The crystal structure of the α-cellobiose•2 NaI•2 H2O complex in the context of related structures and conformational analysis

The crystal structure of beta-D-glucopyranosyl-(1-->4)-alpha-D-glucopyranose (alpha-cellobiose) in a complex with water and NaI was determined with Mo K-alpha radiation at 150 K to R = 0.027. The space group is P2(1) and unit cell dimensions are a = 9.0188, b = 12.2536, c = 10.9016 Angstrom, beta = 97.162degrees. There are no direct hydrogen bonds among cellobiose molecules, and the usual intramolecular hydrogen bond between O-3-H and O-5' is replaced by a bridge involving Na+, O-3, O-5', and 0-6'. Both Na+ have sixfold coordination. One I- accepts six donor hydroxyl groups and three C-H...I- hydrogen bonds. The other accepts three hydroxyls, one Na+, and five C-H...I- hydrogen bonds. Linkage torsion angles phi(O-5) and psi(C-5) are -73.6 and -105.3degrees, respectively (phi(H) = 47.1degrees and psi(H) = 14.6degrees), probably induced by the Na+ bridge. This conformation is in a separate cluster in phi, psi space from most similar linkages. Both C-6-O-H and C-6'-O-H are gg, while the C-6'-O-H groups from molecules not in the cluster have gt conformations. Hybrid molecular mechanics/quantum mechanics calculations show < 1.2 kcal/mol strain for any of the small-molecule structures. Extrapolation of the NaI cellobiose geometry to a cellulose molecule gives a left-handed helix with 2.9 residues per turn. The energy map and small-molecule crystal structures imply that cellulose helices having 2.5 and 3.0 residues per turn are left-handed. (C) 2002 Elsevier Science Ltd. All rights reserved.