Anatole Sarko

Conformational studies of polysaccharide multiple helices

Abstract The possibility of the existence of multiple helices in various homopolysaccharides has been explored by the calculation of conformational-energy contour maps. The structures include homopolymers of d -xylose, d -glucose, d -mannose, and d -galactose, linked α- and β-(1→2), -(1→3), and -(1→4). A number of double parallel-stranded, double antiparallel-stranded, and triple parallel-stranded helices are predicted, all of which are stabilized by interstrand hydrogen-bonds. At least three of the predicted, multiple-helical structures are known to exist. A classification scheme to predict the probabilities of multiple-helix formation is suggested. Possible structure-function relationships of homopolysaccharides are discussed.

Molecular and crystal structure of the regenerated form of (1→3)-α-d-glucan☆

Abstract The crystal structure of a regenerated form of (1→3)-α- d -glucan, obtained by solid state deacetylation of the triacetate derivative, has been determined by combined X-ray diffraction analysis and stereochemical model refinement. The structure crystallizes in an orthorhombic unit cell with parameters a = 16.46 A , b = 9.55 A and c (fibre repeat)=8.44 A, and space group P 2 1 2 1 2 1 . The chain conformation is nearly completely extended and is very close to a 2/1 helix, even though the dimer residue is the crystallographic repeat unit. An intramolecular O (2)   O (4)′ hydrogen bond stabilizes the conformation and extensive intermolecular hydrogen-bonding abilizes the packing. The resulting structure is sheet-like, with an alternating polarity of chain directions within the sheet. In its sheet-like character, extensive hydrogen-bonding, and insolubility in water, this polymorph of (1→3)-α- d -glucan resembles regenerated cellulose. The reliability of the structure analysis is indicated by the X-ray residual R =0.206.

Single crystals of dextran: 1. Low temperature polymorph

Abstract Lamellar single crystals of a synthetic, linear dextran and a slightly branched, bacterial dextran were grown at 95°C, by seeding their metastable solutions in mixtures of polyethylene glycol and water. The crystals gave well-resolved electron diffraction diagrams from which the unit cell parameters a ∼ 2.563 nm and b ∼ 1.021 nm were determined. The patterns displayed Pgg symmetry in a–b projection. A comparison of the elctron and X-ray powder diffraction diagrams indicated either 0.81 or 1.62 nm as the probable value for the c parameter of the unit cell.

Molecular and crystal structure of (1 → 3)-α-d-glucan triacetate

A crystal and molecular structure for GTA I, the low temperature polymorph of (1 → 3)-α-d-glucan triacetate, is proposed on the basis of X-ray diffraction analysis of well-oriented films, combined with stereochemical model refinement. The unit cell is monoclinic with parameters a = 30.17 A, b = 17.42 A, c (fibre axis) = 12.11 A, and β = 90°C. The probable space group is P21 with b axis unique. Six molecular chains pass through the unit cell with alternating polarity and with three independent chains comprising the asymmetric unit. The chain axes are located in a hexagonal packing arrangement. The chain backbone conformation is a left-handed, three-fold helix, but all nine O(6) acetyl groups of the asymmetric unit are in non-equivalent rotational positions. The most probable structure is indicated by X-ray residuals R = 0.261 and R″ = 0.283, based on 62 reflection intensities (41 observed and 21 unobserved).