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Download PDF A tortuous quest involving physicists, chemists and biologists that has endured for over years has finally ended with a paper by Rikken and Raupach 1 on page of this issue. They report the first unequivocal use of a static magnetic field to bias a chemical process in favour of one of two mirror-image products left- or right-handed enantiomers.
The chemistry of life is homochiral, being based almost exclusively on l-amino acids and d-sugars, and the ability of biological molecules to discriminate between enantiomers is vital for living systems. The importance of handedness in nature is such that scientists have long wondered about its origin, and the process demonstrated by Rikken and Raupach may provide a new clue. The quest began in when Faraday made the plane of polarization of a linearly polarized light beam rotate by applying a magnetic field parallel to the beam.
This discovery was of fundamental importance because it demonstrated conclusively the intimate connection between electromagnetism and light. Such natural optical activity is due to the handedness within the microstructure of the crystals and fluids, as Fresnel later showed.
The first to be misled was Pasteur, who in separated crystals of sodium ammonium tartrate into right- and left-handed forms, which gave equal and opposite natural optical rotations in solution. A new twist to the story appeared in This small difference in absorption is completely independent of the polarization state of the light beam and so should work with unpolarized light Fig.
Figure 1: Favouring a lopsided solution. Rikken and Raupach 1 have now exploited this effect to favour the production of one enantiomer, making it a serious candidate for the source of handedness in nature. Full size image Rikken and Raupach 1 have now used magnetochiral dichroism to favour the production of one enantiomer in a photochemical reaction. Their experiment uses the chiral Cr III tris-oxalato complex, which is unstable in solution and spontaneously dissociates and re-associates.
So at equilibrium there are always equal concentrations of the right- and left-handed enantiomers. This dissociation is accelerated by the absorption of light. The authors show that, in the presence of an unpolarized laser beam travelling parallel to a static magnetic field, a small excess of one enantiomer is produced and maintained, and that, on reversing the magnetic field direction, an equal concentration of the mirror-image enantiomer results.
This definition provides a rigorous statement of the fundamental symmetry characteristics that external physical fields and forces must have in order to induce absolute enantioselection in all circumstances.
Including situations where a chemical reaction has reached thermodynamic equilibrium. These are currently the most favoured explanations for the homochirality of life, and enantioselective photochemistry with circularly polarized light has already been observed experimentally.
On both experimental and theoretical grounds, we now have to seriously consider magnetochiral photochemistry in discussions of the possible origins of biological homochirality This is especially pertinent to fashionable theories suggesting that complex organic molecules could evolve in the ice mantles of dust grains in interstellar space 11 , because magnetic fields and unpolarized light are more common in the cosmos than circularly polarized light.
Furthermore, cosmic magnetic fields lead to partial orientation of the dust grains 12 , which may enhance any associated enantioselective chemistry.
Jose Luis Mateos
Cited By This article is cited by publications. DOI: Olga N. Kataeva, Kirill E. Metlushka, Zilya R.
No. 9 Germinal Cocho Gil
Complex Systems Department