Effect of stereochemistry on the electrospray ionization tandem mass spectra of transition metal chloride complexes of monosaccharides

Utilizing ESI-MS/MS and ion mobility spectrometry for structural characterization and isomer differentiation of 1, 2-unsaturated (1-4) linked disaccharide derivatives and their 2-C-functionalized analogues

RATIONALE

The 1, 2-unsaturated derivatives of (1-4) linked disaccharides serve as versatile building blocks for synthesizing biologically active compounds. Distinguishing between four pairs of stereoisomers in mixtures presents a challenging task. In this study, disaccharide derivatives are analyzed as alkali metal adducts using electrospray ionization tandem mass spectrometry (ESI-MS/MS), both as individual compounds and in mixtures by ion mobility mass spectrometry (IMS).

METHODS

Electrospray ionization (ESI) in combination with tandem mass spectrometry (MS/MS) in positive mode was employed to differentiate lithium adduct ions of hexa-acetyl/hexa-benzyl-D-lactals, and hexa-acetyl/hexa-benzyl-D-maltals, along with their corresponding 2-C-branched malonates. The high resolving power of trapped ion mobility spectrometry (TIMS) with the imeX™ functionality rapidly identified different metal adducts (Li, Na, Cs) as individual isomers and separated mixtures of stereoisomers. The measured collisional cross section (CCS) values were analyzed in relation to predicted CCS values.

RESULTS

MS/MS spectra of the [M + Li]+ ions of glycal disaccharide analogues exhibited typical cross-ring and glycosidic bond cleavages. Collision-induced dissociation (CID) spectra provided insights into their fragmentation behavior, allowing differentiation of (1-4) linked disaccharides. TIMS technology delivered adjustable ion mobility resolution for suitable separation of the four sets of stereo isomeric compounds. However, accurately predicting CCS values to differentiate between respective isomeric pairs using the SigmaCCS program for sodium adducts is only partially achievable.

CONCLUSIONS

ESI CID spectra of [M + Li]+ adduct ions for individual glycal disaccharide analogues facilitate the discrimination between alpha and beta (1-4) linked unsaturated disaccharides and their 2-C-branched analogues. Through optimized experimental conditions, complete baseline ion mobility separation of stereoisomer pairs of the Cs adducts is achieved.

Differentiation of underivatized monosaccharides by atmospheric pressure chemical ionization quadrupole time-of-flight mass spectrometry

RATIONALE

Differentiation of underivatized monosaccharides is essential in the structural elucidation of oligosaccharides which are closely involved in many life processes. So far, such differentiation has been usually achieved by electrospray ionization mass spectrometry (ESI-MS). As an alternative to ESI-MS, atmospheric pressure chemical ionization mass spectrometry (APCI-MS) should provide complementary results.

METHODS

A quadrupole time-of-flight (QTOF) mass spectrometer with accurate mass measurement ability was used with an APCI heated nebulizer ion source because we believe that a recently published article using a single quadrupole mass spectrometer assigned incorrect identities for APCI ions from hexoses. Using APCI-QTOF, the MS(2) and pseudo-MS(3) mass spectra of 11 underivatized monosaccharides were obtained under various collision voltages. The mass spectra were carefully interpreted after accurate mass measurement.

RESULTS

Differentiation of three hexoses was achieved by different MS(2) spectra of their [M + NH(4)](+) and [M - H](-) ions. The MS(2) spectra of the [M + NH(4)](+) ions were also used to distinguish methyl α-D-glucose and methyl β-D-glucose, while the pseudo-MS(3) spectra of the [M + H](+) ions were utilized to differentiate the three hexosamine and N-acetylhexosamine stereoisomers. Unique [M + O(2)](-) ions were observed and their distinctive fragmentation patterns were utilized to differentiate the three hexosamine stereoisomers.

CONCLUSIONS

Although ESI coupled with single or triple quadrupole and ion trap mass spectrometers has been widely utilized in the differentiation of monosaccharides, this report demonstrated that APCI-QTOF-MS had its own advantages in achieving the same goal.

Mass spectrometry in India

This review emphasizes the mass spectrometry research being performed at academic and established research institutions in India. It consists of three main parts covering the work done in organic, atomic and biological mass spectrometry. The review reveals that the use of mass spectrometry techniques started in the middle of the 20th century and was applied to research in the fields of organic, nuclear, geographical and atomic chemistry. Later, with the advent of soft and atmospheric ionization techniques it has been applied to pharmaceutical and biological research. In due course, several research centers with advanced mass spectrometry facilities have been established for specific areas of research such as gas-phase ion chemistry, ion-molecule reactions, proscribed chemicals, pesticide residues, pharmacokinetics, protein/peptide chemistry, nuclear chemistry, geochronological studies, archeology, petroleum industry, proteomics, lipidomics and metabolomics. Day-by-day the mass spectrometry centers/facilities in India have attracted young students for their doctoral research and other advanced research applications.

Identification of hexose diastereomers by means of tandem mass spectrometry of oxocarbenium ions followed by neural networks analysis

The ion abundances in the tandem mass spectra of oxocarbenium ions generated from aldohexoses and ketohexoses under electrospray ionization conditions depend on their stereochemistry. The ratios of the abundances of two of the major ions m/z 85 and 91 are different in the aldohexoses and the ketohexoses. Principal component analysis (PCA) allowed visual differentiation of the hexose isomers. However, identification of an individual hexose was difficult. The data were subjected to neural network analysis (NNA) using a multilayer perceptron (MLP) model. The model was first trained using the data from the 11 hexoses and then used to identify the hexoses using fresh data acquired later. There was 100% identification of the various hexoses. The hexose units in methyl glucoside, mannoside and galactoside could also be identified accurately. The method can therefore be used for the characterization of hexose units from monosaccharides and glycosides.

Tandem mass spectra of divalent metal ion adducts of glycosyl sulfides, sulfoxides and sulfones; distinction among stereoisomers

The tandem mass spectra of the divalent metal ion (Mg2+, Ca2+, Sr2+, Ba2+, Mn2+, Ni2+, Co2+ and Zn2+) adducts of acetylated 1,2-trans-glycosyl sulfides, sulfoxides and sulfones were examined using low energy collision-induced dissociation on a Quattro II quadrupole tandem mass spectrometer. Abundant doubly charged ions, such as [3M + Met]2+ and [2M + Met]2+, were observed with alkaline earth metal chlorides. The other ions observed were [M + MetCl]+, [M + MetOAc]+, [M + MetO2SPh]+ and [2M + MetCl]+. The deprotonated metal adducts [M + Met-H]+ were seen only in the sulfones. The divalent metal ion adducts showed characteristic fragmentation pathways for the glycosyl sulfides, sulfoxides and sulfones, depending on the site of metal attachment. The doubly charged metal ion adducts dissociate to two singly charged ions, [M + MetOAc]+ and [M - OAc]+, in the sulfides and sulfoxides. In the sulfones, the adducts dissociate to [M + MetO2SPh]+ and [M - O2SPh]+. In contrast to the alkaline earth metals, which attach to the acetoxy functions, the transition metals attach to the sulfide and sulfoxide functions. The metal chloride adducts display characteristic fragmentation for the sulfides, sulfoxides and sulfones. The glucosyl, mannosyl and galactosyl sulfides, sulfoxides and sulfones could be differentiated on the basis of the stereochemically controlled MS/MS fragmentations of the metal chloride adducts.

Tandem mass spectra of ammonium adducts of monosaccharides: differentiation of diastereomers

Tandem mass spectra of ammonium adducts of monosaccharides gave characteristic fragmentation patterns involving elimination of NH3/H2O followed by multiple eliminations of H2O and cross ring cleavages. Tandem mass spectra were examined over a range of collision energies (1-20 eV) on a triple-quadrupole mass spectrometer. The breakdown behavior of the ammonium adducts revealed patterns that could differentiate diastereomers of monosaccharides.