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Free energy landscapes of the α-d- and β-d-glucopyranose conformations in both vacuum and aqueous solution

The pathway for pyranose ring conformational transitions in both anomers of glucose has been studied using molecular mechanics calculations both in vacuum and in aqueous (SPC/E) solution. The pathway for the transition in vacuum from the 4C1 to the 1C4 chair conformations in α-d-glucopyranose was found to proceed over a low barrier between the 4E and E5 conformers, and centred almost on 4H5, and then downhill through the 1S5 conformation to 1C4. A dead end pathway, with no early barrier, was found to lead to the 5S1 conformation, which thus would represent a kinetic trap impeding interconversion to the opposite chair in vacuum. Hydration was found to significantly lower the transition barriers, and in particular, to make the blocked pathway though 5S1 is feasible at lower temperatures. Transitions between the chair forms for the β-anomer of glucopyranose were in general found to be somewhat more facile, and to exhibit less influence from the presence of solvent.