Space radiation preferentially destroys specific forms of amino acids, the most realistic laboratory simulation to date has found. The work suggests the molecular building blocks that form the “left-handed” proteins used by life on Earth took shape in space, bolstering the case that they could have seeded life on other planets. Amino acids are molecules that come in mirror-image right- and left-handed forms. But all the naturally occurring proteins in organisms on Earth use the left-handed forms – a puzzle dubbed the “chirality problem”. “A key question is when this chirality came into play,” says Uwe Meierhenrich, a chemist at the University of Nice-Sophia Antipolis in France. One theory is that proteins made of both types of amino acids existed on the early Earth but “somehow only the proteins of left-handed amino acids survived”, says Meierhenrich. Meierhenrich and colleagues have a different theory. “We say the molecular building blocks of life were already created in interstellar conditions,” he told New Scientist.
The team believes a special type of “handed” space radiation destroyed more right-handed amino acids on the icy dust from which the solar system formed. This dust, along with the comets it condensed into, then crashed into Earth and other planets, providing them with an overabundance of left-handed amino acids that went on to form proteins. The radiation is called circularly polarised light because its electric field travels through space like a turning screw, and comes in right- and left-handed forms. It is thought to be produced when dust grains become aligned in the presence of magnetic fields threading through regions of space much larger than our solar system. Circularly polarised light is estimated to make up as much as 17% of the radiation at any given point in space.
In 2000, an experiment showed that when circularly polarised ultraviolet light of a particular handedness was shone on an equal mix of right- and left-handed amino acids, it produced an excess of 2.5% by preferentially disintegrating one type. But that experiment was done using amino acids in a liquid solution, which behave differently than those in the solid conditions of icy dust in space. To avoid absorption by water molecules, it was also necessary to use light at a wavelength of 210 nanometres – significantly longer than the peak of 120 nm radiation actually measured in space.
Now, Meierhenrich’s team has performed a similar experiment. The group shone circularly polarised light at a wavelength of 180 nm on a solid film of both right- and left-handed forms of the amino acid leucine. It found that left-handed light produced an excess of 2.6% left-handed amino acids. “Going towards greater realism by exploring another wavelength of light and solid samples is definitely a good thing and a logical step forward,” says chemist Max Bernstein of NASA’s Ames Research Center in California, US, who is not part of the team. He says the research adds to previous measurements of an excess of left-handed amino acids in two meteorites. “If it is thanks to meteorites that our amino acids are left handed, then the same bias should exist at least across our solar system”, he told New Scientist.
But other solar systems may harbour right-handed amino acids if they are subjected to the other type of circularly polarised light, says Meierhenrich. “The chiral amino acids might have been delivered to other planets, to other solar systems,” he adds. “The probability that life arose somewhere else is increased with this experimental result.” Meierhenrich will continue to reduce the wavelength of the experimental radiation by using a synchrotron facility, due to begin operating in 2006. But the real test of his theory may come in 2014, when the European Space Agency’s Rosetta spacecraft lands a probe on Comet 67P/Churyumov-Gerasimenko. He designed an instrument for the lander that will measure the handedness of any amino acids it finds. “If we identify left-handed amino acids on the cometary surface, this would underline the hypothesis that the building blocks of proteins were created in interstellar space and were delivered via comets or micrometeorites to early Earth,” he says.
Journal reference: Angewandte Chemie International Edition (vol 44, p 2)
September 13, 2005
Original web page at New Scientist