Complex Movements in Rotaxanes: Shuttling Coupled with Conformational Transition of Cyclodextrins
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- 作者：Shuangshuang Wang ; Tanfeng Zhao ; Xueguang Shao ; Christophe Chipot ; Wensheng Cai
- 刊名：Journal of Physical Chemistry C
- 出版年：2016
- 出版时间：September 1, 2016
- 年：2016
- 卷：120
- 期：34
- 页码：19479-19486
- 全文大小：530K
- 年卷期：0
- ISSN：1932-7455
文摘
In cyclodextrin (CD)-based rotaxanes, the shuttling rate of the macrocycle along the thread is crucial to characterize their function as molecular machines. In general, the composition of the thread and the environment are considered to be important factors affecting the nature of the movement. Yet, the role of ancillary motions on the shuttling rate remains unclear. In the present contribution, two rotaxanes having the same components, yet significantly different shuttling rates between two stable states in an aqueous environment, have been investigated at the atomic level using numerical simulations. These two rotaxanes consist of an axle with two stations linked by a 2-methylpyridinium group and an α-CD sliding on the axle and assuming two different orientations. We found that a number of cyclodextrin glucopyranose units (GLUs) isomerized during shuttling, which we anticipate to affect the shuttling rate. The two-dimensional free-energy landscapes characterizing the isomerization of the GLUs and the shuttling along the thread were mapped and revealed that the energetic barriers hampering spontaneous transition between the two stations significantly differ for the two rotaxanes. Structural analysis shows that this difference mainly arises from steric hindrances caused by the methyl substituent of the pyridinium group, which leads to a different number of the GLUs experiencing conformational change during shuttling. Moreover, the thermodynamic stability of the complex is found to be distinct between the two rotaxanes. This discrepancy may be ascribed to the dipole moment of the complex, which is sensitive to the orientation of CD. It can be concluded that shuttling in the rotaxanes is not only highly coupled with isomerization of GLUs but also affected by thermodynamic stability, resulting in a shuttling rate sensitive to the orientation of the CD. The present results help understand the complex molecular motion in CD-based molecular shuttles, and are expected to serve in the design of molecular filters for selectively screening molecules with a specific orientation.