Marc Vincendon

13C NMR structural investigation of scleroglucan

Abstract This paper concerns the 13 C NMR signal assignment in the DMSO of a neutral polysaccharide, scleroglucan. The previously proposed chemical structure is confirmed. The 13 C NMR spectrum shows that scleroglucan is a regular poly (A, B, C, D) type glucan. The relaxation times of the different series of carbon atoms demonstrate that a single, pendant glucose group is attached to each third monomer along the main chain of what is a β(1 → 3)-glucan. Partial acid hydrolysis gives a spectrum analogous to that of the β(1 → 3)- d -glucan, curdlan, and confirms the structure of the polymer backbone. In aqueous solution, no signal has been obtained due to the existence of a rigid, ordered conformation as demonstrated by optical rotation; in the presence of sodium hydroxide, a conformational transition is produced just as with curdlan. The conclusion is that the behaviour of scleroglucan in solution is similar to that of other β(1 → 3)- d -glucans even though it is more soluble.

Solid-state 13C-N.M.R. and electron microscopy study on the reversible cellulose I→cellulose IIII transformation in Valonia

An electron microscopy and a high-resolution 13C-n.m.r. study of cellulose is described and a previously unreported solid-state 13C-n.m.r. spectrum of cellulose III is presented. It is shown that a cyclic transformation of highly crystalline cellulose I (Valonia) into cellulose III and back to cellulose I results in a material whose crystallites have a reduced lateral dimension similar to that in cotton and whose electron-diffraction patterns and 13C-n.m.r. spectra are very close to that of cotton. In addition, some new features in the 13C-n.m.r. spectrum of Valonia cellulose are reported.

Spectroscopic characterization and conformation of oligo kappa carrageenans

Abstract Identification of oligo-carrageenans by 13 C n.m.r., infrared spectroscopy and optical rotation is presented. 13 C n.m.r. signals have been assigned for d -galactose 4-sulphate and oligomers of kappa carrageenan on the basis of model spectra of parent polymer and deuterium isotopic effect. Optical rotation measurements give evidence of the conformational transition of such oligomers, depending on their degree of polymerizations.

High resolution nuclear magnetic resonance studies of nigeran

Abstract Polymer motion in solution can be studied by 13CNMR relaxation methods, which provide information about the correlation time for C-H vectors. 13C-Relaxation and Nuclear Overhauser Enhancement (NOE) data may frequently be combined to determine the dipole-dipole relaxation contribution. An alternative method is proposed based on a comparison of the proton spin-lattice relaxation rates of the centre proton resonances of an unlabelled molecule with the relaxation rates of the 13C satellites (from 13C labelled molecules). Selectively labelled nigeran which is an alternating 1 → 3 and 1 → 4 α- d -glucan has been investigated. The discussion in terms of the occurrence of different motions for each of the two units of the polymer requires an unambiguous assignment of the two anomeric carbons. For this reason a detailed assignment of the 1H and 13C Nuclear Magnetic Resonance (NMR) spectra of nigeran in dimethylsulphoxide-d6 is described, based on T1 and NOE measurements in addition to selective homonuclear and heteronuclear spin decoupling experiments. These values are correlated with a conformation estimated by HSEA hard-spheres calculation. The measurements of the relaxation parameters for labelled and unlabelled compounds which provide an alternative determination of the 13C-1H dipole-dipole relaxation contribution in a macromolecule agree well with 13C-{1H} NOE experiments.

Easy identification of hydroxy-bearing carbon atoms in 13C nuclear magnetic resonance spectroscopy: a new method for signal assignment in carbohydrates

By observing the isotopic chemical shifts when OH groups are replaced by OD under conditions of low exchange rate, it is possible to differentiate hydroxy-bearing carbon atoms in carbohydrates using 13C n.m.r. spectroscopy at 62·8 MHz.

Scleroglucan derivatives: Aromatic carbamates

Scleroglucan is a polysaccharide produced industrially by the fungus Sclerotium rolfsii. Aromatic carbamate derivatives of scleroglucan have been prepared via the reaction of phenyl and tolyl isocyanates on the hydroxyl groups of the polymer chain. The reaction conditions have been determined in order to obtain the fully substituted polymer while conserving as much as possible the initial chain length. The reaction in pyridine at 80°C of a large exess of isocyanate (three times the stoechiometric amount) yields the higher degree of substitution (DS) and the higher intrinsic viscosity. These new polymers have been characterized by elemental microanalysis, spectroscopic methods (IR, 1 H-, and 13 C-NMR) and thermal analysis. They are found to be soluble in organic solvents and soften between 200 and 225°C, and are thermally unstable above this temperature.

Étude par r.m.n. de la stéreochimie et de la réactivité des extrémités réductrices de dérivés de polysaccharides

Abstract The CAT (Computer for Average Transients)-assisted n.m.r. spectra of the reducing end-group of cellulose and amylose derivatives were used to determine the specificity of the reaction involving these groups and the selective formation of the α- d -bromide by treatment of cellulose triacetate with acetic acid containing 40% hydrobromic acid. The bromine atom was selectively replaced with a tert -butoxy group to give the β- d -glucoside of the same chain end by treatment with a mixture of anhydrous 2-methyl-2-propanol and silver carbonate. Derivatives of d -glucose and cellobiose were prepared as model compounds.