Hightlight Paper
Working Mechanism for a Redox Switchable Molecular Machine Based on Cyclodextrin: A Free Energy Profile Approach
We report the working mechanism for a redox-responsive bistable [2]rotaxane incorporating α-cyclodextrin (α-CD) ring (J. Am. Chem. Soc. 2008, 130, 11294-11296), based on free energy profiles obtained from all-atom molecular dynamics simulations.
Figure 1. Structure of rotaxane simulated in this study
Employing an umbrella sampling technique, the free energy profiles (potential of
mean force, PMF) were calculated for the shuttling motion of the α-CD ring between a tetrathiafulvalene (TTF) recognition site and a triazole (TZ) unit on the dumbbell of
the rotaxane for three oxidation states (0, +1, +2) of the TTF unit (Figure 2). These calculated free energy profiles verified the experimentally observed binding preference for each state.
Figure 2. The change of PMF as a function of the position of the α-CD ring along the dumbbell for neutral, oxidized +1 and oxidized +2 states in water. Note: the hydrogen
atoms are omitted in the snapshots of energy minima for clarity.
Analysis of the free energy components (Figure 3) reveals that, for these α-CD based rotaxanes with charged TTF units, the real driving force for the shuttling in the oxidized states is actually the interactions between water and the rotaxane components, which overwhelms the attractive interactions between the α-CD ring and the charged dumbbell.
Figure 3. Variation of the intermolecular interaction between the α-CD ring and the dumbbell (ECD-dumbbell), the α-CD ring and H2O (ECD-H2O), and the dumbbell and H2O (Edumbbell-H2O), with the position of the α-CD ring on the dumbbell for the neutral (dotted line), oxidized +1 (dashed line), oxidized +2 (solid line) states in water. The positions where the α-CD ring locates at the TTF (0.2 nm < z < 0.6 nm) and TZ (1.4 nm < z < 1.90 nm) units are marked in cyan. Energies are shown in the units of kJ/mol.
In this work, we put forward a feasible approach to describe correctly the complexation behavior of CD with charged species, that is, free energy profiles obtained from all-atom molecular dynamics simulation.
Reference
J. Phys. Chem. B , 2010, 114, 6561-6566.
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