Journal
RSC ADVANCES
Volume 5, Issue 71, Pages 57309-57317Publisher
ROYAL SOC CHEMISTRY
DOI: 10.1039/c5ra05642j
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Funding
- National Natural Science Foundation of China [21373117]
- MOE Innovation Team of China [IRT13022]
- Natural Science Foundation of Tianjin, China [13JCYBJC18800]
- Cai Yuanpei program
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In water, a remarkable motion can be observed with a [2] rotaxane, wherein the rotor translocates by reeling its axle in the cavity of an altro-alpha-CD stopper. Similarly, in aqueous solution, an alkyl altro-alpha-CD dimer reels its alkyl chain in the altro-alpha-CD cavity to form a pseudo[1] rotaxane dimer. This reeling motion is in fact induced by the tumbling of the altropyranose unit of an altro-alpha-CD, a process shown to be solvent-dependent. Tumbling, however, does not occur in low-polarity solvents such as methanol and DMSO. In the present contribution, the mechanism that underlies solvent-controlled tumbling has been studied at the atomic level by means of molecular dynamics simulations combined with microsecond-timescale free-energy calculations. The free-energy profile delineating the tumbling in water of the altropyranose unit of an alkyl altro-alpha-CD indicates that a 19.8 kcal mol(-1) barrier must be overcome to yield the self-inclusion complex, which is the most stable state available to the supramolecular assembly. In DMSO, the free-energy barrier is about 21.0 kcal mol(-1) higher, and the self-included alkyl altro-alpha-CD corresponds to a metastable state. These results provide new thermodynamic and kinetic insights into solvent-controlled tumbling, and reveal the essence of different experimental observations. Further investigation shows that aside from the polarity of the solvent, tumbling of the altro-alpha-CD derivative stems from the hydrophobicity of the side chain and the propensity of the former to include the latter, which opens perspectives for the design of new, related supramolecular assemblies.
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