Probing Mobility-Selected Saccharide Isomers: Selective Ion-Molecule Reactions and Wavelength-Specific IR Activation

Identifying the D-Pentoses Using Water Adduction to Lithium Cationized Molecule

A method has been developed that is capable of distinguishing an exhaustive list of underivatized D-pentoses with only a mass spectrometer. Electrospray ionization (ESI) of a solution containing a pentose and a lithium salt yields [Pentose + Li]⁺. These lithiated pentoses adduct water in a quadrupole ion trap. The reaction rate of water adduction is unique for several of the pentose isomers. Additionally, there are multiple potential gas-phase lithiation sites to form [Pentose + Li]⁺. A mixture of ions with at least one reactive (water adducting) and at least one unreactive (non-adducting) lithiation site is formed for each pentose. The water adduction reaction rate along with the unreactive fraction of lithiated pentose can be used to completely discriminate all D-pentoses. Graphical Abstract ᅟ.

Bottom-Up Elucidation of Glycosidic Bond Stereochemistry

The lack of robust, high-throughput, and sensitive analytical strategies that can conclusively map the structure of glycans has significantly hampered progress in fundamental and applied aspects of glycoscience. Resolution of the anomeric α/β glycan linkage within oligosaccharides remains a particular challenge. Here, we show that "memory" of anomeric configuration is retained following gas-phase glycosidic bond fragmentation during tandem mass spectrometry (MS²). These findings allow for integration of MS² with ion mobility spectrometry (IM-MS²) and lead to a strategy to distinguish α- and β-linkages within natural underivatized carbohydrates. We have applied this fragment-based hyphenated MS technology to oligosaccharide standards and to de novo sequencing of purified plant metabolite glycoconjugates, showing that the anomeric signature is also observable in fragments derived from larger glycans. The discovery of the unexpected anomeric memory effect is further supported by IR-MS action spectroscopy and ab initio calculations. Quantum mechanical calculations provide candidate geometries for the distinct anomeric fragment ions, in turn shedding light on gas-phase dissociation mechanisms of glycosidic linkages.

Cationized Carbohydrate Gas-Phase Fragmentation Chemistry

We investigate the fragmentation chemistry of cationized carbohydrates using a combination of tandem mass spectrometry, regioselective labeling, and computational methods. Our model system is D-lactose. Barriers to the fundamental glyosidic bond cleavage reactions, neutral loss pathways, and structurally informative cross-ring cleavages are investigated. The most energetically favorable conformations of cationized D-lactose were found to be similar. In agreement with the literature, larger group I cations result in structures with increased cation coordination number which require greater collision energy to dissociate. In contrast with earlier proposals, the B _n -Y _m fragmentation pathways of both protonated and sodium-cationized analytes proceed via protonation of the glycosidic oxygen with concerted glycosidic bond cleavage. Additionally, for the sodiated congeners our calculations support sodiated 1,6-anhydrogalactose B _n ion structures, unlike the preceding literature. This affects the subsequent propensity of formation and prediction of B _n /Y _m branching ratio. The nature of the anomeric center (α/β) affects the relative energies of these processes, but not the overall ranking. Low-energy cross-ring cleavages are observed for the metal-cationized analytes with a retro-aldol mechanism producing the ^0,2 A ₂ ion from the sodiated forms. Theory and experiment support the importance of consecutive fragmentation processes, particularly for the protonated congeners at higher collision energies. Graphical Abstract ᅟ.

Correlation of the structural information obtained for europium-chelate ensembles from gas-phase photoluminescence and ion-mobility spectroscopy with density-functional computations and ligand-field theory

We report a combined investigation of europium(iii)9-oxo-phenalen-1-one (PLN) coordination complexes, [Eu(PLN)₄AE]⁺ with AE = Mg, Ca, and Sr, using gas-phase photoluminescence, trapped ion-mobility spectrometry and density-functional computations. In order to sort out the structural impact of the alkali earth dications on the photoluminescence spectra, the experimental data are compared to the predicted ligand-field splittings as well as to the collision cross-sections for different isomers of [Eu(PLN)₄AE]⁺. The best fitting interpretation is that one isomer family predominantly contributes to the recorded luminescence. The present work demonstrates the complexity of the coordination patterns of multicenter lanthanoid chelates involved in dynamical equilibria and the pertinence of using isolation techniques to elucidate their photophysical properties.

Travelling-wave ion mobility mass spectrometry and negative ion fragmentation of hybrid and complex N-glycans

Nitrogen collisional cross sections (CCSs) of hybrid and complex glycans released from the glycoproteins IgG, gp120 (from human immunodeficiency virus), ovalbumin, α1-acid glycoprotein and thyroglobulin were measured with a travelling-wave ion mobility mass spectrometer using dextran as the calibrant. The utility of this instrument for isomer separation was also investigated. Some isomers, such as Man₃ GlcNAc₃ from chicken ovalbumin and Man₃ GlcNAc₃ Fuc₁ from thyroglobulin could be partially resolved and identified by their negative ion fragmentation spectra obtained by collision-induced decomposition (CID). Several other larger glycans, however, although existing as isomers, produced only asymmetric rather than separated arrival time distributions (ATDs). Nevertheless, in these cases, isomers could often be detected by plotting extracted fragment ATDs of diagnostic fragment ions from the negative ion CID spectra obtained in the transfer cell of the Waters Synapt mass spectrometer. Coincidence in the drift times of all fragment ions with an asymmetric ATD profile in this work, and in the related earlier paper on high-mannose glycans, usually suggested that separations were because of conformers or anomers, whereas symmetrical ATDs of fragments showing differences in drift times indicated isomer separation. Although some significant differences in CCSs were found for the smaller isomeric glycans, the differences found for the larger compounds were usually too small to be analytically useful. Possible correlations between CCSs and structural types were also investigated, and it was found that complex glycans tended to have slightly smaller CCSs than high-mannose glycans of comparable molecular weight. In addition, biantennary glycans containing a core fucose and/or a bisecting GlcNAc residue fell on different mobility-m/z trend lines to those glycans not so substituted with both of these substituents contributing to larger CCSs. Copyright © 2016 John Wiley & Sons, Ltd.

Fragmentation and ion mobility properties of negative ions from N-linked carbohydrates: Part 7. Reduced glycans

RATIONALE

Negative ion collision-induced dissociation (CID) spectra of released N-glycans provide very informative structural information relating to branching patterns and location of residues such as fucose. For some structural studies, particularly those involving chromatography, glycans are often reduced to avoid production of multiple peaks from α- and β-anomers. We examined the effect of reduction on the production of diagnostic fragment ions and on the ion mobility properties of N-glycans.

METHODS

Released N-glycans from the glycoproteins bovine fetuin, ribonuclease B, chicken ovalbumin, and porcine thyroglobulin were reduced with sodium cyanoborohydride and both negative ion CID spectra and ion mobility properties of their phosphate adducts were examined with a Waters Synapt G2Si travelling-wave ion mobility mass spectrometer with electrospray sample introduction. Estimated collisional cross sections were measured with dextran as the calibrant,

RESULTS

Fragment ions were similar to those from the unreduced glycans with the exception that the prominent (2,4) A cleavage ion from the reducing terminus was replaced by a prominent [M-H3 PO4](-) ion. Other ions arising from the chitobiose core were of lower relative abundance than those from the unreduced glycans. Estimated collisional cross sections were similar to those of the unreduced compounds but with symmetrical arrival time distribution (ATD) profiles, unlike those of the unreduced glycans whose peaks often contained prominent asymmetry. This observation showed that this asymmetry was due to anomer separation.

CONCLUSIONS

Reduction of the reducing terminal GlcNAc residue resulted in fewer diagnostic ions from the chitobiose core but fragmentation of the remainder of the molecules generally paralleled that of the unreduced glycans. Thus, most structural information, with the exception of the linkage position of fucose on the core GlcNAc, was available. ATD peaks were symmetrical with the result that cross sections were more appropriate for data-base searching than those from the non-reduced compounds where asymmetry produced lower precision in the measurement.

Travelling-wave ion mobility and negative ion fragmentation of high-mannose N-glycans

The isomeric structure of high-mannose N-glycans can significantly impact biological recognition events. Here, the utility of travelling-wave ion mobility mass spectrometry for isomer separation of high-mannose N-glycans is investigated. Negative ion fragmentation using collision-induced dissociation gave more informative spectra than positive ion spectra with mass-different fragment ions characterizing many of the isomers. Isomer separation by ion mobility in both ionization modes was generally limited, with the arrival time distributions (ATD) often showing little sign of isomers. However, isomers could be partially resolved by plotting extracted fragment ATDs of the diagnostic fragment ions from the negative ion spectra, and the fragmentation spectra of the isomers could be extracted by using ions from limited areas of the ATD peak. In some cases, asymmetric ATDs were observed, but no isomers could be detected by fragmentation. In these cases, it was assumed that conformers or anomers were being separated. Collision cross sections of the isomers in positive and negative fragmentation mode were estimated from travelling-wave ion mobility mass spectrometry data using dextran glycans as calibrant. More complete collision cross section data were achieved in negative ion mode by utilizing the diagnostic fragment ions. Examples of isomer separations are shown for N-glycans released from the well-characterized glycoproteins chicken ovalbumin, porcine thyroglobulin and gp120 from the human immunodeficiency virus. In addition to the cross-sectional data, details of the negative ion collision-induced dissociation spectra of all resolved isomers are discussed.

Photoisomerization action spectroscopy: flicking the protonated merocyanine–spiropyran switch in the gas phase

Molecular ions drifting through buffer gas are exposed to tunable laser light causing a detectable change in their collision cross section and arrival time at the ion detector. The effect can be exploited to obtain action electronic spectra for selected molecular isomers in the gas phase.

Optical spectroscopy coupled with mass spectrometry methods

This themed issue reports on recent progress in the fast developing field of spectroscopy of ions by means of mass spectrometry coupled with optical spectroscopy. It focuses on both the experimental and theoretical aspects of this topic.