A Sharma, Enantiomeric excess of amino acids in interstellar ice analogues-asymmetric photolysis of precursors by circularly polarized UV light, Monthly Notices of the Royal Astronomical Society, Volume 517, Issue 4, December 2022, Pages 6112–6120, https://doi.org/10.1093/mnras/stac2718
Based on the results from UV-irradiated interstellar ice analogues, we describe a two-step phenomenological theory for enantiomer-selective, asymmetric formation of molecules like amino acid precursors: (1) UV photolysis of interstellar ices to molecular fragments/radicals and formation of racemic population of chiral molecules and (2) asymmetric photolysis of precursors by UV-circularly polarized light (UV-CPL), resulting in preferred growth of one enantiomer. Independent of specifics of step (1), when ice-analogue is irradiated with UV-CPL, enantiomeric excess of amino acids following hydrolysis is limited to |EEice(max)|=|(σ+ − σ−)/(σ+ + σ−)|, 0.5 times the anisotropy factor (g) for amino acid precursors (σ+/− is molecular cross-section for absorption of right/left UV-CPL light in cm2). For amino acids, the calculated EEice(max) ∼ 1 per cent agrees with experiments. Over small duration, EE is given by, |EEice(t)|=|(I+ − I−)(σ+ − σ−)|t/4, (I+/− is photon flux of right/left UV-CPL cm−2 s−1) which agrees with the observed dependence of EE on time-integrated flux of UV-CPL light. For ice irradiated with linearly polarized/unpolarized light followed by irradiation of residue with UV-CPL, |EEres(t)|=|tanh[(I+ − I−)(σ+ − σ−)t/2]| and can be much larger than EEice(max) for ice alone. When both ice and residue are irradiated with UV-CPL, EE = EEice + EEres and depends on the durations of irradiation for ice and residue. Similar analysis by assuming asymmetric photosynthesis of precursors as the underlying mechanism does not agree with experimental observations. 3,4-Ethylenedioxythiophene
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