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Abstract
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 gamma = 123.5 degrees, 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 alpha(1----4)
linked. The conformation of the glycosidic linkage is characterized by torsion angles
(phi, psi) which take the values (91.8, -153.2), (85.7, -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.