X-ray structure determination and modeling of the cyclic tetrasaccharide cyclo-{-> 6)-alpha-D-Glcp-(1 -> 3)-alpha-D-Glcp-(1 -> 6)-alpha-D-Glcp-(1 -> 3)-alpha-D-Glcp-(1 ->}

The cyclic tetrasaccharide cyclo-{ --> 6)-alpha -D-Glcp-(1 --> 3)-alpha -D- Glcp-(1 --> 6)-alpha -D-Glcp-(1 --> 3)-alpha -D-Glcp-(1 --> } is the major compound obtained by the action of endo-alternases on the alternan polysacc haride. Crystals of this cyclo-tetra-glucose belong to the orthorhombic spa ce group P2(1)2(1)2(1) with a = 7.620(5), b = 12.450(5) and c = 34.800(5) A ngstrom. The asymmetric unit contains one tetrasaccharide together with fiv e water molecules. The tetrasaccharide adopts a plate-like overall shape wi th a very shallow depression on one side. The shape is not fully symmetrica l and this is clearly apparent on comparing the (Phi, Psi) torsion angles o f the two alpha-(1 -,6) linkages. There is almost 10 degrees differences in Phi and more than 20 degrees differences in Psi. The hydrogen bond network is asymmetric, with a single intramolecular hydrogen bond: O-2 of glucose ring 1 being the donor to O-2 of glucose ring 3. These two hydroxyl groups are located below the ring and their orientation, dictated by this hydrogen bond, makes the floor of the plate. Among the five water molecules, one lo cated above the center of the plate occupies perfectly the shallow depressi on in the plate shape formed by the tetrasaccharide. Molecular dynamics sim ulation of the tetrasaccharide in explicit water allows rationalization of the discrepancies observed between the X-ray structures and data obtained p reviously by NMR. (C) 2000 Elsevier Science Ltd. All rights reserved.