Athens, Ga. – 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 today in the
print version of 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 thymine.
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 paper.
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.”
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