Crystal structure of the koh-amylose complex

Abstract

The crystal structure of potassium hydroxide complexed amylose, obtained by heterogeneous deacetylation of amylose triacetate, has been determined through a combined stereochemical structure-refinement and X-ray diffraction-analysis. The structure crystallizes in an orthorhombic unit-cell with parameters a  8.84, b  12.31, and c (fiber repeat)  22.41 Å, and with P2 12 12 1 symmetry. The conformation of the amylose chain is a distorted, left-handed helix with 6 d-glucose residues per turn. Each three-residue asymmetric unit is complexed with one molecule of potassium hydroxide and three molecules of water. The K + ion coordinates with four oxygen atoms of the amylose chain and with two other oxygen atoms, and this coordination is probably the cause for the more-extended amylose chain-conformation than would be predicted from a φ, ψ map. The distortions in the chain are primarily manifested by different O-6 rotations and by slightly different bridge and φ, ψ angles for the individual residues. The structure is extensively hydrogen bonded, although largely through water molecules, which accounts for its ready water solubility. The left-handed conformation of the chain in this structure is consistent with the conformations of amylose triacetate and V-amylose, both of which are left-handed.