Chemical ionization and electron impact mass spectra of oligosaccharides derived from sphingoglycolipids

Chemical ionization (CI) mass spectra with isobutane and ammonia for the oligosaccharides obtained from sphingoglycolipids were compared with their electron impact (EI) mass spectra. The oligosaccahride moieties were liberated from the parent glycolipids and were further reduced with sodium borohydride. They were analyzed as their permethyl peracetyl and pertrimethylsilyl derivatives. In the CI spectra, peaks corresponding to QM+ and/or [M-59]+ were observed in all of the peracetylated oligosaccharides examined. In CI with ammonia as the reagent, H+ was transferred to nitrogen-containing saccharides to produce [MH]+ and NH4 was transferred to nitrogen-free saccharides to yield [M+NH4]+ as QM+. Non-reducing ends yielded very intense peaks in CI spectra. On the other hand, the reduced end, glucitol, produced rather prominent peaks in EI spectra. Fragment ions due to cleavage of glycosidic bonds were major ones under the CI conditions, and they could be used for elucidating the sugar sequence in the oligosaccharides. An additional characteristic feature in the CI spectra was that ions due to scission of hexosaminyl glycosidic linkages were observed with very high intensities.     

The electron impact, chemical ionization and fast atom bombardment positive ion mass spectra of 1,2-bis(sulfonyl)methylhydrazines.

A series of bis(sulfonyl)-1-methylhydrazines were analyzed by positive ion electron impact (EI), chemical ionization (CI) and fast atom bombardment (FAB) mass spectrometry. Since these compounds showed activity against the L1210 leukemia, an understanding of their mass spectral behavior is important should the structural characterization of metabolites be required. FAB proved to be the most useful technique, generally providing abundant protonated molecule ion peaks, in contrast to the weak peaks observed with CI (ammonia or isobutane) and the total absence of molecular ion peaks in the EI mass spectra. In addition, utilizing FAB eliminated the problem of thermal decomposition, which was very difficult to control under EI and CI experimental conditions. Fragments observed in FAB and CI mass spectra were consistent with protonation at the methyl-bearing nitrogen. One can locate the R1 and R2 moieties relative to the methyl-bearing nitrogen in FAB and CI by assigning that nitrogen as the site of protonation, with subsequent elimination of R2SO2H.     

Syntheses and reactions of 5-O-acetyl-1,2-anhydro-3-O-benzyl-alpha-D-ribofuranose and beta-D-lyxofuranose, 5-O-acetyl-1,2-anhydro-3,6-di-O-benzyl- and 1,2-anhydro-5,6-di-O-benzoyl-3-O-benzyl-beta-D-mannofuranose, and 6-O-acetyl-1,2-anhydro-3,4-di-O-benzyl-alpha-D-glucopyranose and -beta-D-talopyranose

The title compounds 5-O-acetyl-1,2-anhydro-3-O-benzyl-alpha-D-ribofuranose and 5-O-acetyl-1,2-anhydro-3-O-benzyl-beta-D-lyxofuranose, and 6-O-acetyl-1,2-anhydro-3,4-di-O-benzyl-alpha-D-glucopyranose and 6-O-acetyl-1,2-anhydro-3,4-di-O-benzyl-beta-D-talopyranose, and 5-O-acetyl-1,2-anhydro-3,6-di-O-benzyl-beta-D-mannofuranose and 1,2-anhydro-5,6-di-O-benzoyl-3-O-benzyl-beta-D-mannofuranose have each been synthesized from the corresponding 2-O-tosylate and 1-free hydroxyl intermediates by base-initiated intramolecular S(N)2 ring closure in almost quantitative yields. Acetyl and benzoyl groups were not affected in the ring closure reactions. Condensation of 6-O-acetyl-1,2-anhydro-3,4-di-O-benzyl-alpha-D-glucopyranose and 5-O-acetyl-1,2-anhydro-3,6-di-O-benzyl-beta-D-mannofuranose with 1,2:3,4-di-O-isopropylidene-alpha-D-galactopyranose in the presence of ZnCl2 as the catalyst afforded the 1,2-trans-linked 6-O-acetyl-3,4-di-O-benzyl-beta-D-glucopyranosyl-(1-->6)-1,2:3,4-di-O-isopropylidene-alpha-D-galactopyranose and 5-O-acetyl-3,6-di-O-benzyl-alpha-D-mannofuranosyl-(1-->6)-1,2:3,4-di-O-isopropylidene-alpha-D-galactopyranose as the sole products in satisfactory yields, while condensation of 5-O-acetyl-1,2-anhydro-3-O-benzyl-beta-D-lyxofuranose with 3-O-benzyl-1,2-O-isopropylidene-alpha-D-xylofuranose yielded the 1,2-trans-linked 5-O-acetyl-3-O-benzyl-alpha-D-lyxofuranosyl-(1-->5)-3-O-benzyl-1,2-O-isopropylidene-alpha-D-xylofuranose as the sole product in a good yield. The 6-O-acetyl group in the glycosyl donor, 6-O-acetyl-1,2-anhydro-3,4-di-O-benzyl-alpha-D-glucopyranose, did not influence the stereoselectivity of the ring-opening-coupling reaction.     

Negative ion ammonia chemical ionization and electron impact ionization mass spectrometric analysis of steryl fatty acyl esters

Synthesis of steryl palmitates, varied in the nature of the steryl moiety, provided model compounds for investigation of the mass spectrometric behavior of steryl long-chain fatty acyl esters. The structure of the steryl moiety was varied according to: (i) position and degree of unsaturation in the steroid nucleus and C-17 side-chain, (ii) position and degree of methylation, (iii) presence or absence of a 9 beta, 19-cyclopropane ring. Compounds were chosen so as to be representative of biochemically important steryl esters. Electron impact (EI) behavior of steryl palmitate esters closely resembles that of their short-chain (e.g. acetate) counterparts. M+.ions were generally weak or absent and the major high mass ions arose from characteristic fragmentations of the steroid nucleus following loss of the acyl moiety ([M-RCO2H]+.). Fragment ions characteristic of the acyl moiety were lacking. Negative ion chemical ionization (NICI) using ammonia as reagent gas, on the other hand, afforded spectra containing characteristic fragment ions [RCO2]-, [RCO2-18]-, and [RCO2-19]- from which the nature of the fatty acyl moiety can be readily deduced. Hence, NICI and EI provide complementary means of ionization for the mass spectrometric determination of structures of steryl esters.     

Alignment of high resolution mass spectra: development of a heuristic approach for metabolomics

One of the challenges of using mass spectrometry for metabolomic analyses of samples consisting of thousands of compounds is that of peak identification and alignment. This paper addresses the issue of aligning mass spectral data from different samples in order to determine average component m/z peak values. The alignment scheme developed takes the instrument m/z measurement error into consideration in order to heuristically align two or more samples using a technique comparable to automated visual inspection and alignment. The results obtained using mass spectral profiles of replicate human urine samples suggest that this heuristic alignment approach is more efficient than other approaches using hierarchical clustering algorithms. The output consists of an average m/z and intensity value for the spectral components together with the number of matches from the different samples. One of the major advantages of using this alignment strategy is that it eliminates the boundary problem that occurs when using predetermined fixed bins to identify and combine peaks for averaging and the efficient runtime allows large datasets to be processed quickly.     

Chemical ionization mass spectrometry of iprodione and its metabolites

One of the cornerstones of an architectural approach adopted by this laboratory in screening vegetables for pesticides and industrial chemicals is the extensive use of element-selective gas chromatographic detectors followed by gas chromatography/mass spectrometry. In this particular case history, a recently introduced European fungicide, iprodione, was thought to be the first reported incidence in mache imported from France. An analytical protocol involving chemical ionization was devised to confirm this finding as well as search for possible potential metabolites.     

Fragmentation pattern of permethyl 6-C-glycosylflavones in electron impact mass spectrometry

A mass spectral fragmentation pattern of permethyl 6-C-glycosylflavones is proposed from the MS data of permethyl derivatives mono-O-deuteriomethylated in the 2″-, 3″-, 4″- or 6″-positions. The synthesis of these compounds via O″-glycosyl-6-C-glucosylflavones is described.     

Quantification of electron energy-loss spectra with K and L shell ionization cross-sections

Quantitative EELS microanalysis in the analytical electron microscope involves the use of partial scattering cross-sections to relate measured edge intensities of the elements to their composition.To test the accuracy of the presently available quantification procedures for K and L edges, we have quantified a wide range of oxidic compounds by using cross-sections which have been calculated by the efficiency factor method and the hydrogenic approximation.An accuracy for element ratios of the order 10–30% can be obtained with these calculated cross-sections for small collection angles (e.g. 5.9 mrad), whereas quantification of spectra recorded with very large collection angles can result in higher errors.Furthermore, partial scattering cross-sections for L edges have been determined by the use of stoichiometric oxide standards and these cross-sections have been found to provide more accurate quantification results when compared to the calculated cross-section.     

Synthesis of some derivatives of 6-amino-1,5-anhydro-6-deoxy-d-glucitol and 2-amino-1,5-anhydro-2-deoxy-d-glucitol

6-Amino-1,5-anhydro-6-deoxy-d-glucitol (11) was prepared from 2,3,4,6-tetra-O-acetyl-α-d-glucopyranosyl bromide (1) in six steps. Reduction of 1 with tributyltin hydride, followed by deacetylation, monomolar tosylation, and reacetylation, afforded 2,3,4-tri-O-acetyl-1,5-anhydro-6-O-toluene-p-sulfonyl-d-glucitol (9). Alternatively, tritylation of 1,5-anhydro-d-glucitol, followed by acetylation, detritylation, and tosylation, gave 9. Mesylation gave 8. Treatment of 8 or 9 with azide anion afforded the azide 10, reduction of which with tributyltin hydride gave 11, which was mesylated or tosylated, and then deacetylated to give the 6-methane-sulfonamido or 6-toluene-p-sulfonamido derivative. Similarly, mesylation or tosylation of 3,4,6-tri-O-acetyl-2-amino-1,5-anhydro-2-deoxy-d-glucitol (20) gave the 2-methanesulfonamido or 2-toluene-p-sulfonamido derivatives. Treatment of 11 and 20 with sulfur trioxide-pyridine afforded the sulfoamino derivatives, deacetylation of which gave sugar analogs of cyclamate-like compounds.     

Comparison of electron and chemical ionization mass spectrometry of sialic acids

Sialic acids were analyzed as per-O-trimethylsilylated compounds by gas-liquid chromatography/mass spectrometry either on electron or chemical ionization by isobutane. Electron ionization mass spectra of these derivatives are very similar to those of the corresponding methyl esters described earlier whereas chemical ionization mass spectra are characterized in the high mass range by loss of the C-2 and the C-4 substituents from the M + 1 ion. Together with other fragment ions the seven different sialic acids analyzed could be clearly identified.     

Patchwork peptide sequencing: extraction of sequence information from accurate mass data of peptide tandem mass spectra recorded at high resolution

The accurate mass values of all immonium, y(1), y(2), a(2), and b(2) ions of tryptic peptides composed of the 20 standard amino acids were calculated. The differences between adjacent masses in this data set are greater than 10 mDa for more than 80% of the values. Using this mass list, the majority of low mass ions in quadrupole-time of flight tandem mass spectra of peptides from tryptic digests and from an elastase digest could be assigned. Besides the a(2)/b(2) ions, which carry residues 1-2 from the N-terminus, a variety of internal dipeptide b ions were regularly observed. In case internal proline was present, corresponding dipeptide b ions carrying proline at the N-terminal position occurred. By assigning the dipeptide b ions on the basis of their accurate mass, bidirectional or unidirectional sequence information was obtained, which is localized to the peptide N-terminus (a(2)/b(2) ions) or not localized (internal b ions). Identification of the y(1) and y(2) ions by their accurate mass provides unidirectional sequence information localized to the peptide C-terminus. It is shown that this patchwork-type sequence information extractable from accurate mass data of low-mass ions is highly efficient for protein identification.     

Structural characterization of wax esters by electron ionization mass spectrometry

The interpretation of the electron ionization mass spectra of straight-chain and methyl-branched saturated and unsaturated wax esters (WEs) is discussed in this study based on the spectra of 154 standards. The most important fragments indicative of the structure of the acid and alcohol chains are identified and summarized for WEs with various number of double bonds in the chains. Briefly, most WEs provide acylium ions allowing structural characterization of the acid part, whereas the alcohol part gives corresponding alkyl radical cations. The elemental composition of selected important fragments is established from a high-resolution accurate mass analysis. The ion abundances are discussed with respect to the length and unsaturation of the aliphatic chains. The interpretation of the spectra of branched or unsaturated WEs requires the recognition of small but important peaks that are difficult to discern among the other fragments. We demonstrate that such fragments are easily detected in differential mass spectra. This approach requires spectra of WE standards (e.g., straight-chain analogs in the case of branched WEs) recorded under the same experimental conditions. The WEs mass spectral database provided in the supplemental data can be used as a reference for the analysis of the GC/EI-MS data.     

Applications of ion mobility mass spectrometry for high throughput,high resolution glycan analysis

BackgroundDiverse varieties of often heterogeneous glycans are ubiquitous in nature. They play critical roles in recognition events, act as energy stores and provide structural stability at both molecular and cellular levels. Technologies capable of fully elucidating the structures of glycans are far behind the other ‘-omic’ fields. Liquid chromatography (LC) and mass spectrometry (MS) are currently the most useful techniques for high-throughput analysis of glycans. However, these techniques do not provide full unambiguous structural information and instead the gap in full sequence assignment is frequently filled by a priori knowledge of the biosynthetic pathways and the assumption that these pathways are highly conserved.Scope of the reviewThis comprehensive review details the rise of the emerging analytical technique ion mobility spectrometry (IMS) (coupled to MS) to facilitate the determination of three-dimensional shape: the separation and characterization of isobaric glycans, glyco(peptides/proteins), glycolipids, glycosaminoglycans and other polysaccharides; localization of sites of glycosylation; or interpretation of the conformational change to proteins upon glycan binding.Major conclusionsIMS is a highly promising new analytical route, able to provide rapid isomeric separation (ms timescale) of either precursor or product ions facilitating MS characterization. This additional separation also enables the deconvolution of carbohydrate MS(/MS) information from contaminating ions, improving sensitivity and reducing chemical noise. Derivation of collision cross sections (CCS) from IM-MS(/MS) data and subsequent calculations validate putative structures of carbohydrates from ab initio derived candidates. IM-MS has demonstrated that amounts of specific glycan isomers vary between disease states, which would be challenging to detect using standard analytical approaches.General significanceIM-MS is a promising technique that fills an important gap within the Glycomics toolbox, namely identifying and differentiating the three-dimensional structure of chemically similar carbohydrates and glycoconjugates. This article is part of a Special Issue entitled "Glycans in personalised medicine" Guest Editor: Professor Gordan Lauc.     

Mass spectra of complex molecules. I. Chemical ionization of sphingolipids

Methane chemical ionization spectra of acetylated and perdeutero-acetylated ceramides, cerebrosides, and ceramide dihexosides have been analyzed and compared with electron impact ionization spectra of the same compounds. Abundant fragment ions for the loss of acetic acid from the protonated molecular ion in C.I. spectra readily enable the determination of the molecular weights of the principal species of sphingolipid mixtures. Complete structures can be elucidated by the combination of E.I. and C.I. mass spectra.