Most chemicals that have an important role in biology exist as a pair of almost identical twins. These twins, called chiral enantiomers, contain exactly the same atoms, but they appear as mirror images that do not fit on top of each other, like placing your left hand on top of your right hand. Such a trivial difference can, in fact, have a profound effect in living systems; for instance, one form of the common sweetener asparagine is strongly sweet, while its mirror-image form is bitter.
For this reason Nature has evolved ways of selecting which mirror-image version of these chemicals it designs and uses, and the 2001 Nobel Prize in Chemistry rewarded two ways in which chemists finally got to grips with this process. Creating chiral compounds in the laboratory always produced an equal mixture of left- and right-handed molecules, until William Knowles first showed how to tailor the outcome of these reactions. Focusing on a reaction called hydrogenation, in which hydrogen atoms are added to carbon atoms, Knowles discovered that it was possible to use transition metal elements to make a new type of catalyst to assist the reaction that was shaped to suit one of the mirror-image forms; in the way that a left-handed glove is better suited to fit a left hand. Though this only produced a modest excess of one chiral enantiomer in practice, this breakthrough allowed Knowles to develop a similar catalyst that could create L-DOPA, a chiral molecule that became an important treatment for Parkinson’s disease. Ryoji Noyori extended Knowles’ achievements by creating several hydrogen-adding chiral catalysts. Noyori’s catalysts work on a wide range of molecules, many of them important products in the pharmaceutical industry, producing high yields of one mirror-image product.
Barry Sharpless developed chiral catalysts that select the outcome of another class of reaction; one that adds oxygen onto carbon molecules. This oxidation reaction is extremely important for chemists as its products can be easily modified further to create the type of chemical structures found in a wide range of materials and drugs. One of these reactions in particular, which allows the oxidation of particular group of carbon atoms attached to an alcohol group, known as allylic alcohols, is seen by many chemists as one of the most powerful and versatile tools they have to create chemicals in the laboratory.
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Chiral objects, like left and right hands, cannot be superimposed on its mirror image. Find other ‘chiral’ objects in this game.