Origin of Life: Essential Step in Chemistry Unraveled
While scientists believe that water is necessary for the emergence of life on a planet, new findings suggest the water surface may play a more integral role, according to researchers at the Cooperative Institute for Research in Environmental Sciences (CIRES). These scientists have demonstrated that the interface between water and air provides a site for the formation of simple proteins called peptides, one of the key building blocks for life.
“We report unambiguous evidence of peptide-bond formation at the air-water interface, yielding insight into how complex proteins could have emerged on early Earth,” says lead author Elizabeth Griffith, a CIRES doctoral student. CIRES Fellow Veronica Vaida was principal investigator of the study, which appeared in the journal Proceedings of the National Academy of Sciences last fall. Their research is particularly timely because of recent findings by NASA’s Mars rover that habitable water once existed on the planet.
Water-air interfaces are ubiquitous throughout nature, found at the surfaces of lakes, oceans, and atmospheric aerosols (small liquid or solid droplets suspended in air). Of these different surfaces, atmospheric aerosols that emanate from the ocean provide especially advantageous environments for this form of peptide chemistry and the “birth” of life. Collectively, the sea-born aerosols have a much larger surface area than all other water surfaces found on Earth combined and are constantly being generated by breaking waves sending up sea spray.
“The surfaces of these sea-born aerosols could be ‘pre-life reactors’ relevant to many planets,” Vaida said.
To investigate the phenomenon, the scientists observed the surface of a water solution by using very sensitive spectroscopy and other methods and found that the building blocks of peptides, amino acids, reacted—like box cars linking together to form a train—to create peptides at the water surface.
Until now, scientists had not been able to demonstrate how these peptides, which are critical for life, could form spontaneously in water under conditions that were realistic of early Earth. One reason such spontaneous peptide formation in water would be unfavorable has to do with the fact that when two of the building block amino acids join together, they release a water molecule.
“The release of a water molecule into water is highly unlikely—the reaction will not proceed in that direction,” Griffith said. “Thermodynamically, it’s like emptying a bag of marbles at the bottom of a steep hill and then expecting one of them to roll to the top unassisted.”
Additionally, the amino acids have to be oriented exactly with respect to one another and possess a precise charge state to form a peptide bond. These criteria rarely occur in bulk water, Griffith said.
The role of the water-air interface in this process, however, overcomes many of these hurdles. The interface positions the amino acids in the correct orientation and in the right form.
“Water-air interfaces provide an auspicious environment for this type of chemistry through their provision of a water-scarce environment, alteration of the charge state of compounds on the surface, and ability to concentrate and align simple amino acids,” said Vaida.
The study sheds light on the enigma in modern biology of how cells manufacture complex proteins inside structures called ribosomes. It also offers a possible mechanism for how the first bio-polymers could have formed. These “life molecules” are essential for the emergence of organisms.
“This work gives insight into peptide formation en route to the emergence of more complex biomolecules on early Earth, and reinforces the importance of orientation, alignment, and proximity in the functioning of modern protein synthesis,” Vaida said.
The research is funded by a grant from the National Science Foundation, Chemistry Division and a NASA Earth and Space Science Graduate Fellowship. CIRES is a joint institute of the University of Colorado Boulder and NOAA.