Crystal and molecular structure of the amylose–DMSO complex

The crystal and molecular structure of the complex of amylose with dimethyl sulfoxide has been studied by a combination of stereochemical analysis, potential energy, and X-ray diffraction methods. The complex crystallizes in a pseudotetragonal unit cell with a = b = 19.17 Å and c (fiber axis) = 24.39 Å, with two antiparallel chains per unit cell and space group P2₁2₁2₁. The amylose chain is a left-handed 6₁(1.355) helix with three turns per crystallographic repeat. The O(6) rotational position is approximately gt. Dimethyl sulfoxide is located inside the helix with one DMSO molecule for every three glucose residues. An additional four DMSO molecules and eight water molecules each are located in the large interstices between chains, and it is the interaction of these molecules with the helix that results in the pseudotetragonal chain packing. The interstitial DMSO is the source of the previously reported additional layer lines, which are not consistent with the 8.13-Å amylose repeat distance. The final R factor for the layers with amylose contribution to the structure factors was 0.29, while the overall R factor was 0.35. The stereochemical packing analysis provided suitable phasing models for the subsequent X-ray refinement.