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      The lubricating role of water in the shuttling of rotaxanes†


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          The special properties of water make it an effective lubricant in rotaxanes to enhance their shuttling.


          We have investigated at the atomic level amide-based rotaxanes set in motion in four different solvents, namely, ethyl ether, acetonitrile, ethanol and water. In three non-aqueous solvents, shuttling of the macrocycle between two binding sites separated by a free-energy barrier is coupled with a conformational change and rotation, driven primarily by hydrogen-bonding interactions. The mechanism that underlies the shuttling is completely altered when the non-aqueous solvent is replaced by water. In aqueous solution, hydrophobic interactions chiefly control shuttling of the rotaxane, leading to a sharp decrease of the free-energy barrier, thereby speeding up the process. The binding sites and the reaction pathway describing shuttling vary significantly in water compared with in the other three solvents. We found that the high polarity, the hydrogen-bond donor and acceptor ability, and the minimal steric hindrance of water conspire to modify the mechanism. These three physicochemical properties are also responsible for the lubrication by water. That water completely changes the mechanism underlying the shuttling of rotaxanes, is addressed for the first time in this study, and provides valuable guidelines for the de novo design of molecular machines.

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          Making molecular machines work.

          In this review we chart recent advances in what is at once an old and very new field of endeavour--the achievement of control of motion at the molecular level including solid-state and surface-mounted rotors, and its natural progression to the development of synthetic molecular machines. Besides a discussion of design principles used to control linear and rotary motion in such molecular systems, this review will address the advances towards the construction of synthetic machines that can perform useful functions. Approaches taken by several research groups to construct wholly synthetic molecular machines and devices are compared. This will be illustrated with molecular rotors, elevators, valves, transporters, muscles and other motor functions used to develop smart materials. The demonstration of molecular machinery is highlighted through recent examples of systems capable of effecting macroscopic movement through concerted molecular motion. Several approaches to illustrate how molecular motor systems have been used to accomplish work are discussed. We will conclude with prospects for future developments in this exciting field of nanotechnology.
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            Great expectations: can artificial molecular machines deliver on their promise?

            The development and fabrication of mechanical devices powered by artificial molecular machines is one of the contemporary goals of nanoscience. Before this goal can be realized, however, we must learn how to control the coupling/uncoupling to the environment of individual switchable molecules, and also how to integrate these bistable molecules into organized, hierarchical assemblies that can perform significant work on their immediate environment at nano-, micro- and macroscopic levels. In this tutorial review, we seek to draw an all-important distinction between artificial molecular switches which are now ten a penny-or a dime a dozen-in the chemical literature and artificial molecular machines which are few and far between despite the ubiquitous presence of their naturally occurring counterparts in living systems. At the single molecule level, a prevailing perspective as to how machine-like characteristics may be achieved focuses on harnessing, rather than competing with, the ineluctable effects of thermal noise. At the macroscopic level, one of the major challenges inherent to the construction of machine-like assemblies lies in our ability to control the spatial ordering of switchable molecules-e.g., into linear chains and then into muscle-like bundles-and to influence the cross-talk between their switching kinetics. In this regard, situations where all the bistable molecules switch synchronously appear desirable for maximizing mechanical power generated. On the other hand, when the bistable molecules switch "out of phase," the assemblies could develop intricate spatial or spatiotemporal patterns. Assembling and controlling synergistically artificial molecular machines housed in highly interactive and robust architectural domains heralds a game-changer for chemical synthesis and a defining moment for nanofabrication. This journal is © The Royal Society of Chemistry 2012
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              Shuttles and muscles: linear molecular machines based on transition metals.

              Transition-metal-containing rotaxanes can behave as linear motors at the molecular level. The molecules are set into motion either by an electrochemical reaction or using a chemical signal. In a first example, a simple rotaxane is described that consists of a ring threaded by a two-coordination-site axle. The ring contains a bidentate ligand, coordinated to a copper center. The axle incorporates both a bidentate and a terdentate ligand. By oxidizing or reducing the copper center to Cu(II) or Cu(I) respectively, the ring glides from a given position on the axle to another position and vice versa. By generalizing the concept to a rotaxane dimer, whose synthesis involves a quantitative double-threading reaction triggered by copper(I) complexation, a molecular assembly reminiscent of a muscle is constructed. By exchanging the two metal centers of the complex (copper(I)/zinc(II)), a large-amplitude movement is generated, which corresponds to a contraction/stretching process. The copper(I)-containing rotaxane dimer is in a stretched situation (overall length approximately 8 nm), whereas the zinc(II) complexed compound is contracted (length approximately 6.5 nm). The stretching/contraction process is reversible and it is hoped that, in the future, other types of signals can be used (electrochemical or light pulse) to trigger the motion.

                Author and article information

                Chem Sci
                Chem Sci
                Chemical Science
                Royal Society of Chemistry
                1 July 2017
                16 May 2017
                : 8
                : 7
                : 5087-5094
                [a ] Research Center for Analytical Sciences , College of Chemistry , Tianjin Key Laboratory of Biosensing and Molecular Recognition , Nankai University , Tianjin 300071 , China . Email: wscai@ 123456nankai.edu.cn
                [b ] Collaborative Innovation Center of Chemical Science and Engineering (Tianjin) , Tianjin 300071 , China
                [c ] State Key Laboratory of Medicinal Chemical Biology , Nankai University , Tianjin 300071 , China
                [d ] Laboratoire International Associé Centre National de la Recherche Scientifique et University of Illinois at Urbana-Champaign , Unité Mixte de Recherche No. 7565 , Université de Lorraine , B.P. 70239 , 54506 Vandœuvre-lès-Nancy cedex , France
                [e ] Theoretical and Computational Biophysics Group , Beckman Institute , University of Illinois at Urbana-Champaign , Urbana , Illinois 61801 , USA
                [f ] Department of Physics , University of Illinois at Urbana-Champaign , 1110 West Green Street , Urbana , Illinois 61801 , USA
                This journal is © The Royal Society of Chemistry 2017

                This is an Open Access article distributed under the terms of the Creative Commons Attribution 3.0 Unported License ( http://creativecommons.org/licenses/by/3.0/) which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.



                †Electronic supplementary information (ESI) available: Methods, structure of the rotaxane, free-energy calculation characterizing the isomerization of the ring-like molecule, free-energy landscape for the translation and conformational change of the ring in the rotaxane in vacuum, representative three-dimensional arrangements of the rotaxane in vacuum, rotation of the macrocycle along with translation, rotation of the terminal group of the dumbbell-like molecule accompanied with other movements in the rotaxane, one-dimensional free-energy decomposition, committor analysis, hydrogen bond analysis, boat–boat transformation of the macrocycle during the shuttling, solvent-accessible surface area (SASA) of the chain-like molecule along the transition coordinate in different solvents. Simulation parameters. See DOI: 10.1039/c7sc01593c


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