MODELING OF ARABINOFURANOSE AND ARABINAN .2. NMR AND CONFORMATIONAL-ANALYSIS OF ARABINOBIOSE AND ARABINAN

The disaccharide arabinobiose -alpha-L-arabinofuranosyl-alpha-L-arabin ofuranose) constitutes the basic repeating structures found in such po lysaccharides as arabinan or in the side chains of the hairy regions o f pectins. The conformational behavior of aqueous arabinobiose has bee n investigated by high resolution nmr and computerized molecular model ing. The complete conformational analysis of the disaccharide has been achieved with the MM3 molecular mechanics methods using the flexible residue method. In this study, both the puckering of the arabinofurano se rings and the orientations about the glycosidic torsion angles Phi, Psi, and omega were considered. Some insights into conformational tra nsitions were obtained through molecular dynamics simulation using the CHARMM force field. In parallel, transient nuclear Overhauser effects at 400.13 MHz and long-range vicinal homonuclear and heteronuclear co upling constants have been measured. The theoretical nmr data were cal culated taking into;account air accessible conformations and using ave raging methods for both slow and fast internal motions models. The dat a do not support a single conformational model, and only conformationa l averaging yields the excellent agreement between the observed and si mulated parameters. Within the potential energy surfaces computed for the disaccharide, several low energy conformers can be identified. Whe n these conformations are extrapolated to: regular polysaccharide stru ctures, they generate chains of arabinan displaying right- and left-ha nded chirality and a wide range of repeating units per turn of helix. (C) 1994 John Wiley and Sons, Inc.