Three‐dimensional features of the bacterial polysaccharide (1 → 4)‐β‐ D ‐glucuronan: A molecular modeling study

The mutant strain M5N1 CS of Rhizobium meliloti produces, in a Rhizobium complete medium supplemented with fructose and sucrose, a partially acetylated homopolymer of D ‐glucuronic acid residues linked β‐(1 → 4). This polysaccharide forms thermoreversible gels with monovalent salts and thermally stable gels with divalent salts. In order to define the different levels of structural characterization, modeling simulations were performed for both the regular (1 → 4)‐β‐ D ‐glucuronan and the acetylated derivatives. This required the evaluation of the accessible conformational space for the 16 disaccharides. Detailed conformational analysis was accomplished using the flexible residue of the MM3 molecular mechanics procedure and the results were used to access the configurational statistics of representative polysaccharide chains. Within the potential energy surfaces calculated for each disaccharide, several low energy conformers can be identified. When these conformations are extrapolated to regular polysaccharide structures, they generate polymers with right‐ and left‐handed chirality along with a 2‐fold axis. This later arrangement ( n = 2, h = 5.16 Å) closely corresponds to that derived from a fiber x‐ray diffraction investigation. The insertion of acetyl groups induces changes in the helical features of the polymer. As for the simulation of the configurational properties of (1 → 4)‐β‐ D ‐glucuronan, an extended disordered chain having a persistence length of 105 Å (corresponding to 22 monomers) is predicted. This agrees with previous conclusions derived from solution study. The inclusion of varying amounts of acetyl groups only slightly perturbs the calculated persistence length. © 1998 John Wiley & Sons, Inc. Biopoly 45: 165–175, 1998