| Abstract: | By nuclear magnetic resonance (NMR), the four ring forms of psicose, α-and β-pyranose and α- and β-furanose, have almost equal concentrations in aqueous solution. Prediction of this equilibrium by molecular mechanics tests both the balance within the force field and the methods for including the many degrees of freedom in the system. For both the α- and β-furanoses, each of 410 different ring shapes was studied in combination with 37 combinations of staggered side-group orientations. Of these, 48 and 57 initial combinations of side-group orientations contributed to the α- and β-furanose energy surfaces. The pyranoses were analyzed in two steps. First, the 38 characteristic ring conformations were optimized with the 63 staggered side-group combinations. All combinations that gave the lowest energy at one or more of the 38 shapes (20 combinations for α-pyranose, 15 for β-pyranose) were then optimized with 4912 different ring forms. Ring conformations were generated by fixing nonadjacent ring atoms (two for furanose rings, three for pyranoses) at increments of 0.1 Å from the plane of the remaining three atoms. At a dielectric constant of 4.0, prediction was in fair agreement with the NMR results. Model northern and southern conformers contribute to the furanose equilibrium, and both chairs are important for α-psicopyranose. © 1994 by John Wiley & Sons, Inc. |
