Exocyclic Hydroxymethyl Rotational Conformers of β- and α-d-Glucopyranose in the Gas Phase and Aqueous Solution

• Published in: The journal of physical chemistry. A, Molecules, spectroscopy, kinetics, environment, & general theory Vol. 102; no. 26; pp. 5086 - 5092
• Main Authors: Wladkowski, Brian D., Chenoweth, Sarah A., Jones, Kendra E., Brown, James W.
• Format: Journal Article
• Language: English
• Published: American Chemical Society 25.06.1998
• ISSN: 1089-5639; 1520-5215
• DOI: 10.1021/jp980524+

The intrinsic exocyclic hydroxymethyl rotational surface for β-d-glucopyranose as well as the β−α anomer energy difference for d-glucopyranose has been studied using ab initio quantum mechanical methods including continuum solvation. Relevant stationary points, including rotational transition states, have been characterized by full geometry optimization using the 6-31G(d) basis set for the most stable counterclockwise (cc) overall conformation. Effects of dynamic electron correlation on both the geometric structures and the relative energetics of this system are also explored using Møller−Plesset perturbation theory (MP2 through MP4(SDTQ)) and density functional methods (BLYP). A total of six stationary points, including three minima and three transition states, were identified along the exocyclic rotational surface. All three minima were found to be very close in energy with a final order of GG (0.0) < GT (2.84) < TG (3.05) based on the relative free energy, , determined at the MP4(SDTQ)/6-31G(d)//MP2/6-31G(d) level of theory. The rotational transition state free energy differences varied from 18.8 to 28.9 kJ mol-1 at the same level of theory with the transition state connecting the TG and GG minima being the lowest. The intrinsic gas-phase β−α anomer free energy difference for the cc-TG conformer of d-glucopyranose was also determined at various levels of theory. On the basis of the convergence of the MP series, this energetic quantity has been estimated at 8 ± 2 kJ mol-1 favoring the α-anomer, and is insensitive to exocyclic hydroxymethyl rotation. Two different solvation models were used to explore the effects of aqueous solvation on the energetic parameters mentioned above. The Onsager continuum solvation model and the self-consistent isodensity polarized continuum model (SCIPCM) showed significant, yet predictable, effects on the exocyclic hydroxymethyl rotational surface for β-d-glucopyranose. Shifts in the relative energetics compared to those for the gas phase ranged from −0.8 to +2.8 kJ mol-1 for Onsager dipole model and −1.6 to as much as +4.7 kJ mol-1 for the SCIPCM model at the MP2 6-31G(d) level, resulting in a qualitative change in the ordering of the relative stability of the three stable minima. The effects of the solvation models on the β−α anomer energy difference were also significant, showing a relative decrease in the β−α anomer energy difference from the intrinsic gas-phase result. However, it is clear that these specific continuum solvation models alone cannot account for the experimentally observed preference of the β-anomer in aqueous solution.