The double-helical nature of the crystalline part of A-starch

A new three-dimensional structure of the crystalline part of A-starch is described in which the unit cell contains 12 glucose residues located in two left-handed, parallel-stranded double helices packed in a parallel fashion; four water molecules are located between these helices. Chains are crystallized in a monoclinic lattice with a = 2·124 nm, b = 1·172 nm, c = 1·069 nm and γ = 123·5 °, the c axis being parallel to the helix axis. Systematic absences are consistent with the space group B2. The structure was derived from joint use of electron diffraction of single crystals, X-ray powder patterns decomposed into individual peaks and previously reported X-ray fibre diffraction data after adequate re-indexing. The repeating unit consists of a maltotriose moiety where the glucose residues have the 4C1 pyranose conformation and are α(1 → 4) linked. The conformation of the glycosidic linkage is characterized by torsion angles (Φ, ψ) which take the values (91·8,−153·2), (85sd7−145·3) and (91·8,−151·3); all the primary hydroxyl groups exist in a gauche-gauche conformation. There are no intramolecular hydrogen bonds. Within the double helix, interstrand stabilization is achieved without any steric conflict and through the occurrence of O(2)O(6) type hydrogen bonds. The present structure is consistent with both physicochemical and biochemical aspects of the crystalline component of the cereal starch granules.