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Adam Braunschweig
BS Cornell University
Fishing for Pseudorotaxanes
Molecular machines such as catenanes, rotaxanes, and pseudorotaxanes have shown great promise for applications such as molecular electronics and mechanical actuation due to their ability to incur movement in response to chemical, electrical, or photonic stimulus.1. The ability to extrapolate device performance from thermodynamic values such as binding constant and free energy of binding would be a valuable tool to those attempting to design molecular machines. However, the behavior of surface bound molecular machines under mechanical stress can vary drastically from equilibrium dynamics observed typical solution-based experiments.2
Single-molecule force spectroscopy3, in which a scanning-probe microscope (SPM)4 is used to bring together and pull apart single supramolecular complexes,5 provides the ability to directly measure thermodynamic properties of single molecules on surfaces under mechanical stress. This technique has been used to study the forces involved in the supramolecular complexation of interactions including DNA base pairing,6 protein-ligand binding,7 and the inclusion of ferrocene into cyclodextrin.8
This project proposes to apply single-molecule force spectroscopy to the pseudorotaxanes prepared in our group. By tethering the tetracationc host9 cyclo-bis(paraquate-p-phenylene)4+ to a SPM tip and a variety of pi-electron donating guests to a surface through self-assembly, the properties of the suface-bound supramolecular interactions can be determined. Thus, by applying this technique to our systems, (1) a correlation between solution and surface-bound thermodynamic properties can be established, and (2) these parameters can be used to assist in the more effective design of future molecular machinery.
References
1. Balzani, V.; Credi, A.; Raymo, F.M.; Stoddart, J.F. Angew. Chem. Int. Ed. Engl. 2000, 39, 3348-3391.
2. a) Merkel, R.; Nassoy, P.; Leung, A.; Ritchie, K.; Evans, E. Nature 1999, 397, 50-53. b) Evans, E.; Ritchie, K. Biophys. J. 1997, 72, 1541-1555. c) Evans, E. Annu. Rev. Biophys. Biomol. Struct. 2001, 30, 105-128.
3. Florin, E.L.; Moy, V.T.; Gaub, H.E. Science 1994, 264, 415-417.
4. Binning, G.; Quate, C.F.; Gerber, C. Phys. Rev. Lett. 1986, 56, 930-934.
5. Noy, A.; Vezenov. D.V.; Lieber, C.M. Annu. Rev. Mat. Sci. 1997, 27, 381-421.
6. a) Boland, T.; Ratner, B.D. Proc. Natl. Acad. Sci. USA 1995, 92, 5297-5301. b) Lee, G.U.; Chrisey, L.A.; Colton, R.J. Science 1994, 266, 771-773.
7. Dammer, U.; Popescu, O.; Wagner, P.; Anselmetti, D.; Guntherodt, H.J.; Misevic, G.N. Science 1995, 267, 1173-1175.
8. Schonherr, H.; Beulen, M.W.J.; Bugler, J.; Huskens, J.; van Beggel, F.C.J.M.; Reinhoudt, D.N.; Vancso, G.J. J. Am. Chem. Soc. 2000, 122, 4963-4967.
9. a) Collier, C.P.; Wong, E.W.; Belohradsky, M.; Raymo, F.M.; Stoddart, J.F.; Kuekes, P.J.; Williams, R.S.; Heath, J.R. Science, 1999, 285, 391-394. b) Tseng, H.R.; Vignon, S.A.; Stoddart, J.F. Angew. Chem. Int. Ed. Engl. 2003, 42, 1491-1495.
More About AdamAdam Braunschweig has been pursuing graduate studies at UCLA since 2001 after receiving his bachelor’s degree from Cornell University, Ithaca, NY. While at Cornell he worked on the synthesis of amphiphilic block copolymers in the laboratory of Prof. D.Y. Sogah. In his free time, Adam enjoys long walks on the beach and NASCAR.
