François R. Taravel

Carbon-proton coupling constants in the conformational analysis of sugar molecules.

Publisher Summary This chapter examines the carbon–proton coupling constants involved in the conformational analysis of sugar molecules. The indirect spin–spin coupling is independent of molecular rotation. The coupling mechanism is known to involve the electron spins of the bonding electrons and is the result of a weak electron polarization. A proton signal has two 13C satellites, each having an intensity that is 0.5% of that of the corresponding total proton resonance and located symmetrically around that resonance. In addition to providing information about 1H ( 1H couplings) these satellites allow the determination of the 1JC,H couplings with good resolution. With the advent of inverse detection and the corresponding gain in sensitivity, newer schemes for the measurement of long-range coupling constants have been proposed. They are based on the heteronuclear multiple-bond correlation derivative or variants of the heteronuclear multiple-quantum coherence experiment, and also on the heteronuclear single-quantum coherence experiment. The Hamiltonian gives the total electronic kinetic energy and the magnetic interactions between electron orbital motions and nuclear magnetic moments. It is found that two-bond carbon–proton coupling constants are much smaller than one-bond couplings and their signs can be negative or positive.

Extraction of highly gelling pectic substances from sugar beet pulp and potato pulp : influence of extrinsic parameters on their gelling properties

Abstract Sugar beet and potato pulps were considered as potential new sources of pectins because of their high pectin content, from 15 to 30% on a dry matter basis, and their availability in large quantities. Up to now, no pectins with gel forming properties comparable to the pectins extracted from apple pomace and citrus peels have ever been extracted from these by-products. In the present work, an extraction process for each by-product has been developed which maintains the structural integrity of the pectins as much as possible. These procedures yielded products which possessed both a high pectic substance content with a low degree of esterification and a high gelling ability in the presence of calcium. The composition of the extracted products was determined and the effect of extrinsic factors such as concentration of the extracted product, calcium content, sequestrant content and hydration temperature, on gel properties was evaluated by rheological measurements.

Derivatives of 6-deuterio-D-glucose, absolute configuration at C-6, and steric disposition of an acetoxymethyl group

Abstract Borodeuteride reduction of methyl D - gluco -hexodialdo-1,5-pyranoside (2) gave a 3:2 mixture of the 6( S ) and 6( R ) methyl 6-deuterio-α- D -glucopyranosides (3) , converted into other products having the same epimeric distribution at C-6, including the tetraacetate (5) of 3, 6-deuterio- D -glucitol hexaacetate (6), 3,5,6-tri- O -acetyl-6-deuterio-1,2- O -isopropylidene-?α- D -glucofuranose (7), and methyl 2,3-di- O -acetyl-4,6- O -benzylidene-6-deuterio-α- D -glucopyranoside (4) . The deuterium-decoupled p.m.r. spectrum of 4 permitted differentiation of the 6-epimers and assignment of configuration at C-6 for the complete series of derivatives. The favored, but not exclusive, disposition of the 5-acetoxymethyl group in compounds 5 – 7 has the acetoxyl group antiparallel to C-4.

One-bond carbon-proton coupling constants: Angular dependence in β-linked oligosaccharides

SummaryThe angular dependence of1JC,H in model compounds related to β-linked oligosaccharides has been established by FPT INDO quantum chemical calculations. Values calculated for models of (1 → 1)-, (1 → 2)-, (1 → 3)- and (1 → 4)-linked disaccharides were compared, and the effect of the orientation of HO-2 elucidated. The angular dependence of1JC,H on the torsional angles ΦH and ΨH and the solvent dielectric constant (s) was characterized in the form:1JC,H = A cos2ϖ+B cosϖ + C sin2ϖ + D since + E + Fe. The1JC,H values, measured by DEPT methods for C-1-H-1 and C-X′-H-X′ in cellobiose, cyclic trisaccharide and hexopyranoses were used to adjust the calculated angular dependences. Based on the occurrence of the conformers for agarobiose, neoagarobiose, mannobiose and methyl β-xylobioside, the thermodynamically averaged <1JC,H > values were calculated. The results obtained (<1JC-1,H-1 > 162.4, <1JC-4′, H-4′ > 147.6 Hz for methyl β-xylobioside; <1JC-1,H-1 > 162.4 and <1JC-4′,H-4′] > 147.6 Hz for mannobiose; <1JC-1,H-1 > 162.8 Hz for neo agarobiose and <1JC-1,H-1 > 163.2 Hz for agarobiose) agree well with the experimental values of 162.7, 147.5, 160.4, 147.2, 160.9 and 165.7 Hz, respectively.

X-ray diffraction studies on some seed galactomannans from India

Abstract The crystalline structures of four Indian seed galactomannans having mannose to galactose ratios ranging from 1.16 to 2.55 have been investigated by wide-angle X-ray diffraction of oriented films. The orthorhombic lattice constants (A) of the polysaccharides are as follows: galactomannans from Trifolium alexandrium, a = 9.02, b = 30.80, c = 10.27; from Medicago sativa (lucerne) a = 9.00, b = 30.66, c = 10.24; from Cassia siamea, a = 9.00, b = 24.81, c = 10.30; and from Cassia saltiana, a = 8.99, b = 24.75, c = 10.30. In agreement with results already published for other seed gums of commercial importance, the values of the b dimension of the unit cell of galactomannans decrease with decreasing galactose content. The values of the a and c dimensions, as found for other galactomannans, are similar to those of mannan I and mannan II, respectively, and can be understood in terms of conformational restrictions on the mannan backbone imposed by the β-(1→ 4) linkage. This similarity suggests that galactomannans of low and high galactose content have a fundamentally related crystal structure.

Rheological properties of a seed galactomannan from Cassia siamea Lamk

Abstract The rheological properties of a natural polysaccharide isolated from seed endosperms of Cassia siamea Lamk. have been studied for a wide range of concentrations from 2 × 10 −4 to 5 × 10 −2 g/ml. Results showed that the seed gum is a typical galactomannan having a mannose:galactose ratio of 2.55; mol. wt 8.4 × 10 5 , and an intrinsic viscosity of 1166 ml/g. Viscosity observations regarding dependence of flow curves versus shear rate, viscoelastic and critical shear rate behaviour clearly reveal the characteristics of a random coil polymer. The value of the critical concentration Cc * = 2.2/[η] is similar to the values obtained for some other galactomannans and polysaccharides although it is comparatively higher than the value obtained for more rigid polysaccharides. The slope of the log—log plot of specific viscosity versus C at zero shear rate is 4.34 in the concentrated region. This large dependence of the viscosity on the coil overlap parameter is lower than that found for guar with higher mannose proportion. The rheological behaviour of the galactomannan at concentrations higher than 15 g/l is that of a weak gel-like (pseudo network).

Conformation and dynamics of a cyclic (1 → 2)-β-d-glucan

A molecular modelling and nuclear magnetic resonance spectroscopic study was performed in order to gain insight into the conformational preferences of cyclosophoroheptadecaose. MM3 molecular mechanics calculations predicted a non-symmetric conformer with a small cavity of 3.7 A diameter as the lowest energy form. Molecular dynamics simulations gave insight into the dynamics of the free cyclosophoroheptadecaose and also supported the results of molecular mechanics calculations. A fair agreement was found between experimental data and corresponding average values predicted by molecular modelling.

Plant Chemical Defense Induced by a Seed-Eating Pollinator Mutualist

Plant-seed parasite pollination mutualisms involve a specific pollinator whose larvae develop by consuming a fraction of the host plant seeds. These mutualisms are stable only if the plant can control seed destruction by the larvae. Here, we studied the chemical response of the European globeflower Trollius europaeus to infestation by an increasing number of Chiastocheta fly larvae. We used liquid chromatographic analysis to compare the content of phenolic compounds in unparasitized and parasitized fruits collected in two natural populations of the French Alps, and mass spectrometry and nuclear magnetic resonance to elucidate the structure of adonivernith, a C-glycosyl-flavone. This compound is present in many of the organs of T. europaeus, but not found in other Trollius species. Furthermore, it is overproduced in the carpel walls of parasitized fruits, and this induced response to infestation by fly larvae is density-dependent (increases with larval number), and site-dependent (more pronounced in the high-altitude site). Mechanical damage did not induce adonivernith production. This tissue-specific and density-dependent response of T. europaeus to infestation by Chiastocheta larvae might be an efficient regulation mechanism of seed-predator mutualist population growth if it decreases survival or growth of the larvae.

Measurement of long-range heteronuclear couplings: Application to oligosaccharide conformation

3J(C,H) values have been measured using 2D-J heteronuclear-resolved n.m.r. spectroscopy to estimate interglycosidic conformations in various oligosaccharides with a degree of polymerization ranging from 6 to 30.

Biosynthèse de cellulose bacterienne à partir de glucose sélectivement deuterié en position 6: Étude par RMN

D‐glucose specifically deuterated at C‐6 was prepared and used for the biosynthesis of bacterial cellulose with Acetobacter xylinum. The material obtained was converted into glucitol hexaacetate and analyzed by 250 MHz n.m.r. and mass spectrometry. These spectra indicated that about 70% of the starting D‐glucose was incorporated without modification of deuteriation at the C‐6 position. However an explanation is required of the finding that deuterium was also incorporated at the C‐2 and C‐1 positions of the cellulose, arising respectively from the 6 pro‐R and 6 pro‐S positions of the initial D‐glucose.