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Biological physics

Rare returns on lost effort

Nature volume 437, pages 198–199 (2005)Cite this article

How does the size of a system affect its thermodynamic irreversibility? A deft experiment that observes the unfolding and refolding of a single molecule of RNA provides insights into the question at a small scale.

The Crooks fluctuation theorem2 (CFT) describes the exchange of energy between a system and its environment in forward and reverse processes. It generalizes the so-called Jarzynski equality3, which relates equilibrium free energy to the work done in multiple, non-equilibrium measurements. The application of the CFT to a single-molecule experiment can be understood by considering an idealized ‘dashpot’ — a plunger that can move through a viscous fluid held at a constant temperature. The force required to move a large, slow-moving plunger is proportional to its speed; all the mechanical work performed is lost, irretrievably, to the environment as heat. At a given speed, the work required to move the plunger between two well-defined states is always the same: viewed microscopically, the resistance to the motion is just the statistically averaged effect of many collisions between the plunger and the fast-moving molecules of the fluid.

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Figure 1: Stirring stuff.

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References

  1. Collin, D. et al. Nature 437, 231–234 (2005).

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  3. Jarzynski, C. Phys. Rev. Lett. 78, 2690–2693 (1997).

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Authors and Affiliations

  1. Department of Biomedical Engineering, Boston University, 44 Cummington Street, Boston, 02215, Massachusetts, USA

    Wesley P. Wong & Evan Evans

  2. Department of Physics, Harvard University, 17 Oxford Street, Cambridge, 02138, Massachusetts, USA

    Wesley P. Wong & Evan Evans

Authors
  1. Wesley P. Wong
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  2. Evan Evans
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Wong, W., Evans, E. Rare returns on lost effort. Nature 437, 198–199 (2005). https://doi.org/10.1038/437198a

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