Originally posted to 13.7 Cosmos & Culture on 6/22/2016
As Edgar Allan Poe wrote, quoting Lord Bacon in Ligeia: "There is no exquisite beauty without some strangeness in the proportions."
If we consider life to be an exquisite beauty among cosmic creations, it follows this rule quite remarkably. Some biomolecules, including proteins and the sugars that are key to life, may appear in two forms — enantiomers, they are called — which are mirror images of each other. We can call them left- and right-handed. The strangeness in the proportion in biology is that life chooses, overwhelmingly, only one of the two forms: Proteins in living creatures are left-handed, while sugars are right-handed. No one knows why there is this particular "chiral" preference. When such molecules are synthesized in the laboratory, we get a 50-50 mix, no preference.
There are many explanations, including one by me and my then-graduate students Sara Walker and Joel Thorarinson. It claims the choice to be an accident: As we follow the complicated chemical reactions that built longer and longer protein chains in early Earth, we can show that dramatic environmental events could have flipped their chirality randomly. If we are right, if we could sample other worlds with chiral molecules, we would have a 50-50 chance of finding the opposite choice.
A paper published this month indicates that chirality may have its origins in the birth of stars and their associated solar systems. This was the first time that a chiral molecule was found in interstellar space, using a radio telescope pointed toward Saggitarius B2, an active stellar-forming region toward the center of our galaxy.
Brett McGuire and colleagues from the California Institute of Technology and Harvard University found propylene oxide as they studied the absorption spectrum of a giant cloud of gas and dust. As molecules tumble and vibrate in the cold of outer space, they give off and absorb energy at different wavelengths, making for a unique signature, like a molecular thumbprint. Once the data was identified, they knew they had found a chiral molecule. However, the data doesn't allow them to determine whether it was the left or the right-handed form. To do that, scientists will have to investigate how polarized light interacts with the molecules in outer space, a more complex project.
The results support the possibility that chirality is a prebiotic phenomenon — that is, that it happened before life emerged on Earth (and, possibly, elsewhere). If, as the finding indicates, chiral molecules were part of the primal soup that became Earth, life had its biased seeds before it even got going. Although this particular molecule doesn't play a role in life, the fact that it is chiral suggests that other chiral molecules could also have been synthesized in the nascent solar system.
Still, it's important to remember that chirality can go both ways; even if chiral molecules are part of the ingredients of new planets, the choice of chirality may still vary randomly from star-forming region to star-forming region: Other living creatures "out there," belonging to a different stellar nursery, may have the opposite handedness. But we are far away from figuring this one out, unfortunately.