7 January 2011
Chiral molecules are assemblies of identical atoms that exist in two perfectly symmetrical and non-superimposable forms, like our hands for example.
In the living world, these two forms—designated L and D—do not exist at the same time. “The building blocks of life, amino acids, are only present in their L form,” explains Michel Viso, an exobiologist at CNES. “And the sugars of DNA or those used to store energy only exist in their D form. This feature known as homochirality, which excludes one of the two forms of each type of molecule, is considered to be universal.”
Where does this dissymmetry come from? This is what the experiment conducted by French teams from CNRS, the French national scientific research centre, with support from CNES, is trying to find out.
There are several competing hypotheses to explain homochirality in the living world.
Either it is pure coincidence, life having adopted one form rather than another as it evolved on Earth; or it is based on a pre-existing imbalance in the way these building blocks formed on Earth or in space.
First for SOLEIL synchrotron
The team led by Louis d’Hendecourt, director of research at the IAS space astrophysics institute, has succeeded in recreating this imbalance in a particle accelerator, the SOLEIL synchrotron, at Saint-Aubin outside Paris.
“For the first time ever, a mixture of non-chiral molecules was subjected to circularly polarized ultraviolet radiation. These are the conditions that exist in certain regions of the Universe,” says Michel Viso. “The reactions produced organic chiral molecules with an excess of L or D forms depending on the polarization of the light.”
If homochirality is the result of an existing imbalance in the environment, it could be of cosmic origin.
A discovery of major import for planetary exploration.
“Homochirality therefore remains a signature of life. But finding an imbalance, even a large one, between chiral forms of organic substances on Mars one day won’t prove on its own that there was once life on the planet,” concludes Michel Viso.
But that’s not all, as the results of the experiment by Louis d’Hendecourt’s team indicate that these molecules may have formed in regions where massive stars are born, where circularly polarized infrared radiation is frequently observed.