Wavelength-tunable ultraviolet photodissociation (UVPD) of heparin-derived disaccharides in a linear ion trap

Ultraviolet photodissociation and collision-induced dissociation for qualitative/quantitative analysis of low molecular weight compounds by liquid chromatography-mass spectrometry

Collision-induced dissociation (CID) is the most wildly used fragmentation technique for qualitative and quantitative determination of low molecular weight compounds (LMWC). Ultraviolet photodissociation (UVPD) has been mainly investigated for the analysis of peptides and lipids while only in a limited way for LMWC. A triple quadrupole linear ion trap instrument has been modified to allow ultraviolet photodissociation (UVPD) in the end of the q2 region enabling various workflows with and without data-dependent acquisition (DDA) combining CID and UVPD in the same LC-MS analysis. The performance of UVPD, with a 266-nm laser, is compared to CID for a mix of 90 molecules from different classes of LMWC including peptides, pesticides, pharmaceuticals, metabolites, and drugs of abuse. These two activation methods offer complementary fragments as well as common fragments with similar sensitivities for most analytes investigated. The versatility of UVPD and CID is also demonstrated for quantitative analysis in human plasma of bosentan and its desmethyl metabolite, used as model analytes. Different background signals are observed for both fragmentation methods as well as unique fragments which opens the possibility of developing a selective quantitative assay with improved sample throughput, in particular for analytes present in different matrices.

Chemical derivatization strategy for mass spectrometry-based lipidomics

Lipids, serving as the structural components of cellular membranes, energy storage, and signaling molecules, play the essential and multiple roles in biological functions of mammals. Mass spectrometry (MS) is widely accepted as the first choice for lipid analysis, offering good performance in sensitivity, accuracy, and structural characterization. However, the untargeted qualitative profiling and absolute quantitation of lipids are still challenged by great structural diversity and high structural similarity. In recent decade, chemical derivatization mainly targeting carboxyl group and carbon-carbon double bond of lipids have been developed for lipidomic analysis with diverse advantages: (i) offering more characteristic structural information; (ii) improving the analytical performance, including chromatographic separation and MS sensitivity; (iii) providing one-to-one chemical isotope labeling internal standards based on the isotope derivatization regent in quantitative analysis. Moreover, the chemical derivatization strategy has shown great potential in combination with ion mobility mass spectrometry and ambient mass spectrometry. Herein, we summarized the current states and advances in chemical derivatization-assisted MS techniques for lipidomic analysis, and their strengths and challenges are also given. In summary, the chemical derivatization-based lipidomic approach has become a promising and reliable technique for the analysis of lipidome in complex biological samples.

Mass Spectrometry-Based Techniques to Elucidate the Sugar Code

Cells encode information in the sequence of biopolymers, such as nucleic acids, proteins, and glycans. Although glycans are essential to all living organisms, surprisingly little is known about the "sugar code" and the biological roles of these molecules. The reason glycobiology lags behind its counterparts dealing with nucleic acids and proteins lies in the complexity of carbohydrate structures, which renders their analysis extremely challenging. Building blocks that may differ only in the configuration of a single stereocenter, combined with the vast possibilities to connect monosaccharide units, lead to an immense variety of isomers, which poses a formidable challenge to conventional mass spectrometry. In recent years, however, a combination of innovative ion activation methods, commercialization of ion mobility-mass spectrometry, progress in gas-phase ion spectroscopy, and advances in computational chemistry have led to a revolution in mass spectrometry-based glycan analysis. The present review focuses on the above techniques that expanded the traditional glycomics toolkit and provided spectacular insight into the structure of these fascinating biomolecules. To emphasize the specific challenges associated with them, major classes of mammalian glycans are discussed in separate sections. By doing so, we aim to put the spotlight on the most important element of glycobiology: the glycans themselves.

Advanced tandem mass spectrometry in metabolomics and lipidomics-methods and applications

Metabolomics and lipidomics are new drivers of the omics era as molecular signatures and selected analytes allow phenotypic characterization and serve as biomarkers, respectively. The growing capabilities of untargeted and targeted workflows, which primarily rely on mass spectrometric platforms, enable extensive charting or identification of bioactive metabolites and lipids. Structural annotation of these compounds is key in order to link specific molecular entities to defined biochemical functions or phenotypes. Tandem mass spectrometry (MS), first and foremost collision-induced dissociation (CID), is the method of choice to unveil structural details of metabolites and lipids. But CID fragment ions are often not sufficient to fully characterize analytes. Therefore, recent years have seen a surge in alternative tandem MS methodologies that aim to offer full structural characterization of metabolites and lipids. In this article, principles, capabilities, drawbacks, and first applications of these "advanced tandem mass spectrometry" strategies will be critically reviewed. This includes tandem MS methods that are based on electrons, photons, and ion/molecule, as well as ion/ion reactions, combining tandem MS with concepts from optical spectroscopy and making use of derivatization strategies. In the final sections of this review, the first applications of these methodologies in combination with liquid chromatography or mass spectrometry imaging are highlighted and future perspectives for research in metabolomics and lipidomics are discussed.

Investigation of the Experimental Parameters of Ultraviolet Photodissociation for the Structural Characterization of Chondroitin Sulfate Glycosaminoglycan Isomers

Glycosaminoglycans (GAGs) are linear polysaccharides that participate in a broad range of biological functions. Their incomplete biosynthesis pathway leads to nonuniform chains and complex mixtures. For this reason, the characterization of GAGs has been a difficult hurdle for the analytical community. Recently, ultraviolet photodissociation (UVPD) has emerged as a useful tool for determining sites of modification within a GAG chain. Here, we investigate the ability for UVPD to distinguish chondroitin sulfate epimers and the effects of UVPD experimental parameters on fragmentation efficiency. Chondroitin sulfate A (CS-A) and chondroitin sulfate B (CS-B), commonly referred to as dermatan sulfate (DS), differ only in C-5 uronic acid stereochemistry. This uronic acid difference can influence GAG-protein binding and therefore can alter the specific biological function of a GAG chain. Prior tandem mass spectrometry methods investigated for the elucidation of GAG structures also have difficulty differentiating 4-O from 6-O sulfation in chondroitin sulfate GAGs. Preliminary data using UVPD to characterize GAGs showed a promising ability to characterize 4-O sulfation in CS-A GAGs. Here, we look in depth at the capability of UVPD to distinguish chondroitin sulfate C-5 diastereomers and the role of key experimental parameters in making this distinction. Results using a 193 nm excimer laser and a 213 nm solid-state laser are compared for this study. The effect of precursor ionization state, the number of laser pulses (193 or 213 nm UVPD), and the use of the low-pressure versus high-pressure trap are investigated.

Mammal Hyaluronidase Activity on Chondroitin Sulfate and Dermatan Sulfate: Mass Spectrometry Analysis of Oligosaccharide Products

Mammalian hyaluronidases are endo-N-acetyl-D-hexosaminidases involved in the catabolism of hyaluronic acid (HA) but their role in the catabolism of chondroitin sulfate (CS) is also examined. HA and CS are glycosaminoglycans (GAGs) implicated in several physiological and pathological processes, and understanding their metabolism is of significant importance. Data have been previously reported on the degradation of CS under the action of hyaluronidase, yet a detailed structural investigation of CS depolymerization products remains necessary to improve our knowledge of the CS depolymerizyng activity of hyaluronidase. For that purpose, the fine structural characterization of CS oligosaccharides formed upon the enzymatic depolymerization of various CS sub-types by hyaluronidase has been carried out by high resolution Orbitrap mass spectrometry and extreme UV (XUV) photodissociation tandem mass spectrometry. The exact mass measurements show the formation of wide size range of even oligosaccharides upon digestion of CS-A and CS-C comprising hexa- and octa-saccharides among the main digestion products, as well as formation of small quantities of odd-numbered oligosaccharides, while no hyaluronidase activity was detected on CS-B. In addition, slight differences have been observed in the distribution of oligosaccharides in the digestion mixture of CS-A and CS-C, the contribution of longer oligosaccharides being significantly higher for CS-C. The sequence of CS oligosaccharide products determined XUV photodissociation experiments verifies the selective β(1 → 4) glycosidic bond cleavage catalyzed by mammal hyaluronidase. The ability of the mammal hyaluronidase to produce hexa- and higher oligosaccharides supports its role in the catabolism of CS anchored to membrane proteoglycans and in extra-cellular matrix.

Structural Characterization of Glycosaminoglycan Carbohydrates Using Ultraviolet Photodissociation

Structural characterization of sulfated glycosaminoglycans (GAGs) by mass spectrometry has long been a formidable analytical challenge owing to their high structural variability and the propensity for sulfate decomposition upon activation with low-energy ion activation methods. While derivatization and complexation workflows have aimed to generate informative spectra using low-energy ion activation methods, alternative ion activation methods present the opportunity to obtain informative spectra from native GAG structures. Both electron- and photon-based activation methods, including electron detachment dissociation (EDD), negative electron transfer dissociation (NETD), and extreme ultraviolet photon activation, have been explored previously to overcome the limitations associated with low-energy activation methods for GAGs and other sulfated oligosaccharides. Further, implementation of such methods on high-resolution mass spectrometers has aided the interpretation of the complex spectra generated. Here, we explore ultraviolet photodissociation (UVPD) implemented on an Orbitrap mass spectrometer as another option for structural characterization of GAGs. UVPD spectra for both dermatan and heparan sulfate structures display extensive fragmentation including both glycosidic and cross-ring cleavages with the extent of sulfate retention comparable to that observed by EDD and NETD. In addition, the relatively short activation time of UVPD makes it promising for higher throughput analysis of GAGs in complex mixtures.

Two-dimensional mass spectrometry: new perspectives for tandem mass spectrometry

Fourier transform ion cyclotron resonance mass analysers (FT-ICR MS) can offer the highest resolutions and mass accuracies in mass spectrometry. Mass spectra acquired in an FT-ICR MS can yield accurate elemental compositions of all compounds in a complex sample. Fragmentation caused by ion-neutral, ion-electron, or ion-photon interactions leads to more detailed structural information on compounds. The most often used method to correlate compounds and their fragment ions is to isolate the precursor ions from the sample before fragmentation. Two-dimensional mass spectrometry (2D MS) offers a method to correlate precursor and fragment ions without requiring precursor isolation. 2D MS therefore enables easy access to the fragmentation patterns of all compounds from complex samples. In this article, the principles of FT-ICR MS are reviewed and the 2D MS experiment is explained. Data processing for 2D MS is detailed, and the interpretation of 2D mass spectra is described.

Lasers and ion mobility: new additions to the glycosaminoglycanomics toolkit

Glycosaminoglycans are biopolymers present in mammalian cells or in the extracellular matrix. To address their structure, the nature of the hexuronic acids and the position of sulfate groups must be determined. Tandem mass spectrometry using collision induced dissociation or electron-based fragmentation techniques, is a well-established approach for the identification of glycans but suffers from the frequent lack of diagnostic fragments in the case of glycosaminoglycans. This review presents alternative fragmentation techniques, namely photofragmentation in the IR and the UV ranges. Alternative approaches based on the direct analysis of the molecular structure, including ion mobility spectrometry and ion spectroscopies are reviewed. The potential of future multidimensional workflows for glycosaminoglycanomics is discussed.

Analysis of Sulfate Patterns in Glycosaminoglycan Oligosaccharides by MSn Coupled to Infrared Ion Spectroscopy: the Case of GalNAc4S and GalNAc6S

We report distinctive spectroscopic fingerprints of the monosaccharide standards GalNAc4S and GalNAc6S by coupling mass spectrometry and ion spectroscopy in the 3-μm range. The disaccharide standards CSA and CSC are used to demonstrate the applicability of a novel approach for the analysis of sulfate position in GalNAc-containing glycosaminoglycans. This approach was then used for the analysis of a sample containing CSA and CSC disaccharides. Finally, we discuss the generalization of the coupling of mass spectrometry with ion spectroscopy for the structural analysis of glycosaminoglycans on a tetrasaccharide from dermatan sulfate source. Graphical abstract ᅟ.

Effects of wavelength, fluence, and dose on fragmentation pathways and photoproduct ion yield in 213 nm and 266 nm ultraviolet photodissociation experiments

Ultraviolet photodissociation tandem mass spectrometry is a powerful tool to investigate the structure of biomolecules, due to its ability to generate rich fragmentation patterns or bond selective cleavage, as a function of used laser wavelength, laser fluence, dose (number of accumulated laser pulses), and available chromophores. Herein, we report first results obtained with a newly developed two-wavelength (266 nm and 213 nm) ultraviolet photodissociation setup coupled to a Fourier-transform ion cyclotron resonance mass spectrometer. Photoproduct yields for protonated 3-iodo-l-tyrosine were up to ∼75%. Dose and fluence dependent measurements for protonated 3-iodo-l-tyrosine, doubly charged protonated bradykinin and Fe(II) attached to 1,2-dioleoyl-sn-glycero-3-phosphocholine reveal that the ultraviolet photodissociation mechanism for photoproduct formation qualitatively differs between these model systems. Three derived photodissociation models were used to interpret the experimental results and show that while protonated 3-iodo-l-tyrosine and Fe(II) attached to 1,2-dioleoyl-sn-glycero-3-phosphocholine most likely dissociates via a single-photon process, fragmentation of doubly charged bradykinin ions was found to be most consistent with sequential two-photon dissociation (213 nm). The introduced dissociation models present an easy means to study the mechanism of ultraviolet photodissociation processes for a variety of analytes without prior knowledge of their photochemistry or to optimize experimental conditions by adjusting laser fluence or number of laser pulses.

Negative Electron Transfer Dissociation Sequencing of Increasingly Sulfated Glycosaminoglycan Oligosaccharides on an Orbitrap Mass Spectrometer

The structural characterization of sulfated glycosaminoglycan (GAG) carbohydrates remains an important target for analytical chemists attributable to challenges introduced by the natural complexity of these mixtures and the defined need for molecular-level details to elucidate biological structure-function relationships. Tandem mass spectrometry has proven to be the most powerful technique for this purpose. Previously, electron detachment dissociation (EDD), in comparison to other methods of ion activation, has been shown to provide the largest number of useful cleavages for de novo sequencing of GAG oligosaccharides, but such experiments are restricted to Fourier transform ion cyclotron resonance mass spectrometers (FTICR-MS). Negative electron transfer dissociation (NETD) provides similar fragmentation results, and can be achieved on any mass spectrometry platform that is designed to accommodate ion-ion reactions. Here, we examine for the first time the effectiveness of NETD-Orbitrap mass spectrometry for the structural analysis of GAG oligosaccharides. Compounds ranging in size from tetrasaccharides to decasaccharides were dissociated by NETD, producing both glycosidic and cross-ring cleavages that enabled the location of sulfate modifications. The highly-sulfated, heparin-like synthetic GAG, ArixtraTM, was also successfully sequenced by NETD. In comparison to other efforts to sequence GAG chains without fully ionized sulfate constituents, the occurrence of sulfate loss peaks is minimized by judicious precursor ion selection. The results compare quite favorably to prior results with electron detachment dissociation (EDD). Significantly, the duty cycle of the NETD experiment is sufficiently short to make it an effective tool for on-line separations, presenting a straightforward path for selective, high-throughput analysis of GAG mixtures. Graphical Abstract ᅟ.

Glycosaminoglycan Analysis by Cryogenic Messenger-Tagging IR Spectroscopy Combined with IMS-MS

We combine ion mobility spectrometry with cryogenic, messenger-tagging, infrared spectroscopy and mass spectrometry to identify different isomeric disaccharides of chondroitin sulfate (CS) and heparan sulfate (HS), which are representatives of two major subclasses of glycosaminoglycans. Our analysis shows that while CS and HS disaccharide isomers have similar drift times, they can be uniquely distinguished by their vibrational spectrum between ∼3200 and 3700 cm-1 due to their different OH hydrogen-bonding patterns. We suggest that this combination of techniques is well suited to identify and characterize glycan isomers directly, which presents tremendous challenges for existing methods.

Gas-Phase Intercluster Thiyl-Radical Induced C-H Bond Homolysis Selectively Forms Sugar C2-Radical Cations of Methyl D-Glucopyranoside: Isotopic Labeling Studies and Cleavage Reactions

A suite of isotopologues of methyl D-glucopyranosides is used in conjunction with multistage mass spectrometry experiments to determine the radical site and cleavage reactions of sugar radical cations formed via a recently developed 'bio-inspired' method. In the first stage of CID (MS²), collision-induced dissociation (CID) of a protonated noncovalent complex between the sugar and S-nitrosocysteamine, [H₃NCH₂CH₂SNO + M]⁺, unleashes a thiyl radical via bond homolysis to give the noncovalent radical cation, [H₃NCH₂CH₂S^• + M]⁺. CID (MS³) of this radical cation complex results in dissociation of the noncovalent complex to generate the sugar radical cation. Replacement of all exchangeable OH and NH protons with deuterons reveals that the sugar radical cation is formed in a process involving abstraction of a hydrogen atom from a C-H bond of the sugar coupled with proton transfer to the sugar, to form [M - H^• + D⁺]. Investigation of this process using individual C-D labeled sugars reveals that the main site of H/D abstraction is the C2 position, since only the C2-deuterium labeled sugar yields a dominant [M - D^• + H⁺] product ion. The fragmentation reactions of the distonic sugar radical cation, [M - H^•+ H⁺], were studied by another stage of CID (MS⁴). ¹³C-labeling studies revealed that a series of three related fragment ions each contain the C1-C3 atoms; these arise from cross-ring cleavage reactions of the sugar. Graphical Abstract ᅟ.

Differential Fragmentation of Mobility-Selected Glycans via Ultraviolet Photodissociation and Ion Mobility-Mass Spectrometry

The alternative dissociation pathways initiated by ultraviolet photodissociation (UVPD) compared with collision-induced dissociation (CID) may provide useful diagnostic fragments for biomolecule identification, including glycans. However, underivatized glycans do not commonly demonstrate strong UV absorbance, resulting in low fragmentation yields for UVPD spectra. In contrast to UVPD experiments that leverage covalent modification of glycans, we detail the capacity of metal adduction to yield comparatively rich UVPD fragmentation patterns and enhance separation factors for an isomeric glycan set in a drift tube ion mobility system. Ion mobility and UVPD-MS spectra for two N-acetyl glycan isomers were examined, each adducted with sodium or cobalt cations, with the latter providing fragment yield gains of an order of magnitude versus sodium adducts. Furthermore, our glycan analysis incorporated front-end ion mobility separation such that the structural glycan isomers could still be identified even as a mixture and not simply composite spectra of isomeric standards. Cobalt adduction proved influential in the glycan separation by yielding an isomer resolution of 0.78 when analyzed simultaneously versus no discernable separation obtained with the sodium adducts. It is the combined enhancement of both isomeric drift time separation and isomer distinction with improved UVPD fragment ion yields that further bolster multivalent metal adduction for advancing glycan IM-MS experiments. Graphical Abstract ᅟ.

Enhanced Mixture Separations of Metal Adducted Tetrasaccharides Using Frequency Encoded Ion Mobility Separations and Tandem Mass Spectrometry

Using five isomeric tetrasaccharides in combination with seven multivalent metals, the impact on mobility separations and resulting CID spectra were examined using a hybrid ion mobility atmospheric pressure drift tube system coupled with a linear ion trap. By enhancing the duty cycle of the drift tube system using a linearly chirped frequency, the collision-induced dissociation spectra were encoded in the mobility domain according to the drift times of each glycan isomer precursor. Differential fragmentation patterns correlated with precursor drift times ensured direct assignment of fragments with precursor structure whether as individual standards or in a mixture of isomers. In addition to certain metal ions providing higher degrees of separation than others, in select cases more than one arrival time distribution was observed for a single pure carbohydrate isomer. These observations suggest the existence of alternative coordination sites within a single monomeric species, but more interesting was the observation of different fragmentation ion yields for carbohydrate dimers formed through metal adduction. Positive-ion data were also compared with negative-ion species, where dimer formation did not occur and single peaks were observed for each isomeric tetrasaccharide-alditol. This enhanced analytical power has implications not only for carbohydrate molecules but also for a wide variety of complex mixtures of molecules where dissociation spectra may potentially be derived from combinations of monomeric, homodimeric, and heterodimeric species having identical nominal m/z values. Graphical Abstract ᅟ.

Online coupling of high-resolution chromatography with extreme UV photon activation tandem mass spectrometry: Application to the structural investigation of complex glycans by dissociative photoionization

The activation of ions by extreme-energy photons (XUV) produced by a synchrotron radiation beamline is a powerful method for characterizing complex glycans using tandem mass spectrometry (MS). As previously described, this activation method leads to rich fragmentation spectra with many structurally valuable cross-ring cleavages while maintaining labile modifications on the glycan structures. However, until now, the tandem MS event was too long to be compatible with liquid chromatography elution times. In this work, the duty cycle of the activation and detection of fragments was shortened, and the background signal on the spectra was drastically reduced. Both improvements allowed, for the first time, the successful coupling of a UHPLC system to XUV-activated tandem MS. The approach was used to characterize a complex mixture of oligo-porphyrans, which are a class of highly sulfated oligosaccharides, in a fully automated way. Due to an enhanced dynamic range and an increased sensitivity, some hypothetical structures of low abundance have been unequivocally confirmed in this study and others have been revised. Some previously undescribed species of oligo-porphyrans that exhibit lateral branching have been fully resolved. This work contributes to the scarce knowledge of the structure of porphyrans in red algae and pushes the current capacities of XUV-activation tandem MS by demonstrating the possibility of a direct coupling with UHPLC. This study will considerably broaden the applicability and practicality of this method in many fields of analytical biology.

Glycomics and glycoproteomics of membrane proteins and cell-surface receptors: Present trends and future opportunities

Membrane proteins mediate cell-cell interactions and adhesion, the transfer of ions and metabolites, and the transmission of signals from the extracellular environment to the cell interior. The extracellular domains of most cell membrane proteins are glycosylated, often at multiple sites. There is a growing awareness that glycosylation impacts the structure, interaction, and function of membrane proteins. The application of glycoproteomics and glycomics methods to membrane proteins has great potential. However, challenges also arise from the unique physical properties of membrane proteins. Successful analytical workflows must be developed and disseminated to advance functional glycoproteomics and glycomics studies of membrane proteins. This review explores the opportunities and challenges related to glycomic and glycoproteomic analysis of membrane proteins, including discussion of sample preparation, enrichment, and MS/MS analyses, with a focus on recent successful workflows for analysis of N- and O-linked glycosylation of mammalian membrane proteins.

Ion collision cross section measurements in Fourier transform-based mass analyzers

High-vacuum ion collision cross section (CCS) measurements in Fourier transform mass analyzers.

Distinguishing isobaric phosphated and sulfated carbohydrates by coupling of mass spectrometry with gas phase vibrational spectroscopy

An original application of the coupling of mass spectrometry with vibrational spectroscopy, used for the first time to discriminate isobaric bioactive saccharides with sulfate and phosphate functional modifications, is presented. Whereas their nominal masses and fragmentation patterns are undifferentiated by sole mass spectrometry, their distinctive OH stretching modes at 3595 cm(-1) and 3666 cm(-1), respectively, provide a reliable spectroscopic diagnostic for distinguishing their sulfate or phosphate functionalization. A detailed analysis of the 6-sulfated and 6-phosphated d-glucosamine conformations is presented, together with theoretical scaled harmonic spectra and anharmonic spectra (VPT2 and DFT-based molecular dynamics simulations). Strong anharmonic effects are observed in the case of the phosphated species, resulting in a dramatic enhancement of its phosphate diagnostic mode.

Effects of calcium complexation on heparin-like disaccharides. A combined theoretical, tandem mass spectrometry and ultraviolet experiment

RATIONALE

In order to shed light on the influence of the Ca(2+) metal cation on the structure of heparin-like (Hp) disaccharides, we have explored the gas-phase structures of both [Hp, -2H](2-) and [Ca(Hp), -3H](-) ions by coupling experimental and theoretical methods.

METHODS

The goal of this work was to (i) provide new evidence of the metal influence on the Hp structure, which can have important biological consequences, and (ii) to study the usefulness of metal complexation for the analytical distinction of Hp isomers. Collision-induced dissociation (CID) and ultraviolet photodissociation (UVPD) fragments, as well as optical spectra recorded in the gas phase for both [Hp, -2H](2-) and [Ca(Hp), -3H](-) complexes were compared for I-H, II-S and III-S isomers of Hp.

RESULTS

In the case of CID fragmentation, a change in the fragmentation pattern was observed upon calcium complexation, with respect to deprotonated Hp.

CONCLUSIONS

Remarkably, when optical spectra are compared in the UV range, the metal effect on the carboxylic group absorption can be detected by an unambiguous blue-shift (~20 nm).

Linkage and anomeric differentiation in trisaccharides by sequential fragmentation and variable-wavelength infrared photodissociation

Carbohydrates and their derivatives play important roles in biological systems, but their isomeric heterogeneity also presents a considerable challenge for analytical techniques. Here, a stepwise approach using infrared multiple-photon dissociation (IRMPD) via a tunable CO2 laser (9.2-10.7 μm) was employed to characterize isomeric variants of glucose-based trisaccharides. After the deprotonated trisaccharides were trapped and fragmented to disaccharide C2 fragments in a Fourier transform ion cyclotron resonance (FTICR) cell, a further variable-wavelength infrared irradiation of the C2 ion produced wavelength-dependent dissociation patterns that are represented as heat maps. The photodissociation patterns of these C2 fragments are shown to be strikingly similar to the photodissociation patterns of disaccharides with identical glycosidic bonds. Conversely, the photodissociation patterns of different glycosidic linkages exhibit considerable differences. On the basis of these results, the linkage position and anomericity of glycosidic bonds of disaccharide units in trisaccharides can be systematically differentiated and identified, providing a promising approach to characterize the structures of isomeric oligosaccharides.

Anharmonic simulations of the vibrational spectrum of sulfated compounds: application to the glycosaminoglycan fragment glucosamine 6-sulfate

Anharmonic behavior of sulfated glucosamine resolved by hybrid GVPT2 approach.

Radical-directed dissociation of peptides and proteins by infrared multiphoton dissociation and sustained off-resonance irradiation collision-induced dissociation with Fourier transform ion cyclotron resonance mass spectrometry

RATIONALE

Recent experiments utilizing photodissociation in linear ion traps have enabled significant development of Radical-Directed Dissociation (RDD) for the examination of peptides and proteins. The increased mass accuracy and resolution available in Fourier transform ion cyclotron resonance mass spectrometry (FTICR-MS) should enable further progress in this area. Preliminary experiments with photoactivated radicals are reported herein.

METHODS

A 266 nm Nd:YAG laser is coupled to a FTICR or linear ion trap mass spectrometer. Radical peptides and proteins are generated by ultraviolet photodissociation (PD) and further activated by collisions or infrared photons.

RESULTS

A 266 nm UV laser and an IR laser can be simultaneously coupled to a 15 Tesla FTICR mass spectrometer. The ultra-low-pressure environment in FTICR-MS makes collisional cooling less competitive, and thus more secondary fragments are generated by UVPD than in linear ion traps. Activation by sustained off-resonance irradiation collision-induced dissociation (SORI-CID) or infrared multiphoton dissociation (IRMPD) also yields additional secondary fragmentation relative to CID in an ion trap. Accurate identification of RDD fragments is possible in FTICR-MS.

CONCLUSIONS

Relative to linear ion trap instruments, PD experiments in FTICR-MS are more difficult to execute due to poor ion cloud overlap and the low pressure environment. However, the results can be more easily interpreted due to the increased resolution and mass accuracy.

Electron photodetachment dissociation for structural characterization of synthetic and bio-polymer anions

Tandem mass spectrometry (MS-MS) is a generic term evoking techniques dedicated to structural analysis, detection or quantification of molecules based on dissociation of a precursor ion into fragments. Searching for the most informative fragmentation patterns has led to the development of a vast array of activation modes that offer complementary ion reactivity and dissociation pathways. Collisional activation of ions using atoms, molecules or surface resulting in unimolecular dissociation of activated ions still plays a key role in tandem mass spectrometry. The discovery of electron capture dissociation (ECD) and then the development of other electron-ion or ion/ion reaction methods, constituted a significant breakthrough, especially for structural analysis of large biomolecules. Similarly, photon activation opened promising new frontiers in ion fragmentation owing to the ability of tightly controlled internal energy deposition and easy implementation on commercial instruments. Ion activation by photons includes slow heating methods such as infrared multiple photon dissociation (IRMPD) and black-body infrared radiative dissociation (BIRD) and higher energy methods like ultra-violet photodissociation (UVPD) and electron photo detachment dissociation (EPD). EPD occurs after UV irradiation of multiply negatively charged ions resulting in the formation of oxidized radical anions. The present paper reviews the hypothesis regarding the mechanisms of electron photo-detachment, radical formation and direct or activated dissociation pathways that support the observation of odd and even electron product ions. Finally, the value of EPD as a complementary structural analysis tool is illustrated through selected examples of synthetic polymers, oligonucleotides, polypeptides, lipids, and polysaccharides.

Mechanistic study on electron capture dissociation of the oligosaccharide-Mg²⁺ complex

Electron capture dissociation (ECD) has shown great potential in structural characterization of glycans. However, our current understanding of the glycan ECD process is inadequate for accurate interpretation of the complex glycan ECD spectra. Here, we present the first comprehensive theoretical investigation on the ECD fragmentation behavior of metal-adducted glycans, using the cellobiose-Mg²⁺ complex as the model system. Molecular dynamics simulation was carried out to determine the typical glycan-Mg²⁺ binding patterns and the lowest-energy conformer identified was used as the initial geometry for density functional theory-based theoretical modeling. It was found that the electron is preferentially captured by Mg²⁺ and the resultant Mg⁺• can abstract a hydroxyl group from the glycan moiety to form a carbon radical. Subsequent radical migration and α-cleavage(s) result in the formation of a variety of product ions. The proposed hydroxyl abstraction mechanism correlates well with the major features in the ECD spectrum of the Mg²⁺-adducted cellohexaose. The mechanism presented here also predicts the presence of secondary, radical-induced fragmentation pathways. These secondary fragment ions could be misinterpreted, leading to erroneous structural determination. The present study highlights an urgent need for continuing investigation of the glycan ECD mechanism, which is imperative for successful development of bioinformatics tools that can take advantage of the rich structural information provided by ECD of metal-adducted glycans.

Photodissociation mass spectrometry: new tools for characterization of biological molecules

Photodissociation mass spectrometry combines the ability to activate and fragment ions using photons with the sensitive detection of the resulting product ions by mass spectrometry. This combination affords a versatile tool for characterization of biological molecules. The scope and breadth of photodissociation mass spectrometry have increased substantially over the past decade as new research groups have entered the field and developed a number of innovative applications that illustrate the ability of photodissociation to produce rich fragmentation patterns, to cleave bonds selectively, and to target specific molecules based on incorporation of chromophores. This review focuses on many of the key developments in photodissociation mass spectrometry over the past decade with a particular emphasis on its applications to biological molecules.

Combined collision-induced dissociation and photo-selected reaction monitoring mass spectrometry modes for simultaneous analysis of coagulation factors and estrogens

Oral estrogens are directly associated with changes in plasma levels of coagulation proteins. Thus, the detection of any variation in protein concentrations due to estrogen contraceptives, by a simultaneous analysis of both coagulation proteins and estrogens, would be a very informative tool. In the present study, the merit of photo-selected reaction monitoring (SRM), a new analytical tool, was evaluated towards estrogens detection in plasma. Then, SRM and photo-SRM detection modes were combined for the simultaneous analysis of estrogen molecules together with heparin co-factor and factor XIIa, two proteins involved in the coagulation cascade. This study shows that photo-SRM could open new multiplexed analytical routes.

Vacuum ultraviolet action spectroscopy of polysaccharides

We studied the optical properties of gas-phase polysaccharides (maltose, maltotetraose, and maltohexaose) ions by action spectroscopy using the coupling between a quadrupole ion trap and a vacuum ultraviolet (VUV) beamline at the SOLEIL synchrotron radiation facility (France) in the 7 to 18 eV range. The spectra provide unique benchmarks for evaluation of theoretical data on electronic transitions of model carbohydrates in the VUV range. The effects of the nature of the charge held by polysaccharide ions on the relaxation processes were also explored. Finally the effect of isomerization of polysaccharides (with melezitose and raffinose) on their photofragmentation with VUV photons is presented.

UV photodissociation action spectroscopy of haloanilinium ions in a linear quadrupole ion trap mass spectrometer

UV-vis photodissociation action spectroscopy is becoming increasingly prevalent because of advances in, and commercial availability of, ion trapping technologies and tunable laser sources. This study outlines in detail an instrumental arrangement, combining a commercial ion-trap mass spectrometer and tunable nanosecond pulsed laser source, for performing fully automated photodissociation action spectroscopy on gas-phase ions. The components of the instrumentation are outlined, including the optical and electronic interfacing, in addition to the control software for automating the experiment and performing online analysis of the spectra. To demonstrate the utility of this ensemble, the photodissociation action spectra of 4-chloroanilinium, 4-bromoanilinium, and 4-iodoanilinium cations are presented and discussed. Multiple photoproducts are detected in each case and the photoproduct yields are followed as a function of laser wavelength. It is shown that the wavelength-dependent partitioning of the halide loss, H loss, and NH3 loss channels can be broadly rationalized in terms of the relative carbon-halide bond dissociation energies and processes of energy redistribution. The photodissociation action spectrum of (phenyl)Ag2 (+) is compared with a literature spectrum as a further benchmark.

Improved detection specificity for plasma proteins by targeting cysteine-containing peptides with photo-SRM

Targeted mass spectrometry using selected reaction monitoring (SRM) has emerged as an alternative to immunoassays for protein quantification owing to faster development time and higher multiplexing capability. However, the SRM strategy is faced with the high complexity of peptide mixtures after trypsin digestion of whole plasma or the cellular proteome that most of the time causes contamination, irremediably, by interfering compounds in the transition channels monitored. This problem becomes increasingly acute when the targeted protein is present at a low concentration. In this work, the merit of laser-induced photo-dissociation in the visible region at 473 nm implemented in an hybrid quadrupole linear ion-trap mass spectrometer (photo-SRM) was evaluated for detection specificity of cysteine-containing peptides in a group of plasma proteins after tagging with a dabcyl chromophore. Compared with conventional SRM, photo-SRM chromatograms have improved detection specificity for most of peptides monitored. Comparison of the signals obtained for the best proteotypic peptides in SRM mode and those recorded by photo-SRM of cysteine-containing peptides for the same proteins reveals either increased (up to 10-fold) or similar signal to photo-SRM detection. Finally, photo-SRM has extended response linearity across a calibration plot obtained by diluting human plasma in rat plasma, down to the lowest concentrations. Hence, photo-SRM may advantageously complement conventional SRM in assay of proteins in complex biological matrices.

Formation and fragmentation of radical peptide anions: insights from vacuum ultra violet spectroscopy

We have studied the photodissociation of gas-phase deprotonated caerulein anions by vacuum ultraviolet (VUV) photons in the 4.5 to 20 eV range, as provided by the DESIRS beamline at the synchrotron radiation facility SOLEIL (France). Caerulein is a sulphated peptide with three aromatic residues and nine amide bonds. Electron loss is found to be the major relaxation channel at every photon energy. However, an increase in the fragmentation efficiency (neutral losses and peptide backbone cleavages) as a function of the energy is also observed. The oxidized ions, generated by electron photodetachment were further isolated and activated by collision (CID) in a MS(3) scheme. The branching ratios of the different fragments observed by CID as a function of the initial VUV photon energy are found to be independent of the initial photon energy. Thus, there is no memory effect of the initial excitation energy on the fragmentation channels of the oxidized species on the time scale of our tandem MS experiment. We also report photofragment yields as a function of photon energy for doubly deprotonated caerulein ions, for both closed-shell ([M-2H](2-)) non-radical ions and open-shell ([M-3H](2-•)) radical ions. These latter ions are generated by electron photodetachment from [M-3H](3-) precursor ions. The detachment yield increases monotonically with the energy with the appearance of several absorption bands. Spectra for radical and non-radical ions are quite similar in terms of observed bands; however, the VUV fragmentation yield is enhanced by the presence of a radical in caerulein peptides.

Structure and physicochemical characterisation of heparin

Heparin is a member of the heparan sulphate family of glycosaminoglycans, a linear polysaccharide with a complex sequence resulting from the action of post-polymerisation enzymes on a regular repeating disaccharide background. Its overall conformation is rod-like in solution as well as in the solid state, but the conformational fluctuations of iduronate residues give rise to considerable internal motion and variation in local three-dimensional structure. Structure/function relationships and their relation to sequence are still the subject of argument, but new methodologies to tackle the subject are emerging. Heparin as a therapeutic agent and as the object of research may be characterised by numerous physico-chemical techniques. These include chromatographic methods for measurement of molecular weight; a variety of spectroscopic techniques; separation methods for whole polysaccharides, as well as for oligo- and monosaccharides; and mass spectrometric methods for mapping and sequence analysis. The impetus provided by the discovery of heparin contamination with oversulphated chondroitin sulphate has been influential in bringing combinations of many old and new techniques into use to ensure that heparin is sufficiently consistent and pure to be used safely. Synthetic and semi-synthetic heparins are in development and may become reality in the relatively near future.

Photo-SRM: laser-induced dissociation improves detection selectivity of Selected Reaction Monitoring mode

Selected Reaction Monitoring (SRM) carried out on triple-quadrupole mass spectrometers coupled to liquid chromatography has been a reference method to develop quantitative analysis of small molecules in biological or environmental matrices for years and is currently emerging as a promising tool in clinical proteomic. However, sensitive assays in complex matrices are often hampered by the presence of co-eluted compounds that share redundant transitions with the target species. On-the-fly better selection of the precursor ion by high-field asymmetric waveform ion mobility spectrometry (FAIMS) or increased quadrupole resolution is one way to escape from interferences. In the present work we document the potential interest of substituting classical gas-collision activation mode by laser-induced dissociation in the visible wavelength range to improve the specificity of the fragmentation step. Optimization of the laser beam pathway across the different quadrupoles to ensure high photo-dissociation yield in Q2 without detectable fragmentation in Q1 was assessed with sucrose tagged with a push-pull chromophore. Next, the proof of concept that photo-SRM ensures more specific detection than does conventional collision-induced dissociation (CID)-based SRM was carried out with oxytocin peptide. Oxytocin was derivatized by the thiol-reactive QSY® 7 C(5)-maleimide quencher on cysteine residues to shift its absorption property into the visible range. Photo-SRM chromatograms of tagged oxytocin spiked in whole human plasma digest showed better detection specificity and sensitivity than CID, that resulted in extended calibration curve linearity. We anticipate that photo-SRM might significantly improve the limit of quantification of classical SRM-based assays targeting cysteine-containing peptides.

Ultraviolet photodissociation of chromophore-labeled oligosaccharides via reductive amination and hydrazide conjugation

The fragmentation patterns of hydrazide-conjugated and reductively aminated oligosaccharides, including lacto-N-fucopentaoses and lacto-N-difucohexaoses, produced on collisionally induced dissociation (CID) and ultraviolet photodissociation (UVPD) in a quadrupole ion trap are presented. The two derivatization methods generate different cross-ring cleavages on UVPD and CID. UVPD of hydrazide-conjugated oligosaccharides yield predominant (2, 4)A-type cross-ring cleavage ions. In contrast, UVPD of aminated oligosaccharides results mainly in (0, 1)A-type ions. Moreover, more extensive dual-cleavage pathways (i.e. internal fragment ions) were observed on UVPD.

Shedding light on the frontier of photodissociation

The development of new ion activation/dissociation methods is motivated by the need for more versatile ways to characterize structures of ions, especially in the growing arena of biological mass spectrometry in which better tools for determining sequences, modifications, interactions, and conformations of biopolymers are essential. Although most agree that collision-induced dissociation (CID) remains the gold standard for ion activation/dissociation, recent inroads in electron- and photon-based activation methods have cemented their role as outstanding alternatives. This article will focus on the impact of photodissociation, including its strengths and drawbacks as an analytical tool, and its potential for further development in the next decade. Moreover, the discussion will emphasize photodissociation in quadrupole ion traps, because that platform has been used for one of the greatest arrays of new applications over the past decade.

Photoinduced dissociation of heparin-derived oligosaccharides controlled by charge location

The development of strategies based on mass spectrometry to help for deep structural analysis of acidic oligosaccharides remains topical. We thus examined the dissociation behavior of deprotonated ions of heparin-derived di- to tetra-saccharides under UV irradiation at 220 nm. Depending on the ionization state of the carboxylic groups, an oxidized species issued from electron photodetachment was observed in complement to photoinduced fragmentation of precursor ions. The influence of the charge location in the oligosaccharide dianions on the balance between photodissociation and electron photodetachment is examined and a way to direct the relaxation pathways, (i.e., dissociation versus electron detachment), is proposed using sodium adducts. The oxidized species was subjected to activated-electron photodetachment (activated-EPD) leading to complementary informative fragment ions to those issued from photodissociation. Directed photoinduced dissociation at 220 nm and activated-EPD should complement the more conventional CAD and IRMPD activation modes for deeper structural analysis of acidic oligosaccharides-derived anions.