Abstract
M. H. Engel and S. A. Macko1 tentatively ascribe the enantiomeric excess of L-amino acids in extraterrestrial sources to the circular polarization of synchrotron radiation, from a neutron star, incident on the interstellar molecular cloud from which the Solar System formed (see also the accompanying News and Views article by C. F. Chyba2). But the Kuhn-Condon zero-sum rule3,4 for the rotational strengths of a chiral molecule requires that broad-band circularly polarized radiation cannot discriminate between the enantiomers of a racemic substance in photochemical reactions.
Main
Kuhn first demonstrated photochemical optical resolution by showing that monochromatic circularly polarized light of a given handedness, tuned to the frequency of a specific circular dichroism absorption of an enantiomer, preferentially photolysed that enantiomer in a racemic mixture3. The mirror-image enantiomer was favoured if the radiation was tuned to a circular dichroism absorption of opposite sign, under the same conditions. Kuhn used coupled-oscillator theory to account for the observation that the circular dichroism bands of an enantiomer alternate in sign along the wavelength ordinate, and so cancel out over its electronic spectrum. Therefore the optical rotatory power of a given enantiomer, and its susceptibility to differential photochemical change with circular radiation, sum to zero over the electromagnetic spectrum as a whole3. Kuhn's classical result was confirmed by Condon4, who demonstrated quantum-mechanically that an enantiomer's rotational strengths (measured by the circular dichroism band areas) sum to zero over the spectrum.
The zero-sum rule does not exclude a photochemical origin for biomolecular homochirality under severely restrictive initial conditions, covering time, place, radiation filters, and so on, for example by a prebiotic pool of racemic amino acids on an east-facing slope exposed to solar radiation only at dawn on the early Earth5. But such suggestions are essentially ad hoc.
References
Engel, M. H. & Macko, S. A. Nature 389, 265–268 (1997).
Chyba, C. F. Nature 389, 234–235 (1997).
Kuhn, W. Trans. Faraday Soc. 26, 293–310 (1930).
Condon, E. U. Rev. Mod. Phys. 9, 432–457 (1937).
Mason, S. F. Chem. Soc. Rev. 17, 347–359 (1988).
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Mason, S. Extraterrestrial handedness. Nature 389, 804 (1997). https://doi.org/10.1038/39777
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DOI: https://doi.org/10.1038/39777
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