MORPHOLOGY OF GALACTOMANNANS - AN X-RAY STRUCTURE-ANALYSIS OF GUARAN

The three-dimensional structure of a galactomannan has been determined by careful analysis of the X-ray data obtained from polycrystalline a nd well oriented fibers of guaran. The polymer forms a flat 2-fold hel ix of pitch 10.38 Angstrom. The galactosyl side-chain hydrogen bonds t o the mannan backbone intramolecularly and provides structural stabili ty. Four helices are packed in an orthorhombic unit cell of dimensions a = 9.26, b = 30.84 and c (fiber axis) = 10.38 Angstrom according to space group symmetry I222. Each chemical repeat consisting of 1.0 mann osyl and 0.6 galactosyl units is associated with two structured water molecules which are responsible for the formation and stability of she ets of guaran helices in the crystalline lattice. The crystallographic R-value is 0.185 for the best model. The guaran molecular structure a nd packing arrangement presented here are readily adoptable to the ent ire galactomannan family. (C) 1998 Elsevier Science Ltd. All rights re served.