Analysis of N-acetylated hexosamine monosaccharides by ferrocenyl boronation and tandem electrospray ionization mass spectrometry

Molecular identification in metabolomics using infrared ion spectroscopy

Small molecule identification is a continually expanding field of research and represents the core challenge in various areas of (bio)analytical science, including metabolomics. Here, we unequivocally differentiate enantiomeric N-acetylhexosamines in body fluids using infrared ion spectroscopy, providing orthogonal identification of molecular structure unavailable by standard liquid chromatography/high-resolution tandem mass spectrometry. These results illustrate the potential of infrared ion spectroscopy for the identification of small molecules from complex mixtures.

Single electron redox via an ion-neutral complex in the fragmentation of protonated benzoylferrocenes

RATIONALE

Ferrocene derivatives have become very popular molecules for biological applications. Although considerable experimental and theoretical calculation studies have demonstrated that ferrocene derivatives are easily oxidized during electrospray ionization (ESI), the details of the single electron redox reaction for protonated benzoylferrocenes in collision-induced dissociation (CID) mass spectrometry (MS) has not been obtained. Characterizing this mechanism is useful for further understanding the properties of ferrocene-containing biomaterials.

METHODS

All CID MS experiments were carried out using ESI ion trap mass spectrometry in positive ion mode. In addition, the accurate mass of fragments was measured on a ESI quadrupole time-of-flight (QTOF) mass spectrometer in positive ion mode. Theoretical calculations were carried out by the density functional theory (DFT) method with a hybrid basis set consisting of 6-31G (d) and ECP LanL2DZ in the Gaussian 03 program.

RESULTS

In the fragmentation of protonated benzoylferrocenes, the characterized ferrocinium cation was observed, which was proposed from the cleavage of the bond between the ipso-carbon atom and the carbonyl carbon followed by a single electron redox in [substituted benzoyl/ferrocene] complexes. Moreover, when the complex contained an oxidant (substituted benzoyl cation) with higher electron affinity, the single electron redox reaction was more efficient. A correlation was established between the intensities of the two competitive product ions and the electron affinities of substituted benzoyl cations.

CONCLUSIONS

The single electron redox reaction by the [substituted benzoyl/ferrocene] complexes was proposed by CID MS and theoretical calculations, which provided potential evidence to further understand the reversible reduction characteristics of ferrocene-containing biomaterials.

Furanose C-C-linked γ-lactones: a combined ESI FTICR MS and semi-empirical calculations study

Sugars that incorporate the unsaturated carbonyl motif have become important synthetic targets not only as a result of their potential biological properties but also as precursors in the synthesis of many bioactive products. Moreover, little is known about the influence of the γ-lactone moiety in the fragmentation pattern of furanose rings. Therefore, two α,β-unsaturated γ-lactones (butenolides) and two β-hydroxy γ-lactones, C-C linked to a furanose ring were studied using electrospray ionization FTICR mass spectrometry. The behaviour of the protonated and sodiated forms of the compounds under study has been compared considering their structural features. Fragmentation mechanisms were established and ion structures were proposed taking into account the MS(2) and MS(3) experiments, accurate mass measurements and semi-empirical calculations. These inexpensive methods proved to be a valuable resource for proposing protonation sites and for the establishment of fragmentation pathways.

Enhanced carbohydrate structural selectivity in ion mobility-mass spectrometry analyses by boronic acid derivatization

The boronic acid derivatization of carbohydrates is demonstrated as an ion mobility shift strategy to improve confidence in the identification and characterization of carbohydrate assignments using ion mobility-mass spectrometry.

Sequencing and characterization of oligosaccharides using infrared multiphoton dissociation and boronic acid derivatization in a quadrupole ion trap

A simplified method for determining the sequence and branching of oligosaccharides using infrared multiphoton dissociation (IRMPD) in a quadrupole ion trap (QIT) is described. An IR-active boronic acid (IRABA) reagent is used to derivatize the oligosaccharides before IRMPD analysis. The IRABA ligand is designed to both enhance the efficiency of the derivatization reaction and to facilitate the photon absorption process. The resulting IRMPD spectra display oligosaccharide fragments that are formed from primarily one type of diagnostic cleavage, thus making sequencing straightforward. The presence of sequential fragment ions, a phenomenon of IRMPD, permit the comprehensive sequencing of the oligosaccharides studied in a single stage of activation. We demonstrate this approach for two series of oligosaccharides, the lacto-N-fucopentaoses (LNFPs) and the lacto-N-difucohexaoses (LNDFHs).

The distinction of underivatized monosaccharides using electrospray ionization ion trap mass spectrometry

A convenient method for distinguishing underivatized isomeric monosaccharides has been established using electrospray ionization ion trap mass spectrometry (ESI-ITMS). Mass spectra of hexoses (glucose, galactose, and mannose), N-acetylhexosamines (N-acetylglucosamine, N-acetylgalactosamine, and N-acetylmannosamine) and hexosamines (glucosamine, galactosamine, and mannosamine) dissolved in solvent containing 1 mM ammonium acetate were obtained in the positive ion mode. Glucose was distinguished from galactose and mannose in the MS(2) spectrum of the [M+NH(4)](+) ion at m/z 198. The MS(3) spectra generated from [M+NH(4)-H(2)O-NH(3)](+) at m/z 163 showed that galactose and mannose could be distinguished by the ratio of peak intensities at m/z 145 and 127, while the three N-acetylhexosamine and hexosamine stereochemical isomers could be identified by the relative abundance ratios of product ions observed in MS(3) spectra. The investigation of MS and MS(2) spectra from complexes of these monosaccharides with Na(+) and Pb(2+) failed to distinguish these monosaccharide isomers. Therefore, multiple stage mass analysis by ESI-ITMS using either [M+NH(4)](+) or [M+H](+) was useful to distinguish between the isomers of monosaccharides.

Derivatization in mass spectrometry--8. Soft ionization mass spectrometry of small molecules

This is the first of two reviews devoted to derivatization approaches for "soft" ionization mass spectrometry (FAB, MALDI, ESI, APCI) and deals, in particular, with small molecules. The principles of the main "soft" ionization mass spectrometric methods as well as the reasons for derivatizing small molecules are briefly described. Derivatization methods for modification of amines, carboxylic acids, amino acids, alcohols, carbonyl compounds, monosaccharides, thiols, unsaturated and aromatic compounds etc. to improve their ionizability and to enhance structure information content are discussed. The use of "fixed"-charge bearing derivatization reagents is especially emphasized. Chemical aspects of derivatization and "soft" ionization mass spectrometric properties of derivatives are considered.

Letter: Influence of experimental conditions on electrospray ionization mass spectrometry of ferrocenylalkylazoles

Influence of experimental conditions on electrospray/ionization (ESI) mass spectra of ferrocene derivatives FcCHRAz (Fc = eta5-C5H5-Fe-eta5- C5H4; R = H, Az = benzimidazole; R = Ph, Az = 2-methylimidazole) has been investigated. The spectra of all the compounds revealed [M]+*, product of its fragmentation [FcCHR]+ as well as products of ion/molecular interactions (protonated molecule [MH]+, binuclear ion [(FcCHR)2 Az]+, dimeric ion [M2]+* and its protonated form [M2H]+). Relative abundances of these ions appreciably (more than one order) depend on experimental conditions: analyte concentration, temperature of heated capillary, spray voltage, flow rate of mobile phase and polarity of solvents. Established correlations allow the selection of optimum experimental conditions for registration of ESI mass spectra, as required by the application. If an unknown compound is to be identified, it is necessary to operate by using polar solvents, small concentration, low temperature of heated capillary, high spray voltage and flow rates. There are high-intensity binuclear and protonated dimeric ions in mass spectra under other conditions. It can give rise to wrong interpretation of the structure of investigated compound. At the same time, for study of ion-molecular processes by ESI-MS it is necessary to use concentrated samples in non-polar solvents. In this case the dependence of reaction products yields on temperature and flow rate of mobile phase must be investigated.

Structural characterization of a series of 10-carbon sugar derivatives by electrospray-ionization MSn mass spectrometry

Electrospray-ionization MSn mass spectrometry (ESI-MSn) with low-energy, collision-induced dissociation (CID) was used to establish the fragmentation behavior of sodium ion adducts of higher-carbon amino spiro-sugar derivatives. Their fragmentation pathways are proposed on the basis of the MSn studies and deuteration experiments. Some of the rings of these derivatives opened under the conditions of electrospray ionization. Novel fragmentations were observed and their mechanisms are proposed. This study demonstrates the power of modern mass spectrometry for rapid elucidation of the structure of higher-carbon sugar derivatives.

Differentiation of Underivatized Diastereomeric Hexosamine Monosaccharides and Their Quantification in a Mixture Using the Kinetic Method under Electrospray Ionization Conditions

A simple method to differentiate underivatized diastereomeric hexosamine monosaccharides, glucosamine, galactosamine, and mannosamine is reported by applying the kinetic method using N-acetylhexosamines or naturally occurring amino acids as reference bases under electrospray ionization conditions. The observed differences to distinguish the diastereomeric hexosamines are found mainly due to the proton affinity (PA) differences between the analyte and the reference base. The PA values of the hexosamines are not available in the literature, and hence, we estimated them by the kinetic method using N-acetylhexosamines as reference bases. The determined PA values are 223.97 kcal/mol for glucosamine, 224.99 kcal/mol for mannosamine, and 224.71 kcal/mol for galactosamine. The similar PA values were also obtained by using amino acids as reference bases. We have applied the same methodology to quantify these hexosamines in a mixture following the three-point calibration method suggested in the literature.

Review: Derivatization in mass spectrometry--4. Formation of cyclic derivatives

This fourth in a series of reviews describes a further common derivatization approach, namely, the formation of cyclic derivatives (cyclic acetals and ketals, boronates, siliconides, carbonates and other miscellaneous derivatives) that can be used to increase volatility and to improve chromatographic and, if possible, the mass spectral properties of various di- and polyfunctional compounds. Some chemical aspects of this type of derivatization are briefly discussed. Characteristic mass spectral features of various cyclic derivatives that are helpful in the structure determination, profiling and quantitation of multifunctional organic compounds are presented. Some recent analytical applications of mass spectrometry in conjunction with preliminary cyclic derivative formation are given.

Ratiometric analysis of the ferrocene boronate esters of 2- and 4-hydroxyestradiol by tandem electrospray mass spectrometry

A method has been developed for the semiquantitative analysis of the catechol estrogens, 2- and 4-hydroxyestradiol, using tandem electrospray mass spectrometry in a quadrupole ion trap mass analyzer. The implication of catechol estrogens in the biogenesis of breast and prostate cancer makes these labile lipophilic compounds important analytical targets. Ferrocene boronic acid is reacted with 2- and 4-hydroxyestradiol to form their cyclic boronate esters. Sample ionization is accomplished during the electrospray process by a one-electron oxidation of the ferrocene functionality to form the radical cation. The analysis depends on a non-aqueous solvent system consisting of 90% acetonitrile and 10% dichloromethane with 100 microM lithium triflate as the supporting electrolyte. The sensitivity of the analysis is greatly increased by the use of a novel electrospray interface with a large surface area stainless steel electrode coupled to a pulled fused-silica needle. Collision-induced dissociation of the selected molecular ion within the ion trap produces fragment ion spectra that can be used to distinguish between the two isobaric isomers and ultimately determine the relative amounts in mixtures containing both components. The method is sensitive to analyte concentrations in the low nM range.

Analysis of neutral isomeric low molecular weight carbohydrates using ferrocenyl boronate derivatization and tandem electrospray mass spectrometry

A new analytical technique for small carbohydrates utilizing the cyclic ferrocenyl boronic esters (FcBors) of several neutral mono- and disaccharides is demonstrated. Distinction between the diastereomers of mono- and disaccharides is obtained. Analysis is by tandem electrospray-mass spectrometry (ES-MS) using a modified ion-source that promotes the preformation of ions. Selection of the molecular ion produced during single-electron oxidation of the ferrocene moiety of a specific population of saccharide isomers permits a variety of collisionally induced dissociation (CID) experiments. The resultant MS2/MS3 spectra reflect the ensemble of possible cyclic esters in equilibrium. An array of stable cross pyranose ring fragment ions representing sequential carbon loss as 30 u is observed. Consequently, the system provides an information-rich set of MSn spectra containing large amounts of structural information. Identification of D-glucose (D-Glc), its two commonly found epimers (D-mannose and D-galactose), and the two major L-diastereomers of 6-dideoxymonosaccharides, L-fucose (L-Fuc) and L-rhamnose (L-Rhm), are demonstrated. Selected pairs of disaccharides can be distinguished in terms of their anomeric linkage by reference to their MS3 spectra.