The origin of the loss of mirror symmetry between enantiomers

Y. Paltiel, D. Goldberg, N. Yuran, S. Yochelis, J.H. Soh, C. Seibel, J. Gauss, S. Zilberg, S.F. Ozturk, J. Fransson, A.I. Krylov, and R. Naaman
Science Advances , submitted (2025)

Two fundamental questions have puzzled scientists for more than 150 years. The first is, "how did life become homochiral?" The second, related question is, "why was this specific handedness selected?" By homochiral, we mean that all living organisms contain chiral molecules, i.e., molecules that exist in two forms (enantiomers), like left and right hands. However, in life on Earth, we predominantly find only one form: the L-form for amino acids, and the D-form for sugars and nucleic acids such as RNA and DNA. Some models have been proposed to explain the development of homochirality during evolution. Recently, it has been shown that homochirality could have emerged through the enantioselective interactions of molecules with magnetic substrates due to the asymmetric crystallization of an RNA precursor on a magnetite substrate, abundant on early Earth. This phenomenon is based on the chirality-induced spin selectivity (CISS) effect. Despite its robustness, this model, like some other previously proposed ones, could not provide an answer to the second question: why one specific handedness (D for RNA) was selected. This difficulty stemmed from the assumption that two mirror-imaged forms of a chiral molecule are perfectly identical. Here we demonstrate that spin-involving process can have different outcomes in the two enantiomers of chiral molecules. In chiral molecules with unpaired electrons or while electron is passing through them, the total angular momentum vector which is the sum of the orbital and spin angular momentum, J, is aligned along the "easy axis", which is defined by the magnetic anisotropy induced by the spin-orbit coupling and asymmetry of the molecular field. The magnitude J is the same for both enantiomers, but the vectors may be aligned differently relative to the molecular frame in the two enantiomers. This difference can be quantified by, for example, by the angle between J and electric dipole moment of the molecule, mu. This is what causes the difference in spin-polarization that can be observed between the enantiomers for a specific current direction. Different alignment of J can also result in different interactions with other chiral objects (fields, light, nuclear vibrations, polarized electrons, etc.), which can result in different outcomes for the two enantiomers We show by direct measurements, theory, and ab initio calculations that dynamic spin processes in chiral molecules could result in different efficiencies of spin-related phenomena, including the interaction of chiral molecules with magnetic surfaces. The findings may lead to new venue for control enantiospecific interactions, applying electrons’ spin polarization, and may provide an explanation for the specific homochirality in nature.

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