ANALYSIS OF THE RING-FORM TAUTOMERS OF PSICOSE WITH MM3(92)

By nuclear magnetic resonance (NMR), the four ring forms of psicose, a lpha- and beta-pyranose and alpha- and beta-furanose, have almost equa l concentrations in aqueous solution. Prediction of this equilibrium b y molecular mechanics tests both the balance within the force field an d the methods for including the many degrees of freedom in the system. For both the alpha- and beta-furanoses, each of 410 different ring sh apes was studied in combination with 3(7) combinations of staggered si de-group orientations. Of these, 48 and 57 initial combinations of sid e-group orientations contributed to the alpha- and beta-furanose energ y surfaces. The pyranoses were analyzed in two steps. First, the 38 ch aracteristic ring conformations were optimized with the 6(3) staggered side-group combinations. All combinations that gave the lowest energy at one or more of the 38 shapes (20 combinations for alpha-pyranose, 15 for beta-pyranose) were then optimized with 4912 different ring for ms. Ring conformations were generated by fixing nonadjacent ring atoms (two for furanose rings, three for pyranoses) at increments of 0.1 An gstrom from the plane of the remaining three atoms. At a dielectric co nstant of 4.0, prediction was in fair agreement with the NMR results. Model northern and southern conformers contribute to the furanose equi librium, and both chairs are important for alpha-psicopyranose. (C) 19 94 by John Wiley & Sons, Inc.