In another step toward understanding the origin of Earth’s biological molecules, two independent laboratory experiments have produced amino acids–the building blocks of proteins–by simulating conditions in icy, interstellar space.
The results, published in the March 28 Nature, suggest that some amino acids could have formed in giant clouds of icy particles and then hitched rides on comets and asteroids to planets throughout the universe, says Max Bernstein of NASA’s Ames Research Center in Mountain View, Calif.
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In the search for precursors to life on Earth, various researchers have created amino acids and other organic molecules in lab experiments that simulate specific environments, such as those on early Earth or asteroids.
Researchers have also speculated that amino acids might form on interstellar ice particles that are exposed to ultraviolet light, yet until now, no one had shown that this could actually happen, says Bernstein.
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“I think it’s a really exciting justification of the notion that these building blocks can come from outer space,” comments Jennifer Blank of the Lawrence Livermore National Laboratory in California (SN: 5/19/01, p. 317: Cosmic Chemistry Gets Creative).
In each of the new experiments, performed at very low temperatures and pressures, scientists chose small molecules found in space and deposited them on a surface while irradiating them with ultraviolet light. Bernstein and his colleagues made an icy layer of water, methanol, ammonia, and hydrogen cyanide. The other team, which includes scientists from four European institutions, used less water than the NASA team did. It also included carbon monoxide and carbon dioxide in its mix but no hydrogen cyanide.
After warming the samples to room temperature, the NASA group detected three amino acids: glycine, alanine, and serine. The European group identified 16 amino acids, including 6 that appear in life’s proteins.
Uwe Meierhenrich of Bremen University in Germany says the creation of amino acids in the European group’s experiment was a surprise. The team had been preparing interplanetary ice samples to test equipment for the upcoming Rosetta space probe, which will analyze a comet’s chemistry.
To rule out contamination by natural amino acids, both research teams used a heavy isotope of carbon in their initial chemicals and found that same isotope in the final amino acids. Moreover, some amino acids have both a left- and a right-handed form, but living organisms contain almost exclusively the left-handed variety. So, most amino acids on Earth are in that form. The researchers produced a 50-50 mix of left-handed and right-handed forms in their samples, a confirmation that the amino acids derived from the experiment.
These studies may help explain how some amino acids formed initially, but they do not indicate why life incorporates only left-handed amino acids. One popular theory is that certain naturally occurring forms of UV radiation, called circularly polarized light, preferentially produce left-handed varieties in interstellar space. By running their experiments again with such UV light, the NASA group is now pursuing this possibility, says Bernstein.
Blank says that another question is, How long can such amino acids survive in their icy birthplaces?
“Really rich, robust chemistry taking place in the cosmos” contributes to making the molecules necessary for life, says Jeffrey Bada of the University of California at San Diego. He notes that interstellar ice chemistry is just one of many processes–including chemical reactions on asteroids or the early Earth–that could form amino acids.