In order for life to emerge, both peptides and nucleic acids must have appeared under "prebiotic" conditions. Despite numerous efforts, the formation of these macromolecules without the help of modern synthetic reagents has not been achieved in a laboratory.
Now, for the first time, researchers have proposed a mechanism by which the formation of peptides could have occurred under prebiotic conditions.
Reporting their findings in the July issue of the UK Society of Chemical Industry (SCI) journal Polymer International, they describe a molecular engine mechanism which could have taken place on primitive beaches in the Hadean age.
(The Hadean began approximately 4.6 billion years ago with the creation of the Earth and ended 3.8 billion years ago.)
The molecular engine (the primary pump) relies on a reaction cycle made up of several successive steps, fed by amino acids and fueled by nitrous oxide (NO) species.
French researchers repeatedly cycled through the reaction steps using models of the primitive ocean, and each time peptides were formed, showing that the primary pump works at ambient temperatures and it continuously generates, elongates and complexifies sequential peptides.
Only a few of all the possible peptide sequences were formed, demonstrating that the primary pump should be able to select particular peptide sequences.
Further, the primary pump should be able to drive the peptide pool towards homochirality (same-handedness of chemical structures that can be left- or righthanded) through the amplification from a starting small excess of left- or rigthhanded forms.
For the proposed mechanism to work, one assumes that there was a buffered ocean, emerged land and an atmosphere that included nitrogen. The researchers show that the primitive Earth during the Hadean may have satisfied all these requirements.
It was during the Hadean that the Earth surface cooled and solidified. The oldest terrestrial rocks are from this age and their chemical character demonstrates that a stable continental crust existed.
Analysis has also shown that huge volumes of liquid water must have been available on the surface of the primitive Earth, and as the moon was already formed, this would have tidal properties.
According to lead researcher Auguste Commeyras of the University of Montpellier, "The primary pump could have worked as soon as the pH of the oceans rose to 4-5. We consider it reasonable to postulate that the primitive ocean was initially acidic due to the presence of large amounts of CO2, and that its pH gradually increased to its current level through extraction of alkaline materials from reductive rocks."
The most recent works on the primitive atmosphere of the Earth suggest that its main components were CO2 and N2. Calculations show that sufficient nitrogen oxides would have been available to supply the primary pump and act as a driving force to the mechanism.
"The role of NO in the metabolisms of current living organisms might be a remnant of such a prebiotic chemistry," said Commeyras.
In conclusion, Commeyras says, "Our primary pump scenario appears to be the first that is capable of supplying sequential peptides under realistic prebiotic conditions.
"Maintained out of thermodynamic equilibrium, this system had the ability to recycle its reagents and to cause the products to evolve and increase in complexity. The emerging peptides would quickly have begun to act as catalysts, which may have helped the emergence of autoreplicant systems."
(Reference: "Prebiotic Synthesis of Sequential Peptides on the Hadean Beach by a Molecular Engine Working with Nitrogen Oxides as Energy Sources," by A. Commeyras et. al. Polymer International, 51, 7.)
Polymer International is a SCI journal published by John Wiley & Sons on behalf of the Society of Chemical Industry, and is available in print (ISSN: 0959-8103) and online (ISSN: 1097-0126) via Wiley InterScience.