The basic building blocks of life could have an interstellar origin

Chiral molecules exist in two forms: left, or levo- (L) and right, or dextro- (D), known as enantiomers. Like a pair of hands, they are non-superposable, but are mirror images of each other (Figure 1). This type of system is commonly found in the biosphere, and many fundamental molecules such as amino acids, protein monomers, and the sugars of nucleic acids, are chiral. A more surprising fact is that the amino acids used in living organisms are all L-type, but when synthesized in the laboratory they tend to be in racemic form, 50% L and 50% D. This is the homochirality of life, also known as biomolecular asymmetry, which has been puzzling scientists ever since it was discovered in the mid-nineteenth century by Pasteur, who saw it as a signature of life, a boundary between living matter and inert matter. What selection process is at work here? Is it a deterministic process? What happened to the D enantiomer?

Figure 1: The two enantiomers of a chiral molecule, in this case a generic amino acid, are mirror images one of the other.

There is very extensive literature describing the large number of possible scenarii that can help to explain the origin of the homochirality of life: a central question in astrobiology that remains open. Most authors agree on so-called abiotic group of scenario, in which homochirality, or at least an initial enantiomeric excess (e.e.), is a necessary condition for the development of life.

Moreover, many amino acids have been discovered in carbonaceous meteorites, with enantiomeric excesses and isotopic compositions indicating an extraterrestrial origin of the prebiotic organic matter, and probably also of its homochirality. Certain amino acids have also been produced in racemic fashion in the laboratory by simulating UV photochemistry on interstellar/circumstellar ice analogs. This means that, to confirm the theory of the interstellar or circumstellar origin of the elementary building blocks of life such as amino acids, it is necessary to show that there is a chiral, i.e. asymmetric, bias, to which these amino acids were exposed during their journey towards the primitive Earth, and which could have led to a notable e.e. Note that it is not necessary to reach 100% e.e. (pure homochirality), because laboratory tests have shown that auto-catalytic reactions can amplify a weak (but significant) e.e., leading to quasi-homochirality.

These deterministic abiotic scenarii (which are therefore accessible to experimentation) include the influence of external chiral fields such as the one associated with circularly polarized radiation that can photo-induce asymmetric processes. This theory is supported by the fact that a partially circularly polarised infrared radiation has been observed in the Orion nebula. This radiation probably also exists in the VUV range over a large region of space.
This is the line of research we have been following for the past decade using synchrotron radiation on beamlines SU5 (LURE) and now DESIRS at SOLEIL: with their exotic undulator, these can produce any type of polarisation, particularly nearly-perfect, calibrated left and right circular polarisations  (refer to the DESIRS 2008 polarimetry highlight) and can therefore simulate inter/circumstellar VUV radiation in the laboratory.

After examining the idea of an enantioselective photo-degradation of initially racemic amino acids deposited on a substrate in the form of solid films, in collaboration with Uwe Meierhenrich (University of Nice) and André Brack (CBM-Orléans) [U. J. Meierhenrich, L. Nahon, C. Alcaraz, J. H. Bredehoft, S. V. Hoffmann, B. Barbier, and A. Brack, "Asymmetric vacuum UV photolysis of the amino acid leucine in the solid state," Angew. Chem.-Int. Edit. 44, 5630-5634 (2005).], providing an e.e. of 2.6% on leucine, and after proposing an alternative photophysical scenario in the gaseous phase based on photoelectron circular dichroism [L. Nahon, G. Garcia, I. Powis, U. Meierhenrich, and A. Brack, "Advanced search for the origin of life's homochirality: asymmetric photon induced processes on chiral compounds with far UV circularly polarized synchrotron radiation," SPIE Proceedings : "Intruments, Methods, and Missions for Astrobiology X" 6694, 69403-69403 (2007).], we have recently obtained within an experiment conducted in collaboration with Louis D’Hendecourt (IAS Oray) and Uwe Meierhenrich, spectacular results in a (phenomenologically) direct photochirogenesis experiment with the formation of amino acids showing an enantiomeric excess.

This is the Chiral MICMOC (Interstellar and Cometary Matter, Complex Organic Molecules) pluridisciplinary experiment, which is based upon the use of CPL VUV light supplied by DESIRS to irradiate simple achiral molecules, to the so-called interstellar ice analogs, a state of matter the IAS’s ‘Astrochemistry and Origins’ team has been studying for a long time.  

Figure 2: Chiral-MICMOC apparatus on the DESIRS beamline at SOLEIL.

Thanks to the performance achieved by DESIRS in terms of purity of polarisation and flux, as well as to the new analytical chromatography methods in the multidimensional gas phase chromatography coupled with mass spectrometry (GCxGC-MS) implemented by Uwe Meierhenrich and his team at the University of Nice (LCMBA), a spectacular result has been obtained: the detection of enantiomeric excesses (Figure 3) on alanine (the simplest proteic chiral amino acid) with values (up to 1.3 %) compatible with those measured in meteorites. This constitutes a major result in terms of comprehension of a possible source of asymmetry in organic matter considered as prebiotic in the context of astrophysics.