Oligosaccharide sequence determination using B/E linked field scanning or tandem mass spectrometry of phosphatidylethanolamine derivatives

Rapid and sensitive differentiation of anomers, linkage, and position isomers of disaccharides using High-Field Asymmetric Waveform Ion Mobility Spectrometry (FAIMS)

A challenging aspect of structural elucidation of carbohydrates is gaining unambiguous information for anomers, linkage, and position isomers. Such isomers with identical mass can't be easily distinguished in mass spectrometry and a separation step is required prior to mass spectrometry identification. In our laboratory, gas-phase separation and differentiation of anomers, linkage, and position isomers of disaccharides was achieved using High-Field Asymmetric Waveform Ion Mobility Spectrometry (FAIMS). The FAIMS method responds to changes in ion mobility at high field rather than absolute values of ion mobility, and was shown to provide efficient separation and identification of disaccharide isomers at high sensitivity. Separation of analyzed disaccharide isomers can be accomplished at low nM level in a matter of seconds without sample purification or fractionation. Capability for examining a large population of ionic species of disaccharides by this method allowed for correlating structural details of disaccharide isomers with their separation properties in FAIMS. Results for disaccharide isomers indicate that this method could be applied to a larger group of carbohydrates.

Ranking of gas-phase acidities and chloride affinities of monosaccharides and linkage specificity in collision-induced decompositions of negative ion electrospray-generated chloride adducts of oligosaccharides

Negative ion electrospray-tandem mass spectrometry has been employed to study chloride adducts of saccharide molecules. Decompositions of [M + Cl]- obtained under identical low-energy collision conditions allow the approximate ranking of chloride affinities and gas-phase acidities of a series of isomeric monosaccharides. The ketohexoses are found to be more acidic than the aldohexoses. Chloride adduct decompositions are examined for a glucopyranosyl fructose and a glucopyranosyl glucose series. For each disaccharide series, the linkage position is shown to markedly influence the favored pathways of [M + Cl]- decompositions, initiated either by loss of neutral HCl to form [M - H]- and possibly leading to further (consecutive) decompositions, or by loss of M to form Cl-. Upon formation of [M - H]-, both cross-ring cleavages and glycosidic bond decompositions were observed in varying degrees for the two series of disaccharides. Remarkably, for three non-reducing polysaccharides that each contain a terminal sucrose group at the "downstream" end, chlorine-containing product ions arising from cleavage of the Glcalpha-2Fru linkage have been observed. Apart from Cl-, chlorine-containing product ions are not observed for any of the other disaccharides investigated, and they appear to be specifically diagnostic of a terminal Glcalpha-2Fru linkage. Their appearance is rationalized based upon a substantially reduced tendency for HCl loss from these non-reducing polysaccharides.

Structural analysis of the O-antigen polysaccharide from the Shiga toxin-producing Escherichia coli O172

The structure of the O-antigen polysaccharide from Escherichia coli O172 has been determined. In combination with sugar analysis, NMR spectroscopy shows that the polysaccharide is composed of pentasaccharide repeating units. Sequential information was obtained by mass spectrometry and two-dimensional NMR techniques. An O-acetyl group was present as 0.7 equivalent per repeating unit. Treatment of the O-deacetylated polysaccharide with aqueous 48% hydrofluoric acid rendered cleavage of the phosphodiester in the backbone of the polymer and the pentasaccharide isolated after gel permeation chromatography was structurally characterized. Subsequent NMR experiments on polymeric materials revealed the structure of the repeating unit of the O-polysaccharide from E. coli O172 as:-->P-4)-alpha-D-Glcp-(1-->3)-alpha-L-FucpNAc-(1-->3)-alpha-D- GlcpNAc-(1-->3)-alpha-L-FucpNAc-(1-->4)-alpha-D-Glcp6Ac-(1-->

Core-branching pattern and sequence analysis of mannitol-terminating oligosaccharides by neoglycolipid technology

The occurrence of mannitol-terminating oligosaccharides (2-substituted or 2,6-disubstituted) among the O-glycans released by alkaline borohydride treatment from glycoproteins of the nervous system has prompted the development of a microscale method to analyze the core-branching pattern and sequence by the neoglycolipid (NGL) technology, analogous to a method previously described for GalNAcol-terminating oligosaccharides (M. S. Stoll, E. F. Hounsell, A. M. Lawson, W. Chai, and T. Feizi, Eur. J. Biochem. 189, 499-507, 1990). The approach involves the selective cleavage at the core mannitol by mild periodate treatment and analysis of the reaction products as NGLs by in situ TLC/liquid secondary ion mass spectrometry. Oxidation conditions have been optimized using as reference compounds 2-, 3-, 4-, or 6-monosubstituted mannobi-itols, 3,6-disubstituted mannitol-terminating pentasaccharides, and 2-mono- and 2,6-disubstituted mannitol-terminating neutral and sialylated oligosaccharides isolated from brain glycopeptides. When a 2:1 molar ratio of periodate to alditol is used, the core mannitol is cleaved at the C3-C4 threo-diol bond and in the absence of a threo-diol cleavage occurs to a lesser extent at erythro-diols. Saccharide ring diols are not cleaved under these conditions, and it is also shown that the side chain of sialic acid on the oligosaccharide is largely unaffected. Substituents at 2- and 6-positions of the core mannitol can be identified, and the method is applicable to neutral and sialylated oligosaccharide alditols. Typically, the starting material is 5 nmol of oligosaccharide and 0.5-1 nmol of derivatives is applied for analysis. By this strategy, the core-branching pattern and position of sialic acid of two branched monosialylated mannitol-terminating oligosaccharide isomers have been determined.

High-energy collision-induced dissociation mass spectrometry of synthetic mannose-6-phosphate oligosaccharides

The high-energy collision-induced dissociation spectra of a series of linear and branched synthetic mannosyl oligosaccharides that contain 6-phosphate substituents on either or both non-reducing terminal or penultimate residues have been studied. These phosphorylated structures were designed to mimic those of naturally derived N-glycans (Man-6-PO4) on lysosomal enzymes and to probe the minimally required binding motif for the Man-6-PO4 receptors. When a phosphate group was present, the spectra were dominated by ions that arise from cleavages at the glycosidic bonds (single and double) with charge retention on the phosphate-containing fragments. The spectra of linear structures that bear the nonreducing terminal Man-6-phosphate residues were devoid of Y-type ions, unlike those with similar phosphorylation at the penultimate residue. The location of the phosphorylated residue was deduced from the presence or absence of unique B and Y ions. In neutral branched structures, the ions were formed by cleavage at the glycosidic bond at either one or both of the branch points and the aglycon, which was attached to the disubstituted mannosyl residue. Branched oligosaccharides that contained one or two terminal Man-6-PO4 residues also showed double cleavages with charge retention on the phosphate-containing fragment. Our investigation shows that positive mode high energy collision-induced dissociation mass spectrometry can determine the location-terminal or penultimate-of Man-6-PO4 residues in N-linked type oligosaccharides.

TLC-LSIMS of neoglycolipids of glycosaminoglycan disaccharides and of oxymercuration cleavage products of heparin fragments that contain unsaturated uronic acid

Heparin and chondroitin sulfate disaccharides have been investigated by high-performance (HP) TLC and liquid secondary-ion mass spectrometry (LSIMS) after conversion to neoglycolipid derivatives by reductive-amination with an aminolipid (dihexadecyl phosphatidylethanolamine, DHPE). Mobility on HPTLC was largely determined by the number of sulfate groups present, but was also influenced by the position of sulfate, monosaccharide composition and linkage. The mass spectra acquired directly from the TLC plate provided quasimolecular and fragment ions from which composition, including sulfate content, and sequence information was obtained at high sensitivity. Lipid DHPE conjugation and TLC-LSIMS were performed to analyse products of the oxymercuration reaction used to cleave unsaturated uronic acid (delta UA) residues from glycosaminoglycan (GAG) fragments produced by enzymatic degradation with glycan lyases. Previously the identification of the product from delta UA and the integrity of the remaining structures from oligosaccharides larger than disaccharide have not been made. Multiple and characteristic products of the cleaved delta UA were detected and these can be used for identification of terminal delta UA and its sulfate content. It was established with several disaccharides and a tetrasaccharide that glycosidic linkages and O- and N-sulfate groups are preserved in the remaining structures after removal of delta UA. These results indicate that the oxymercuration reaction will be applicable to generating series of GAG fragments containing unmodified sequences for biological activity studies.