Flexibility of the pyranose ring in α- and β-D-glucoses

Abstract

Conformational energies of α - and β -D-glucopyranoses were computed by varying all the ring bond angles and torsional angles using semiempirical potential functions. Solvent accessibility calculations were also performed to obtain a measure of solvent interaction. The results indicate that the ⁴C₁ (D) chair is the most favored conformation, both by potential energy and solvent accessibility criteria. The ⁴C₁ (D) chair conformation is also found to be somewhat flexible, being able to accommodate variations up to 10° in the ring torsional angles without appreciable change in energy. Observed solid-state conformations of these sugars and their derivatives lie in the minimum-energy region, suggesting that the substituents and crystal field forces play a minor role in influencing the pyranose ring conformation. Theory also predicts the variations in the ring torsional angles, i.e., CCCC < CCCO < CCOC, in agreement with the experimental results. The boat and twist-boat conformations are found to be at least 5 kcal mol^-1 higher in energy compared to the ⁴C₁ (D) chair, suggesting that these forms are unlikely to be present in a polysaccharide chain. The ¹C₄ (D) chair has energy intermediate between that of the ⁴C₁ (D) chair and that of the twist-boat conformation. The calculated energy barrier between ⁴C₁ (D) and ¹C₄ (D) conformations is high-about 11 kcal mol^-1.