Life's asymmetry may come from space
Jan 14, 2011 19 comments
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.
Result of evolution
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.
D'Hendecourt and colleagues found what they were looking for. Irradiation of the interstellar-like matter created an organic residue that contained a noticeable asymmetry in the chiral amino acid alanine. Specifically, they found a 1.3% reduction or a 0.7% increase in the amount of left-handed alanine, depending on the orientation of the light polarization (with the lower magnitude in the second case, they say, being explained by a lower concentration of photons arriving at the sample). Using linearly polarized light, in contrast, they found no noticeable asymmetry.
Conjuring up chirality
The researchers therefore conclude that it is possible to create "asymmetrical molecules of life" in space-like conditions from a mixture that does not initially contain any chiral substances. Team member Laurent Nahon, who works at SOLEIL, points out that the figure of 1.3% is of the same order of magnitude as the asymmetric fraction of amino acids discovered in primitive meteorites and so lends further weight to the idea that chiral asymmetry originated in space.
Nahon says that previous laboratory experiments have shown how a slight imbalance such as this can then lead to 100% asymmetry in a chiral substance, but says it is too early to pin down exactly the mechanism that creates the initial imbalance. It is not clear, he points out, whether the polarized radiation creates more of one kind of handedness than the other or whether it creates equal quantities of both and then destroys one of them more readily, but adds that his group is carrying out additional experiments to try and resolve this.
Laurence Barron, a chemist at Glasgow University, believes that D'Hendecourt and co-workers have carried out a "most interesting" experiment but points out that circularly polarized light is not the only mechanism that has been put forward to explain living matter's asymmetry. Indeed, he notes, there are a number of other candidates, including the combined effect of unpolarized light and a static magnetic field, spin-polarized electrons from beta decay, and even charge–parity violation. "Whether this latest work has anything to do with the origin of biological chirality is not clear," he says. "But it certainly merits inclusion in future discussions of the problem."
About the author
Edwin Cartlidge is a science writer based in Rome