Theoretical Study of Alternative Ring Forms of α-l-Fucopyranose

Abstract

The ring puckering of α-l-fucopyranose was studied by the MM2*-LMOD (low mode) conformational search technique built into the Macromodel program. The molecular geometries of lowest energy (within a 10 kcal/mol energy window) were analyzed at the HF/3-21G, HF/6-31G(d), and generalized gradient approximation (GGA) levels of density functional theory (DFT). Our results show that the MM2* method tends to overestimate the number of stable conformers in the high-energy region. The MM2* method detected 33 stable conformers within the aforementioned energy window, and this number was reduced to 25 after re-minimization by the HF and GGA DFT methods. The low-energy conformers yielded by the MM2* method are in qualitative agreement with the ab initio results. In these conformers, intramolecular chains of hydroxyl groups are formed in counterclockwise or clockwise directions. The best calculations predict the most stable 1C4 chair conformer to be lower in energy by ∼6 kcal/mol than the most stable non-1C4 conformer....