Feeling the force of a bistable molecular shuttle

The widespread utilization of sub-molecular motion in key biological processes is inspiring chemists that have been trying to synthesize molecular machines able to imitate the machinery of biological world. In recent years, it has been proved possible to design synthetic molecular systems in which positional displacements of sub-molecular components occur upon the application of external stimuli.[1-4] The architecture of synthetic systems is crucial to translate molecular level effect into a useful response exploitable in the macroscopic world. The pioneering work of Stoddart, Sauvage and others[1-3] has shown that molecular machines with mechanically interlocked architecture are particularly suited for these sorts of applications, because they permit the controlled, large amplitude, movement and positioning of one mechanically interlocked component with respect to another. Among these architectures, rotaxanes -i.e. molecules consisting of a ring threaded on a linear molecule capped with bulky end stoppers- are a particularly promising kind of synthetic 'molecular shuttles'.

Truly functional systems based on synthetic molecular machines have not yet been proposed because some key questions remain unanswered:
What are the structural features necessary for molecules to convert this controlled motion into useful function? At what level (single molecule, nanoscopic, microscopic, macroscopic) can this be done? Can we address and utilize the induced-motion in a single molecular machine?
To answer those questions we are advocating the use of molecular shuttles coupled to a polymeric scaffold and interfaced with AFM. We are convinced that this is an efficient route to translate the sub-molecular motion into a useful response that can be exploited to perform physical tasks

Our objective is to demonstrate the feasibility of transducing sub-molecular movements into mechanical work by combining the controlled translational motion of the ring in a rotaxane coupled to a polymer chain, and the ability of AFM-based single molecule force spectroscopy to be used as a mechanical device.[5] For that purpose, a bistable hydrogen-bonded rotaxane with one fumaramide and one succinic amide ester station was synthesized. The equilibrium distribution of the ring between the two stations is in favour of the fumaramide station (>95%).[6] If an external force forces the ring to leave the preferred binding site, it will move back to this preferred binding site through biased Brownian motion. We have attached a poly-ethylene oxide (PEO) chain to the ring and the resulting rotaxane-polymer compound was grafted onto gold substrates. We then fished the PEO chain with an AFM tip. The applied force exerted on the ring when pulling on the polymer chain causes the H bonds linking the ring to the fumaramide station to break. When trying to move away the ring, it shuttles back to its station in the opposite direction of the pulling force, doing work against the AFM cantilever. We have estimated the work done by the ring and show that the value is in good agreement with the theoretical value predicted by Altieri et. al.[6]

[1] Special Issue on Molecular Machines, Acc. Chem. Res. 2001, 34, p. 409-522.
[2] V. Balzani, A. Credi, M. Venturi, Molecular Devices and Machines – A Journey into the Nano World, Wiley-VCH, Weinheim, Germany, 2003.
[3] E R Kay, D A Leigh and F Zerbetto, Angew. Chem. Int. Ed. 2007, 46, 72.
[4] a) V Bermudez, N Capron, T Gase, F G Gatti, F Kajzar, D A Leigh, F Zerbetto and S Zhang, Nature, 2000, 406, 608. b) A M Brouwer, C Frochot, F G Gatti, D A Leigh, L Mottier, F Paolucci, S Roffia, G W H Wurpel, Science , 2001, 291, 2124. c) D A Leigh, J K Y Wong, F Dehez and F Zerbetto, Nature, 2003, 424, 174. d) J V Hernandez, E R Kay and D A Leigh, Science, 2004, 306, 153. e) E R Kay and D A Leigh, Nature, 2006, 440, 286. f) V Serreli, C-F Lee, E R Kay and D A Leigh, Nature, 2007, 445, 523.
[5] T. Hugel, N. B. Holland, A. Cattani, L. Moroder, M. Seitz, H. E. Gaub, Science 2002, 296, 1103.
[6] A. Altieri, G. Bottari, F. Dehez, D A Leigh, J.K.Y Wong, F Zerbetto, Angew. Chem. Int. Ed. 2003, 42, 2296.