Conformational analysis of 1-kestose by molecular mechanics and by n.m.r. spectroscopy.

Models of the trisaccharide, 1-kestose [beta-D-fructofuranosyl-(2----1)-beta-D-fructofuranosyl-(2 in equilibrium with 1)-alpha-D -glucopyranoside], were analyzed with the molecular mechanics computer program MM2(87) to ascertain their inter-ring torsion angles, primary alcohol side-group orientations, and ring puckering. The most striking result was that the modeling predicted and n.m.r. spectroscopy corroborated that the central fructofuranose ring takes a different form from that previously observed in the crystal. No other studies of fructofuranoses have observed that crystallographic form, thus suggesting that the 18 hydrogen bonds created upon crystallization of 1-kestose support the ring deformation. Because this trisaccharide is too complex for a complete study of conformation space, only structures having inter-ring conformations that were at energetic valleys in previous studies of the constituent disaccharides were analyzed. The model of the model disaccharides, although they were generally close to the linkage conformations observed in the crystal structure, differing by an average of 19 degrees.