Crystalline conformation and structure of lichenan and barley β-glucan

Abstract

An investigation based on X-ray fiber diffraction and conformational analysis methods provided a proposed chain conformation and crystalline structure for lichenan, a poly .beta.(1 .fwdarw. 3) cellotriose. The analysis of the fiber diagram of the hydrated form of lichenan led to a trigonal unit cell of dimension a = b = 9.92 .ANG. and c (fiber axis) = 42.03 .ANG., with cellotriose as the asymmetric unit and 6 cellotriose residues/unit cell. A unit cell of the dry form is also trigonal with a = b = 9.36 .ANG. amd c = 42.03 .ANG.. A right-handed 3-fold helix made up of 3 cellotriose residues was derived from a comparison of experimental helical parameters with isoenergy conformational maps made by plotting dihedral angles (.phi., .psi.) at the .beta.(1 .fwdarw. 3) linkage. A virtual bond approach was also used to arrive at the proposed conformation which is stabilized by intramolecular hydrogen bonds between all carbohydrate units. The refinement of geometrical and packing parameters produced results consistent with an antiparallel arrangement of the 2 chains corresponding to a space group symmetry of P31. The naturally occurring copolysaccharides known as mixed (1 .fwdarw. 3) and (1 .fwdarw. 4) .beta.-glucans from the cell walls of barley endosperm crystallize in the same fashion as lichenan.