Experiments show that simple molecules can combine chemically rather
than biologically to form the building blocks of DNA, the key component
of all life forms. These processes might have taken place on primitive
earth, but how they occur is an unsolved puzzle.
Chemists at the University of Georgia have now proposed the first
detailed, feasible mechanism to explain how adenine, one of the four
building blocks of DNA, might be built up from the combination of five
cyanide molecules. The investigation is based on extensive quantum
chemical computations over several years.
"Just where these biomolecules originated isn’t known," said Paul
von Ragué Schleyer, Graham Perdue Professor of Chemistry at the
University of Georgia. "One can only speculate. They could have formed
from smaller molecules present on primitive Earth, either very slowly
over millions of years or rapidly before the Earth cooled down.
Asteroids may have brought them from outer space, but how did
biomolecules form there?"
The newly proposed mechanism for the formation of adenine gives a
clear picture of how it could have become one of the building blocks
essential for the formation of DNA. The research was published in the
Proceedings of the National Academies of Science. Schleyer’s coworkers
were Ph. D. candidate Debjani Roy, the first author of the paper, and
Katayoun Najafian, his former student from Iran.
DNA is the nucleic acid blueprint of life that is passed on from
generation to generation. First isolated in 1869 from the pus of
discarded surgical bandages by Friedrich Miescher, a Swiss doctor,
DNA’s double helix structure was solved by Watson and Crick in 1953.
DNA is shaped somewhat like a twisted ladder with the rungs anchored by
matching pairs of only four bases: adenine, guanine, cytosine and
The UGA chemists focused on adenine because of its relative
prevalence on Earth and its formation in the dark in from simple
components. Along with other fundamental building blocks, adenine has
even been detected extraterrestrially. Still, the vast distance between
the smaller molecules required to form adenine in outer space precludes
its formation, unless some nucleation centers, like specks of
interstellar dust, are present.
"Numerous experiments have demonstrated that amino acids,
nucleotides, carbohydrates and other essential compounds form under
simulated primitive Earth conditions," the authors write in their
Remarkably, a solution of highly poisonous cyanide in ammonia,
frozen solid in a refrigerator for 25 years, produced adenine, a
necessary component of life. A substantial amount of adenine also was
formed in a high-temperature experiment designed to simulate early
volcano-like environments. But the question is how.
The Georgia researchers arrived at an answer by solving a series of
key riddles. They worked out the processes in which five cyanide
molecules might combine to form adenine under terrestrial conditions.
Their predictions are based on extensive computations of sequences of
reaction steps along possible mechanistic routes.
"Finding a viable, thermodynamically feasible, step-by-step
mechanism that can account for the formation of adenine was far from
straightforward," the authors said. "Our report provides a more
detailed understanding of some of the chemical process involved in
chemical evolution, and a partial answer to the fundamental question of
molecular biogenesis. Our investigation should trigger similar
investigations of the abiotic formation of the remaining nucleic acid
bases as well as other biologically relevant molecules."
University of Georgia. October 2007.