Evaluation of synthetic linear motor-molecule actuation energetics

Abstract

By applying atomic force microscope (AFM)-based force spectroscopy together with computational modeling in the form of molecular force-field simulations, we have determined quantitatively the actuation energetics of a synthetic motor-molecule. This multidisciplinary approach was performed on specifically designed, bistable, redox-controllable [2]rotaxanes to probe the steric and electrostatic interactions that dictate their mechanical switching at the single-molecule level. The fusion of experimental force spectroscopy and theoretical computational modeling has revealed that the repulsive electrostatic interaction, which is responsible for the molecular actuation, is as high as 65 kcal⅐mol ؊1 , a result that is supported by ab initio calculations.computational modeling ͉ force spectroscopy ͉ molecular motors ͉ switchable rotaxanes M olecular motors have recently garnered considerable interest within the domains of microsciences and nanosciences (1, 2). Harnessing the ability to selectively, cooperatively, and repeatedly induce structural changes in molecules may hold the promise of engineered systems that operate with the same complexity, elegance, and efficiency as biological motors function in the human body. In natural systems, it has become apparent that both macro and micro processes are initiated and controlled by nanoscale molecular motors (3). For example, myosin and kinesin are associated with muscle contraction and intracellular trafficking, respectively, and have recently found their ways into engineered devices (4, 5). Moreover, initial work has been performed that demonstrates the ability of natural nanoscale molecular motors to power microfabricated systems (6).Synthetic motor-molecules (7-11), which are designed to excel where their biological counterparts fall short, also have been investigated. Whereas devices powered by biological molecules require (4-6) chemical diffusion for actuation stimulus, synthetic molecules have been shown (2, 9) to operate with a variety of different stimuli, thereby lending much greater flexibility to a particular system's design. Moreover, a synthetic nanoscale actuating molecule carries with it an inherent ability to be modified and optimized precisely for a specific task.Switchable, bistable rotaxanes (2, 9), compounds comprised of a dumbbell-shaped component containing two different recognition sites for an encircling ring-shaped component, show particular promise as molecular actuators, given their ability to undergo controllable, reversible mechanical switching with the appropriate chemical, electrochemical, or photochemical stimulus in solution. Toward the goal of device applications, switching has been shown to operate in condensed phases such as in a polymer electrolyte gel (12), on a self-assembled monolayer (SAM) (13), on the solid supports of engineered systems (14), and in molecular switch tunnel junctions (15). Bistable rotaxanes benefit from their synthesis being highly modular, a virtue that allows for a considerable degree of flexibility in their des...

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