Design principles for Brownian molecular machines: how to swim in molasses and walk in a hurricane

Protein molecular motors—perfected over the course of millions of years of evolution—play an essential role in moving and assembling biological structures. Recently chemists have been able to synthesize molecules that emulate in part the remarkable capabilities of these biomolecular motors (for extensive reviews see the recent papers: E. R. Kay, D. A. Leigh and F. Zerbetto, Angew. Chem., Int. Ed., 2006, 46, 72–191; W. R. Browne and B. L. Feringa, Nat. Nanotechnol., 2006, 1, 25–35; M. N. Chatterjee, E. R. Kay and D. A. Leigh, J. Am. Chem. Soc., 2006, 128, 4058–4073; G. S. Kottas, L. I. Clarke, D. Horinek and J. Michl, Chem. Rev., 2005, 105, 1281–1376; M. A. Garcia-Garibay, Proc. Natl. Acad. Sci., U. S. A., 2005, 102, 10771–10776)). Like their biological counterparts, many of these synthetic machines function in an environment where viscous forces dominate inertia—to move they must “swim in molasses”. Further, the thermal noise power exchanged reversibly between the motor and its environment is many orders of magnitude greater than the power provided by the chemical fuel to drive directed motion. One might think that moving in a specific direction would be as difficult as walking in a hurricane. Yet biomolecular motors (and increasingly, synthetic motors) move and accomplish their function with almost deterministic precision. In this Perspective we will investigate the physical principles that govern nanoscale systems at the single molecule level and how these principles can be useful in designing synthetic molecular machines.