PUMP a few thousand hydrogen atoms into a box and they won鈥檛 come to life. Throw in a few thousand carbon or nitrogen atoms and still nothing weird will happen. But if you find just the right combination of inanimate atoms and arrange them in a particular way, strange properties 鈥 even life 鈥 will emerge. Work out how that happens and you could make the world a very different place.
An influential group of researchers suspect that nature鈥檚 assembly manual 鈥 the instructions for arranging atoms to bring out strange, 鈥渆mergent鈥 properties such as superconductivity or life 鈥 is almost within their grasp. They believe they have caught a fleeting glimpse of the contents, and are calling for a concerted effort to read the rest.
There鈥檚 nothing special about biology, the researchers say. Life may arise from the same set of organising principles that determine the arrangement of atoms or electrons in materials with unusual properties. If they can learn the assembly tricks behind such phenomena, they reckon they could design materials with new and unexpected traits: superconductivity at room temperature, or drugs targeted to fight specific diseases. They may even go on to trump biology by creating an entirely artificial system that outperforms living things at certain tasks.
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Welcome to the world of 鈥渃omplex adaptive matter鈥. About two years ago, some of the most respected physicists and chemists in the US formed a research network 鈥 the Institute for Complex Adaptive Matter (ICAM) 鈥 spearheaded by the University of California and based at Los Alamos National Laboratory in the mountains above Santa Fe, New Mexico. These researchers are convinced that most of their colleagues have ignored a vital gap in our understanding of the world around us: we know little about how organisation and assembly arises in nature.
杏吧原创s have known for decades that unexpected properties emerge from biological systems 鈥 the way cell membranes form, for example, or the way slime mould colonies ebb and flow in response to changing nutrients. But nobody knows where such self-organisation comes from. This, the researchers believe, should be the main thrust of science in the 21st century. 鈥淭he discoveries that matter are the grand surprises that occur when matter organises itself,鈥 says Nobel prize-winning physicist Robert Laughlin of Stanford University, California.
The frontier where these surprises occur, Laughlin says, is known as the mesoscopic scale, from the Greek 鈥渕esos鈥 meaning 鈥渋n the middle鈥. Objects ranging from a few nanometres to a tenth of a micrometre show special properties that do not occur on microscopic or macroscopic scales. At this size threshold, adding more particles often makes the material not just bigger, but entirely different. Groups of hundreds to thousands of particles 鈥 dumb, inanimate matter 鈥 suddenly begin to organise themselves into patterns that make them seem capable of responding to their surroundings. This mysterious self-organisation eventually leads to the emergence of life itself.
Finding what causes emergence is going to be extraordinarily difficult, however. The ICAM researchers are groping in the dark, because objects on this scale are fiendishly difficult to see. They are just beyond the reach of visible light waves, and too fragile to survive most of the X-rays and other probes used to expose atoms and crystals.
But although they have no theory to guide them and little means of doing experiments, ICAM researchers believe they have already found some tantalising hints of nature鈥檚 assembly methods. The first key principle 鈥 which seems to apply to both living and inanimate matter 鈥 is 鈥渇rustration鈥.
Emergent behaviour appears most likely to arise from systems where the building blocks are competing against each other in two or three different ways 鈥 often with simultaneous repulsion and attraction. Such constant struggles can force clumps of material into patterns that grow and mutate in unstable ways. And then, under the right conditions, ensembles of particles may snap into one dominant state with new and unexpected properties.
Frustration may be behind the puzzling emergent phenomenon of high-temperature superconductivity. Researchers in physicist Seamus Davis鈥檚 lab at the University of California, Berkeley, have found evidence that a frustrating tussle can lead a superconductor鈥檚 electrons to arrange themselves into patterns on the mesoscopic scale. This organisation may allow electrons to move without resistance.
Their findings have yet to be published, but Davis and his colleagues have seen 3-nanometre blobs of altered electron density in the thin planes of superconducting copper oxide. A constant tug-of-war between the electrical and magnetic forces acting on the electrons probably leads to this pattern.
David Pines of Los Alamos National Laboratory, New Mexico, a co-director of ICAM, is excited by Davis鈥檚 results because they suggest that frustration could be a universal organising principle in nature. Some biological structures are known to arise from the same kind of frustrated tussling. In water, fatty molecules called phospholipids assemble into double layers with their water-hating tails huddled together inside and their water-loving heads facing outwards, making structures like cell membranes. Tiny spheres or 鈥渕icelles鈥 made from single layers of phospholipid molecules spontaneously assemble within the protoplasm of cells and have distinct boundaries similar to the electron blobs seen by Davis.
Indeed, Pines wonders whether the electron arrangements in high-temperature superconductors are the hard-matter equivalent of micelles 鈥 a few elements pulled together by competing forces into an arrangement that works in a whole new way. 鈥淎re these really electronic micelles?鈥 he asks. 鈥淒oes one find this kind of organisation elsewhere in nature?鈥 Davis鈥檚 results may show that the organising principles behind hard matter and biology are closely linked 鈥 if not exactly the same.
It wouldn鈥檛 be the first indication that hard matter and biological matter follow the same principles. Last year Nobel prize-winning physicist Philip Anderson of Princeton University published a paper in Science (vol 288, p 480) on cooperation between electrons. Anderson had seen evidence that electrons in high-temperature superconductors seemed to arrange themselves so as to protect the superconducting state from the effects of external disturbances. It鈥檚 not dissimilar to the flocking behaviour of birds or the swarming of bacteria.
Unexpected self-organisation arising from the frustrations of inanimate particles has also been witnessed in the lab of chemist George Whitesides of Harvard University. His work shows self-organisation can happen at a much larger scale, suggesting that in some physical systems the number of particles involved is the key factor, rather than their sizes. He and his team have taken to dumping lots of millimetre-sized iron balls into a plastic Petri dish, putting a rotating bar magnet under the dish, and watching what happens. The results are startling.
The balls swarm around inside the plastic dish as the magnet rotates. At first the swarm is disordered. But after a minute, it breaks up into a set of concentric rotating rings. Within each ring, the balls follow one another along precise tracks, as if hugging the rim of an invisible roulette wheel. Soon the balls in each track are perfectly equidistant. Finally, one ball in each ring comes to a dead stop. The other balls in each track line up behind this leader in a tiny arc and stay there, even though the magnet is still whirling away below. The patterns, says Whitesides, have something to do with 鈥渢ribocharging鈥 鈥 an accumulation of electrostatic charge on the balls due to friction with the dish. But he has no formal explanation for their emergence. He has seen similar self-organisation emerge from the interactions of tiny magnetised discs.
Because they have observed frustration in a number of systems with emergent properties, the ICAM researchers believe that it may be written on the first page of nature鈥檚 assembly manual. Design something that contains a combination of competing forces, such as simultaneous attraction and repulsion, and you鈥檒l get new, emergent properties in your material. It鈥檚 only a vague clue to the principles behind self-organisation, but it鈥檚 a start.
However, frustration on its own won鈥檛 always do the job. Superconducting electrons and the steel balls achieve the same organised outcome from any random initial conditions. Whatever the situation, matter in these self-organised systems is always guided to the same conclusion. Something, somehow, works alongside frustration to guide its path.
Unfolding the secrets
To find what that something might be, the ICAM researchers have turned to proteins, the kings of self-assembly. In order to repair DNA or cart oxygen around the body, the string of hundreds of amino acids that constitutes a protein has to fold into a characteristic shape. This folding seems to be another protected property: like the arrangement of electrons in a superconductor, it always reaches the same end point, unfailingly settling into a precise formation. When one imagines the number of ways to wind and knot a necklace with more than a hundred beads on it, the process of arriving at a set of unique shapes again and again boggles the mind. And this is the only shape that will allow the protein to do its job.
Los Alamos biophysicist Hans Frauenfelder and chemist Peter Wolynes of the University of California, San Diego 鈥 two of the driving forces behind ICAM 鈥 think they have discerned a principle behind this remarkable ability: funnelling.
Each amino-acid chain has an 鈥渆nergy landscape鈥 associated with the various shapes it can adopt. The overall energy of a protein depends on how its atoms are arranged, and the associated attractive and repulsive forces. Alter its shape 鈥 even slightly 鈥 and its energy changes. When the scientists make a mathematical plot of this energy landscape, they see a tortured 鈥渟urface鈥 consisting of innumerable peaks and valleys. The peaks represent high-energy, strained alignments of amino acids that force parts of the chain to unfold. Valleys, on the other hand, represent favourable alignments that reduce the overall strain 鈥 and thus the energy 鈥 of the amino-acid chain.
The key, say Frauenfelder and Wolynes, is that all functional proteins have funnel-shaped energy landscapes. 鈥淭here are valleys within valleys within valleys,鈥 says Frauenfelder. Random motions of the chain keep bumping it from one valley down to the next until it reaches the lowest energy. Because of this funnel, the protein ultimately settles at the lowest, most stable position, like a properly folded road map instead of a crumpled one. 鈥淣ature has selected only these funnel-shaped energy landscapes,鈥 Frauenfelder says. 鈥淚f there鈥檚 no funnel, the chain wouldn鈥檛 fold properly.鈥
The ICAM researchers say it could take years of research to learn whether funnel-shaped energy landscapes are really written into nature鈥檚 assembly manual. However, the idea may already have found a practical application in medicine. At the end of May, some of ICAM鈥檚 protein researchers met with representatives from pharmaceuticals companies to see whether it鈥檚 possible to harness the principles behind protein folding to design new drugs. Just as the landscapes encourage proteins to fold into a specific, functional shape, pharmaceuticals companies are interested in designing drugs with energy landscapes that funnel the drug molecules towards a specific target inside cells. That strategy might create a drug that only binds to the disease-causing prion proteins in vCJD, for example, leaving normal proteins unaffected.
Other applications of the ICAM research will take longer to emerge. David Pines sees great promise in using competing interactions within superconductors as a basis for designing new ones that, eventually, will work at room temperature. Materials scientists ought to look for compounds in which magnetic and electrical interactions are maximised, he believes. 鈥淜nowing which classes of compounds can do this will suggest other materials that might behave in a similar way,鈥 Pines says. Today鈥檚 temperature limit for superconductivity is still far too cold for widespread use but, with an increase of 100 degrees or so, any number of technologies could be revolutionised 鈥 from transportation to medical imaging.
For his part, Whitesides thinks it may be feasible to shrink his systems of strangely self-organising particles down to the mesoscopic 鈥渟cale of life鈥. There, he believes, they could form strange new kinds of machines that self-assemble in the lab. 鈥淭hese are machines in which the parts sit in a semi-frictionless world, interacting with one another, but driven with no connecting parts,鈥 Whitesides says.
It鈥檚 still a distant dream, but Laughlin believes that designing the energy landscape of materials, and creating novel characteristics in the process, will have far-reaching consequences. It may even be possible to design an entirely artificial system that shows the adaptive behaviour seen in living things.
For instance, tiny 鈥渞obot bacteria鈥 could be designed to clean pipes and degrade pollutants more efficiently than swarms of biological bacteria. But because the robot bugs would respond to changes in their environment by obeying the same self-organising rules that are evident in biology, there would be no need for prescriptive internal programming.
Laughlin and Pines are still not sure, though, that the principles of self-organisation will prove transparent enough for us to outdo biology. And Whitesides doubts the ICAM researchers will ever be able to create things with the complexity needed to make them truly 鈥渁live鈥.
But another form of life might emerge soon 鈥 without ICAM鈥檚 help. Whitesides suggests that the organising principles that give matter emergent properties might also take effect on the Web. 鈥淚t has enormous complexity 鈥 it鈥檚 a very rich environment for unexpected things to happen,鈥 he says. Hunting down its emergent properties will take someone with an altogether different expertise, however. 鈥淚 don鈥檛 know that I would recognise life in the World Wide Web,鈥 Whitesides admits. 鈥淥r that it would recognise me.鈥
