Physicists and chemists are familiar with the concept of chirality or "handedness". The influence of chirality is also felt in biology, where scientists have long sought to discover the reason why the amino acids that make up the proteins in the human body are all left-handed. A new clue to the "homochirality" of life has now been offered by researchers in Grenoble.
Following a prediction in 1982, the team were the first to demonstrate, in 1997, the phenomenon of magnetochiral dichroism, whereby light is absorbed differently by a solution of chiral molecules according to whether the light beam travels parallel or antiparallel with an external magnetic field. The difference in absorption is independent of the polarization of the light and works even with unpolarized light. The effect represents a subtle interplay of the natural optical activity of chiral molecules and the induced magnetic optical activity.
Now the researchers have exploited their discovery to bias a chemical process in favour of one of two mirror-image products (left- or right-handed molecules, called enantiomers). They studied a complex chiral molecule that spontaneously disassociates and re-associates in solution: at equilibrium there are always equal amounts of left-handed and right-handed molecules. If the sample is illuminated, the rate of disassociation increases. Unpolarized laser light travelling parallel with an applied magnetic field produced and maintained an excess of one enantiomer in the solution. Reversing the magnetic field direction produced an equal concentration of the mirror-image enantiomer.
Now the definition of chirality goes beyond the original static mirror-image idea of Kelvin to allow for motion-dependent effects. Magnetochirality is therefore akin to polarized light and the electroweak interaction in its ability to select a certain handedness, and it thus merits consideration as an explanation for homochirality. Nature