Modelling of arabinofuranose and arabinan. Part 1: conformational flexibility of the arabinofuranose ring

The conformational behaviour of methyl β- d- and methyl α- l-arabinofuranosides have been assessed through computations performed with the molecular mechanics program MM3 using the flexible residue method. Energies for various envelope and twist conformers were calculated as a function of the puckering parameters Q and Φ. The gauche-gauche, gauche-trans, and trans-gauche orientations of the primary hydroxyl groups at C-5 were accounted for. Our calculations provide some insight into extensive conformational flexibility that both molecules display and shed light on the possible pathways for conformational interconversions. Both molecules display very characteristic conformational behaviour. The conformations of methyl β- d-arabinofuranoside, which have been determined by single-crystal X-ray diffraction studies, are distributed in two fairly deep low-energy wells. These conformations are no more than 1.5 kcal/mol above the respective energy minima. In the case of methyl α- l-arabinofuranoside, crystallographic data are lacking and only 1H- 1H coupling constants are available. Using suitable equations for the H-C-C-H segments, the theoretical 3J H-H coupling constants were calculated as a function of the two puckering parameters, taking into account all the accessible conformations. Agreement with the experimentally reported data confirms the high flexibility of furanose rings in solution.