BETA-D-CELLOTETRAOSE HEMIHYDRATE AS A STRUCTURAL MODEL FOR CELLULOSE .2. AN X-RAY-DIFFRACTION STUDY

beta-D-Cellotetraose crystallizes in the triclinic space group P1 with two independent molecules A and B and one water molecule W in the uni t cell with dimensions a = 8.023(1), b = 8.951(2), c = 22.445(2) Angst rom, alpha = 89.26(2), beta = 85.07(1), gamma = 63.93(1)degrees. The c rystal structure was determined by Patterson search and direct methods and refined to R = 6.2% using 3124 reflections F greater than or equa l to 1 sigma F collected with X-rays from rotating anode generator and synchrotron. Of the expected 28 O-H hydroxyl hydrogens from molecules A and B, 12 could be located from difference Fourier maps; configurat ional disorder is observed for anomeric O1 of molecule A, 75% beta and 25% alpha. beta-D-Cellotetraoses A and B are arranged antiparallel, t he water W connects molecules B through hydrogen bonds. The orientatio ns of all C6-O6 bonds are gauche, trans; D-glucopyranoses are more str ained in B than in A, with average Cremer-Pople parameters Q(2);Theta for A, 0.61 Angstrom; 2 degrees and for B 0.59 Angstrom; 13 degrees. T he conformations of A and B are stabilized by intramolecular three cen ter hydrogen bonds O3H ... O5',O6' and the crystal structure shows sys tematic intermolecular two-center hydrogen bonds involving O2H and O6H , and, most notably, systematic C-H ... O interactions to all O4 oxyge n atoms, viz. C5b-H ... O4a and C3b-H ... O4b. If a subcell is constru cted of the two central D-glucopyranoses 2 and 3 in molecules A and B, the obtained cell dimensions are identical to those of polymer cellul ose II. Based on this subcell we propose a new model for cellulose II which has all O6 groups in gt position and agrees with spectroscopic d ata.