Life’s asymmetry may come from space

Processes taking place in outer space, and not on Earth, are likely to have led to the biological molecules found exclusively in either a left-handed or right-handed form. That is the conclusion drawn from recent experiments carried out at the SOLEIL synchrotron facility near Paris in which a number of simple molecules found in star-forming regions exposed to polarized radiation created amino acids with an imbalance of left- and right-handed molecules.

So-called chiral molecules can exist in two forms, with one being the non-superimposable mirror-image of the other, even though both have the same chemical make-up. Although laboratory experiments will tend to produce equal quantities of the left- and right-handed versions of a given chiral molecule, many of the chiral molecules found in living organisms come in only one variety. For example, the amino acids that make up proteins only exist in the left-handed form, while the sugars found in DNA are exclusively right-handed.

Scientists have long debated the reasons for this asymmetry in living matter. Some have argued that equal numbers of both versions of each chiral molecule were present at the onset of life and that it was only during biological evolution that the imbalance occurred. That view has become increasingly unpopular, however, with the realization that the fundamentally important process of protein folding seems to require chiral imbalances, while for nature to have selected the left- or right-handedness of each molecule during evolution would involve extraordinarily complex processes.

The latest work, published in Astrophysical Journal Letters, provides further backing for the alternative view, that the asymmetry existed before life got going. A group of astrophysicists, physicists and chemists in France, led by Louis le Sergeant d’Hendecourt of the University of Paris South, irradiated molecules of water, ammonia and methanol at low temperatures using circularly polarized ultraviolet light at SOLEIL. The idea was to recreate the conditions found in star-forming regions, where partially circularly polarized light has been observed, and to test the hypothesis that this polarization could induce an imbalance in the creation of left- and right-handed versions of certain amino acids. Other researchers have previously shown experimentally that chiral organic molecules can be created in space-like conditions, and that organic matter might therefore have its origins in space, but could not induce any asymmetry because they lacked a suitable source of radiation

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