Theoretical study of alternative ring forms of α-L-fucopyranose

The ring puckering of alpha-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 C-1(4) chair conformer to be lower in energy by similar to 6 kcal/mol than the most stable non-C-1(4) conformer. The zero-point energy differences are very small. We provide a detailed study of the basis set (up to 6-311+G-(2d,p) basis set) and correlation effects (MP2 and hybrid functional study) on the relative energies and molecular geometries. The converged results are presented wherever it is possible. The largest difference between the various methods is similar to 2.7 kcal/mol; however, agreement well within 1 kcal/mol is experienced for most of the conformers. We discuss the geometric consequences of the exoanomeric effect, and show how the calculated and X-ray experimental results can be brought into accordance with each other. The geometric aspects of the O ... H interactions (hydrogen bonding) are described with the aid of a polar coordinate system centered at the acceptor oxygen atom. The equatorial-equatorial and the equatorial-axial type of OH interactions can be readily distinguished using the proposed geometric parameters. We present the shape of the contour surface of the Laplacian of the electron density around the oxygen atom and show how this shape influences the various types of O ... H interactions. The strength and weakness of the Van den Enden's pi-, sp(3)-, and sigma-type interactions were readily explained using the aforementioned surface.