Theoretical study of the conformations of pustulan [(1----6)-beta-D-glucan].

The total potential energy including nonbonded, torsional and hydrogen bond contributions has been computed for pustulan, a (1----6) linked beta-D-glucan polysaccharide, as a function of rotational angles phi, psi, and omega. The (phi,psi,omega)-space contains many local minima and at least three distinct deep minima. Two minima at (phi,psi,omega) = (25 degrees, 190 degrees,gg) and (phi,psi,omega) = 65 degrees, 150 degrees,gg) of almost equal energies have helical parameters (n = 5.2, h = 1.0A) and (n = 3.2, h = 1.5A), respectively. A third minimum at (phi,psi,omega) = (40 degrees, 70 degrees gt) leads to an extended zig-zag structure (n = 2.2, h = 2.2A). Energy maps obtained for gentiobiose, the disaccharide of pustulan, also reveal many local minima and the small energy differences among them indicate that gentiobiose is extremely flexible. Gentiodextrins, a family of cyclic molecules of (1----6)-beta-D-glucose residues, were also studied. Conformations free from steric hindrance were found for cyclic molecules with three to six glucose residues.