Molecular Dynamics Simulations of some Small Organic Molecules

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Abstract
Free energy differences between different conformers of D-ribofuranose, L-malic acid and meso-tartaric acid in solution were calculated using Molecular Dynamics simulations. In case of ribose the α → β transition was studied. For the acids attention was focussed on the transitions between the three possible staggered conformers with respect to the central C-C bond. In all cases a thermodynamic integration method was employed to evaluate the free energy difference. The use of an alternative technique, umbrella sampling, for ribose did not give promising results. It was shown that one needs a fairly accurate picture of the accessible conformational space in case of flexible molecules like the ones considered here before one can determine meaningful free energy differences. Large hysteresis effects between forward and reverse simulated transitions were observed, but contrary to the general belief they are no direct measure of the accuracy of the calculated ΔG values. In all cases the ΔG values resulting from the simulations and from NMR experiments agree within the, considerable, error limits and for the different forms of D-ribose, L-malic acid and L-tartaric acid the relative order of their populations is also correctly reproduced.