Structural components of alginic acid. II. The crystalline structure of poly‐α‐ L ‐guluronic acid. Results of X‐ray diffraction and polarized infrared studies

A structural investigation of the marine algal polysaccharide poly‐α‐L‐guluronic acid is described. The molecular chains consist of 1 → 4 diaxially linked L‐guluronic acid residues in the 1C chair conformation and are stabilized in a twofold helix conformation by an intra‐molecular O(2)H … O(6)D hydrogen‐bond. The X‐ray fiber diffraction photograph has been indexed to an orthorhombic unit cell in which a = 8.6 Å, b (fiber axis) = 8.7 Å, c = 10.7 Å. A structure corresponding to the space group P212121 is proposed, in which all intermolecular hydrogen bonds interact with water molecules and in which all oxygen atoms except for the inaccessible bridge oxygens are involed. The relationship between the shape and structure of the polyguluronic acid molecule and its biological function is discussed.