No abstract

Xylose is the optimal substrate for β4GalT7, an essential enzyme in GAG biosynthesis, but analogs act as effective inhibitors.

(1)H and (13)C NMR chemical shift data are used by the computer program CASPER to predict chemical shifts of oligo- and polysaccharides. Three types of data are used, namely, those from monosaccharides, disaccharides, and trisaccharides. To improve the accuracy of these predictions we have assigned the (1)H and (13)C NMR chemical shifts of eleven monosaccharides, eleven disaccharides, twenty trisaccharides, and one tetrasaccharide; in total 43 compounds. Five of the oligosaccharides gave two distinct sets of NMR resonances due to the α- and β-anomeric forms resulting in 48 (1)H and (13)C NMR chemical shift data sets. In addition, the pyranose ring forms of Neu5Ac were assigned at two temperatures, due to chemical shift displacements as a function of temperature. The (1)H NMR chemical shifts were refined using total line-shape analysis with the PERCH NMR software. (1)H and (13)C NMR chemical shift predictions were subsequently carried out by the CASPER program (http://www.casper.organ.su.se/casper/) for three branched oligosaccharides having different functional groups at their reducing ends, namely, a mannose-containing pentasaccharide, and two fucose-containing heptasaccharides having N-acetyllactosamine residues in the backbone of their structures. Good to excellent agreement was observed between predicted and experimental (1)H and (13)C NMR chemical shifts showing the utility of the method for structural determination or confirmation of synthesized oligosaccharides.

The general molecular properties and in particular, the molar mass of lignin are of central importance for industrial applications, as these data govern important thermal and mechanical characteristics. The focus of the present paper is pulsed field gradient-nuclear magnetic resonance (PFG-NMR), which is suitable for determination of lignins’ weight-average molar mass, based on diffusion constants. The method is calibrated by lignin fractions characterized by matrix-assisted laser desorption/ionization time of flight mass spectrometry (MALDI-TOF-MS). It could be demonstrated on a set of softwood kraft lignins that the PFG-NMR approach gives results in very good agreement with those obtained using conventional size exclusion chromatography (SEC).

No abstract

The predominantly populated conformation of carbohydrates in solution does not necessarily represent the biologically active species; rather, any conformer accessible without too large an energy penalty may be present in a biological pathway. Thus, the conformational preferences of a naphthyl xyloside, which initiates in vivo synthesis of antiproliferative glycosaminoglycans, have been studied by using NMR spectroscopy in a variety of solvents. Equilibria comprising the conformations (4)C1, (2)SO and (1)C4 were found, with a strong dependence on the hydrogen bonding ability of the solvent. Studies of fluorinated analogues revealed a direct hydrogen bond from the hydroxyl group at C2 to the fluorine atom at C4 by a (1h)JF4,HO2 coupling. Hydrogen bond directionality was further established via comparisons of fluorinated levoglucosan molecules.

Mannopyranosyluronic acids display a very unusual conformation behavior in that they often prefer to adopt a (1)C4 chair conformation. They are endowed with a strikingly high reactivity when used in a glycosylation reaction as a glycosyl donor. To investigate the unusual conformational behavior a series of mannuronic acid ester derivatives, comprising anomeric triflate species and O-methyl glycosides, was examined by dynamic NMR experiments, through lineshape analysis of (1)H and (19)F NMR spectra at various temperatures from -80 °C to 0 °C. Exchange rates between (4)C1 and (1)C4 chair conformations were found to depend on the electronic properties and the size of the C2 substituent (F, N3 or OBn) and the aglycon, with higher exchange rates for the glycosyl triflates and smaller C2 substituents. Low temperature (19)F exchange spectroscopy experiments showed that the covalently bound anomeric triflates did not exchange with free triflate species present in the reaction mixture. To relate the conformational behavior of the intermediate triflates to their reactivity in a glycosylation reaction, their relative reactivity was determined via competition reactions monitored by (1)H NMR spectroscopy at low temperature. The 2-O-benzyl ether compound was found to be most reactive whereas the 2-fluoro compound - the most flexible of the studied compounds - was least reactive. Whereas the ring-flip of the mannuronic acids is important for the enhanced reactivity of the donors, the rate of the ring-flip has little influence on the relative reactivity.

Carbohydrates, also known as glycans in biological systems, are omnipresent in nature where they as glycoconjugates occur as oligo‐ and polysaccharides linked to lipids and proteins. Their three‐dimensional structure is defined by two or three torsion angles at each glycosidic linkage. In addition, transglycosidic hydrogen bonding between sugar residues may be important. Herein we investigate the presence of these inter‐residue interactions by NMR spectroscopy in D2O/[D6]DMSO (70:30) or D2O and by molecular dynamics (MD) simulations with explicit water as solvent for disaccharides with structural elements α‐d‐Manp‐(1→2)‐d‐Manp, β‐d‐GlcpNAc‐(1→2)‐d‐Manp, and α‐d‐Glcp‐(1→4)‐β‐d‐Glcp, all of which have been suggested to exhibit inter‐residue hydrogen bonding. For the disaccharide β‐d‐GlcpNAc‐(1→2)‐β‐d‐Manp‐OMe, the large extent of O5′⋅⋅⋅HO3 hydrogen bonding as seen from the MD simulation is implicitly supported by the 1H NMR chemical shift and 3JHO3,H3 value of the hydroxy proton. In the case of α‐d‐Glcp‐(1→4)‐β‐d‐Glcp‐OMe, the existence of a transglycosidic hydrogen bond O2′⋅⋅⋅HO3 was proven by the presence of a cross‐peak in 1H,13C HSQC‐TOCSY experiments as a result of direct TOCSY transfer between HO3 of the reducing end residue and H2′ (detected at C2′) of the terminal residue. The occurrence of inter‐residue hydrogen bonding, albeit transient, is judged important for the stabilization of three‐dimensional structures, which may be essential in maintaining a conformational state for carbohydrate–protein interactions of glycans to take place in biologically important environments.