides. Radical-driven dissociation induced by He-CTD can thus expand MS/MS to carbohydrate linkage analysis, as demonstrated by the comprehensive fragment ion spectra on native oligosaccharides. The determination of connectivity in true unknowns would benefit from the separation of isobaric precursors, through UPLC or IMS, before linkage determination via He-CTD.

Characterization of New Oligosaccharides Obtained by An Enzymatic Cleavage of the Exopolysaccharide Produced by the Deep-Sea Bacterium Alteromonas infernus Using its Cell Extract

Bacteria from deep-sea hydrothermal vents constitute an attractive source of bioactive molecules. In particular, exopolysaccharides (EPS) produced by these bacteria become a renewable source of both biocompatible and biodegradable molecules. The low molecular weight (LMW) derivatives of the GY785 EPS produced by the deep-sea hydrothermal vent strain Alteromonas infernus have previously displayed some biological properties, similar to those of glycosaminoglycans (GAG), explored in cancer and tissue engineering. These GAG-mimetic derivatives are obtained through a free radical depolymerization process, which could, however, affect their structural integrity. In a previous study, we have shown that A. infernus produces depolymerizing enzymes active on its own EPS. In the present study, an enzymatic reaction was optimized to generate LMW derivatives of the GY785 EPS, which could advantageously replace the present bioactive derivatives obtained by a chemical process. Analysis by mass spectrometry of the oligosaccharide fractions released after enzymatic treatment revealed that mainly a lyase activity was responsible for the polysaccharide depolymerization. The repeating unit of the GY785 EPS produced by enzyme cleavage was then fully characterized.

Structure Determination of Large Isomeric Oligosaccharides of Natural Origin through Multipass and Multistage Cyclic Traveling-Wave Ion Mobility Mass Spectrometry

Carbohydrate isomers with identical atomic composition cannot be distinguished by mass spectrometry. By separating the ions according to their conformation in the gas phase, ion mobility (IM) coupled to mass spectrometry is an attractive approach to overcome this issue and extend the limits of mass spectrometry in structural glycosciences. Recent technological developments have significantly increased the resolving power of ion mobility separators. One such instrument features a cyclic traveling-wave IM separator integrated in a quadrupole/time-of-flight mass spectrometer. This system allows for multipass ion separations and for pre-, intra-, and post-IM fragmentation. In the present study, we utilize this system to explore a complex mixture of oligoporphyrans derived from the enzymatic digestion of the cell wall of the red alga P. umbilicalis. We are able to deduce their complete structure using IM arrival times and the m/z of specific fragments. This approach was successfully applied for sequencing of oligoporphyrans of up to 1500 Da and included the positioning of the methyl ether and sulfate groups. The structures defined in this study by IM-MS/MS agree with those found in the past but use much more time-consuming analytical approaches. This study also revealed some so far undescribed structures, present at very low abundance. In addition, the results made it possible to compare the abundance of the different isomers released by the enzyme and to draw further conclusions on the specificity of β-porphyranase and more particularly on its accommodation tolerance of anhydro-bridges in subsites. Finally, a separation of two isomers with very similar mobility was obtained after 58 passes around the cIM, with an estimated resolving power of 920 for these triply charged species, confirming the structures attributed to these two isomers.

Efficient Mass Spectrometric Dissection of Glycans via Gold Nanoparticle-Assisted in-Source Cation Adduction Dissociation

Structural identification of glycans is important but remains challenging, for which tandem mass spectrometry has evolved as an indispensable tool. However, it requires additional complex hardware and extra time for ion extraction. Herein, we report a straightforward approach called gold nanoparticles (AuNPs)-assisted in-source cation adduction dissociation (isCAD) for efficient mass spectrometry (MS) dissection of glycans. Although AuNPs have been employed as an inorganic matrix for MALDI MS, this is the first report of AuNP-induced fragmentation. In this approach, AuNPs were employed as an energy absorber for laser ionization as well as a trigger for fragmentation, while residual or deliberately added sodium ions acted as a cationizing agent. The addition of sodium ions induced intensive fragmentation, but the addition of protons suppressed the fragmentation, allowing for facile tuning of the degree of fragmentation. In addition, it was found that larger oligosaccharides and glycans were much easier to fragment as compared with their smaller counterparts, and the use of high-concentration AuNPs effectively suppressed the degree of fragmentation and thereby provided abundant molecular ions. Without any extra hardware and ion extraction, this approach provides a straightforward, cost-efficient and tunable fragmentation for efficient MS dissection of saccharides, including monosaccharides, oligosaccharides, and glycans. Thus, it opens new access to efficient MS dissection of glycans.

Top-Down Charge Transfer Dissociation (CTD) of Gas-Phase Insulin: Evidence of a One-Step, Two-Electron Oxidation Mechanism

Top-down analyses of protonated insulin cations of charge states of 4+, 5+, or 6+ were performed by exposing the isolated precursor ions to a beam of helium cations with kinetic energy of more than 6 keV, in a technique termed charge transfer dissociation (CTD). The ~100 ms charge transfer reaction resulted in approximately 20% conversion efficiency to other intact charge exchange products (CTnoD), and a range of low abundance fragment ions. To increase backbone and sulfide cleavages, and to provide better structural information than straightforward MS2 CTD, the CTnoD oxidized products were isolated and subjected to collisional activation at the MS3 level. The MS3 CTD/CID reaction effectively broke the disulfide linkages, separated the two chains, and yielded more structurally informative fragment ions within the inter-chain cyclic region. CTD also provided doubly oxidized intact product ions at the MS2 level, and resonance ejection of the singly oxidized product ion revealed that the doubly oxidized product originates directly from the isolated precursor ion and not from consecutive CTD reactions of a singly oxidized intermediate. MS4 experiments were employed to help identify potential radical cations and diradical cations, but the results were negative or inconclusive. Nonetheless, the two-electron oxidation process is a demonstration of the very large potential energy (>20 eV) available through CTD, and is a notable capability for a 3D ion trap platform. Graphical Abstract ᅟ.