Significant\nChiral Asymmetry Observed in Neutral Amino\nAcid Ultraviolet Photolysis

The\norigin of homochirality in biological organisms remains an\nopen question. Some suggest that its origin might be extraterrestrial,\nspecifically due to the exposure of chiral molecules to circularly\npolarized photons in interstellar space, which could cause an initial\npopulation imbalance leading to the homochirality observed today.\nHowever, this extraterrestrial hypothesis has not been widely accepted,\nlargely due to the belief that molecular optical rotatory dispersion\nis too insignificant to create the substantial imbalance required\nfor homochirality. Here we report experimental evidence that specific\nconformers of neutral amino acids exhibit significant asymmetry in\nthe chiral destroying dissociation rate induced by circularly polarized\nphotons. The observed anisotropy factor for the lowest energy conformer\nof leucine was remarkably large, reaching 0.1a factor of 13\ntimes larger than observed for zwitterionic leucine in solid films,\nand nearly 40 times greater than the anisotropy reported in the electronic\nabsorption spectrum of gas-phase leucine ensembles at room temperature.\nThis significant finding indicates that even if reported anisotropy\nvalues in the electronic absorption spectrum are low, the dissociation\nasymmetry of certain conformers can still be substantial. An anisotropy\nfactor of 0.1 could result in an initial enantiomeric excess exceeding\n10%, even with a 90% extent of reaction. This discovery suggests that\nasymmetric photodissociation of amino acids may have been a crucial\nfactor in the emergence of biological homochirality.