A new study of interstellar ice analogs gives insight to explain the biomolecular asymmetry at the surface of primitive Earth.
Living organisms use amino acids exclusively in their L enantiomeric form, a fact known as homochirality. However, the abiotic synthesis of these molecules to produce racemic mixtures of them (equal amount of L and D forms) and the origin of the asymmetry remain hypothetical. A consortium gathering scientists from the Astrochimie et Origines team of IAS (Orsay), the Institut de Chimie de Nice and SOLEIL synchrotron performed experiments to shed light on the photochemistry of ices as templates of interstellar/pre-cometary ices. These ices may be irradiated by a VUV asymmetric light, the circularly polarized one, as delivered by the DESIRS beamline that promotes enantioselectivity in chiral molecules, ie transferring chirality from photons to matter. New results show that enantioselectivity obtained on five different amino acids allows to confirm previous results and may help to further precise the astrophysical scenario that may be responsible for the origin of the biomolecular asymmetry at the surface of primitive Earth.
Since 2003, the team « Astrochimie et Origines » at IAS has developed an experimental apparatus to reproduce some characteristics of the evolution of interstellar/pre-cometary ices that are widely observed in the spectra of dense molecular clouds out of which stars, disks, planets and various debris such as comets and asteroids do form. Most of the work concentrates on the photochemical evolution of these ices that lead to some soluble complex organic material that may present some analogy with similar materials found in meteorites. These experiments performed with a laboratory unpolarized VUV lamp, known under the name MICMOC does indeed produce, in the form of an organic residue, many molecules such as amino acids (racemic) that may be significant for the starting of prebiotic chemistry at the surface of telluric planets.
A slight modification of this experiment, named Chiral-MICMOC, is performed on the synchrotron SOLEIL facility, using the beamline DESIRS. This beamline provides circularly polarized light in the UV (UV-CPL) that is connected to the MICMOC apparatus (see Figure 1). The goal of this experiment is to observe a transfer of the asymmetry of the light (the chiral photons) to the produced organic matter and in particular to the amino acids produced by the photochemistry. The performances of DESIRS are unique gathering over a broad UV-VUV range: photon flux density, spectral purity and tailored calibrated polarization states (including of course the linear and left- as well as right-CPL). Samples are extracted from their supporting window at room temperature, carefully kept under argon to avoid oxidation and sent to the Nice team to be analyzed by multidimensional gas chromatography techniques (GCxGC-MS). To avoid any possible contamination during handling, laboratory samples are labelled with 13C so that enantiomeric excesses (e.e’s) are performed only on amino acids containing 13C. New results have recently been achieved on five different amino acids to generalize results previously obtained on only one (alanine) in 2011.
Enantiomeric excesses of the same sign have been obtained on 5 amino acids (see Figure 2 and 3): α-alanine, valine (proteinaceous ones), 3-diaminopropionic acid, 2-aminobutyric acid, and norvaline (non proteinaceous). The measured excesses are always quite small (2% max) but comparable to the L-excesses observed in a few carbonaceous primitive meteorites. These results allow the strengthening of an astrophysical scenario leading to a possible source of asymmetry that is necessary in prebiotic chemistry to further lead to homochirality via a still unknown amplification process. The fact that these five amino acids show the same behavior (excess of the same sign with the same helicity of the irradiating UV-CPL) is absolutely necessary for the astrophysical scenario to be considered. Photochemistry of achiral ices do indeed lead to chiral molecules but, in the presence of UV-CPL, the amino acids detected do show some enantioselective effect leading to small excesses (in L or D, depending on the helicity orientation of the light).
For alanine, several experiments have been performed in which the helicity of the UV-CPL and the energy of the chiral photons (6.6 and 10.2 eV) have been changed. A direct connection has been observed between the sign of the helicity and the sign of the measured excesses, suggesting an effect mediated by circular dichroism. Moreover, the irradiation of original achiral ices and of the organic residue only at room temperature does lead to similar results. This result clearly shows the formation of prochiral entities within the achiral ices as well as the possibility to transfer the asymmetry to the organic residue itself. These organic residues could be thus present in grains at late stages of the molecular cloud and then accumulating in planetesimals and various debris (comets, asteroids) before delivery to planets (such as the Earth).
High degrees of CPL (up to 22% in the infrared domain) have been observed in regions of high mass star formation such as the Orion Nebula and NGC 6334. The spatial extension of the same helicity of the observed CPL is much larger (a factor 100) than the size of the Solar System which allows the possibility to keep excesses of a constant sign (L in the case of our Solar System) and may explain excesses observed in meteorites.
This experiment of laboratory astrophysics may thus help to validate a very old hypothesis in which the enantio-selectivity in biological molecules, first discussed by Pasteur in 1848, is the result of a fully deterministic pathway, later discussed by Pierre Curie in 1897 and involving the action of asymmetric (or chiral) photons.
Indirect implications of this result may be that the solar nebula was indeed formed in a region of massive stars. This idea was suggested thirty years ago to account for the observation of the presence of extinct radionuclides in primitive meteorites as a consequence of the supernovae considered to be responsible for the collapse of the molecular cloud from which the Sun was formed.
This experiment is now shifting towards the study of other molecules of relevance for prebiotic chemistry such as sugars, recalling that biological sugars have are D-homochiral (opposite to the amino acids).
Figure 1 : the Chiral MICMOC experimental set-up connected to the VUV DESIRS beamline at SOLEIL
Figure 2:. Multidimensional enantioselective gas chromatographic resolution of 13C enantiomers of alanine (Ala), 2-aminobutyric acid (2-Aba), valine (Val), norvaline (Norval), and 2,3-diaminopropionic acid (DAP) produced by 10.2 eV UV photo-irradiation with (a) L CPL, (b) UPL, and (c) R CPL.
Figure 3 : L-enantiomeric excess (eeL), determined by enantioselective GC×GC-TOFMS), measured in five different amino acids labeled with 13C. Experimental values are obtained from three residue-enlarged samples of initially achiral circumstellar analogs at the stage of residue irradiated by CPL at 10.2 eV. Blue squares refer to eeL induced by R CPL, red triangles refer to eeL induced by l-CPL, and white circles refer to eeL measured after unpolarized/linearly polarized light. The eeL are of the same sign in all five amino acids for a given helicity of CPL