Chemical predecessors of modern biomolecules may have originated not just in the deep oceans at hydrothermal vents, but also in warm ponds on Earth's surface. An international team lead by academics from Friedrich Schiller University Jena, Germany, has now replicated the chemical processes that may have occurred in this "primordial soup" in tests. They even discovered that one of the nucleobases, which encode the genetic code, might have formed on our planet's surface.
The Earth is about 4.6 billion years old and was not always suitable to life. Our planet's atmosphere was predominantly composed of nitrogen, carbon dioxide, methane, hydrogen sulphide, and hydrogen cyanide, sometimes known as hydrocyanic acid, for the first hundred million years. There was no such thing as free oxygen. Under these circumstances, iron sulphide, which when exposed to oxygen transforms into iron oxide, is stable. However, physiologically significant events can occur on the surface of iron sulphide, comparable to those seen in some iron and sulfur-based enzymes such as nitrogenases and hydrogenases.
It was made feasible through an unintentional rediscovery.
"We questioned ourselves: what happens when iron sulphide in this primordial environment comes into touch with hydrocyanic acid?" asks Prof. Wolfgang Weigand of the University of Jena's Institute of Inorganic and Analytical Chemistry.
"We were fortunate to have identified a very reactive type of iron sulphide by accident during a successful partnership with my colleague Prof. Christian Robl. This form had already been found twice in history, but had been lost both times: first in 1700 and again in the 1920s. So, for the third time, the two PhD students at the time, Robert Bolney and Mario Grosch, uncovered it "he adds.
When iron powder is mixed with sulphur in water and gently heated, iron sulphide is created as mackinawite in an explosive reaction, according to the two chemists. In the "primordial soup" experiment, this mineral acted as a catalyst.
This might have resulted in the creation of a letter of the genetic code.
"In a nitrogen environment, we added potassium cyanide, phosphoric acid, and water to the iron sulphide and heated the combination to 80 degrees Celsius. The potassium cyanide is converted into hydrocyanic acid by the phosphoric acid. We then collected and examined gas samples from the atmospheres of the individual boats "Weigand says. The researchers discovered compounds that might have been chemical antecedents of today's biomolecules.
The team verifies, among other things, the finding of thiols, which exist as lipids in cell membranes, as well as acetaldehyde, which is required as a precursor for DNA building components, in the journal ChemSystemsChem (called nucleosides). "It was especially exciting that we were able to detect adenine, a nucleobase that is one of the five letters of the genetic code," Weigand says.
The scientists used isotope tagging to establish that the cyanide did actually provide the carbon for the compounds they discovered. Weigand elaborates: "The potassium cyanide in this experiment did not include the isotope carbon-12, which accounts for 98.9 percent of the carbon naturally found in the environment. Instead, it was carbon-13, a heavier and more stable isotope. This isotope was discovered in the reaction products. We were able to demonstrate clearly that the carbon atoms in the compounds we discovered came from isotope-labeled potassium cyanide."
Decades of creativity and perseverance
Weigand is especially appreciative for the multinational team's cooperation: "You truly need imagination and patience for such task," he adds. "And Robert Bolney and Mario Grosch have demonstrated this. Our collaboration with partners at the University of California, Irvine, and LMU Munich was equally outstanding."
Wolfgang Weigand exemplifies the value of imagination and, especially, patience in science. Because, in 2003, he and Prof. Günter Kreisel of the University of Jena and Dr. Willi Brand of the Max Planck Institute for Biogeochemistry Jena got the Thuringian Research Prize for their study "A probable prebiotic synthesis of ammonia from molecular nitrogen on iron sulphide surfaces."
Now, some 20 years later, Weigand has demonstrated that the earliest carbon molecules, from which life subsequently evolved, might have originated from cyanide on the Earth's surface under these conditions.
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