CHIRGEN

Deoxyribonucleic acid (DNA) is the genetic source code for all known living organisms. DNA is composed of oligonucleotides that are chemically bound together via phosphate bridges, Watson-Crick base-pairing and pi-pi interactions to form a right-handed double helix (P-helix) in most biological organisms. The chirality of this double helix structure is determined solely by the asymmetric sugar subunit within the nucleotide structure. Deoxyribofuranose has been observed exclusively in D configuration in biological materials. It is currently thought that DNA was not actually the original genetic material and that our present DNA genomic composition evolved from a primordial RNA World state (with D ribofuranose sugar subunits dictating genetic stereochemistry). However the ultimate origin of the asymmetry of the ribose sugar subunit essential to RNA structure still remains unknown. Deterministic hypotheses for the origin of such genetic asymmetry include the absolute asymmetric photochemistry model: circularly polarized light (cpl) can induce an enantiomeric excess (e.e.) in chiral organic molecules exposed to it. This model has recently been strengthened by the observation of cpl in the star-forming region of Orion and the detection of enantioenriched organic molecules in meteorites. In 2009 Sutherland et al published results indicating that we have been looking for the incorrect genetic precursors (sugars plus nucleotides): these data strongly suggest that glyceraldehyde and its derivates were the fundamental asymmetric building blocks from which enantiomerically enriched oligonucleotide intermediates were synthesized in a prebiotic RNA World. This bilateral CHIRGEN-project proposes to discover the origin of genetic asymmetry as the initial and crucial step toward the origin of life by utilizing both interstellar asymmetric photochemical processes (France) and autocatalytic amplification (Mexico). Samples of simulated interstellar ices created at IAS, Orsay, will be transported to UNS, Nice, where extracted and derivatized RNA World precursors such as glyceraldehyde will be resolved into enantiomers and quantified using an entirely new and ultra-modern multidimensional GCxGC TOF-MS system. Simultaneously, racemic mixtures of glyceraldehyde and its derivatives will be subjected to UV circularly polarized synchrotron radiation, simulating interstellar cpl at the synchrotron SOLEIL on the DESIRS beamline in Orsay, France; these irradiated samples will then also be analyzed at UNS. The asymmetric photochemistry of nucleotide precursor molecules at synchrotron SOLEIL is dependent upon on molecular chiroptical properties and will most certainly yield only small e.e.s (as previously observed for amino acids). It is therefore imperative that any induced asymmetry present in these photochemical samples will require enantiomeric enhancement in order to model further evolution towards pre-genetic oligonucleotides in a prebiotic RNA World. The bilateral CHIRGEN proposal therefore also involves our partners at the Universidad Autónoma del Estado de Morelos (UAEM) and the Universidad Nacional Autónoma de México (UNAM) in Mexico for theoretical modelling and experimental evaluation of autocatalytic reactions that afford significant enhancement of initially small e.e.s observed post-photoinduced condensation of formaldehyde to glyceralde¬hyde, a key step in ribose synthesis. A number of autocatalytic scenarios will be investigated and their chemical kinetics will be determined by our Mexican partners. The exchange of cpl-irradiated samples containing RNA World precursor molecules between France and Mexico will then allow us to decipher the original photochemical and autocatalytic processes that resulted in the asymmetric formation of today’s genetic material. Our collaborative research will therefore result in a revolutionary understanding of the origin of the homochirality of our genes.