A revisit to the three‐dimensional structure of B‐type starch

Abstract

A new three‐dimensional structure of B‐starch is proposed in which the unit cell contains 12 glucose residues located in two left‐handed, parallel‐stranded double helices packed in a parallel register; 36 water molecules are located between these helices. Chains are crystallized in the hexagonal space group P6₁, with lattice parameters a = b = 1.85 nm, c = 1.04 nm. The space group symmetry was derived from an exhaustive analysis of the large body of structural studies published so far. Diffraction data used in this work were taken from the previously reported x‐ray fiber diffractogram [H.C. Wu and A. Sarko (1978), Carbohydrate Research, 61, 7–25] after adequate reindexing. The final R factor is 0.145 for the three‐dimensional data. The repeating unit consists of a maltose molecule 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) that take the values (83.8°, −144.6°) and (84.3°, −144.1°), whereas the valence angles at the glycosidic bridge have a magnitude of 115.8° and 116.5°, respectively. The primary hydroxyl groups exist in a gauche–gauche conformation. There is no intramolecular hydrogen bond. Within the double helix, interstrand stabilization is achieved without any steric conflict and through the occurrence of O(2)…O(6) type of hydrogen bonds. The model presented here, with an hydration around 27% w/w, corresponds to a well‐ordered crystalline sample, since all the water molecules could be located with no apparent sign of a disorder. Half of the water molecules are tightly bound to the double helices; the remainder forms a complex network centered around the sixfold screw axis of the unit cell. The consistency of the present structural model, with both physicochemical and biochemical aspects of the crystalline component of tuber starch granules, is analyzed.