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February 2001
Vol. 10, No. 02, pp.50–58.
Focus: Separation Sciences

FEATURE

Amino Acid Racemization

Chiral Origins of Life
In the Beginning…
Geert Rikkens and his colleagues have introduced an artificial enantiomeric bias into a solution of Cr(III)tris-oxalato by introducing a magnetic field (B) that is parallel (
two up arrows) to a beam of unpolarized light (bottom). If the field is antiparallel (up arrowdown arrow) to the beam, the solution stays racemic. In doing so, they may have mimicked the earliest moments of nature’s enantiomeric choice.
Biochemical systems are remarkable in their ability to distinguish between enantiomers. For life emerging from the primordial soup, it might seem that the choice of using only
L-amino acids and D-sugars was perhaps arbitrary. New findings from Geert Rikken and colleagues at the Grenoble High Magnetic Field Laboratory/CNRS (France) may indicate that the basis for homochirality might not have been simply a roll of the dice (Rikken, G.L.J.A.; Raupach, E. Nature 2000, 405, 932–935). The combination of light and magnetic fields may be the factors that tipped the balance. Sunlight and the Earth’s magnetic field could have provided these factors.

Their research is based on Faraday’s 1846 observation that a magnetic field is capable of rotating plane-polarized light. This led Pasteur and many others over the past 150 years to try to exploit magnetism to induce chirality in crystals and molecules. The next major advance came with the relatively recent discovery that a solution of an enantiomeric compound placed in a magnetic field absorbed light differently, depending on the orientation of the light within the magnetic field (see figure). Extending this finding, Rikken’s research demonstrated for the first time that a chiral compound can give rise to an enantiomeric excess when subjected to unpolarized light and a parallel magnetic field.

“The underlying basis of the enantioselectivity is the optical effect called magnetochiral anisotropy,” says Rikken. “The sign of this effect changes with the handedness of the molecules. In general, this will result in enantiomeric excess.”

The compound that they examined was Cr(III)tris-oxalato, because it is a chiral complex for which the photochemistry is well-defined. The compound is unstable, spontaneously dissociating and associating into right- and left-handed forms. Depending on the orientations of the light and the magnetic field, one enantiomer will absorb more light than the other, destabilizing it, and lead to a preference for the lower-absorbing enantiomer. Rikken speculates that this process could bias the synthesis of organic compounds and that life may have been founded on preferences for certain stereochemistry.

He explains how these studies of Cr(III)tris-oxalato may be extrapolated to include other chiral compounds. Norden first showed photodestruction or photoresolution of amino acids by circularly polarized light in 1977 (Norden, B. Nature 1977, 266, 567–568). Therefore, unpolarized light in a magnetic field must also produce enantiomeric excess. He postulated at that time that this might be a reason for optically active life.

Back to: Amino Acid Racemization