Uncovering the chiral bias of meteoritic isovaline through asymmetric photochemistry

Abstract

Systematic enrichments of l-amino acids in meteorites is a strong indication that biological homochirality originated beyond Earth. Although still unresolved, stellar UV circularly polarized light (CPL) is the leading hypothesis to have caused the symmetry breaking in space. This involves the differential absorption of left- and right-CPL, a phenomenon called circular dichroism, which enables chiral discrimination. Here we unveil coherent chiroptical spectra of thin films of isovaline enantiomers, the first step towards asymmetric photolysis experiments using a tunable laser set-up. As analogues to amino acids adsorbed on interstellar dust grains, CPL-helicity dependent enantiomeric excesses of up to 2% were generated in isotropic racemic films of isovaline. The low efficiency of chirality transfer from broadband CPL to isovaline could explain why its enantiomeric excess is not detected in the most pristine chondrites. Notwithstanding, small, yet consistent l-biases induced by stellar CPL would have been crucial for its amplification during aqueous alteration of meteorite parent bodies.

Excess of l-amino acids in meteorites suggests an extraterrestrial origin of biomolecular homochirality, which may stem from chiral light-matter interactions. Here the authors support this hypothesis with asymmetric photolysis experiments on racemic isovaline films, showing that circularly polarized starlight can produce l-enantiomeric excesses that can be amplified during parent bodies’ alteration.