Rapid identification of disaccharides by tandem mass spectrometry

Quantification of disaccharides in solution using isomer-selective ultraviolet photodissociation of hydrogen-bonded clusters in the gas phase

Chemical properties of gas-phase hydrogen-bonded clusters were investigated as a model for interstellar molecular clouds. Cold gas-phase hydrogen-bonded clusters of tryptophan (Trp) enantiomers and disaccharide isomers, including d-maltose and d-cellobiose, were generated by electrospray ionization and collisional cooling in an ion trap at 8 K. Product ion spectra in the 265-290 nm wavelength range were obtained using tandem mass spectrometry. NH₂CHCOOH loss via the C_α-C_β bond cleavage of Trp occurred frequently in homochiral H⁺(d-Trp)(d-maltose) compared with heterochiral H⁺(l-Trp)(d-maltose) at 278 nm, indicating that an enantiomeric excess of l-Trp was formed via the enantiomer-selective photodissociation. The photoreactivity differed between the enantiomers and isomers contained in the clusters at the photoexcitation of 278 nm. A calibration curve for the quantification of disaccharide isomers in solution was constructed by photoexcitation of the hydrogen-bonded clusters of disaccharide isomers with H⁺(l-Trp) at 278 nm. A linear relationship between the natural logarithm of the relative product ion abundance and the mole fraction of d-maltose to d-cellobiose ratio in the solution was obtained, indicating that the mole fraction could be determined from a single product ion spectrum. A calibration curve, for quantification of Trp enantiomers, was also obtained using d-maltose as a chiral auxiliary.

Differentiation of disaccharide isomers via a combination of IR and UV photodissociation mass spectrometry

RATIONALE

The challenge of glycan identification due to their structural complexity and diversity has profited enormously from recent developments in mass spectrometry (MS)-related methods. For photodissociation MS, infrared (IR) and ultraviolet (UV) lasers can generate complementary fragment ions, so an effective combination of the two methods may provide rich and valuable fragmentation patterns for glycan analysis.

METHODS

A 7.0 T Fourier transform ion cyclotron resonance (FTICR) mass spectrometer equipped with a double-beam laser system was applied for the experiments. 3,5-Diiodo-L-tyrosine was selected as the assistant molecule to form complex ions with ten isomeric disaccharides through electrospray ionization. The complex ions were further isolated and irradiated by IR and UV lasers separately or continuously in the FTICR cell.

RESULTS

By combining the two complementary fragment spectra generated from the IR and UV lasers, a clear identification of all the ten isomers was achieved using their binary codes based on their fragmentation patterns. The double-beam method simplifies the experiment by introducing the two lasers sequentially in one experiment, providing richer fragmentation patterns and making the full discrimination easier.

CONCLUSIONS

This study demonstrates the capabilities of the combination of IR and UV photodissociation MS in the identification of diverse glycan isomers. The double-beam photodissociation method described here distinguished compositional, configurational and connectivity disaccharide isomers successfully. Compared with the data accumulation method based on separate IR and UV experiments, this method is simpler, faster, more flexible and also characterized by richer fragmentation patterns.

Separations of Carbohydrates with Noncovalent Shift Reagents by Frequency-Modulated Ion Mobility-Orbitrap Mass Spectrometry

An increased focus on characterizing the structural heterogeneity of carbohydrates has been driven by their many significant roles in extant life and potential roles in chemical evolution and the origin of life. In this work, multiplexed drift tube ion mobility-Orbitrap mass spectrometry methods were developed to analyze mixtures of disaccharides modified with noncovalent shift reagents. Since traditional coupling of atmospheric pressure drift tube ion mobility cells with Orbitrap mass analyzers suffers from low duty cycles (<0.1%), a frequency modulation scheme was applied to improve the signal-to-noise ratios (SNR). Several parameters such as the resolution setting and maximum injection time of the Orbitrap analyzer and the magnitude and duration of the frequency sweep were investigated for their impact on the sensitivity gains and resolution of disaccharide-shift reagent adducts. The sweep time and disaccharide concentration had a positive correlation with SNR. The magnitude of the frequency sweep had a negative correlation with SNR. However, increasing the frequency sweep improved the resolution of mixtures of disaccharide analytes. Application of frequency-modulated ion mobility-Orbitrap mass spectrometry to four noncovalently modified glucose dimers allowed for the differentiation of three out of these four analytes.

Organic acid shift reagents for the discrimination of carbohydrate isobars by ion mobility-mass spectrometry

Traveling wave and drift tube ion mobility were utilized to separate isomeric disaccharides. Organic acid shift reagents were necessary to increase the resolution of these separations for mixture analysis.

Carbohydrate isomer resolutionviamulti-site derivatization cyclic ion mobility-mass spectrometry

Cyclic ion mobility-tandem mass spectrometry enhances the separation and identification of small carbohydrate isomers.

Theoretical investigation of low detection sensitivity for underivatized carbohydrates in ESI and MALDI

Electrospray ionization (ESI) and matrix-assisted laser desorption/ionization (MALDI) mainly generate protonated ions from peptides and proteins but sodiated (or potassiated) ions from carbohydrates. The ion intensities of sodiated (or potassiated) carbohydrates generated by ESI and MALDI are generally lower than those of protonated peptides and proteins. Ab initio calculations and transition state theory were used to investigate the reasons for the low detection sensitivity for underivatized carbohydrates. We used glucose and cellobiose as examples and showed that the low detection sensitivity is partly attributable to the following factors. First, glucose exhibits a low proton affinity. Most protons generated by ESI or MALDI attach to water clusters and matrix molecules. Second, protonated glucose and cellobiose can easily undergo dehydration reactions. Third, the sodiation affinities of glucose and cellobiose are small. Some sodiated glucose and cellobiose dissociate into the sodium cations and neutral carbohydrates during ESI or MALDI process. The increase of detection sensitivity of carbohydrates in mass spectrometry by various methods can be rationalized according to these factors. Copyright © 2016 John Wiley & Sons, Ltd.

Specific interactions of leucine with disaccharides by electrospray ionization mass spectrometry: application to rapid differentiation of disaccharide isomers in combination with statistical analysis

The identification of carbohydrate isomers, including mono units, linkage positions and anomeric configurations, remains an arduous subject. In this study, the natural amino acid leucine (Leu) was found to specifically interact with cellobiose (Cello) to form a series of potassium adducts as [Cello + Leu + K](+), [Cello + 2Leu + K](+), and [2Cello + Leu + K](+) in the gas phase using mass spectrometry. By using CID-MS/MS, these complexes produced specific fragmentation patterns from the sugar backbone cleavage instead of non-covalent interactions. Moreover, their fragment distributions were dependent on the ratios of Cello-to-Leu in the complexes and the fragmentation pathways of potassium-cationized disaccharides (Dis) were remarkably changed with leucine binding. It should be pointed out that the ternary complex [2Cello + AA + K](+) was unique for leucine among all the twenty natural amino acids. The [2Dis + Leu + K](+) complex produced the most informative fragments by tandem mass spectrometry, which was successfully applied for rapid and efficient discrimination of twelve glucose-containing disaccharide isomers in combination with statistical analyses including PCA and OPLS-DA. The methodology developed here not only provides a novel analytical approach for the differentiation of disaccharide isomers, but also brings new sight towards the interactions of amino acids with disaccharides.

Distinguishing isomeric aldohexose-ketohexose disaccharides by electrospray ionization mass spectrometry in positive mode

RATIONALE

The identification of the structure of carbohydrates is challenging because of their complex composition of monosaccharide units, linkage position and anomeric configuration. We used a combination of principle component analysis (PCA) and tandem mass spectrometry (MS/MS), including collision-induced dissociation (CID) and higher energy collision dissociation (HCD), to distinguish four aldohexose-ketohexose isomers, sucrose, turanose, maltulose, and palatinose, which are composed of glucose and fructose.

METHODS

The electrospray ionization (ESI)-MS/MS spectra of the lithium and sodium adducts of the glucopyranosyl fructose (Glc-Fru) isomers were recorded on two independent mass spectrometers using CID (MicroTOF QII) and HCD (Q-Exactive Orbitrap). The differences between the fragment ions were evaluated by the PCA models. The glycosidic bond cleavage mechanism of lithiated sucrose was verified by a deuterium-labeling experiment combined with density functional theory calculations (Gaussian 09).

RESULTS

The main fragment ions in the MS/MS spectra from the glycosidic bond decomposition, cross-ring cleavage (-90 Da), and dehydration of the precursor ions of m/z 349 ([M+Li](+)) and m/z 365 ([M+Na](+)) were observed. Surprisingly, cross-ring cleavage and dehydration of the precursor ions were rarely observed in both lithiated and sodiated sucrose. There were significant differences in the fragmentation patterns and relative abundances of fragment ions in second-order mass spectrometry, which allowed discriminant models to be constructed for the alkali adducts and collision modes.

CONCLUSIONS

Glc-Fru isomers were discriminated in the PCA score plots for their lithium and sodium adducts by using different collision modes. The results showed that HCD-MS/MS is an ideal tool for differentiating lithium adducts, whereas, CID-MS/MS is better for discriminating sodium adducts. The hydrogen migration of the hydroxyl group at C3 of the fructose unit caused the glycosidic bond decomposition of lithiated sucrose.