Left-handed people may be in the minority, but left-handed amino acids rule the Earth. Researchers have long known that the building blocks of proteins can be constructed in either “left-handed” or “right-handed” versions that are mirror images of each other, but that almost every living organism on Earth uses left-handed amino acids. Now, a new study gives weight to a theory of how that preference came to pass. NASA researchers examined meteorites that predate the Earth’s formation, and say that those early rocks also have a preponderance of left-handed molecules. “Meteorites would have seeded the Earth with some of the prebiotic compounds like amino acids that are needed to get life started, and also biased the origin of life to the left-handed amino acid form,” says [study coauthor] Daniel Glavin [New Scientist].
Researchers note that if you make amino acids from scratch in a lab using their chemical components, you inevitably get half of the right-handed version and half of the left handed version. So it might be expected that if nature makes amino acids in space using similar chemistry, you’d also get a fifty-fifty mixture [CBC]. Yet that’s not what Glavin and his colleagues found when they studied the molecular deposits in six meteorites that are more than 4.5 billion years old. Instead, they found the ratio of amino acids tilted toward left-handedness in all six specimens. In one of the rocks, the imbalance was 18%, the largest ever reported for a meteorite. “I have to admit I didn’t believe it at first,” Glavin says [ScienceNOW Daily News].
The new study, published in the Proceedings of the National Academy of Sciences, also suggests various methods by which lefty molecules may have gained the upper hand. Glavin says the process might have started when the amino acids made contact with melting ice inside the meteorites’ parent asteroids–water tends to help left-handed amino acids multiply and dominate. But that’s only part of the answer, he says. Polarized ultraviolet radiation in space might also have helped shift the balance toward southpaw molecules [ScienceNOW Daily News].
Not everyone is convinced by the new findings. Astrobiologist Sandra Pizzarello argues that most meteorites quickly become contaminated when they land here. “Researchers have seen bacteria happily growing inside them,” notes Pizzarello…. She says that if these bugs prefer to eat right-handed amino acids, they could skew the ratio [ScienceNOW Daily News].
Still, if the results bear up under future scrutiny, it suggests that we humans would have something in common with any extraterrestrial life that might exist in our solar system. Studyd coauthor Jason Dworkin says that “if the bias toward left-handed amino acids began in space, it likely extends across the solar system, so any life we may find on Mars, for example, will also be left-handed” [Astrobiology Magazine].
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Image: NASA/Pat Rawlings
March 17th, 2009 at 11:44 am
There is no credible explanation for biological homochirality: chiral L-protein amino acids and chiral D-sugars. Weak interaction arguments (Mendeleev Commun.< .I> 13(3) 129 (2003), Angew. Chem. Int. Ed. 41(24) 4618 (2002)) are woefully insufficient (maximum 8×10^(-12) eV PVED versus ambient kT = 0.0257 eV. An obvious source is a chiral vacuum background in the massed sector (not interactive with EM) consistent with teleparallel gravitation, testable on a lab bench.
Crystallographic space groups P3(1)21 and P3(2)21 are enantiomorphic. All formula units configure into homochiral 3-fold helices (right-handed and left-handed, respectively). There are no conflictng or racemic screw axes. Quartz, berlinite and analogues, tellurium, selenium… single crystals thus furnish atomic-scale left and right “shoes” to test for a vacuum left “foot”. P3(1)21 versus P3(2)21 quartz is particularly good for its commercial production and a control “sock” – amorphous fused silica.
Load an Eotvos balance with macroscropically and chemically indistinguishable single crystal quartz test masses, space group P3(1)21 opposing P3(2)21. 90 days later the net output is either zero (as with all Equivalence Principle tests since Galileo and Stevin in the late 1500s) or non-zero. Observation trumps dialectic. Somebody should look. A non-zero output is followed by each space group run against fused silica. A quartz enantiomorph homochiral to the vacum background gives a smaller net signal. The two hemiparity experiments’ outputs sum to the full parity experiment’s output as a final validation.
March 17th, 2009 at 11:58 am
Now, if life on earth is prone to left-handed molecules, why would bacteria grow on downed meteorites that PREFERRED right-handed molecules, since they can’t be found anywhere on earth. They shouldn’t really even know what they are, let alone have a preference for them.
March 17th, 2009 at 1:14 pm
It is puzzling to me that organisms that can consume only half the contents of a primordial soup would survive and prosper over the possibility of one that can utilize twice as much. But then the building blocks it would create for itself might be just too complicated, and not hang together.
Thank goodness for such puzzles.
eogord
April 14th, 2011 at 12:28 am
Dear Sir/Madam, I would like to use your image of homocirality in one of my lectures:
http://blogs.discovermagazine.com/80beats/files/2009/03/left-handed-molecule.jpg