The appearance of life in a “warm little lake” about 4.5 billion years ago is a relatively solid foundation of modern biology.
Despite water’s vital role in facilitating early organic reactions on Earth, one of the most basic components would not form in aqueous environments, raising the question of how life got them in the first place.
A new experiment reveals how these crucial chemical reactions may have taken place.
Amide bonds are the links in the amino acid chains that form the basis of so many critical components of life, including peptides (short strings of amino acids) and proteins (long strings of amino acids that can function as enzymes).
The problem is that the amide bonds are actually blocked by water, which is something of a problem on an oceanic world like ancient Earth. Something else must have come into play, scientists believe, and the new study suggests it was at the boundary of water and air that the magic happened.
“Here, we report a unique reactivity of free amino acids at the air-water interface of micron-sized water droplets that leads to the formation of peptide isomers on the millisecond time scale,” Purdue University chemist Dylan Holdena and colleagues write in their published paper . .
“This reaction takes place under ambient conditions and requires no additional reagents, acid, catalysts or radiation.”
The team sprayed microdroplets of water containing two amino acids, glycine and L-alanine, into a mass spectrometer device for detailed chemical analysis. A chain of two amino acids, a dipeptide, was shown to form in the droplets.
Since dipeptides are able to form further amino acid chains, the results suggest that airborne microdroplets could have accelerated the early construction of peptide chains by exposing dissolved amino acids to the air.
Billions of years ago, such microdroplets may have been produced as sea spray washed up from the ocean, creating the chemical bonds necessary for life to develop.
Furthermore, the reaction observed in these experiments occurred without the addition of other chemical agents, catalysts, or radiation sources, making it more likely that it could have occurred billions of years ago on Earth.
“The observed production of peptides from free amino acids at the air-water interface of pure water droplets, the simplest of all prebiotic systems, suggests that settings such as atmospheric aerosols or sea spray may have provided a unique and ubiquitous environment to overcome the energetic barriers associated with the condensation and polymerization of biomolecules in water,” the researchers write.
If the team is right, where tiny water droplets hit the air, at the smallest scale the environment may be dry rather than wet – meaning it would provide conditions where dipeptides can be synthesized.
Scientists have been busy considering all sorts of explanations for how amino acid chains could have formed in oceanic environments. Hydrothermal vents may have played a role, for example, or perhaps a visiting asteroid. Now, there is a new option.
However, it is still a hypothesis for now, and future studies will be needed to investigate how these amino acid chains are assembled – and how these basic chemical building blocks led to life on Earth as we know it today.
“This reactivity provides a plausible pathway for the formation of the first biopolymers in aqueous environments,” the researchers write.
The research has been published in PNAS.