Using directed evolution, researchers say they’ve “bred” protein molecules from an unusual type of bacteria to create chemical bonds between silicon atoms and carbon atoms efficiently.
Chemists have been able to do that in the lab, but it’s not been done biologically before.
“No living organism is known to put silicon-carbon bonds together, even though silicon is so abundant, all around us, in rocks and all over the beach,” Caltech researcher Jennifer Kan, the lead author of a report on the experiment published in the journal Science, said in a news release.
Silicon is the second most abundant element in Earth’s crust (after oxygen), and the idea of living organisms based on silicon rather than carbon has been a science-fiction standard for decades. The best-known example is the Horta, the rock-eating creature on the planet Janus VI in the original “Star Trek” TV series.
More recently, astrobiologists (including the University of Washington’s Peter Ward) have speculated that if life were to arise on Titan, a smog-shrouded moon of Saturn, it could make use of silicon-based molecules known as silanes and silanols.
For the experiment described in Science, the researchers started out with a bacterium that grows in hot springs in Iceland, known as Rhodothermus marinus. They focused a protein called cytochrome c, which was previously known to be capable of acting chemically on silica nanoparticles.
Kan and her colleagues mutated the bacteria’s DNA coding for cytochrome c, in a region that was thought to play a part in the protein’s ability to form silicon-carbon bonds. They tested the mutant molecules for their ability to make the bonds, and selected the best strains for further breeding.
“It’s like breeding a racehorse,” said Caltech chemical engineer Frances Arnold, the principal investigator for the project. “A good breeder recognizes the inherent ability of a horse to become a racer, and has to bring that out in successive generations. We just do it with proteins.”
After just three rounds of breeding, the resulting protein could selectively make silicon-carbon bonds 15 times more efficiently than the best catalyst invented by chemists, the researchers reported.
“This iron-based, genetically encoded catalyst is nontoxic, cheaper, and easier to modify compared to other catalysts used in chemical synthesis,” Kan said.
The biologically based catalyst could be used to produce the types of organosilicon compounds used in pharmaceuticals, agricultural and industrial chemicals, semiconductors and display screens. And theoretically, it could be part of the molecular machinery for silicon-based life.
“This study shows how quickly nature can adapt to new challenges,” Arnold said. “The DNA-encoded catalytic machinery of the cell can rapidly learn to promote new chemical reactions when we provide new reagents and the appropriate incentive in the form of artificial selection. Nature could have done this herself, if she cared to.”
In addition to Kan and Arnold, the authors of “Directed Evolution of Cytochrome c for Carbon-Silicon Bond Formation: Bringing Silicon to Life” include Russell Lewis and Kai Chen. The research is funded by the National Science Foundation, the Caltech Innovation Initiative program and the Jacobs Institute for Molecular Engineering for Medicine at Caltech. The project recently won Caltech’s Dow Sustainability Innovation Student Challenge Award grand prize.