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, n.m.r. spectroscopy, and molecular mechanics calculations.

The crystal structure of neocarrabiose monohydrate, 3-O-(3,6-anhydro-alpha-D-galactopyranosyl)-beta-D-galactopyranose (C12H20O10.H2O) belongs to the monoclinic space group P2(1), and has a unit cell of dimensions a = 6.351(1), b = 7.675(2), c = 15.096(8) A, and beta = 91.11(1) degree. 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 a gauche-trans conformation. The orientation about the (1----3) linkage is defined by phi = 94.5 degrees and psi = 141.9 degrees. There is an intramolecular hydrogen bond (O-5'...O-2 = 2.777A). The conformation 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.