Probing sub-molecular motion in a bistable molecular shuttle

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] Through the pioneering work of Stoddart, Sauvage and others[1-3] it has become clear that the remarkable ability of rotaxanes to change the relative positions of their interlocked components in response to various external stimuli (light, electrons, heat, pH, nature of the environment) should one day find practical uses. However, truly functional systems based on synthetic molecular machines are still to be produced.
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 machines coupled to a polymeric scaffold. 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. Molecular machines with mechanically interlocked architectures 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 macrocycle threaded on a linear molecule capped with bulky end stoppers- are a particularly promising kind of synthetic molecular 'shuttles'.

Our objective is to demonstrate the feasibility of transducing the controlled translational motion of the ring in a rotaxane into mechanical work using AFM-based single molecule force spectroscopy 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] We have attached a poly-ethylene oxide (PEO) chain to the ring by 'click chemistry' 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, but when trying to move away the ring, it shuttles back to its station in the opposite direction of the pulling force. We estimated that the work done by the ring against the pulling force was about 3 kcal mole-1. This result 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.