In their quest to understand how life on Earth formed, scientists long ago determined that amino acids—simple yet important building blocks within all organisms—can form under conditions mimicking those of ancient Earth. But the question remains: How might those amino acids combine to form more complex molecules such as peptides?
A team of researchers associated with the NSF/NASA Center for Chemical Evolution proposes one possible pathway. The team, led by Nicholas V. Hud and Facundo M. Fernandez of Georgia Tech and Ramanarayanan Krishnamurthy of Scripps Research Institute, California, demonstrates that under plausible prebiotic conditions, α-hydroxy acids and α-amino acids react to form depsipeptides: oligomers containing a mix of ester and amide bonds (Angew. Chem. Int. Ed. 2015, DOI: 10.1002/anie.201503792).
Specifically, the team mixed lactic acid and the amino acid glycine in a 1:1 ratio and subjected the mixture to dry and wet cycles designed to simulate day and night on early Earth. During the dry phase, the mixture was heated to 85 °C for 18 hours. Then, the researchers rehydrated the concoction for 30 minutes and held it at 65 °C for 5.5 hours during the wet phase.
Using mass spectrometry, the team revealed that after four dry/wet cycles, the product mixture included depsipeptides made of up to 10 units. The researchers propose that the reaction starts with the formation of lactic acid dimers and oligoesters. An amino acid then displaces a lactic acid unit through an ester-amide exchange reaction. The oligomer grows through repeated addition of lactic acid and subsequent ester-amide exchanges.
Pointing out a common drawback of origin-of-life studies, Matthew Powner of University College London, says, “No one knows what exact conditions are prebiotic.” But Powner, who was not involved in the new study, looks forward to seeing whether longer peptides can be formed, chirality can be induced, and other amino acids can be incorporated.
Hud and his collaborators plan to use the system to study whether they can form peptides of a single chirality starting from racemic mixtures, an important step toward understanding the origins of the chirality of life.