Enantiomeric Excesses of Aminonitrile Precursors Determine the Homochirality of Amino Acids

High enantiomeric excesses (ee's) of L-amino acids, including non-proteinogenic amino acid isovaline (Iva), were discovered in the Murchison meteorite, but the detailed molecular mechanism responsible for the observed ee of amino acids remains elusive and inconsistent, because Iva has an inverted circular dichroism (CD) spectrum with respect to a-H amino acids, e.g., alanine. To address this issue, we resort to accurate ab initio calculations for amino acids and their precursors in the Strecker synthesis. We evaluated their photolysis-induced ee in the range 5-11 eV including the Lyman alpha emission line (Lya), the typical intensive 10.2 eV radiation ascribed to the early phase of galactic evolution. We show that only the aminonitrile precursors are characterized by positive ee in the Lya region, explaining why right-handed circularly polarized Lya (R-CP-Lya) induces homologous L-amino acids. This study shows that the homochirality of amino acids is produced at the aminonitrile precursors stage.

Automated summary

High enantiomeric excesses (ee's) of L-amino acids, including non-proteinogenic amino acid isovaline (Iva), were discovered in the Murchison meteorite. However, the molecular mechanism responsible for the observed ee of amino acids remains elusive. Accurate ab initio calculations and evaluation of photolysis-induced ee were conducted for amino acids and their precursors. It was found that only the aminonitrile precursors have positive ee in the Lyman alpha region, explaining the homochirality of L-amino acids induced by right-handed circularly polarized Lyman alpha.