Identification and relative-quantification of glycans by matrix-assisted laser desorption/ionization in-source decay with hydrogen abstraction

Nitro Indole Derivatives as Novel Dual-Polarity Matrices for MALDI Mass Spectrometry and Imaging with Broad Applications

A new matrix framework is presented in this study for the improved ionization efficiency of complex mixtures by matrix-assisted laser desorption ionization (MALDI) mass spectrometry/imaging. Five nitro indole (NI) derivatives [3-methyl-4-nitro-1H-indole (3,4-MNI), 3-methyl-6-nitro-1H-indole (3,6-MNI), 2,3-dimethyl-4-nitro-1H-indole (2,3,4-DMNI), 2,3-dimethyl-6-nitro-1H-indole (2,3,6-DMNI), and 4-nitro-1H-indole (4-NI)] were synthesized and shown to produce both positive and negative ions with a broad class of analytes as MALDI matrices. NI matrices were compared to several common matrices, such as 2,5-dihydroxybenzoic acid (DHB), alpha-cyano-4-hydroxylcinnamic acid (CHCA), sinapinic acid (SA), 1,5-diaminonaphthelene (1,5-DAN), and 9-aminoacridine (9-AA), for the analysis of lipid, peptide, protein, glycan, and perfluorooctanesulfonic acid (PFOS) compounds. 3,4-MNI demonstrated the best performance among the NI matrices. This matrix resulted in reduced ion suppression and better detection sensitivity for complex mixtures, for example, egg lipids/milk proteins/PFOS in tap water, while 2,3,6-DMNI was the best matrix for blueberry tissue imaging. Several important aspects of this work are reported: (1) dual-polarity ion production with NI matrices and complex mixtures; (2) quantitative analysis of PFOS with a LOQ of 0.5 ppb in tap water and 0.05 ppb in MQ water (without solid phase extraction enrichment), with accuracy and precision within 5%; (3) MALDI imaging with 2,3,6-DMNI as a matrix for plant metabolite/lipid identification with ionization enhancement in the negative ion mode m/z 600-900 region; and (4) development of a thin film deposition under/above tissue method for MALDI imaging with a vacuum sublimation matrix on a high-vacuum MALDI instrument.

In-Source Decay MALDI and High-Energy Collision-Induced Dissociation Mass Spectrometry of Alkali Metal-Adducted Underivatized Oligosaccharides

In-source decay (ISD) and high-energy collision-induced dissociation (HE-CID) were explored to provide structural information on alkali metal-adducted linear and stacked oligosaccharides (oligosaccharides with increased flexibility due to linkage type). These oligosaccharides include isomeric tetrasaccharides, maltoheptaose, and several human milk oligosaccharides (HMOs). Matrix-assisted laser desorption ionization (MALDI) ion production efficiency, as well as the product ion intensities, and the number of product ions formed in ISD and HE-CID of these oligosaccharides were influenced by the matrix, the ionic radius of the metal ion used for adduction, and the affinity of metal ions for specific functional groups in the oligosaccharides. 2,4,6-Trihydroxyacetophenone (THAP) was the best matrix for HE-CID of oligosaccharides, 4-dimethylaminobenzaldehyde (DMABA) worked best for ISD of tetrasaccharides and pentasaccharides, while 2,5-dihydroxybenzoic acid (DHB) was the best matrix for ISD and HE-CID of long chain oligosaccharides. In general, the number of product ions formed followed the trend Li+ > Na+ > K+ > Rb+ > Cs+, except for HMOs where Na+ ≥ Li+ > K+ > Rb+ > Cs+ occurred. The type of product ions formed and their intensities varied based on the position of the glycosidic bond linkage and the content of the monosaccharide. ISD and HE-CID produced diagnostic ions that could structurally differentiate isomers. Overall, HE-CID of alkali-metal adducted oligosaccharides produces intense glycosidic bond cleavages and low intensity cross-ring and internal cleavages. In contrast, ISD generates mainly cross-ring cleavages and internal cleavages at intensities higher than in HE-CID. In addition, ISD produced unique product ions that complement results from HE-CID.

Structural identification of carbohydrate isomers using ambient infrared-assisted dissociation

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

Analysis of Synthetic Monodisperse Polysaccharides by Wide Mass Range Ultrahigh-Resolution MALDI Mass Spectrometry

Carbohydrates, such as oligo- and polysaccharides, are highly abundant biopolymers that are involved in numerous processes. The study of their structure and functions is commonly based on a material that is isolated from complex natural sources. However, a more precise analysis requires pure compounds with well-defined structures that can be obtained from chemical or enzymatic syntheses. Novel synthetic strategies have increased the accessibility of larger monodisperse polysaccharides, posing a challenge to the analytical methods used for their molecular characterization. Here, we present wide mass range ultrahigh-resolution matrix-assisted laser desorption/ionization (MALDI) Fourier transform ion cyclotron resonance (FT-ICR) mass spectrometry (MS) as a powerful platform for the analysis of synthetic oligo- and polysaccharides. Synthetic carbohydrates 16-, 64-, 100-, and 151-mers were mass analyzed and characterized by MALDI in-source decay FT-ICR MS. Detection of fragment ions generated from glycosidic bond cleavage (or cross-ring cleavage) provided information of the monosaccharide content and the linkage type, allowing for the corroboration of the carbohydrate compositions and structures.

NEGATIVE ION MASS SPECTROMETRY FOR THE ANALYSIS OF N-LINKED GLYCANS

N-glycans from glycoproteins are complex, branched structures whose structural determination presents many analytical problems. Mass spectrometry, usually conducted in positive ion mode, often requires extensive sample manipulation, usually by derivatization such as permethylation, to provide the necessary structure-revealing fragment ions. The newer but, so far, lesser used negative ion techniques, on the contrary, provide a wealth of structural information not present in positive ion spectra that greatly simplify the analysis of these compounds and can usually be conducted without the need for derivatization. This review describes the use of negative ion mass spectrometry for the structural analysis of N-linked glycans and emphasises the many advantages that can be gained by this mode of operation. Biosynthesis and structures of the compounds are described followed by methods for release of the glycans from the protein. Methods for ionization are discussed with emphasis on matrix-assisted laser desorption/ionization (MALDI) and methods for producing negative ions from neutral compounds. Acidic glycans naturally give deprotonated species under most ionization conditions. Fragmentation of negative ions is discussed next with particular reference to those ions that are diagnostic for specific features such as the branching topology of the glycans and substitution positions of moieties such as fucose and sulfate, features that are often difficult to identify easily by conventional techniques such as positive ion fragmentation and exoglycosidase digestions. The advantages of negative over positive ions for this structural work are emphasised with an example of a series of glycans where all other methods failed to produce a structure. Fragmentation of derivatized glycans is discussed next, both with respect to derivatives at the reducing terminus of the molecules, and to methods for neutralization of the acidic groups on sialic acids to both stabilize them for MALDI analysis and to produce the diagnostic fragments seen with the neutral glycans. The use of ion mobility, combined with conventional mass spectrometry is described with emphasis on its use to extract clean glycan spectra both before and after fragmentation, to separate isomers and its use to extract additional information from separated fragment ions. A section on applications follows with examples of the identification of novel structures from lower organisms and tables listing the use of negative ions for structural identification of specific glycoproteins, glycans from viruses and uses in the biopharmaceutical industry and in medicine. The review concludes with a summary of the advantages and disadvantages of the technique. © 2020 John Wiley & Sons Ltd. Mass Spec Rev.

Characterization of glycan isomers using magnetic carbon nanoparticles as a MALDI co-matrix

Matrix-assisted laser desorption ionization-in source decay (MALDI-ISD) analysis is a useful technique in the structural analysis of glycans. Our recent publication demonstrated that magnetic carbon nanoparticles (MCNPs), used as a MALDI co-matrix, significantly enhanced ISD efficiency for glycomic analysis by MALDI-TOF. In this study, MCNPs were used for the structural study of isomeric glycans. Results from the standard glycans confirmed easy distinction of positional and linkage isomers without the need for further derivatization of glycan molecules. Extensive glycosidic and cross-ring fragmented ions provided different fragment patterns for various glycan isomers. Core- and branch-fucosylated isomers were distinguished by several unique ions, and pseudo-MS³ data were used to recognize the fucosylated branch. Although no diagnostic fragment ion was observed for 2,3- and 2,6-linked sialic acid isomers, their MALDI-ISD patterns were found to be significantly different (P < 0.05). Furthermore, the method introduced in this study could not only be used for the identification of glycan isomers but has also proved effective for the isomeric structural confirmation of gangliosides. GD1a and GD1b gangliosides were easily distinguished by the diagnostic ion originated from GD1a, produced by Z_4αZ_2β cleavages. Moreover, liquid chromatography coupled with MALDI-TOF was applied to analyze N-glycan isomers derived from a pooled human blood serum sample, providing an alternative method of isomeric glycomic analysis of biological specimens.

Magnetic carbon nanoparticles as a MALDI co-matrix enable isomeric characterization of glycans in biological samples.

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.

Carbon Nanoparticles and Graphene Nanosheets as MALDI Matrices in Glycomics: a New Approach to Improve Glycan Profiling in Biological Samples

Glycomics continues to be a highly dynamic and interesting research area due to the need to comprehensively understand the biological attributes of glycosylation in many important biological functions such as the immune response, cell development, cell differentiation/adhesion, and host-pathogen interactions. Although matrix-assisted laser desorption ionization (MALDI) mass spectrometry (MS) has proven to be suitable for glycomic profiling studies, there is a need for improved sensitivity in the detection of native glycans, which ionize inefficiently. In this study, we investigated the efficiencies of graphene nanosheets (GNs) and carbon nanoparticles (CNPs) as MALDI matrices and co-matrices in glycan profiling. Our results indicated an enhancement of signal intensity by several orders of magnitude upon using GNs and CNPs in MALDI analysis of N-glycans derived from a variety of biological samples. Interestingly, increasing the amounts of CNPs and GNs improved not only the signal intensities but also prompted in-source decay (ISD) fragmentations, which produced extensive glycosidic and cross-ring cleavages. Our results indicated that the extent of ISD fragmentation could be modulated by CNP and GN concentrations, to obtain MS² and pseudo-MS³ spectra. The results for glycan profiling in high salt solutions confirmed high salt-tolerance capacities for both CNPs and GNs. Finally, the results showed that by using CNPs and GNs as co-matrices, DHB crystal formation was more homogeneous which improved shot-to-shot reproducibility and sensitivity. Graphical Abstract ᅟ.

Matrix-Assisted Laser Desorption/Ionization Mass Spectrometry: Mechanistic Studies and Methods for Improving the Structural Identification of Carbohydrates

Although matrix-assisted laser desorption/ionization (MALDI) mass spectrometry is one of the most widely used soft ionization methods for biomolecules, the lack of detailed understanding of ionization mechanisms restricts its application in the analysis of carbohydrates. Structural identification of carbohydrates achieved by MALDI mass spectrometry helps us to gain insights into biological functions and pathogenesis of disease. In this review, we highlight mechanistic details of MALDI, including both ionization and desorption. Strategies to improve the ion yield of carbohydrates are also reviewed. Furthermore, commonly used fragmentation methods to identify the structure are discussed.

Analysis of carbohydrates and glycoconjugates by matrix-assisted laser desorption/ionization mass spectrometry: An update for 2011-2012

This review is the seventh update of the original article published in 1999 on the application of MALDI mass spectrometry to the analysis of carbohydrates and glycoconjugates and brings coverage of the literature to the end of 2012. General aspects such as theory of the MALDI process, matrices, derivatization, MALDI imaging, and fragmentation are covered in the first part of the review and applications to various structural types constitute the remainder. The main groups of compound are oligo- and poly-saccharides, glycoproteins, glycolipids, glycosides, and biopharmaceuticals. Much of this material is presented in tabular form. Also discussed are medical and industrial applications of the technique, studies of enzyme reactions, and applications to chemical synthesis. © 2015 Wiley Periodicals, Inc. Mass Spec Rev 36:255-422, 2017.

Principles of hydrogen radical mediated peptide/protein fragmentation during matrix-assisted laser desorption/ionization mass spectrometry

Matrix-assisted laser desorption/ionization in-source decay (MALDI-ISD) is a very easy way to obtain large sequence tags and, thereby, reliable identification of peptides and proteins. Recently discovered new matrices have enhanced the MALDI-ISD yield and opened new research avenues. The use of reducing and oxidizing matrices for MALDI-ISD of peptides and proteins favors the production of fragmentation pathways involving "hydrogen-abundant" and "hydrogen-deficient" radical precursors, respectively. Since an oxidizing matrix provides information on peptide/protein sequences complementary to that obtained with a reducing matrix, MALDI-ISD employing both reducing and oxidizing matrices is a potentially useful strategy for de novo peptide sequencing. Moreover, a pseudo-MS(3) method provides sequence information about N- and C-terminus extremities in proteins and allows N- and C-terminal side fragments to be discriminated within the complex MALDI-ISD mass spectrum. The combination of high mass resolution of a Fourier transform-ion cyclotron resonance (FTICR) analyzer and the software suitable for MALDI-ISD facilitates the interpretation of MALDI-ISD mass spectra. A deeper understanding of the MALDI-ISD process is necessary to fully exploit this method. Thus, this review focuses first on the mechanisms underlying MALDI-ISD processes, followed by a discussion of MALDI-ISD applications in the field of proteomics. © 2014 Wiley Periodicals, Inc., Mass Spec Rev 35:535-556, 2016.

Hydrazinonicotinic acid as a novel matrix for highly sensitive and selective MALDI-MS analysis of oligosaccharides

A new matrix that enables highly sensitive and selective as well as straightforward glycan analysis is reported.