By Marc Kaufman
Washington Post Staff Writer
Monday, October 6, 2008
For more than 150 years, scientists have known that the most basic building blocks of life -- chains of amino acid molecules and the proteins they form -- almost always have the unusual characteristic of being overwhelmingly "left-handed." The molecules, of course, have no hands, but they are almost all asymmetrical in a way that parallels left-handedness.
This observation, first made in the 1800s by French chemist Louis Pasteur, is taught to introductory organic chemistry students -- until recently with the caveat that nobody knew how this came to be.
But research into the question has picked up in recent years, focusing on a 200-pound chunk of rock found 40 years ago in Murchison, Australia. A meteorite that broke off an asteroid long ago, it brought to Earth a rich collection of carbon-based material from far away in the solar system.
While the Murchison meteorite does not have any once-living material, it is telling researchers new things about how life may have started on Earth, and how that almost universal protein left-handedness came to be.
The answer they believe they have found is that 3 billion to 4 billion years ago, before life on Earth began, similar meteorites crashed regularly into the planet -- delivering the amino acids that would later be incorporated into all living things. The meteorites did this by providing building blocks with a slight preponderance of that handedness (known scientifically as chirality) that makes life possible.
"We know that all amino acids start mirror-image the same, but in living things they have this handedness," said Ronald Breslow, a Columbia University researcher who published recently on the topic. "This change doesn't happen spontaneously, and we've never been able to reproduce it in the laboratory" under conditions similar to early Earth.
"The answer to where it comes from looks increasingly like meteorites," he added, "from extraterrestrial bodies falling to Earth. It's a complex story, but we're beginning to understand it better."
Breslow and his colleagues made significant progress recently when they proved that the Murchison amino acids could transfer their left-handedness to otherwise symmetrical amino acids. They then found that small degrees of chirality could be dramatically amplified in a water solution under conditions similar to the early Earth.
Their conclusion: Even the relatively limited number of additional left-handed amino acids in the meteorites could, under the right conditions, lead to a world where almost all amino acids and proteins end up left-handed. Their papers appeared in the journal Organic Letters and in the Proceedings of the National Academy of Sciences.
This transformation is essential to life because if all the amino acids and proteins -- which in time became the basic substance of the RNA and DNA that organize life through genes -- were equally left- and right-handed, they could never have bonded into the stable compounds needed for the infrastructure of living things.
The story is made even more complex by the likelihood that most or all of the amino acids in asteroids, meteors and comets traversing the solar system -- molecules that can be part of a living entity or not -- were initially evenly right- or left-handed. So how did meteorites bring in slightly more left-handed amino acids?
The most common theory is that some were transformed by radiation from a certain kind of faraway neutron star, the dense and very highly charged remnant of a massive star that had collapsed. The ultraviolet, circular-polarized light from these stars hit the asteroids as they sped through space and caused a disproportionately large number of left-handed amino acids to form before they hit Earth as meteorites. (In other solar systems bathed by different neutron stars, the effect of the polarized light would be to turn more amino acids right-handed, potentially leading to right-handed molecular worlds.)
Daniel Glavin, an astrobiologist at the Goddard Space Flight Center in Greenbelt, said he and his colleagues recently discovered that a particular amino acid in the Murchison meteorite, isovaline, was disproportionately left-handed at a level of almost 18 percent. Other left-over-right imbalances had mostly been in the single digits. The logical conclusion, Glavin said, is that the imbalance arose on a water-containing asteroid or comet well before it broke up.
"There are signs that this compound existed in a watery environment and that the change in handedness happened because of that," he said.
Chirality is a simple concept that is hard to fully grasp. A "chiral" molecule is one that cannot be superimposed on its mirror image. Like left and right hands that have a thumb and fingers in the same order but are mirror images, chiral molecules have the same things attached in the same order but are mirror images and not the same. To make things a bit more complex, while almost all proteins are left-handed, almost all sugars are right-handed.
When researchers initially reported the higher proportion of left-handed amino acids in the Murchison meteorite -- which now resides at the Smithsonian's National Museum of Natural History -- the news was met with skepticism. The main criticism was that the meteorite was no doubt contaminated by earthly chirality when it struck, accounting for the overabundance of left-handedness.
Further study, however, has shown that some of the disproportionately left-handed Murchison amino acids are rare on Earth and contain much more of the heavy isotope of carbon (with an extra neutron) than is found in organic carbon on Earth.
Some researchers conclude that chirality and its complex origins make it less likely that life exists on other planets. But Breslow and Glavin hold the opposite view.
"I think this kind of chemistry could exist on many other planets and asteroids," Breslow said. "Meteorites crash into celestial bodies all the time. Given at all similar conditions that early Earth had, I don't see why some of those meteorites couldn't have the same effect when they hit other planets, too."