Disaccharide conformational flexibility. I. An adiabatic potential energy map for sucrose.

Constrained conformational energy minimizations have been used to calculate an adiabatic (phi, psi) potential energy surface for the disaccharide sucrose. The inclusion of molecular flexibility in the conformational energy analysis of this disaccharide was found to have a significant effect upon the allowed conformational space of the molecule. Three low-energy regions were identified on the adiabatic energy surface, and two of these regions were found to contain two related local minimum-energy conformations, with similar energies, differing only in the directionality of the intra-residue hydrogen bonds of the glucose portion of the molecule. The known crystal structures of seven molecules containing the sucrose moiety all fall within the region of the primary allowed minimum and are consistent with the relaxed energy map, while these crystal conformations could not be rationalized using energy maps for rigid residue geometries. The greater flexibility of the furanoid ring relative to that of the pyranoid ring contributed significantly to the enlargement of the low-energy region on the adiabatic map. However, in spite of the importance of limited flexibility in understanding the conformation and fluctuations of sucrose, this molecule was found to be considerably more rigid that some other disaccharides, such as maltose and cellobiose, in accord with experimental studies.