Inner circles

By Jeff Hecht POLARISED light from space could solve a mystery about the origin of life. According to an international team of astronomers, a rare form of ultraviolet light from the Orion Nebula might account for the chemical structure of the building blocks of proteins and carbohydrates. Life on Earth uses amino acids and sugars in only one of two possible mirror-image forms—amino acids are left-handed and sugars right-handed. The choice was vital, because proteins can fold consistently into their complex shapes only if made entirely of one form or another, but not both. Yet most inorganic chemical reactions form equal numbers of left and right-handed molecules. So where did this bias come from? Reports last year of excess left-handed amino acids in the meteorite that fell near Murchison, Australia, in 1969 suggested the choice originated in outer space, but did not reveal how the excess came about (In Brief, 20 September, p 26). Now James Hough, dean of natural sciences at the University of Hertfordshire, says that the answer may be a strong dose of circularly polarised light, which could have produced an excess of particular kinds of amino acids in the dust that formed the Solar System. In circularly polarised light, the direction of polarisation rotates continuously. Ultraviolet wavelengths of such light can force chemical reactions to make molecules of mostly one-handedness when they would otherwise have churned out half of one and half of the other. Right-handed light destroys right-handed molecules, leaving an excess of left-handed ones. Circular polarisation is rare and hard to spot, so Hough and his colleagues built a special instrument for the Anglo-Australian Telescope near Coonabarabran in New South Wales. With it, they found that as much as 17 per cent of the light reflected from gas clouds where stars are forming in Orion was circularly polarised. Although they could only observe in the infrared, they say in Science (vol 281, p 672) that ultraviolet light—obscured by the clouds—should have the same polarisation. If the Solar System formed in a similar environment, Hough says, one would expect a 5 to 10 per cent excess in the handedness of molecules, the same range found in the Murchison meteorite. If that excess persisted on the young Earth, it could have tipped the scales in selecting building blocks for early life. All those “ifs” bother Jeff Bada of the Scripps Institution of Oceanography in La Jolla, California. “It’s just a series of maybe steps,” he says. “To me that makes the whole thing a big maybe.” But Hough says what’s appealing about his theory is that in the regions of Orion much larger than our Solar System, the polarisation was all right-handed, which could account for the left-handed bias of amino acids on earth. Sugars, on the other hand, are too unstable to have arrived directly from space, says Hough. Their right bias may be the byproduct of the early biological synthesis of amino acids. Bada and Hough agree that life on Mars or Europa could test the theory. “If it has [right-handed] amino acids,” Bada says,
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