PACKING ANALYSIS OF CARBOHYDRATES AND POLYSACCHARIDES .20. MOLECULAR AND CRYSTAL-STRUCTURE OF (1-]3)-ALPHA-D-GLUCAN TRIACETATE

A crystal and molecular structure for GTA I, the low temperature polymorph of (1 --> 3)-alpha-D-glucan triacetate, is proposed on the basis of X-ray diffraction analysis of well-oriented films, combined with stereochemical model refinement. The unit cell is monoclinic with parameters a = 30.17 angstrom, b = 17.42 angstrom, c (fibre axis) = 12.11 angstrom, and beta = 90-degrees-C. The probable space group is P2(1) with b axis unique. Six molecular chains pass through the unit cell with alternating polarity and with three independent chains comprising the asymmetric unit. The chain axes are located in a hexagonal packing arrangement. The chain backbone conformation is a left-handed, three-fold helix, but all nine O(6) acetyl groups of the asymmetric unit are in non-equivalent rotational positions. The most probable structure is indicated by X-ray residuals R = 0.261 and R" = 0.283, based on 62 reflection intensities (41 observed and 21 unobserved).