Such an early phase of galaxies is categorized as Lyα emitters (LAEs). In particular, in the early evolutionary stage, the Milky Way is thought to have been be embedded in intense pan-galactic Lyα light. It is known that strong Lyman-α (Lyα) radiation, peaked at 10.2 eV, is emitted from ionized hydrogen (HII) in star-forming regions. (25,26) Hence, to produce ee on AAs, the spectrum of UV CPL must be narrowed to a single band. (20) Furthermore, CD alternates in sign and sum to zero over the whole spectrum according to the Kuhn–Condon sum rule. (19−24) However, the CD spectra of Iva shows a reversed CD sign, suggesting that the production of d-Iva instead of l-Iva is induced by the FUV. (12−18) Based on photolysis experiments and circular dichroism (CD) measurement of l-AAs in the solid state, it has been proposed that the far-ultraviolet (FUV) region between 170 and 200 nm (7.3–6.2 eV) is the effective wavelength range to induce the homochirality, since the CD spectra have equally the positive and large magnitude for several key amino acids such as Ala, Aba and Val. (9−11) Laboratory experiments have shown that circular polarized light (CPL) irradiation produces an asymmetric photosynthesis and decomposition of AAs eventually leading to ee. (9) In star-forming regions, strong infrared CPL has been detected. It has been proposed that ultraviolet (UV) circularly polarized light (CPL) in star-forming regions might have played a significant role in inducing ee in interstellar organic molecules which, in turn, were probably subsequently delivered to the early Earth by meteorites. (6) These results have opened a novel paradigm about the cosmic origin of the homochirality. The observed %ee values for AAs in the Murchison meteorite are 1.2, 0.4, 2.2, and 8.4 for alanine (Ala), 2-aminobutyric acid (Aba), valine (Val) and Iva, respectively. Isotopic ratios of meteoritic AAs are different from the ratios of terrestrial ones, and high %ee values were also measured in abiotic AAs such as isovaline (Iva). (5) The detected AAs turn out to be not only proteinogenic but also abiotic, including β- and γ-amino acids, and their %ee values were generally positive, where the %ee is defined as ( C L – C D)/( C L + C D) × 100 and C L and C D are the amounts of each enantiomer. Their presence and related ee of AAs in the Murchison meteorite, that fell in Australia in 1969, provided an unprecedented clue about a cosmic origin of homochirality. (2−4) Since the first chiral symmetry-breaking event determines the selection of l-AAs or d-AAs, namely, an l- or d-world on Earth, the origin of ee achieved by an asymmetrical event is of crucial importance. Even if the early enantiomeric excess (ee) is very low, it can be enhanced through recrystallizations and polymerizations. (1) This AA homochirality is thought to have originated in the early stage of chemical evolution when the AAs were synthesized nonbiologically. However, l-AAs are almost exclusively used in living systems. Their enantiomeric forms, l- and d-AAs, are equally produced by nonbiological chemical syntheses. This study shows that the homochirality of amino acids is produced at the aminonitrile precursors stage.Īmino acids (AAs) are the fundamental building blocks of proteins in living organisms on Earth, and AAs, with the only exception of glycine, have a chiral center. We show that only the aminonitrile precursors are characterized by positive ee in the Lyα region, explaining why right-handed circularly polarized Lyα (R-CP-Lyα) induces homologous l-amino acids. We evaluated their photolysis-induced ee in the range 5–11 eV including the Lyman alpha emission line (Lyα), the typical intensive 10.2 eV radiation ascribed to the early phase of galactic evolution. To address this issue, we resort to accurate ab initio calculations for amino acids and their precursors in the Strecker synthesis. 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 α-H amino acids, e.g., alanine.
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