Did lefty molecules seed life?

The molecular orientation of compounds brought to Earth by meteorites could have determined the world's chemistry long before life began, according to a new linkurl:study;http://www.pnas.org/cgi/doi/10.1073/pnas.0811618106 published online today (Mar. 16) in the __Proceedings of the National Academy of Sciences__.

Artist's impression of the origins of
left-handedness (click to enlarge)
Image: NASA/Pat Rawlings

Amino acids come in left-handed and right-handed forms, which, like a pair of human hands, are mirror images that cannot be superimposed onto each other. Yet living organisms use only the left-handed version, which presents a conundrum: There's no biochemical reason why one mirror image should be better than the other, so scientists have long debated whether life's left-handed leaning arose because of random processes or whether rocks from outer space seeded a southpaw solar system. The current study argues for the latter possibility by showing that some extraterrestrial meteorites contain an abundance of left-handed molecules. "The implications are that all life in our solar system could be the same handedness as life on Earth," linkurl:Jeffrey Bada,;http://www-mpl.ucsd.edu/MarChem/Main_Site/Teaching_Staff/Bada.html a geochemist at the Scripps Institution of Oceanography in La Jolla, Calif., who was not involved in the research, told __The Scientist__. linkurl:Daniel Glavin;http://astrobiology.gsfc.nasa.gov/analytical/glavin.html and linkurl:Jason Dworkin,;http://astrobiology.gsfc.nasa.gov/analytical/dworkin.html astrobiologists at NASA's Goddard Space Flight Center in Greenbelt, MD, compared the ratio of left- and right-handed 5-carbon amino acids found in six primitive, carbon-rich meteorites that have an elemental composition similar to that presumably found in the early solar system. Three of these rocks were heavily left-skewed, while the remaining three showed equal handedness, or chirality, the researchers found. Of the lefty rocks, the linkurl:meteorite that fell on Murchison, Australia,;http://en.wikipedia.org/wiki/Murchison_meteorite in 1969 -- arguably the most widely studied carbonaceous meteorite in the world -- contained the largest imbalance ever observed: a 18.5% excess of the left-handed form of the amino acid isovaline. "There really is a large, 15 to 20% excess for this particular amino acid, and it has important implications for homochirality [single-handedness] and the origins of life," Glavin told __The Scientist__.

Fragment of the Murchison meteorite
Image: US Department of Energy

"Maybe life was biased toward left-handedness in our solar system" said Dworkin. The possibility that left-handed amino acids are so prevalent in our solar system is "bad news in looking for independent origins of life," he noted, because it decreases the chances of researchers stumbling upon an organism that uses only right-handed amino acids -- a clear trademark of alien life. "But it's also good news" for the possibility of a second origin, because single-handedness is essential for biotic chemistry as we know it. Thus, a meteorite-driven imbalance could have helped "jumpstart" early life, he said. linkurl:Paul Davies,;http://cosmos.asu.edu/ a theoretical physicist and astrobiologist at Arizona State University (ASU) in Tempe who was not involved in the study, was not convinced by this argument. "Even if there's this slight excess at the outset there has to be some sort of mechanism that's going to amplify that to make it 100%," he said. The mechanism that Glavin and Dworkin propose to explain the observed left-handed excess is that polarized light -- which is twisted and can rotate molecules -- probably set the imbalance in motion. Then, once the balance was slightly askew, water within the meteorites further drove an enrichment of left-handed amino acids in the liquid phase and relegated right-handed molecules to the solid phase. "The whole amplification is due to this process of aqueous alteration," said Dworkin. But the link between water-bearing rocks and a left-handed skew is just a correlation, said ASU biochemist linkurl:Sandra Pizzarello,;https://webapp4.asu.edu/directory/person/274781 who was also not involved with the work. "It's just a supposition," she said. "I would have liked them to back it up with physico-chemical possibilities." Glavin pointed to the work of Columbia University's linkurl:Ronald Breslow;http://www.columbia.edu/cu/chemistry/breslow/ and Imperial College's linkurl:Donna Blackmond,;http://www3.imperial.ac.uk/people/d.blackmond which has demonstrated that this so-called "enantiomeric enrichment" can occur in a liquid phase, such as the one found during the melting of ice inside the meteorites' parent asteroid. Still, even if aqueous alteration can explain the build-up of left-handed molecules, it doesn't explain the disappearance of their mirror images, noted linkurl:Robert Hazen,;http://hazen.gl.ciw.edu/ a geochemist at the Carnegie Institution in Washington, DC. "Where did the right-handed amino acids go? They had to go somewhere else... There has to be a destruction process." Bada also noted that the left-skewed, 5-carbon amino acids described by the authors are not the same molecules that are used by life on Earth. The 20-odd protein building blocks that living beings rely on showed no such bias in the meteorites, the NASA researchers found, so to get homochirality in life there would need to be some mechanism of transferring the single-handedness between different types of amino acids. The origins of life remain "one of those bewildering things," Bada said. "If there was a straight-forward answer for the homochirality of amino acids, I think we would have found it."
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[22nd June 1998]