Murchison amino acids could transfer theirleft-handedness to otherwise

Tag : Organic Acid
Research into the question has picked up in recent years,focusing on a 200-pound chunk of rock found 40 years ago inMurchison, Australia. A meteorite that broke off an asteroid longago, it brought to Earth a rich collection of carbon-based materialfrom far away in the solar system.
While the Murchison meteorite does not have any once-livingmaterial, it is telling researchers new things about how life mayhave started on Earth, and how that almost universal proteinleft-handedness came to be.
The answer they believe they have found is that 3 billion to 4billion years ago, before life on Earth began, similar meteoritescrashed regularly into the planet -- delivering the amino acidsthat would later be incorporated into all living things. Themeteorites did this by providing building blocks with a slightpreponderance of that handedness (known scientifically aschirality) that makes life possible.
"We know that all amino acids start mirror-image the same, butin living things they have this handedness," said RonaldBreslow, a Columbia University researcher who published recently on the topic. "This changedoesn't happen spontaneously, and we've never been able toreproduce it in the laboratory" under conditions similar toearly Earth.
"The answer to where it comes from looks increasingly likemeteorites," he added, "from extraterrestrial bodies falling toEarth. It's a complex story, but we're beginning to understand itbetter."
Breslow and his colleagues made significant progress recently whenthey proved that the Murchison amino acids could transfer theirleft-handedness to otherwise symmetrical amino acids. They thenfound that small degrees of chirality could be dramaticallyamplified in a water solution under conditions similar to the earlyEarth.
Their conclusion: Even the relatively limited number of additionalleft-handed amino acids in the meteorites could, under the rightconditions, lead to a world where almost all amino acids andproteins end up left-handed. Their papers appeared in the journalOrganic Letters and in the Proceedings of the National Academy of Sciences .
This transformation is essential to life because if all the aminoacids and proteins -- which in time became the basic substance ofthe RNA and DNA that organize life through genes -- were equallyleft- and right-handed, they could never have bonded into thestable compounds needed for the infrastructure of living things.
The story is made even more complex by the likelihood that most orall of the amino acids in asteroids, meteors and comets traversingthe solar system -- molecules that can be part of a living entityor not -- were initially evenly right- or left-handed. So how didmeteorites bring in slightly more left-handed amino acids?
The most common theory is that some were transformed by radiationfrom a certain kind of faraway neutron star, the dense and veryhighly charged remnant of a massive star that had collapsed. Theultraviolet, circular-polarized light from these stars hit theasteroids as they sped through space and caused adisproportionately large number of left-handed amino acids to formbefore they hit Earth as meteorites. (In other solar systems bathedby different neutron stars, the effect of the polarized light wouldbe to turn more amino acids right-handed, potentially leading toright-handed molecular worlds.)