An interesting paper was published in the online advanced issue of PNAS. Titled, “Molecular Asymmetry in Extraterrestrial Chemistry,” the researchers looked at the ratios of chemical isomers found on a “pristine meteorite.” Because the isomeric ratios of biomolecules on earth (especially amino acids and sugars) are very specific, finding different ratios could help us learn a) how life might have started and evolved and b) what life might look like on other planets.
On Earth, biomolecules such as amino acids and sugars have different handedness; that is, the molecules can be mirror images of each other. With the two amino acids in this picture, you can kind of imagine a mirror in between the two of them. Yet no matter how you flip them around, the hydrogen atom and the amino group will not match up. Chemically, these compounds are exactly the same. If you ran the same chemical reactions side by side, you wouldn’t be able to tell the difference. However, the enzymes in your body are a little more discerning. They only like the left-handed alanine molecule (on, ironically enough, the right-hand side of the image). The D-alanine molecule? The enzymes couldn’t be bothered.
With sugars, the enzymes prefer the right-hand stereoisomer.
Why? There’s a lot of hand-waving that goes along with those answers. What many scientists think is that it was likely just some of the chance happenings that occur with natural selection. As far as any biochemists know, there is no biological reason one stereoisomer should be preferred more than any other. And when experiments have been done to try and create biomolecules in “primitive earth” conditions, there has been no selectiveness in terms of isomers. Both left-handed and right-handed molecules were produced in a 50:50 ratio.
The researchers who wrote this paper, however, have a different theory. They believe that the selectiveness of the isomers might have been due to the molecules that arrived on meteorites and other debris from outer space.
On meteorites, there are relatively large numbers of aldehyde molecules (like formaldehyde from your biology dissection labs- the fumes always made me throw up). The researchers found a “pristine meteorite” in Antarctica and analyzed some of the carbon-based molecules on it. They looked at two amino acids- alloisoleucine and isoleucine, which are mirror images of each other, and found that their precursor molecules — the aldehydes — were likely not in a 50:50 ratio of stereoisomers.* In fact, they found a 14% excess of the left-handed aldehydes.
This paper, like any research paper should, raises more questions than it answers. What is a pristine meteorite and how can we tell? Are these good ways to look for biomolecules? What does this tell us about how life may have started? Could the excess of the left-handed isomers be a fluke? Does this make life from outer space any more or less likely? Does life have to have left-handed amino acids and right-handed sugars?
And for further reading, check out Robert Hazen’s book Genesis. Don’t worry, it’s not religious at all.
I can’t get the DOI information right now- I’m working on it. I’ll post it as soon as I get it! Image credit: Tainano Nanobiotechnology Glossary *Fun chemical vocabulary: racemization (ray-SUH-muh-zay-shun) is when you start out with an excess of one stereoisomer, and over time, it turns into a 50:50 ratio.