Chiral Fields Light Magnetism and Chirality

In this chapter we will ask the inspiring question: "Can the asymmetry of a chiral field - such as a specifically oriented magnetic field, radiation field, or the combination of both - be transferred to racemic or prochiral organic molecules in a manner that an enantiomeric enhancement is induced, that is to say, transferred from the mass-less chiral field into the organic molecule?" And indeed, the answer to this question is "yes". In order to understand systematically such phenomena, we will discriminate between (a) the combination of a magnetic field with unpolarized light, known as magneto-optical effects and (b) circularly polarized light as a chiral field to induce enantiomeric enhancement by a process often called photochirogenesis.

To date, four absolute asymmetric photochemical pathways for the generation of the enantiomeric enrichment have been postulated (Fig. 6.1). In this chapter, each

Barron 1987, 1994b Kuhn & Braun 1929 Inoue et al. 1996 Moradpour et al. 1971

Rikken & Raupach Balavoine et al. 1974 Bernstein et al. 1972,1973

1998, 2000 Inoue & Ramamurthy 2004 Nuevo et al. 2006 Meierhenrich et al. 2005b

Barron 1987, 1994b Kuhn & Braun 1929 Inoue et al. 1996 Moradpour et al. 1971

Rikken & Raupach Balavoine et al. 1974 Bernstein et al. 1972,1973

1998, 2000 Inoue & Ramamurthy 2004 Nuevo et al. 2006 Meierhenrich et al. 2005b

Amplification of the obtained enantiomeric enhancement

Kondepudi & Nelson 1985 Fenwick & Kagan 1999 Soai et al. 2000

Fig. 6.1 Chiral fields as vehicles for asymmetry: four pathways to induce an enantiomeric enhancement via absolute asymmetric photochemistry. We distinguish between magneto-optical effects and pure photochirogenesis. Circularly polarized light is a true chiral field and offers possibilities to induce the e.e. via well-defined processes called photolysis, isomerization, and synthesis

U. Meierhenrich, Amino Acids and the Asymmetry of Life. Advances in Astrobiology 103

and Biogeophysics, © Springer-Verlag Berlin Heidelberg 2008

of them will be explained and discussed individually. I would like to mention that new concepts and recent results on absolute asymmetric reactions induced by photochemical processes appear to be particularly encouraging, and modern theories suggest that complex organic molecules could evolve in the ice mantles of dust grains in interstellar and circumstellar space, where magnetic fields and electromagnetic radiation are abundant. Consequently, we will take a close look at this aspect.

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