Molecular handedness found in space

Propylene oxide’s two mirror-image forms could be clue to biological chirality

gas cloud with molecular structure

MOLECULE CLUE  A gas cloud (Sagittarius B2) near the center of the galaxy (Sagittarius A*) is loaded with propylene oxide, a molecule that comes in mirror-image configurations.

B. Saxton, NRAO/AUI/NSF from data provided by N.E. Kassim, Naval Research Laboratory, Sloan Digital Sky Survey

SAN DIEGO — A clue about why life on Earth chooses only one mirror-image form of certain molecules might lie in a gas cloud tens of thousands of light-years away.

For the first time, researchers have detected a chiral molecule, propylene oxide, in interstellar space. Chiral molecules, which come in two mirror-image versions, show up in many of life’s building blocks, such as the amino acids that make up proteins as well as sugars. The finding may be a step toward understanding why life prefers one of these versions over another.

The results were presented June 14 at a meeting of the American Astronomical Society and published online the same day in Science.

Chiral molecules are like opposing hands. Left hands and right hands mirror each other, but no amount of turning will get them to match when overlaid. Matching configurations of a chiral molecule are labeled as either left-handed or right-handed.

Amino acids and sugars come in both styles of handedness. But life on Earth exclusively uses left-handed amino acids and right-handed sugars. “This is one of the longest standing mysteries in the origin of life,” Brett McGuire, a chemist at Caltech, said at a news briefing.

Chiral molecules have shown up in meteorites with a slight preference for one configuration. McGuire and colleagues went looking for chiral molecules in space to see whether some interstellar intervention could preferentially seed a solar system with one handedness. The researchers sifted through radio observations from the Green Bank Telescope in West Virginia of a gas cloud dubbed Sagittarius B2. The nebula sits near the center of the galaxy and has historically been a rich hunting ground for interstellar molecules.

McGuire and colleagues found that the cloud was loaded with the chiral molecule propylene oxide. The stockpile has a mass equal to about 80 percent of Earth’s mass, said McGuire, and if compressed into a liquid blob, it would occupy a volume over five times that of our planet. The observations don’t reveal whether the cloud has a preference for one handedness over another; that will have to wait for future observations. But “we’re in the best position we could possibly be,” said McGuire, to figure out if life’s chiral exclusivity has an interstellar origin. 

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