V. Michon

Conformational analysis of nitrilium intermediates in glycosylation reactions

In order to shed light on the stereoselectivity of acetonitrile-mediated glycoslation reactions, the nitrilium intermediates have been genegrated at —30°C in acetonintrile-d3 and studied by 1H, 13C, and 15N NMR spectroscopy. Conjointly, molecular modeling of the intermediates by molecular mechanics has been performed with the CHARMm force field. All the evidence suggests that only nitrilium derivatives with the α configuration are formed.

New investigations of the structure of grape arabino-galactan-protein

The structure of an arabinogalactan-protein (AGP) isolated from grape juice was studied by methylation analysis, n.m.r. spectroscopy, and interactions with peanut lectin, after specific degradation with purified enzymes and/or Smith degradation. AGP appeared to be homogeneous with a weight-average molecular weight of 110,000. Treatment of AGP with arabinofuranosidase released 88% of the arabinose and left GP1. Hydrolysis of GP1 with an endo-(1----6)-beta-D-galactanase removed 50% of the galactose and left GP2. Smith degradation of GP1 gave a 3-linked galactan that still contained 3,6-linked residues. Endogalactanase- and Smith-degraded GP1, but not AGP and GP1, reacted strongly with peanut lectin. Thus, AGP is a 3-linked galactan cross-linked at positions 6. The core also carries, at positions 6, 6-linked galactan chains heavily 3-substituted with arabinofuranose residues.

Preparation, structural analysis and anticonvulsant activity of 3- and 5-aminopyrazole N-benzoyl derivatives

Summary Some unsymmetrical N -exocyclic and N -endocyclic derivatives from benzoylation of 3- and 5-aminopyrazole were prepared with the aim of comparing their anticonvulsant activity towards the MES and scMET tests. Unambiguous proof of their structure was obtained from heteronuclear long-range correlation spectroscopy and NOE difference spectra. Only the N - exo -pyrazole benzamides showed good protection with respect to these tests.

Douglas-fir polyphenols and heartwood formation

Abstract Phenolic extractives have been isolated from Douglas-fir sapwood and heartwood and identified by NMR and mass spectroscopy. Pinocembrin, [5,5′]-bisdihydroquercetin and two lignans, pinoresinol and 2,3-dihydro-2-(4′- O -β-glucopyranosyl-3′-methoxyphenyl)-3-hydroxymethyl-5-(3-hydroxypropyl)-7-methoxy-benzofuran are reported for the first time in these tissues. The chemical structures of these phenolic extractives show that heartwood formation is characterized by both primary (biosynthetic) and secondary reactions. Secondary reactions include hydrolysis of phenol glucosides and oxidative polymerization.

ROBURIN A, A DIMERIC ELLAGITANNIN FROM HEARTWOOD OF QUERCUS ROBUR

Abstract Three ellagitannins from Quercus robur wood have been studied by high resolution 1H and 13C NMR. Two are the epimeric 1,2,3,5-nonahydroxytriphenoyl-4,6-hexahydroxydiphenoyl-glucoses, castalagin and vescalagin. The third is a dimeric compound, roburin A, composed of two vescalagin subunits probably linked through an ether bond between the diphenoyl group of one subunit and the triphenoyl moiety of the other one.

Polyphenols isolated from the bark of Castanea Sativa. Mill. Chemical structures and auto-association

Abstract Eight phenolic compounds (castalin, castalagin, vescalagin, kurigalin, 5-O-galloylhamamelose, (3′, 5′-dimethoxy-4′-hydroxyphenol)-1-O-β- d -(6-O-galloyl)glucose, chestanin, and acutissimin A) were isolated from chestnut bark and their structures elucidated on the basis of spectroscopic analyses. Evidence for strong intermolecular associations of 5-O-galloylhamamelose is presented on the basis of NMR and mass spectroscopic data.

Pectin changes in samples containing poplar cambium and inner bark in relation to the seasonal cycle

Biochemical changes occurring during the transition between meristematic activity and rest were studied in samples containing cambial cells and their phloem derivatives from Populus x euramericana. Uronic acids represented around 9% of the cell-wall dry matter in spring and 7% in summer and winter. In contrast, a higher content of methylated galacturonic acids was observed during the rest period. The degree of esterification increased from 2% in spring to 35% in winter, indicating an important accumulation of acidic pectins during the active season although the cation content was always very low. Nuclear magnetic resonance spectroscopy of neutral polysaccharides solubilized with boiling water showed that in winter arabinans and xylans were the main carbohydrates. By contrast, in spring and in summer the xylans were very scarce, arabinans being the major neutral polysaccharide, indicating that important modifications occur during the autumn. Histochemical observations of material treated with hot water and EDTA confirmed the low relative pectin content during the rest period. Calcium ions, detected as antimonate salt were scarce. In the cambium, they were located mainly in cell junctions whereas in phloem derivatives these cations were distributed throughout the whole cell wall.

Structural analysis of cyclamen seed xyloglucan oligosaccharides using cellulase digestion and spectroscopic methods

Xyloglucan polymers have been isolated from cyclamen seeds and characterized by both liquid and CP-MAS 13C NMR spectroscopy. Treatment of the polysaccharides with cellulase from Trichoderma viride afforded XG oligomers which have been studied with both mass spectrometry and NMR spectroscopy. The repeating unit in the intact polymers and the most abundant hydrolysis product correspond to the Gal1 Xyl3 Glc4 (XXLG) fragment. However, detection of notable amounts of Xyl3 Glc4 (XXXG) and Gal2 Xyl3 Glc4 (XLLG) indicates that the galactose distribution in xyloglucan from cyclamen is irregular. FAB-MS analysis of a new derivative, prepared by forming the glycosamine of m-tetrafluoroethoxy aniline, has led to unambiguous sequence information for the XXLG oligomer.

Internal motion in carbohydrates as probed by n.m.r. spectroscopy

In the present study a combination of proton and carbon relaxation rates have been measured for several oligosaccharides at different temperatures. Correlation times, which have been calculated from both sets of data, have been compared in an attempt to establish the relative rate of internal motion. All the data suggest that these motions are not slow with respect to the overall tumbling.

Dynamic behavior of a perfunctionalized β-cyclodextrin as probed by NMR and molecular modeling

Abstract Conformational analysis of a perfunctionalized β-cyclodextrin by NMR and molecular modeling has revealed that this compound exists as an equilibrium mixture of C1 and C7 conformers in slow exchange. From carbon chemical shifts, interglycosidic vicinal heteronuclear coupling constants and NOESY volumes, it has been demonstrated that the macrocyclic conformation is responsable for the asymmetry in the C1 form. Several dynamic processes such as rapid conformational averaging, localized chemical exchange and the C 1 C 7 equilibrium have been shown to occur on widely separated timescales. In parallel, a molecular modeling study has revealed that the accessible conformational space of β-CDs is not significantly reduced with respect to that of the related disaccharide, β-maltose. All of the dynamic phenomena appear to be related to the macrocylic puckering and two distinct modes of variation in puckering have emerged from MD trajectories: transitions are either auto-compensated without perceptible change in macrocycle puckering or irreversible leading to considerable variation in macrocycle puckering. Optimization of the NMR-defined theoretical structures leads to auto-complexation of an exocyclic substituent.