Magnetic circular dichroism in Archean atmosphere and asymmetric photolysis of biomolecules: enantiomeric excess of preb
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Magnetic circular dichroism in Archean atmosphere and asymmetric photolysis of biomolecules: enantiomeric excess of prebiotic sugar A. Sharma 1 Received: 10 March 2020 / Accepted: 21 June 2020/ # Springer Nature B.V. 2020
Abstract
In the terrestrial dipolar magnetic field, magnetic circular dichroism (MCD) of UV sunlight by paramagnetic O2 in an Archean atmosphere (mostly CO2 and N2) results in circular polarization anisotropy (~ 10−10). This is used to calculate enantiomeric excess (EE~10−13) of glyceraldehyde (3-carbon sugar) with a model that includes racemic production and asymmetric photolysis of its enantiomers. The sign and magnitude of enantiomeric excess (EE) vary with the Earth’s latitude. Unlike random noise fluctuation in spontaneous mirror symmetry breaking (SMSB) models, the sign of EE is deterministic and constant over large areas of prebiotic Earth. The magnitude is several orders greater than the mean amplitude of stochastically fluctuating EE. MCD could provide the initial EE for growth of homochirality by asymmetric autocatalysis. Keywords Homochirality . Enantiomeric excess . Prebiotic sugar . Magnetic circular dichroism . Asymmetric photolysis . Archean atmosphere . Spontaneous mirror symmetry breaking . Asymmetric autocatalysis
1 Introduction Homochirality of biomolecules remains an unsolved mystery [1, 2]. Thus, all nucleic acids have the right-handed D-ribose/deoxyribose sugar as a building block, and amino acids (except achiral glycine) are largely left (L)-handed. Starting from a racemic prebiotic state, the growth of homochirality is believed to have happened in two steps [3]: (a) appearance of a small initial enantiomeric excess (EE) of prebiotic biomolecules and (b) amplification of this EE to a state of 100% homochirality in biopolymers. Plausible scenarios have been investigated for creating the initial seed EE. One of these involves an extraterrestrial source of prebiotic organic molecules. Several investigators have reported EE of 50% and even higher for L-amino-acids [4–6] and Dderivatives of sugar [7, 8] including sugar acids and sugar alcohols in some meteorites. However, * A. Sharma [email protected]
1
Department of Physics, Alabama A&M University, Huntsville, AL 35762, USA
A. Sharma
questions remain about possible contributions from experimental artifacts and contamination [9] from the terrestrial biosphere. It is not clear how the flux of extraterrestrial amino acids and sugar derivatives compared with the rate for their racemic production on the Archean Earth itself. Promising routes for terrestrial production of prebiotic amino acids [10] as well as simple sugars [11] have been demonstrated. Alternate mechanisms to explain homochirality includes a large body of theoretical work based on spontaneous mirror symmetry breaking (SMSB) [12–18]. Here, spontaneous fluctuation of EE is amplified by asymmetric autocatalysis to near 100% homochirality [19]. Such a mechanism has been observed experimentally [20] in a reaction involving pyrimidine compounds and does not requir
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