Polysaccharide structures from powder diffraction data: molecular models of arabinan.

X-ray intensity data from a polycrystalline sample of debranched arabinan, -->5)-alpha-L-Ara(f)-(1-->, have been obtained using a powder diffractometer in order to determine its three-dimensional structure. The observed peaks index on a monoclinic cell with a=5.444(7), b=6.395(10), c=8.680(5) A, and gamma=99.6(3) degrees , V=298 A3. One 2-fold helix along the c-axis can be accommodated in the unit cell. Molecular and packing models have been analyzed using the seven C-2'-endo/C-3'-endo allomorphs originally proposed by Radha and Chandrasekaran [Carbohydr. Res. 1997, 298, 105]. The generated powder pattern matches closely with the observed diffraction only for one C-2'-endo model. In this structure, the three main chain conformation angles are in the trans domains, there are no intra-chain hydrogen bonds, and the packing arrangement is stabilized by inter-chain O-3-H...O-2 bonds.