Packing analysis of carbohydrates and polysaccharides. 16. The crystal structures of celluloses IVI and IVII

The crystal structures of cellulose polymorphs IV I and IV II have been determined by X-ray fiber diffraction analysis combined with stereochemical model refinement. Both structures crystallize in a two-chain unit cell of essentially identical parameters. The most probable space group in both cases is P1. The chain conformations, although close to two-fold helical, are marked by unequal rotational positions of the O(6) hydroxyl groups in adjacent residues. Despite identical unit cell parameters, the structures differ in chain polarity: in cellulose IV I both chains of the unit cell are parallel, whereas in cellulose IV II they are antiparallel. The difference in polarity is further substantiated by the results of chemical conversions which show that cellulose IV I is converted to cellulose I, and cellulose IV II is converted to cellulose II, via parallel and antiparallel cellulose triacetates, respectively. The reliability of the structure analyses is indicated by the residual R″ = 0.115 for cellulose IV I and 0.094 for cellulose IV II , for data sets of 41 and 43 reflections, respectively.