MOLECULAR-STRUCTURE OF 3-O-(3,6-ANHYDRO-ALPHA-D-GALACTOPYRANOSYL)-BETA-D-GALACTOPYRANOSE (NEOCARRABIOSE) IN THE SOLID-STATE AND IN SOLUTION - AN INVESTIGATION BY X-RAY CRYSTALLOGRAPHY, NMR-SPECTROSCOPY, AND MOLECULAR MECHANICS CALCULATIONS

The crystal structure of neocarrabiose monohydrate, 3-O-(3,6-anhydro-α-d-galactopyranosyl)-β-d-galactopyranose (C12H20O10·H2O) belongs to the monoclinic space group P21, and has a unit cell of dimensions a = 6.351(1), b = 7.675(2), c = 15.096(8)Å, and β = 91.11(1)°. The reducing unit is in the 4C1 conformation, the non-reducing residue is 1C4, with the 3,6-anhydro bridge in an E conformation, and HO-6 is in gauche-trans conformation. The orientation about the (1→3) linkage is defined by φ{symbol} = 94.5° and ψ = 141.9°. There is an intramolecular hydrogen bond (O-5′⋯O-2 = 2.777Å). The conformations of the pyranose rings in solution, derived from 3JH,H values, were not significantly different from those in the crystal, but the 3,6-anhydro bridge assumed a half-chair conformation. All these features have been rationalised through molecular modelling and computation of potential energy surfaces. © 1990.