Silicon, although it is second to oxygen in abundance in the Earth’s crust, does not occur in pure form in nature. It’s highly reactive and instantly bonds with other elements to form compounds. Hence, pure silicon can only exist by creating it in the lab.
The same is true with silicon-carbon bonds, known as organosilicon: this is a laboratory-made compound that has a vital role in a number of fields—it has uses in medicine, manufacturing enterprises, electronics, and a host of other areas.
A new study, however, has successfully tweaked living cells and allowed them to produce organosilicon for the first time in history. Caltech scientists used a protein called cytochrome c, which was harvested from a bacterium found in hot springs in Iceland, and they mutated its DNA. After three rounds, the tests resulted in an enzyme that selectively creates silicon-carbon bonds—and it’s 15 times more efficient than the best catalysts chemists have. There were also less undesirable byproducts, minimizing the need for more chemical processes to weed them out and, thereby, cutting down costs.
Production of silicon alone produces carbon monoxide, while synthesizing silicon-carbon compounds involves toxic solvents. Gaining the ability to produce essential compounds naturally means environmental hazards are significantly reduced. "We decided to get nature to do what only chemists could do--only better," says principal investigator Frances Arnold.
Arnold pioneered directed evolution in the early 1990s, a method of “breeding” enzymes as if you were breeding animals or crops. And it is this same method used for the study.
"It's like breeding a racehorse," she says. "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."
Ultimately, this work is the very first to reveal that nature can adapt to incorporate silicon into carbon-based molecules—the building blocks of life. Notably, scientists previously speculated that life on our planet very well could have evolved to be based on silicon instead of carbon. This would result in entirely different lifeforms on Earth, and it seems this study has just proven that it could have happened.
However, we've never found any silicon-based life. To that extent, these new cells open a number of new doors—they could help us understand more about the possibility of silicon-based life on our own planet and elsewhere in the Universe.
Carbon and silicon are chemically very similar. They both can form bonds to four atoms simultaneously, making them well suited to form the long chains of molecules found in life, such as proteins and DNA. Silicon and carbon are very similar in chemical properties, so there may be an untold number of worlds with silicon life.
“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," says Arnold.