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United States Department of Agriculture

Agricultural Research Service


item Bradbrook, Gail
item Gessler, Katrin
item Cote, Gregory - Greg
item Momany, Frank
item Biely, Peter
item Bordet, Pierre
item Perez, S
item Imberty, Anne

Submitted to: Carbohydrate Research
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 7/6/2000
Publication Date: N/A
Citation: N/A

Interpretive Summary: The gum known as alternan is of considerable interest due to its unusual physical and chemical properties. We have recently discovered several types of bacteria that digest alternan into smaller sugars, one of which has been patented. This sugar has been called cyclo-alternan due to its unique molecular structure. In this collaborative study with French scientists, the exact structure of cyclo-alternan was determined by irradiating crystals of the sugar with X-rays. Knowledge of its structure at the atomic level will enable us to predict the ability of cyclo-alternan to react with other materials such as enzymes, metals, salts, etc. Direct users of this information include scientist working on carbohydrate structures. This information should also lead to derivatives of cyclo-alternan which can be used by the pharmaceutical and medical diagnostics industries.

Technical Abstract: 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 activity of endo-alternases on the alternan polysaccharide. Crystals of this cyclo-tetra-glucose belong to the orthorhombic space group P212121 with a = 7.620(5), b = 12.450(5) and c = 34.800(5) Angstrom. The asymmetric unit contains one tetrasaccharide together with 5 water molecules. The tetrasaccharide adopts a plate-like overall shape with a very shallow depression on one side. The shape is not fully symmetrical and this is clearly apparent on comparing the (Phi, Psi)torsion angles of the two alpha(1-6) linkages. There is almost 10 deg differences in Phi and more than 20 deg differences in Psi. When analyzing the hydrogen bond network, it appears that its symmetry is indeed broken by an intramolecular hydrogen bond:O2 of glucose ring 1 being the donor to O2 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 located above the center of the plate occupies perfectly the shallow depression in the plate shape formed by the tetrasaccharide. Molecular dynamics simulation of the tetrasaccharide in explicit water allows rationalization of the discrepancies observed between the X-ray structures and data obtained previously by NMR.

Last Modified: 07/25/2017
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