Article: How did chemical constituents essential to life arise on primitive Earth?

How did chemical constituents essential to life arise on primitive Earth?

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."

Source: University of Georgia
http://www.physorg.com/news112977065.html

Posted by
Robert Karl Stonjek


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