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 and well oriented fibers of guaran. The polymer forms a flat 2-fold helix of pitch 10.38 Å. The galactosyl side-chain hydrogen bonds to the mannan backbone intramolecularly and provides structural stability. Four helices are packed in an orthorhombic unit cell of dimensions a=9.26, b=30.84 and c (fiber axis)=10.38 Å according to space group symmetry I222. Each chemical repeat consisting of 1.0 mannosyl and 0.6 galactosyl units is associated with two structured water molecules which are responsible for the formation and stability of sheets of guaran helices in the crystalline lattice. The crystallographic R-value is 0.185 for the best model. The guaran molecular structure and packing arrangement presented here are readily adoptable to the entire galactomannan family.