Key To Life Before Its Origin On Earth May Have Been Discovered (Handedness)

Fragment of the Murchison meteorite (at right) and isolated individual particles (shown in the test tube). (Credit: DOE/Argonne National Laboratory)

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.

Why we are made of only left-handed amino acids: the origin of chirality on Earth and in the solar system in beyond ABSTRACT: Most biomolecules are "chiral" or handed, that is to say they exist in two left and right-handed mirror image forms. But biology only uses one hand, i.e. it is "homochiral". One of the greatest puzzles in biophysics is the question of why life on Earth is based on left-handed (L) amino acids and right-handed (D) sugars - why not "mirror life" based on right-handed (D) amino acids and left-handed (L) sugars? The answer may lie in fundamental physics: the parity-violating weak neutral current produces a very slight energy difference between left and right handed molecules, which may become amplified over an evolutionary timescale, and our calculations of this energy difference show that the natural L-amino acids are indeed more stable than their "unnatural" D mirror images. This parity-violating energy difference or "PVED" between mirror image molecules is important not only in biology but also as a "molecular footprint" of fundamental physics: future measurements of the PVED could in effect give us "table-top particle physics", yielding values of the Weinberg angle and other important parameters of fundamental physics much more cheaply with a new generation of spectrometers rather than a new generation of particle accelerators. Homochirality is such a characteristic signature of life that finding molecules all of one hand on other planets could be a signature of life or prebiotic chemistry, and we are building a polarimeter to Search for Extra-Terrestrial Homochirality (SETH) on missions to Mars and other solar system bodies. We are also looking to Search for EXtra-SOlar Homochirality (SEXSOH) by looking for circular polarization in light reflected from planets round other suns in the next century.

We are interested in the origin of biomolecular chirality. Dr Alexandra MacDermott - It is well known that biomolecules are all of one hand, but what determines which hand? Why are animals made of L-amino acids and not D-amino acids? This asymmetry in biology may be a feature of fundamental physics, because it turns out that the "natural" L-amino acids are slightly more stable than their "unnatural" D mirror images, due to the weak force. The weak force, carried by the Z boson recently discovered at CERN, is one of the four forces of nature - electromagnetic, weak, strong and gravity - and it is the only one of the four which can tell the difference between left and right. Due to the weak force, L and D molecules have slightly unequal energies because they are not in fact true mirror images: the true enantiomer of an L-amino acid is the D-amino acid made of anti-matter. We calculate these small energy differences between enantiomers using ab initiomolecular orbital methods. In most cases our calculations do indeed predict the correct sign: not only are the L-amino acids more stable than the D, but the natural D-sugars are more stable than the L, and the right-hand DNA double helix is also more stable than its left-hand mirror image. We believe the slight enantiomeric excess from these "parity-violating energy differences" could be amplified kinetically in the pre-biotic soup to preferentially select today's L-amino acid/D-sugar biochemistry over D-amino acid/L-sugar "mirror life". The parity-violating energy difference between enantiomers is not the only way in which the weak force could select biomolecular chirality. Radioactive beta decay is mediated by the weak force, and this causes a polarization of the electrons emitted in beta decay, which could produce selective destruction of one enantiomer. We are currently starting to develop the theory of this enantioselective beta-radiolysis. Chirality is a characteristic signature of life, and we are collaborating with experimentalists and space engineers to develop a small polarimeter to detect optical rotation as the signature of life on Mars. We also hope to detect the chiral signature of life using polarimetry on the future Darwin space telescopes which will catch light from planets around otherstars: light from an Earth-like planet will show a small circular polarization due to the highly chiral chlorophyll molecules in vegetation cover. Finding molecules of the same hand on many different extra-solar planets would lend support to the weak force theory of the origin of chirality. Some of our team are Co-Investigators on the COSAC experiment of the Rosetta mission, launch 2003, which will use chiral gas chromatography to identify enantiomeric excesses on Comet Wirtanen. We are collaborating with Glaxo Wellcome to use chiroptically detected HPLC to identify chiral molecules in the Murchison and Mars meteorites.

In the words of a physician I once knew: “A universal law is that stool gets everywhere.”

And while he was talking about routine contamination, he was also correct that it is a universal situation. Or at least, from our point of view, a galactic one.

That is, in our galaxy, there are truly vast distances between planetary bodies. But these distances can be reduced by having a lot of time to travel between such bodies. For example, the nearest star to our own is Alpha Centauri, just down the block from Sol at about 4 light years distance.

Since physical objects can’t travel that fast, at least normally, even if it was zipping right along, say at 36,000 miles an hour, it would still take about 150,000 years just to get from there to here.

However, The Milky Way galaxy may be 8 billion or more years old. Its oldest star is estimated to be around 13 billion years old.

So if time is no object, a lot of “stool”, or space junk, can get circulated around the galaxy at a leisurely pace.

This is important, if you consider that during the lifetime of Earth, a mere 4.5 billion years, only a tiny fraction of that time has had life here, of any kind. Life was almost wiped out many times.

However, while Earth was teeming with life, had another major object struck it, blasting pieces of Earth into space, while the life would not have survived, its component organizational blocks, such as amino acids, might have traveled vast distances to perhaps encounter another planetary body.

And other planets organizational structures might have done the same to Earth.

Or as the doctor said, “Stool gets everywhere.”

Disclaimer: Opinions posted on Free Republic are those of the individual posters and do not necessarily represent the opinion of Free Republic or its management. All materials posted herein are protected by copyright law and the exemption for fair use of copyrighted works.