Light on Chirality: Absolute Asymmetric Formation of Chiral Molecules Relevant in Prebiotic Evolution

Abstract

Homochirality is a distinct property of living matter manifested by the unichiral molecular units of genetic material and of proteins, namely, d-deoxyribose and l-amino acids. These molecular subunits or their precursors have been shown to form under various prebiotic conditions in racemic form. However, the nature of the chiral influence, which results in the first breakage of molecular symmetry, remains unclear. In this respect, the photochemical model of enantioselection has gained particular importance in recent years. In this model, the interaction of circularly polarized light with racemic molecules generated in the interstellar medium is considered to be the main driving force of enantiomeric discrimination in early prebiotic evolution. Cometary ice simulations, l-enantio-enriched amino acids in meteorites, and the detection of circularly polarized electromagnetic radiation in star-forming regions provide evidence for photochirogenesis. The recent discovery of aldehydes in the cometary nucleus of 67P/Churyumov-Gerasimenko by means of the Cometary Sampling and Composition (COSAC) experiment on board the Rosetta lander Philae, along with the detection of aldehydes and aldopentoses (including chiral ribose) in simulated interstellar ice analogues, provide a direct link between laboratory simulations and cometary composition. In the context of these new findings, this review will provide an overview of the role of chiral molecules in prebiotic evolution with special emphasis on their chiroptical properties and absolute asymmetric photosynthesis.

Keywords: amino acids; anisotropy; asymmetric photolysis; chirality; circular dichroism.