Robert H. Marchessault

Solid-state and solution conformation of scleroglucan

Abstract Scleroglucan is a neutral polysaccharide composed of a linear chain of (1→3)-linked β- d -glucopyranosyl residues with (1→6)-linked β- d -glucopyranosyl groups attached to every third residue. The conformational behaviour of scleroglucan has been investigated in solution and in the solid state. Order-disorder transitions in aqueous solution were studied by measurement of intrinsic viscosity. The results indicate the occurrence of such a transition at a pH /gt; 12, whereas gel formation under 10° is observed. X-Ray diffraction experiments performed on oriented fibers indicate that the backbone conformation is similar to that previously observed for curdlan, i.e. , a triple helix. The pendant (1→6)-linked β- d -glucopyranosyl residues protrude from the outside of the triplex, causing an expansion of the base plane parameters of the unit cell and further hampering lateral packing of the scleroglucan chains. The observed behaviour can be rationalized on the basis of a conformational analysis involving molecular modelling. As for the gentiobiose residue, extreme conformational flexibility about the (1→6)-β-linkage is disclosed. This conformational freedom is not significantly altered for the rotations about the (1→6)-β-linkage in the scleroglucan repeating-unit. Combination of solution and solid-state investigations provides insight into the aqueous gel-forming characteristics of scleroglucan.

Correlation of 13C chemical shifts with torsional angles from high-resolution, 13C-C.P.-M.A.S. N.M.R. studies of crystalline cyclomalto-oligosaccharide complexes, and their relation to the structures of the starch polymorphs

The chemical shifts and multiplicities of the resonances in high-resolution, 13C-c.p.-m.a.s. n.m.r. spectra of cyclomalto-oligosaccharide inclusion complexes are characteristic of the crystalline structure of the different complexes. In particular, the 13C chemical shifts of C-1 and C-4 correlate with the torsion angles φ2and φ1 respectively (related to Ψ and φ, respectively, in an alternative terminology), which describe the orientation of the D-glucosyl residues about the α-D-(1→ 4) glycosidic linkage. The 13C chemical shift of C-6 correlates with the torsion angle x, which describes the orientation of O-6 about the exocyclic, C-5-C-6 bond. The cyclomalto-oligosaccharide inclusion complexes are good models for the interpretation of the characteristic chemical shifts and multiplicities previously observed in the 13C-c.p.-m.a.s. n.m.r. spectra of the natural starch polymorphs. From these chemical-shift correlations, values for the torsion angles φ2, φ1 and X are predicted for starches that crystallize as “A” and “B” structures. These predicted values are in agreement with the limited data currently available from X-ray fiber diffraction studies.

Application of High-Resolution Solid-State NMR with Cross-Polarization/Magic-Angle Spinning (CP/MAS) Techniques to Cellulose Chemistry

Abstract Vegetation is thought to produce an estimated 100 billion tons of cellulose a year, that is, approximately 25 tons of cellulose for every person on earth This vast quantity of naturally occurring, readily available fiber has helped to maintain cotton as the most important textile fiber in the world and made wood the commodity material of the world. Enormous research effort has been directed toward the study of cellulosic materials and has led to a wealth of information in the literature pertaining to the morphology, biosynthesis, and varied reactions of this important substance.

High-resolution electron microscopy of poly(β-hydroxybutyrate)

Abstract High-resolution electron microscopy of poly(β-hydroxybutyrate) (PHB) single crystals has allowed direct visualization of the lattice planes of this highly beam-sensitive thermoplastic polyester from bacteria. From lattice images having a resolution of 0.35 nm, a simple Fourier averaging performed optically has generated an image of the crystal projected along its fibre ( c ) axis. The lattice image provides a molecular-level picture of the elliptical PHB cross-section with rows of alternating orientation clearly identifiable, as in the X-ray unit cell packing.

Insights into the Lattice Structure of Cellulose II From the High Resolution CP/MAS Solid State 13C NMR Spectrum of Cellotetraose

Abstract The chemical shifts and multiplicities of the high-resolution 13C CP/MAS NMR spectrum of cellulose II are quite diagnostic of the lattice structure of this polymorph. Particularly important is the chemical shift of C-1 and its clear splitting into two lines of equal intensity. Similar chemical shifts and multiplicities are seen in the spectrum of cellotetraose. Thus cellotetraose is considered to be a good model for the lattice structure of the polymer. A detailed investigation of the multiplicity of the C-1 resonance of cellotetraose shows that the two peaks are of equal intensity in this case also. Because of the limited number of repeat units in the tetramer, this observation implies that the unit cell contains two independent chains rather than a “double” repeat unit. This gives support for a similar lattice structure, with two independent chains, for cellulose II itself.

Crystalline conformation of homo- and regular heteroglucan chains

Abstract Crystalline polysaccharides with 1 → 3− β and 1 → 4− β glycosidic linkages are the most prevalent ones in nature. An interpretation of recent X-ray data on 1 → 3− α glucan shows that it has a ribbon-like crystalline conformation similar to cellulose. Comparison of the crystalline conformation of the four principal homoglucans shows that they fall either in the ‘ribbon-like’ or ‘large amplitude’ helix class. Heteroglucans with a regular sequence of glucosidic linkages show characteristics of the ‘extended conformation’ rather than the ‘coiled conformation’ even when there is 50% of a linkage which in a homoglucan leads to a large amplitude helix. It is concluded that X-ray diffraction analysis fully establishes the hypothesis that the glycosidic linkage type is the determinant of polysaccharide conformation. In this respect, polysaccharides are more like synthetic polymers than proteins or nucleotides; in the latter, it is variation in the substituents which are responsible for the conformational diversity.