Conformational Free Energy Landscape of β-Glucose in the Gas Phase and Aqueous Solution: Energetic, Structural, and Electronic Changes

The conformational flexibility of β-glucose is critical for the enzymatic breakdown of carbohydrates such as cellulose and starch. Detailed knowledge of its ring conformations supports the rational design of therapeutic agents and functional molecules, including glucosidase activity-based probes. Although quantum mechanical methods have been employed to study β-glucose conformations, a comprehensive analysis of the Cremer-Pople conformational space, particularly accounting for solvent effects, remains incomplete. Using density functional theory (DFT), we systematically characterize β-glucose conformations in both gas and aqueous phases. We apply three metadynamics approaches  standard, well-tempered, and parallel bias  using Cremer-Pople polar coordinates and ring dihedral angles as collective variables. Consistent conformational stability trends are observed across methods and environments. In both gas and aqueous phases, the free energy landscape (FEL) identifies the ⁴ <i>C</i> ₁ chair as the global minimum, followed by equatorial conformers and the inverted ¹ <i>C</i> ₄ chair, which is less stable in solution than in the gas phase. In the gas phase, the most stable distorted conformers (in the ² <i>S</i> _O -<i>B</i> _3,O - ¹ <i>S</i> ₃ region) exhibit structural and electronic features characteristic of an oxocarbenium ion, including a high C1-O1/C1-O5 bond length ratio, a pronounced anomeric effect, and negative charge accumulation at O1 and O5. These features are significantly diminished in aqueous solution, suggesting that the gas-phase FEL better reflects the conformational preferences of the saccharide at the <i>-1</i> subsite in enzyme-substrate complexes of glucosidases. These findings provide a valuable framework for investigating saccharide conformations, establishing β-glucose as a model system for computational and methodological benchmarking.