The crystal structure of the α-cellobiose·2 NaI·2 H2O complex in the context of related structures and conformational analysis Original Research Article

The crystal structure of β-d-glucopyranosyl-(1→4)-α-d-glucopyranose (α-cellobiose) in a complex with water and NaI was determined with Mo Kα radiation at 150 K to R=0.027. The space group is P21 and unit cell dimensions are a=9.0188, b=12.2536, c=10.9016 Å, β=97.162°. 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 O-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 φO-5 and ψC-5 are −73.6 and −105.3°, respectively (φH=47.1° and ψH=14.6°), probably induced by the Na+ bridge. This conformation is in a separate cluster in φ, ψ 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.