CONFORMATION OF AN ARABINOXYLAN ISOLATED FROM THE RICE ENDOSPERM CELL-WALL BY X-RAY-DIFFRACTION AND A CONFORMATIONAL-ANALYSIS

The chain conformation of an arabinoxylan isolated from the rice endos perm cell wall, which was composed of the linear backbone of a 1,4-lin ked beta-xylose residue highly substituted at the C3 position with alp ha-L-arabinofuranose, was studied by X-ray fiber diffraction and a con formational analysis, From the X-ray results the polysaccharide was su ggested to take an extended left-handed, three-fold helical structure which is not a desirable conformation to make a complex firmly associa ted with cellulose or xyloglucan that was elucidated to be present in the cell wall. A conformational analysis, using the PPOS force field f or the O-alpha-L-arabinofuranose-(1 --> 3)-beta-D-xylose residue and O -alpha-L-arabinofuranose-(1 --> 3)-[(1 --> 4)-beta-D-xylose residue](3 ), in which the arabinose residue was linked to the C3 position of the middle residue of the xylotriose residue, and using the PS79 computer program for the arabinoxylan indicated that the backbone xylan chain could not take a two-fold helix similar to that of cellulose because o f significant steric interaction between the arabinose side chain and the backbone. Thus, the left-handed, three-fold helix was found to be reasonable.