Modelling of carbohydrate conformations is a challenging task for force field developers. Three carbohydrate force fields, namely GLYCAM06, GROMOS 45a4 and OPLS were evaluated. Free energies of different ring conformations of beta-D-glucopyranose were calculated using metadynamics in vacuum as well as in explicitly modelled water. All three force fields model the (4)C(1) conformation as the most stable by at least 6kJ/mol, as compared to other conformations. Interconversion from the (4)C(1) to any other conformation is associated with a barrier of no lower than 26kJ/mol. The free energy surface calculated in the GLYCAM06 force field is in remarkably good agreement with the recent Car-Parrinello metadynamics study. The effect of a water environment is relatively low and analogous in all tested force fields. Namely, the presence of water stabilizes the upper-left ((3,O)B) versus bottom-right (B(3,O)) area of Stoddard's plot, relative to the situation in vacuum. Comparison of free and potential surfaces is also provided for vacuum calculations.

Department of Biochemistry and Microbiology Institute of Chemical Technology Prague Technická 3 Prague Czech Republic

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