Abstract
One of the most attractive hypothesis for the origin of homochirality in terrestrial bioorganic compounds is that a kind of “chiral impulse” as an asymmetric excitation source induced asymmetric reactions on the surfaces of such materials such as meteorites or interstellar dusts prior to the existence of terrestrial life (Cosmic Scenario). To experimentally introduce chiral structure into racemic films of amino acids (alanine, phenylalanine, isovaline, etc.), we irradiated them with linearly polarized light (LPL) from synchrotron radiation and circularly polarized light (CPL) from a free electron laser. After the irradiation, we evaluated optical anisotropy by measuring the circular dichroism (CD) spectra and verified that new Cotton peaks appeared at almost the same peak position as those of the corresponding non-racemic amino acid films. With LPL irradiation, two-dimensional anisotropic structure expressed as linear dichroism and/or linear birefringence was introduced into the racemic films. With CPL irradiation, the signs of the Cotton peaks exhibit symmetrical structure corresponding to the direction of CPL rotation. This indicates that some kinds of chiral structure were introduced into the racemic film. The CD spectra after CPL irradiation suggest the chiral structure should be derived from not only preferential photolysis but also from photolysis-induced molecular structural change. These results suggest that circularly polarized light sources in space could be associated with the origin of terrestrial homochirality; that is, they would be effective asymmetric exciting sources introducing chiral structures into bio-organic molecules or complex organic compounds.
Highlights
The origin of homochirality in terrestrial bioorganic compounds (L-amino acid, D-sugar dominant) is one of the most mysterious issues that remain unresolved in the study of chemical evolution prior to the origin of terrestrial life
To introduce optical asymmetry into the racemic film deposited from DL-amino acids, we irradiated them with linearly polarized light (LPL) introduced from synchrotron radiation (SR) from the normal-conducting accelerator ring (NAR) of SR facilities of Nippon Telegraph and Telephone Co. (NTT), Atsugi, Japan [19], and with circularly polarized light (CPL) introduced from the free electron laser (FEL) of the UVSOR-II ring of Institute for Molecular Science (IMS), Okazaki, Japan [20]
When we used the aromatic amino acids phenylalanine and tryptophan as the starting materials, two-dimensional anisotropic structure expressed as linear dichroism (LD) and/or linear birefringence (LB) was introduced into the racemic films by LPL irradiation
Summary
The origin of homochirality in terrestrial bioorganic compounds (L-amino acid, D-sugar dominant) is one of the most mysterious issues that remain unresolved in the study of chemical evolution prior to the origin of terrestrial life. Because bioorganic compounds synthesized in abiotic circumstances are intrinsically racemic mixtures of equal amounts of L- and D-bodies, it is hypothesized that chiral products originated from asymmetric chemical reactions induced by a “chiral impulse”. These types of asymmetric reactions could have possibly been derived from physically asymmetric excitation sources in space and the chiral products would have been transported to primitive Earth resulting in terrestrial homochirality (Cosmic Scenario) [1,2,3]. The polarized electrons are derived from parity-violating radioactive β-decay involving weak nuclear interaction (
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