Published online 22 June 2000 | Nature | doi:10.1038/news000622-10


A handle on handedness

Why are the molecules of life not ambidextrous? The interaction of light with magnetic fields might explain why life has a mysterious twist at its core, reports Philip Ball.

The most important molecular building blocks of life -- amino acids and sugars -- come in left-handed and right-handed varieties which, like a pair of gloves, are mirror images of one another. Living organisms use only left-handed amino acids and right-handed sugars. Now there is a new theory as to why, at some point in life's origin, this choice was made.

Geert Rikken and E. Raupach of the CNRS's High Magnetic Field Laboratory in Grenoble, France, show that a magnetic field can interact with light to induce an imbalance in the handedness or 'chirality' of molecules produced in a light-driven chemical reaction.

Molecular chirality was discovered in the nineteenth century by the French scientist Louis Pasteur. He noticed that crystals of tartaric acid came in two mirror image shapes. Solutions of these rotated the plane of polarized light in opposite directions. (Light consists of wave-like electric and magnetic fields, which oscillate in a plane rather like a shaken string tied at one end to a pole.) Pasteur supposed that the plane of polarization was being altered by the interaction of the light with the molecules of tartaric acid, and that the different directions of rotations were due to a handedness inherent in the structure of the molecules.

Pasteur found that many natural molecules, such as tartaric acid, display chirality. He suggested that chirality was a property of living organisms only. We now know that this is not so, although it can be tricky to synthesize only one type of handedness in chiral products.

In the drug industry, such specificity can be crucial, since the differently handed forms (called 'enantiomers') of chiral molecules can have very different physiological effects. A harmless example is glucose: both enantiomers taste sweet, but the body can only metabolize the (natural) right-handed form. The left-handed form, which is manufactured artificially, is used as a calorie-free sweetener.

Amino acids are the raw materials of proteins -- but our bodies make these molecules from only the left-handed enantiomers. If provided with the right-handed versions, our cells ignore them, just as they ignore left-handed sugars. The protein-making machinery is like a left-hand glove, which right-handed amino acids do not fit. In principle, an entire 'alternative biology' using right-handed amino acids is possible: but life has selected the left-hand path for proteins. No one knows why.

Several explanations have been proposed. As life began amidst a primordial brew of complex chemicals, either the initial choice was arbitrary or some factor tipped the balance in favour of left-handed amino acids. People have speculated, for example, that circularly polarized sunlight, in which the plane of polarization rotates in a screw-like manner, may have created an imbalance of enantiomers in light-induced reactions on the early Earth. Sunlight has a preferential direction of circular polarization near dusk.

But there is some reason to believe that homochirality (same chirality) may have originated far from Earth on meteorites. An excess of 'left-handed' amino acids has been reported on at least one carbon-rich meteorite, suggesting that the delivery of these materials to Earth by impacts might have 'seeded' the preference.

This is one of the scenarios in which Rikken and Raupach's work is relevant. They have shown that the interaction of chiral molecules with unpolarized light (much more common in the Universe than polarized light) is affected by a magnetic field. If the light beam travels parallel to the field in one direction, one enantiomer absorbs the light slightly more strongly than the other. If the poles of the field are reversed, the other enantiomer absorbs preferentially. If absorption breaks the molecules apart, then the combination of magnetism and light can create an imbalance in an initially equal mixture of the two enantiomers.

This kind of process, occurring, say, on a meteorite or asteroid surface in the presence of the magnetic field of a star or galaxy, could tip the scales to produce an excess of one enantiomer in small, chiral organic molecules. Alternatively, perhaps sunlight and the Earth's magnetic field would be sufficient to generate the same on our planet's surface. The results suggest a new set of hypotheses about why life has a handedness. Comparing these with earlier ideas could, however, be tremendously difficult given that these are events that happened at least four billion years ago. 

  • References

    1. Rikken,G. L. J. A. & Raupach, E. Enantioselective magnetochiral photochemistry. Nature 405, 932 - 935 2000. | Article | PubMed | ISI | ChemPort |