Depicting the MM3 potential energy surfaces of trisaccharides by single contour maps: application to beta-cellotriose and alpha-maltotriose

The adiabatic potential energy surfaces (PES) of two trisaccharides (beta-cellotriose and alpha-maltotriose) were obtained using the MM3 force field. Each PES can be described by a single 3D contour map for which the energy is plotted against the two psi glycosidic angles. Given the usually small variations of the phi glycosidic torsional angle in the low-energy regions of disaccharide maps (at least with MM3), it is valid to leave both phi glycosidic angles to relax in the process of building the conformational map of trisaccharides. The surfaces are those expected from the map of disaccharides containing the same linkages and monosaccharide units (i.e., beta-cellobiose and alpha-maltose), with second-order factors altering the 'symmetry' of both linkages. A large low-energy region appears for beta-cellotriose, comprising four minima in close proximity, with barriers between them below 0.6 kcal/mol. On the other hand, for alpha-maltotriose a main global minimum is observed, with several surrounding local minima. The surfaces obtained agree with single-crystal X-ray data on these trisaccharides and derivatives. A reduction of the linkage flexibilities is observed when passing from the disaccharides to the trisaccharides. Furthermore, the linkage closer to the reducing end appears to be less flexible than the linkage closer to the non-reducing end.