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From Steam Engines to Life?

What is the state of thermodynamics on the 100th anniversary of the death of Lord Kelvin?

Mark Haw

Thermodynamics Under the Microscope

Part%20of%20enzyme%20cytochrome%20P450Click to Enlarge ImageProteins are incontestably beyond the limits of Kelvin's thermodynamics, for several reasons. Proteins are not at all "equilibrium" machines; the inside of a cell is a profoundly heterogeneous place of concentration gradients and complex spatial structures. The size of a protein also limits the reach of Kelvin's laws: A typical protein molecule contains a few thousand atoms and is only tens of nanometers long. Precisely because of this scale, proteins are inextricably part of the chaotic molecular sea that surrounds them—one that could never be called a closed system.

Fluctuation is the key to understanding thermodynamics in small systems. The Second Law as developed by Kelvin tells us that, on average, entropy must always increase. But when a system such as a protein engine is made from a small number of components—and compared to the billion billion billion molecules of gas in a steam engine, even a ten-thousand-atom protein qualifies as small—you can't ignore fluctuations about that average. The water molecules surrounding the protein are subject to Brownian motion, which sends them crashing into the working parts of the protein engine. Not surprisingly, the energy supplied by the engine tends to fluctuate by an appreciable amount.

Over the past decade and a half, scientists have developed theories about thermodynamics at a microscopic scale. Christopher Jarzynski at the University of Maryland, Denis Evans at the Australian National University and Carlos Bustamante at the University of California, Berkeley, have all studied the application of the Second Law to the world of microscopic engines. Through experiments that tracked the motion of individual molecules, Bustamante's team in 2002 watched how a stretched RNA molecule relaxed back to its equilibrium shape. Two years later, Evans' group used a laser trap to tug on tiny particles in the grip of Brownian motion. Such experiments provide spectacular views of microscopic thermodynamics in the real world.

The primary conclusion of such studies is that biological engines such as proteins—not to mention manufactured engines on the same nanoscale—are indeed caught in a world of inevitable fluctuation. Understanding their biophysical function means figuring out how such engines not only cope with fluctuations, but use them to maximize efficiency. Scientists such as Alan Cooper of Kelvin's own University of Glasgow and Hans Frauenfelder at Los Alamos National Laboratory have spent years investigating this topic. Data from Frauenfelder's group show that a protein's fluctuating shapes form a complex hierarchy that may explain the versatility and efficiency of such machines in a difficult environment.

Understanding these microscopic protein engines at a thermodynamic level would mark a huge achievement in science and, potentially, a revolution in medicine. Imagine curing disease by reengineering the microscopic workings of the cell rather than by prescribing some chemical cocktail. Of course, such ability raises serious ethical dilemmas. However, ignorance is no protection: Our growing understanding of microscopic thermodynamics is the key to safe, ethical use of nanotechnology.

In Kelvin's time, thermodynamics enabled the industrial revolution—a historical period that produced its own ethical and social questions. Similarly, microscopic thermodynamics will enable a new revolution and prompt more discussion about the appropriate uses of technology.

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