Magnets help tell left from right
Jun 22, 2000
Many molecules come in two forms or "enantiomers" that are mirror images of each other. Most chemical reactions produce equal numbers of so-called right- and left-handed enantiomers. For some reason, however, the amino acids which make up the proteins in the human body are all left-handed. Understanding the reasons for this is one of the oldest challenges in biology. Now Geert Rikken and E Raupach of the Grenoble High Magnetic Field Laboratory in France have shown for the first time that the combined effect of a magnetic field and light can lead to an excess of one type enantiomer over its mirror image (G Rikken and E Raupach 2000 Nature 405 932).
Like these so-called chiral molecules, circularly polarized light also comes in right and left-handed varieties, and can be used to produce an excess of one type of molecule. However, circularly polarized light is quite rare in nature and is not thought to be the source of the "homochirality of nature". This problem greatly puzzled Louis Pasteur, who thought that magnetic fields might be responsible because they can rotate the polarization of light. And despite forceful opposition from Lord Kelvin, who first introduced the word "chirality" to science, many scientists followed Pasteur's lead.
Rikken and Raupach study a molecule called Cr(III)tris-oxalato which exists in left- and right-handed forms and is unstable in solution. They show that the combined effect of a magnetic field and a beam of unpolarized light travelling parallel to the magnetic field can lead to a build-up of one of the enantiomers. When the magnetic field is reversed, an excess of the other enantiomer is produced. The use of polarized light has no effect on the process. However, when the magnetic field and light are at right angles to each other, no excess is observed.
According to Laurence Barron of the University of Glasgow, Rikken and Raupach's work means that "a tortuous quest involving physicists, chemists and biologists that has endured for over 150 years has finally ended."