Molecular asymmetry in extraterrestrial organic chemistry: An analytical perspective

The enantiomeric excesses determined for eight amino acids and one hydroxy acid of carbonaceous chondrite meteorites represent to date the only case of molecular asymmetry measured outside the biosphere. Because of the chiral homogeneity of life’s structures and functions, the findings have been debated for the possible relevance that a-biotic chiral symmetry-breaking might have had in the origin of terrestrial homochirality. While the many unknowns surrounding the origin of life have inevitably hindered the inquiries raised in this discourse, the hypotheses put forward in regard to the origin of extraterrestrial chiral asymmetry, which is a defined physico-chemical phenomenon, have been approached analytically and their scrutiny has aided the understanding of pre-biotic chemical evolution. We report here on our current knowledge of the asymmetric effects that could have influenced the chiral symmetry breaking of molecules in cosmochemical environments and how they correlate with the data obtained from meteorite analyses. We also address recent proposals that aqueous processes might have influenced the chirality of amino acids in meteorites and show that the crystallization behavior of isovaline, the most abundant non-racemic amino acid in the Murchison meteorite, excludes its attainment of enantiomeric excesses via phase changes such as crystallization or sublimation.