Asymmetric photoreactions as the origin of biomolecular homochirality: a critical review.

The role of asymmetric photoreactions (occurring in space or on the primitive Earth) in the origin of biomolecular homochirality is critically reviewed. A general description of the various possible ways for light to interact with chiral molecules is first presented on the basis of a series expansion of the dielectric constant: natural, magnetic and magnetochiral circular dichroism are identified with the first three terms in this development. Natural and magnetochiral circular dichroism may cause, through asymmetric photolysis, an enantiomeric excess in a racemic mixture of chiral molecules irradiated, respectively, by circularly polarized ultraviolet light, or by unpolarized ultraviolet light in the presence of a magnetic field non-perpendicular to the light beam. Terrestrial and extraterrestrial sites matching these conditions are then critically reviewed. Finally, we stress the possibility to arrive at the homochirality of amino acids through a path involving D-ribose during RNA world as an alternative to the usual scenarios operating directly on amino acids.