Photochirogenesis

Is it reasonable to wonder if a collection of 'chiral photons' -sometimes loosely referred to as 'chiral light' - is capable of inducing enantioenrichments into prochiral or racemic molecules? Or, in other words, can one imagine a migration of chiral information from pure and mass-less electromagnetic radiation to matter?

In theory, the effect of circularly polarized light represents indeed a "true-chiral" influence on racemates of organic molecules (MacDermott 1993; Barron 1994a, 1994b) and hence photochemical reactions applying circularly polarized light have been examined extensively in many systems in the laboratory.

Reactions that induce an enantiomeric enhancement in prochiral or racemic systems with circularly polarized light are often called absolute asymmetric, as there is no net chirality in the molecular reactants. This discriminates asymmetric photochemistry from ordinary photochemistry of chiral molecules, which can simply be induced by non-polarized light (Rau 2004). Absolute asymmetric photochemical reactions induce enantiomeric enhancement by an absorption process. This requires that two different circularly polarized light absorbing ground state species, i.e., enantiomers or diastereoisomers, are present. Here, we will concentrate on enan-tiomers. Treatment of photoreactions of diastereoisomers will not be covered, because (a) non-polarized light is sufficient to modify the optical activity of a sample and (b) such reactions are not of relevance for the origin of biomolecular asymmetry on Earth.

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