11:51 14 September 2011 by Katharine Sanderso Magazine issue 2830
He has managed to build cell-like bubbles from giant metal-containing molecules and has given them some life-like properties. He now hopes to induce them to evolve into fully inorganic self-replicating entities.
"I am 100 per cent positive that we can get evolution to work outside organic biology," says Lee Cronin at the University of Glasgow. His building blocks are large "polyoxometalates" made of a range of metal atoms – most recently tungsten – linked to oxygen and phosphorus. By simply mixing them in solution, he can get them to self-assemble into cell-like spheres.
Cronin and his team begin by creating salts from negatively charged ions of the large metal oxides bound to a small positively charged ion such as hydrogen or sodium. A solution of this salt is squirted into another salt solution made of large, positively charged organic ions bound to small negative ones.
When the two salts meet, they swap parts and the large metal oxides end up partnered with the large organic ions. The new salt is insoluble in water: it precipitates as a shell around the injected solution.
Cronin calls the resulting bubbles inorganic chemical cells, or iCHELLs, and says they are far more than mere curiosities. By modifying their metal oxide backbone he can give the bubbles some of the characteristics of the membranes of natural cells. For example, an oxide with a hole as part of its structure becomes a porous membrane, selectively allowing chemicals in and out of the cell according to size, just like the walls of biological cells. This property gives the membrane control over the range of chemical reactions that can happen within – a key feature of specialised cells (Angewandte Chemie, DOI: 10.1002/anie.201105068).
The team has also made bubbles within bubbles (see images), creating compartments that mimic the internal structure of biological cells. Better yet, they have started imbuing the iCHELLs with the equipment for photosynthesis by linking some oxide molecules to light-sensitive dyes. Cronin says early results suggest he can create a membrane that splits water into hydrogen ions, electrons and oxygen when illuminated – the initial step of photosynthesis.
"We've [also] got an indication that we can pump protons across the membrane" to set up a proton gradient, says Cronin – another key stage in harnessing energy from light. If he can assemble all these steps, Cronin could create a self-powered cell with elements of plant-like metabolism.
It's early days; other synthetic biologists are reserving judgement for now. Cronin's bubbles are never going to be truly life-like until they carry something like DNA to drive self-replication and evolution, says Manuel Porcar of the University of Valencia in Spain. That is theoretically possible, he says, "but I cannot imagine what kind of system they would implement". Cronin isn't sure yet either, but last year he showed that he could get polyoxometalates to use each other as templates to self-replicate (Science, DOI: 10.1126/science.1181735).
In an ambitious seven-month experiment, Cronin is now mass-producing bubbles and injecting them into an array of tubes and flasks filled with different chemicals at different pH levels. He hopes that the mix of environments will allow only the fittest bubbles to survive. "If the pH is too low and [some bubbles] dissolve then those droplets will have died." Others may persist and accumulate. In the long run, the real test will be whether the cells can modify their own chemistry to adapt to different environments. Cronin hints that his latest work may show this, but is unwilling to give details as yet. "I think we have just shown the first droplets that can evolve" is all he will say.
If Cronin is right, then the possible range of extraterrestrial life is blown wide open. "There is every possibility that there are life forms out there which aren't based on carbon," he says. Tadashi Sugawara of the University of Tokyo, Japan, doesn't see why not. "On Mercury, the materials are all different. There might be a creature made of inorganic elements." Cronin may be some way from proving this, says Sugawara, but "he has pointed out a new direction".