University of California, Los Angeles

A physicist links magnetism, force and fatigue.

If a metal bar is repeatedly stretched and released it becomes fatigued and, eventually, ruptures. The latter can occur suddenly and unexpectedly: sometimes materials scientists can find no obvious thermodynamic hint that a steel rod is about to break. I am interested in fatigue because it parallels other phenomena that concentrate energy density, such as triboluminescence, whereby diffuse stress makes a crystal glow.

In both triboluminescence and fatigue, applied forces cause molecular rearrangements. But fatigue also involves nanometre-sized defects that accumulate during the useful life of a piece of metal and organize themselves into a soft spot. Recently, Sidney Guralnick and his colleagues at the Illinois Institute of Technology in Chicago measured how much work is needed to complete each 'stretch and release' cycle in rods of AISI 1018 steel, a common low-carbon steel that is used in vehicle parts such as gears (S. A. Guralnick et al. J. Phys. D Appl. Phys. 41, 115006; 2008). This allowed them to follow changes in the material's response to force as it fatigued.

A shift occurred at merely 12.3% of the time to rupture. What is happening inside the steel at this point is mysterious, but the number holds true even when the useful life of identically manufactured rods varies by a factor of 200.

Further clues will no doubt come from steel's piezomagnetism — the fact that its magnetism varies with the degree of stretch it experiences. This relationship is complex: even when the metal is so slightly strained that it goes back to its original shape on release, its magnetic field does not return to the pre-stretched state. One day investigations into this property may uncover the organizing principle of the nanometre-sized defects that underlie metal rupture.

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Semmelweis University, Budapest, Hungary

A network scientist highlights active sites of enzymes, cells, brains and society.

For proteins, chemical binding is a tricky business. Special signals must be sent across a sea of water molecules to the desired partner, and complex mutual structural adjustments (a fluctuation fit) must be completed before each successful binding event.

I have long taught that a protein at its lowest-energy conformation still has regions of higher energy. But I've always been intrigued: how is the extra energy of the active sites preserved? And why do we need such big enzymes when their active sites occupy only a tiny region?

Piazza and Sanejouand found part of the answer by identifying special energy-preserving segments of proteins (F. Piazza and Y.-H. Sanejouand Phys. Biol. 5, 026001; 2008). Taking into account the effect of the surrounding water, they modelled proteins with a computer program that arranges oscillating elements in the same pattern as amino acids in real proteins. In most of these proteins, they identified a few easily excitable segments that collected and harboured long-lived, localized vibrations. An analysis of 833 enzymes showed that these segments co-occur with the catalytic active sites; are located on the stiffest parts of the proteins; and have many connections but are surrounded by a less well-connected environment.

The generality of many network properties prompts me to ask: can we find 'active sites' of cells, brains, ecosystems and societies? Piazza and Sanejouand's segments correspond to Ronald Burt's "structural holes" in social networks — whereby areas of greatest economic potential are areas of low connectedness, where brokers can make new connections. Indeed, not only amino acids, but people may also act as brokers, mediators and catalysts. It may be worthwhile to think about creative, broker proteins as drug targets. One could even imagine creative sets of neurons.

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