The conformational flexibility of //-gentiobiose has been studied by using convergent energy minimisation in a new force-field, with relaxation of all degrees of freedom. Twenty-four local minima are found in the q~,~,co-space. The free-cnthalpy differences are 1.7, 3.7, 5.1, 5.3, and from 5.9 to 29.4 kJ.mol-~ above the lowest minimum, corresponding to a distribution of 40 : 20 : 9 : 5 : 5' 21 at 298 K. Each minimum is surroundcd by a manifold of minimum conformers that differ only in exocyclic torsions. The average conformation of/~-gentiobiose is not fully extended, but to some degree coiled. Three conformers are shown in stereo.
INTRODUCI"ION
In the previous papers ~'2 of this series, conformations and conformational interchange of/:;-maltose and /3-cellobiose were dcscribed. In those studics, we used a simple force-tield, where thc parameters were taken from similar studies of ~and fi-D-glucopyranose ~'~. From theoretical considerations and the experience gained from the calculations on the disaccharidcs, we have now improved the force field. The three-parameter Buckingham potential is substituted by the two-parameter Lennard-Jones potential, and electrostatic interactions are included; reliable netcharges for mono-and di-saccharides are available from ab hfftio computations on D-glucopyranoses and //-maltose 5. All parameters in the force-tield are littcd to a scries of test molecules consisting of normal and cyclic alkancs and ethers, alkanols, and saccharides. A detailed documentation of this new, consistent force-field is given elsewhere 6.