[meteorite-list] Key To Life Before Its Origin On Earth May Have Been Discovered

[Darren Garrison]

http://www.sciencedaily.com/releases/2008/02/080228174823.htm

Key To Life Before Its Origin On Earth May Have Been Discovered
ScienceDaily (Feb. 29, 2008) — An important discovery has been made with respect
to the mystery of "handedness" in biomolecules. Researchers led by Sandra
Pizzarello, a research professor at Arizona State University, found that some of
the possible abiotic precursors to the origin of life on Earth have been shown
to carry "handedness" in a larger number than previously thought.

Pizzarello, in ASU's Department of Chemistry and Biochemistry, worked with
Yongsong Huang and Marcelo Alexandre, of Brown University,  in studying the
organic materials of a special group of meteorites that contain among a variety
of compounds, amino acids that have identical counterparts in terrestrial
biomolecules. These meteorites are fragments of asteroids that are about the
same age as the solar system (roughly 4.5 billion years.)

Scientists have long known that most compounds in living things exist in
mirror-image forms. The two forms are like hands; one is a mirror reflection of
the other. They are different, cannot be superimposed, yet identical in their
parts.

When scientists synthesize these molecules in the laboratory, half of a sample
turns out to be "left-handed" and the other half "right-handed." But amino
acids, which are the building blocks of terrestrial proteins, are all
"left-handed," while the sugars of DNA and RNA are "right-handed." The mystery
as to why this is the case, "parallels in many of its queries those that
surround the origin of life," said Pizzarello.

Years ago Pizzarello and ASU professor emeritus John Cronin analyzed amino acids
from the Murchison meteorite (which landed in Australia in 1969) that were
unknown on Earth, hence solving the problem of any contamination. They
discovered a preponderance of "left-handed" amino acids over their
"right-handed" form.

"The findings of Cronin and Pizzarello are probably the first demonstration that
there may be natural processes in the cosmos that generate a preferred amino
acid handedness," Jeffrey Bada of the Scripps Institution of Oceanography, La
Jolla, Calif., said at the time.

The new PNAS work* was made possible by the finding in Antarctica of an
exceptionally pristine meteorite. Antarctic ices are good "curators" of
meteorites. After a meteorite falls -- and meteorites have been falling
throughout the history of Earth -- it is quickly covered by snow and buried in
the ice. Because these ices are in constant motion, when they come to a
mountain, they will flow over the hill and bring meteorites to the surface.

"Thanks to the pristine nature of this meteorite, we were able to demonstrate
that other extraterrestrial amino acids carry the left-handed excesses in
meteorites and, above all, that these excesses appear to signify that their
precursor molecules, the aldehydes, also carried such excesses," Pizzarello
said. "In other words, a molecular trait that defines life seems to have broader
distribution as well as a long cosmic lineage.""

This study may provide an important clue to the origin of molecular asymmetry,"
added Brown associate professor and co-author Huang.

*The work is being published in the Early Edition of the Proceedings of the
National Academy of Sciences. The paper is titled, "Molecular asymmetry in
extraterrestrial chemistry: Insights from a pristine meteorite," and is
co-authored by Pizzarello, Huang and  Alexandre.

Adapted from materials provided by Arizona State University, via EurekAlert!, a
service of AAAS.