One of the biggest biological mysteries is how life originally sparked out of non-living molecules. The answer likely lies in the primordial soup, a mineral-rich mixture that was lapping against the shorelines on early Earth, but there's still a big question mark hovering over just how these molecules managed to link up to form life-giving proteins. To find out, researchers at Georgia Tech have recreated some of the conditions common to a pre-life Earth, and discovered that drying and rewetting this primordial soup lets these key peptides form quickly and relatively easily.

Amino acids are often called the building blocks of life: these organic compounds link up in chains to form peptides, polypeptides and eventually, proteins. Proteins are the workhorses of living cells, performing specific functions that allow vital biological processes to occur and life to exist. Amino acids themselves are fairly common and have even been found in space, but those all-important links can be fragile, and it's not known just how they formed chains long enough and stable enough for life to arise.

It's been thought that energetic catalysts like volcanic eruptions or lightning strikes were needed to kickstart life, but the Georgia Tech team hypothesized that more gentle conditions might suffice. Something as simple and common as a pond drying out and refilling over time could be all that's needed, and to test the idea, the researchers experimented with the most likely and plausible conditions of an early Earth.

"We want to stay away from scenarios that are not readily possible," says Facundo Fernández, principal investigator on the study. "Don't deviate from conditions that would have been realistic and reasonably common on prebiotic Earth. Don't invoke any unreasonable chemistry."

First, the team mixed three amino acids with three hydroxy acids in a watery solution. Since they're very similar to amino acids, hydroxy acids can act as stand-ins to form peptide chains with amino acids through what are called ester bonds. These bonds generally form more easily than all-amino acid bonds, and may act as a stepping stone to more complicated chains.

Sure enough, the hydroxy acids helped form chains that would normally be fairly difficult. These precursor molecules, known as depsipeptides, were created relatively quickly and in high numbers.

Next, the researchers mimicked the gentle catalyst they believe might have kicked life into gear: the regular cycles of drying out and rewetting of the primordial soup as it lapped against rocks, dried out in the sun and was washed back into the soup by rain.

In their experiments, the team dried out the mix at temperatures up to 85° C (185° F), but the same reactions have been seen to work as low as 55° and 65° C (131° and 149° F). Even under the blazing sun of the Sahara, temperatures would struggle to soar to those heights nowadays, but the researchers point out that early Earth was a very different place.

"We call it an environmental cycling approach to making these early peptides," says Fernández. "If you think about early Earth having a lot of volcanic activity and an atmospheric mix that promoted warming, those temperatures are realistic on many parts of an early Earth."

Through the cycles of drying and wetting, the depsipeptides formed more complicated chains than they would normally be capable of. The stand-in hydroxy acids played a crucial part in the process: while they form chains more easily, they also break apart more easily, but by the time they break, they've already formed a framework to allow the hardier amino acid bonds to form longer peptides. The end result is longer, more complicated depsipeptides that form relatively easily. The researchers also found that over a few days, some even began to "evolve" into different varieties.

More than 650 different depsipeptides formed through the experiments, and the researchers used mass spectrometry and ion mobility techniques to identify them, with algorithms helping process the data. And that's just using a recipe of three amino acids and three hydroxy acids – the team says that if they started with 10 of each, some 10 trillion depsipeptides could theoretically have been formed.

"Ease and bounty are key," says Fernández. "Chemical evolution is more likely to progress when components it needs are plentiful and can join together under more ordinary conditions. Now we know how peptides can form easily. Next, we want to find out what's needed to get to the level of a functional protein."

The research was published in the journal Proceedings of the National Academy of Sciences.

Source: Georgia Tech

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