MODELING OF ARABINOFURANOSE AND ARABINAN .1. CONFORMATIONAL FLEXIBILITY OF THE ARABINOFURANOSE RING

The conformational behaviour of methyl beta-D- and methyl alpha-L-arab inofuranosides 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 phi. The gauche-gauche, gauche-trans, and trans-gauche orientations of the primary hydroxyl gr oups 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 interconver sions. Both molecules display very characteristic conformational behav iour. The conformations of methyl beta-D-arabinofuranoside, which have been determined by single-crystal X-ray diffraction studies, are dist ributed in two fairly deep low-energy wells. These conformations are n o more than 1.5 kcal/mol above the respective energy minima. In the ca se of methyl alpha-L-arabinofuranoside, crystallographic data are lack ing and only H-1-H-1 coupling constants are available. Using suitable equations for the H-C-C-H segments, the theoretical 3J(H-H) coupling c onstants 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 fur anose rings in solution.