Enantiomeric excess of amino acids in interstellar ice analogues-asymmetric photolysis of precursors by circularly polarized UV light

Abstract

ABSTRACT 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.