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

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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.

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