Why did life on Earth evolve with "one handedness" in its chiral molecules? | Science
Chiral molecules have two forms that are identical in structure, but are mirror images of one another. They are important on Earth because most biomolecules found in living organisms are chiral. Now, for the first time, these molecules have been detected in interstellar space, a new study in the 17 June issue of Science reports after the results were announced on 14 June at the American Astronomical Society meeting in San Diego, California.
These enigmatic molecules are identical but not interchangeable — imagine a left-handed and right-handed glove, for example. On Earth, a puzzling phenomenon exists where life is overwhelmingly made up of just one type of chirality or "handedness," a phenomenon known as homochirality. For example, the amino acids that make up proteins in our bodies are all "left-handed." Why one handedness is dominant over the other is a great mystery of the origin of life.
"To discover how this came about, we need to look as far back in evolutionary time as possible, to see how far back a preference for one handedness over the other is seen," explained Brett McGuire of the National Radio Astronomy Observatory and a lead author of the study.
First hints of a chiral molecule in space came from publicly available data from the Green Bank Telescope (GBT), in West Virginia; however, the data were not sufficient to absolutely confirm the finding, prompting McGuire and his colleague Brandon Carroll of the California Institute of Technology to explore the finding further.
"Brett and I traveled to the Parkes Telescope in Australia to confirm our findings from the Green Bank Telescope, and were in the middle of our third all-night observing session when the signal finally became clear," said Carroll. "We were incredibly excited."
By analyzing radio frequencies picked up by the Parkes Telescope, they confirmed the presence of chiral molecules, in the form of propylene oxide, in Sagittarius B2 North, a cloud of gas and dust roughly three million times the mass of the Sun that's located in the center of the Milky Way galaxy.
"The most important result of this finding is that we now have a laboratory of sorts in space to explore how chiral molecules form and evolve in space," Carroll said.
The next steps, Carroll said, will involve both laboratory studies and astronomical observations to try to determine if there is an excess of one chiral version of propylene oxide over the other. "Many studies have proposed various mechanisms for generating such an excess in regions like Sgr B2(N), and if we can detect that excess, we can start to determine which of these mechanisms is critical to the [phenomenon]."
[Credit for associated teaser image: Photo of the Milky Way and Galactic Center over Lake Waiau, Hawaii by Brett A. McGuire. Foreground: P. Brandon Carroll]