Did a deadly poison lead to life on earth?
Compounds between carbon and nitrogen are called cyanides. The carbon atom is triple-bonded to the nitrogen atom. It therefore has one valence left to become hydrocyanic acid with hydrogen, for example, or potassium cyanide (“cyanide”) with potassium. These are usually highly toxic. however, there are more complex molecules in which the cyanide is so tightly bound that it is no longer toxic. For example, the additive E536 (potassium hexacyanidoferrate(II)) is approved as a food additive.
Cyanides could also have prepared the way for the emergence of life in the early days of the Earth, four billion years ago. This is shown by chemists from Scripps Research in a paper in the journal Nature Chemistry. That’s because before life could be born at all, the necessary basic molecules had to be created first.
Some bacteria still living on Earth today use a series of chemical reactions known as the reverse tricarboxylic acid cycle (r-TCA cycle) to convert carbon dioxide and water into chemical compounds necessary for life. Many scientists suspect that the r-TCA cycle also occurred on the surface of the early, still inanimate Earth, where it produced the molecules necessary for life. But there is a problem with this: today’s r-TCA cycle relies on a series of complex proteins that could not have existed before the emergence of life. However, other scientists have been able to prove that it is possible to do without these proteins. In the primordial soup four billion years ago, certain metals would then have triggered the same reactions without today’s proteins. However, this would have required extremely acidic and hot conditions, which – according to today’s knowledge – probably did not exist on the early Earth.
Ramanarayanan Krishnamurthy, associate professor of chemistry and lead author of the new study, and his colleagues wondered if another molecule could stimulate the same reactions under more temperate conditions. Knowing that cyanides were present in the atmosphere of the early Earth, they thought of ways to use them to make organic molecules from carbon dioxide. Then they recreated these reactions in a test tube. It worked: cyanides, like proteins or metals, are able to transfer electrons between molecules.
“It was startling how simple it was,” Krishnamurthy says. “We really didn’t have to do anything special; we mixed these molecules together, waited, and the reaction happened spontaneously.” Unlike the earlier r-TCA versions, which used metals, the cyanide-based cycle worked at room temperature and in a broad pH range that corresponded to conditions likely to have prevailed on early Earth. In addition, the team was able to show that cyanides enable an even simpler version of the r-TCA cycle – one that bypasses some of the steps and the less stable intermediate molecules of the present-day cycle. This subset of reactions may have preceded the full r-TCA cycle in the origin of life, Krishnamurthy speculates.
There is, of course, no way to prove beyond a doubt what chemistry took place on early Earth. But the discovery of the new reactions is exciting because it will allow us to better search for other homes of life in the cosmos. “It frees us from the assumption that there must be metals and extreme conditions,” Krishnamurthy says. “There could be life that evolves from this cyanide-based chemistry.”