PubMed:1663000 JSONTXT 24 Projects

Structure and solution conformations of a cyclic trisaccharide from high-resolution n.m.r. spectroscopy and molecular modelling. The conformational properties of a cyclic trisaccharide: [O-beta-D-glucopyranosyl-(1----6)]3 1,6"-anhydride nonacetate (C36H48O24, 1) have been established by high-resolution 1H- and 13C-n.m.r. spectroscopy in conjunction with potential-energy and molecular-mechanics calculations. The n.m.r. parameters used were nuclear Overhauser enhancements (n.O.e.) and coupling constants. From theoretical models of the trisaccharide, a statistical-mechanics approach was used to compute an ensemble average-relaxation matrix from which the n.O.e. were calculated. The observed nuclear Overhauser enhancements as measured by n.m.r. spectroscopy may be satisfactorily modelled if averaging over two conformational states is considered. In solution, both conformations of the molecule exhibit three-fold symmetry; the beta-linked glucopyranose rings have the 4C1 conformation. In one conformer, the orientation about the (1----6) linkage is characterized by torsion angles phi = 79.5 degrees, psi = 143.5, and omega = -64.3. For the other conformer, these values are phi = -137.7, psi = 68.2, and omega = 45.6. The existence of such a conformer shows that solution behaviour is not dominated by the stabilizing influence of the exoanomeric effect.