Torgny Rundlöf

Long‐range proton–carbon coupling constants in conformational analysis of oligosaccharides

Long‐range heteronuclear coupling constants, 3J(C,H), were measured across glycosidic linkages in β‐linked disaccharides and also for vicinally disubstituted trisaccharides. One‐dimensional multiple‐site 13C excitation experiments using Hadamard spectroscopy and band‐selective 1H decoupling during the acquisition period were used in conjunction with pulsed field gradients for coherence selection in the 1H detected experiment. For one dihedral angle the 3J(C,H) value could only be unambiguously determined utilizing the band‐selective 1H decoupling. The 2D EXSIDE was also used for the measurement of 3J(C,H) values and the results showed excellent agreement with those of the 1D measurements. The influence of solvent, water and water–dimethyl sulfoxide (7:3), on conformation was investigated, but showed only small changes in 3J(C,H) values. In general, the long‐range heteronuclear coupling constants related to the ϕ dihedral angles (3.6–4.3 Hz) were smaller than those of the ψ dihedral angles (4.3–5.3 Hz). A comparison with 3J(C,H) values from Monte Carlo or Langevin dynamics simulations showed better agreement for the ψ dihedral angles than for the ϕ dihedral angles as calculated using a Karplus relationship.

NMR INVESTIGATION OF OLIGOSACCHARIDE CONFORMATION USING DIPOLAR COUPLINGS IN AN AQUEOUS DILUTE LIQUID CRYSTALLINE MEDIUM

The tetrasaccharide lacto‐N‐neotetraose β‐D‐Galp‐(1→4)‐β‐D‐GlcpNAc‐(1→3)‐β‐D‐Galp‐(1→4)‐D‐Glcp was dissolved in a dilute mixture of dimyristoyl‐ and dihexanoylphosphatidylcholine in deuterium oxide and 1J(C,H) spin–spin coupling constants were measured in an isotropic solution at 25°C. On raising the temperature to 38°C, an ordering of the system occurs which permits determination of dipolar couplings. An energy mini‐mized molecular mechanics model was generated showing that the molecule is anisotropic and can be treated as a cylinder. A fit of the expression for the carbon–proton dipolar couplings in a uniaxial molecule in a uniaxial liquid crystal revealed good agreement with experimental data, emphasizing the future potential of the method in studies of carbohydrate conformation.

Structural studies of the enteroinvasive Escherichia coli (EIEC) O28 O-antigenic polysaccharide

The structure of the O-specific side-chain of the lipopolysaccharide from Escherichia coli O28 has been investigated. NMR spectroscopy has been the main method used, complemented with sugar and methylation analyses. The polysaccharide contains one equivalent of O-acetyl groups per repeating unit. Selective cleavage of the O-deacetylated polymer was performed by treatment with aqueous hydrofluoric acid, and resulted in a trisaccharide-glycerol. The polysaccharide thus is of the teichoic acid type and composed of repeating units in which the trisaccharide-glycerol residues are joined by phosphodiester linkages. The O-antigen polysaccharide has the following structure. [sequence: see text] The absolute configuration of the glycerol moiety as R, )i.e., D-glycerol 1-phosphate) was determined by a new method based on TEMPO oxidation of the polysaccharide, followed by GLC analysis of the (+)-2-butyl ester of the resulting glyceric acid.

Synthesis, NMR spectroscopy and conformational studies of two vicinally disubstituted trisaccharides

Two trisaccharides with 2,3-vicinal disubstitution have been synthesized and analysed by NMR spectroscopy and Monte Carlo simulations. When vicinal disubstitution occurs in trisaccharides, experimental NMR spectra often differ significantly from those calculated by simple additivity schemes. In the present study, small deviations from additivity occurred for the carbons at the glycosylation positions of the disubstituted sugar residues in β-D-Glcp(1→2)[β-D-Glcp(1→3)]α-D-Glcp-OMe (1) and β-D-Glcp(1→2)[β-D-Glcp(1→3)]α-D-Manp-OMe (2). Monte Carlo simulations, using the HSEA force field, showed a change in the average values and in the rms fluctuations of the ψ-dihedral angles of 2 compared to the corresponding disaccharides, whereas for 1 smaller changes were observed.

Structure of the acidic microcapsular glycan from the reference strain (C.D.C. 6320-58) for Serratia marcescens serotype O14:K12.

The acidic polysaccharide from Serratia marcescens serogroup O14:K12 was analyzed by means of chemical studies and NMR spectroscopy and its repeating unit structure found to be carbohydrate sequence [see text] O-Acetyl groups are proposed to be present in non-stoichiometric amounts on O-6 on one of the hexose residues in the main chain.