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Drying puddles may hold clues about origins of life on Earth

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Hunt for clues about origin of life on Earth has brought scientists to a rather unexpected location – drying puddles – as they believe that the environment within drying puddles may be home to chemical reactions that were critical to the formation of life on the early Earth.

In a paper published in the journal Angewandte Chemie International Edition, researchers have demonstrated how drying puddles could be ideal locations for formation of polypeptides – important molecules of contemporary life. Researchers say that these molecules can easily be formed by mixing amino and hydroxy acids – ingredients which are believed to have existed together on the early Earth – then subjecting them to cycles of wet and dry conditions.

Researchers say that the cycling of wet and dry conditions is something that is possible in puddles drying out because of sun and then reforming with the next rain.

This cyclic nature of formation of the important molecules of contemporary life then supports the theory that life could very well have begun on dry land, perhaps even in the desert, where the conditions of repetitive drying and moistening – through daytime heating and evaporation, nighttime cooling and early morning dew formation – could have been ideal.

Researchers were able to demonstrate in lab that just 20 day-night, wet-dry cycles were enough to form a complex mixture of polypeptides in the lab indicating that even the unlikeliest of places like dry puddles may have been the location of initial life formation.

Researchers were also able to use the wet and dry cycles to breakdown and reassemble organic materials to form random sequences that could have led to the formation of the polypeptide chains that were needed for life.

Nicholas Hud, a professor in the School of Chemistry and Biochemistry at the Georgia Institute of Technology, and director of the NSF Center for Chemical Evolution said that a process as simple as hydration-dehydration cycles to drive the kind of chemistry you need for life is really appealing. He added that dry land would have provided a very favorable environment for getting the chemistry necessary for life started.

Sheng-Sheng Yu, a graduate student, put the amino and hydroxy acid mixtures through 20 wet-dry cycles to produce molecules that are a mixture of polyesters and peptides, containing as many as 14 units. After just three cycles, and at temperatures as low as 65 degrees Celsius, peptides consisting of two and three units began to form. Postdoctoral fellow Jay Forsythe confirmed the chemical structures using NMR mass spectrometry.

“We allowed the peptide bonds to form because the ester bonds lowered the energy barrier that needed to be crossed,” Hud added.

Researchers say that on the early Earth, those cycles could have taken about 20 days and nights – or perhaps much longer if the heating and drying cycles corresponded to seasons of the year.

Beyond the easy formation of polypeptides, the wet-dry cycle process has another advantage as well. Researchers found that this process allows compounds like peptides to be regularly broken apart and reformed, creating new structures with randomly-ordered amino acids. This ability to recycle the amino acids not only conserves organic material that may have been in short supply on the early Earth, but also provides the potential for creating more useful combinations.

A combination of hydroxy and amino acids likely existed in the prebiotic soup of the early Earth, but analyzing such a “messy” reaction was challenging, Hud said. “We were led into this idea that a mixture might work better than separate components,” he explained. “It might have been messy at the start, but it’s easier to get going than a pristine chemical reaction.”

Beyond helping explain how life might have started, the wet-dry cycles could also provide a new way to synthesize polypeptides. Existing techniques produce the chemicals through genetic engineering of microorganisms, or through synthetic organic chemistry. The wet-dry cycling could provide a simpler and more sustainable water-based process for producing these chemicals.

The demonstration of peptide formation opens the door to asking other questions about how life may have gotten going in prebiotic times, said Ramanarayanan Krishnamurthy, a member of the research team and an associate professor of chemistry at the Scripps Research Institute. Future studies will include a look at the sequences formed, whether there are sequences favored by the process, and what sequences might result. The process could ultimately lead to reactions able to continue without the wet-dry cycles.

“If this process were repeated many times, you could grow up a peptide that could acquire a catalytic property because it had reached a certain size and could fold in a certain way,” Krishnamurthy said. “The system could begin to develop certain emergent characteristics and properties that might allow it to self-propagate.”

One of the reasons why dry-wet cycles as a theory of origin-of-life is appealing is because previous studies have all involved creation of polypeptides from amino acids by heating them well past the boiling point of water, or by driving polymerization with activating chemicals. However, if we take a practical look at the process, it isn’t ideal for creation of life, as life can’t survive at high temperatures and the robust availability of activating chemicals on the early Earth is questionable as well.