Photis Dais

High-field, 13C-N.M.R. spectroscopy of β-d-glucans, amylopectin, and glycogen

Abstract 13 C-N.m.r. spectra of several d -glucans, recorded at 100 MHz, have afforded information about structural detail not previously accessible at lower frequencies. Spectra of (1→4)- and (1→3)-linked β- d -glucans of oats, barley, and lichenan of Iceland moss demonstrate the presence, in each, of three, non-equivalent, 4- O -substituted residues, that the ratio of these to 3- O -substituted residues averages 2.4–2.5, and hence that the patterns of repeating sequences in the three polymers are essentially the same. A comparison of wheat amylopectin with a minor, amylopectin-like fraction of wheat starch indicates that they are strictly analogous in basic structure, and differ only in that the average length of branches in the minor fraction is 20–25% shorter. By combining the advantages of high-field operation with the use of dimethyl sulfoxide as solvent, a large number of distinctive resonances have been observed, representing end-units, branch-points, and residues adjacent to branch-points. Accordingly, these signals are even more prominent in the spectrum of glycogen, reflecting the higher incidence of branching in this polymer. At 100 MHz, the excellent resolution and sensitivity afforded constitute a potent basis for assessing the purity of polysaccharide preparations, as illustrated with wheat amylose and barley β- d -glucans.

Proton spin-lattice relaxation rates in the structural analysis of carbohydrate molecules in solution.

Publisher Summary This chapter focuses on proton spin–lattice relaxation rates in the structural analysis of carbohydrate molecules in solution. It gives an account of the simple theory related to spin-lattice relaxation-rates; cautions against the uncritical use of this simple theory for systems that are strongly coupled, or undergoing anisotropic reorientation, or both; introduces the pulse nuclear magnetic resonance (NMR) experiments that are used to measure spin–lattice relaxation-rates and to stress the precautions necessary for accurate measurements; discusses the types of experiments that reveal stereo-specific dependencies of spin–lattice relaxation-rates, along with concrete examples from the literature; and assesses the reliability of these experiments and their relative merits as to accuracy and technical simplicity. The proton spin-lattice relaxation-rate (R,) is a well established, NMR parameter for the structural, configurational, and conformational analysis of organic molecules in solution. However, its utility has received little attention in the field of carbohydrate chemistry and very few studies have been reported in which relaxation rates were used to extract qualitative or quantitative information, or both, about the geometry of carbohydrate molecules in solution. The chapter also provides details on the stereochemical implications of relaxation rates and explains the limitations and relative merits of relaxation methods.

Stabilization of the β-furanose form, and kinetics of the tautomerization of d-fructose in dimethyl sulfoxide

Abstract An explanation is offered for the marked increase observed in the relative stability of the β-furanose form of d -fructose in dimethyl sulfoxide as compared to that in water. Evidence obtained at 400 MHz, over a range of temperatures, indicates that HO-3 and HO-4 are hydrogen-bonded to primary hydroxyl groups HO-6 and HO-1, respectively. It is proposed that this intramolecular association between pairs of secondary and primary hydroxyl groups is a major source of the enhanced stabilization of the β-furanose form relative to the other tautomeric forms of the ketose. The kinetics of this equilibrium have been studied by 1 H saturation-transfer n.m.r. spectroscopy. As the acyclic form, through which the interconversions are believed to occur, was not detected, the measured rate constants represent apparent rate constants. Their magnitudes, which are a measure of the direction towards equilibrium of the four tautomers of d -fructose, are in accord with the relative proportions of these forms as measured from the spectra.

A relationship between 13C-chemical-shift displacements and counterion-condensation theory, in the binding of calcium ion by heparin

Characteristics of the interaction between heparin and calcium ion in the presence of sodium ion have been examined by monitoring the 13C-chemical shift changes as a function of the calcium ion concentration and the total ionic strength. The results indicated that the association between the polyanion and the divalent cation is a delocalized process, as opposed to one involving specific binding. The correspondence found between chemical shift and the number of Ca2+ ions bound per charged group, as derived from the Manning counterion-condensation model, showed that the stoichiometry is not a constant quantity but, rather, varies throughout the titration, and approaches a limiting value of 2 at high dilution. Additional measurements of T1 and line-width were consistent with an intramolecular order-disorder conformational process induced by the binding of calcium ion. Moreover, binding does not occur or is relatively weak with N-desulfated heparin, or chondroitin 4-sulfate and 6-sulfate, each of which possesses fewer sulfate groups than heparin. These differences serve to emphasize the importance of the charge-density parameter in the control of counterion condensation according to the Manning model, and suggest that the spacing between the negatively charged groups is an associated factor.

Carbon-13 magnetic relaxation and local chain motion of amylose in dimethyl sulfoxide

Abstract The nature of internal and overall modes of reorientation in amylose dissolved in dimethyl sulfoxide at 80° is examined by a variety of dynamic models using multifield relaxation parameters T 1 , T 2 , and n.O.e. The overall motion appears to be that of an axially symmetric cylinder, with rotational correlation times in good agreement with those calculated from hydrodynamic theory, assuming helical segments comprising 40–50 monomer residues. The internal motion can best be described as restricted-amplitude internal-diffusion of individual C-H vectors, with conic boundary conditions. A bistable jump model is a less favorable alternative, as it requires a physically unrealistic angle between the C-1, H-1 vector and the jump axis. The critical role of chain conformation and chain stiffness on dynamic modelling is discussed, as well, on the basis of earlier studies.

Chemical shifts of the methyl groups in di-O-isopropylidene furanoses, and their relationship to molecular conformation and site of ring fusion. Spin-lattice relaxation measurements, and motional characteristics

Abstract Aspects of the conformation and mobility of a series of di- O -isopropylidene furanose derivatives in solution have been examined by 1 H- and 13 C-n.m.r. spectroscopy. Attention has been focused particularly on the chemical shifts of the methyl substituents of the acetal rings, in an attempt to extract information about molecular conformation and site of fusion. Obtained unambiguously by a combination of chemical and spectroscopic technqiues, the methyl chemical shifts were found not to reflect consistently the site of fusion to the furanose ring nor the overall molecular conformation, although they represent useful probes for ring-size determination. This inconsistency may be related to the flexibility of the fused ring systems and associated individual differences in the conformations of both the O -isopropylidene and sugar rings. Through measurements of 13 C spin—lattice relaxation times, a number of noteworthy motional characteristics related to overall molecular tumbling, hydrogen bonding, and internal mobility, were detected.

Proton spin-lattice relaxation-rates and nuclear Overhauser enhancement, in relation to the stereochemistry of β-d-mannopyranose 1,2-orthoacetates

Abstract Data are reported on spin-lattice relaxation-rates and nuclear Overhauser enhancement of protons of exo and endo diastereoisomers of 1,2-O-(1-methoxyethylidene) and 1,2-O-(1-benzyloxyethylidene) derivatives of 3,4,6-tri-O-acetyl-β- d -mannopyranose, and of some specifically deuterated analogs of these derivatives. The results verified assignments of the orientation at the quaternary carbon atom of the acetal ring, and yielded information about the orientations favored by the exocyclic C-methyl and benzyloxy substituents.

Polymerization of Allyldimethyl-Dodecylammonium Bromide Liquid Crystalline Monomer to its Liquid Crystalline Polymer

Recently we have introduced1 a novel class of thermotropic liquid crystalline compounds comprised of quaternary ammonium salts, bearing the dimethyldodecylamine moiety as the basic unit.