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What is chirality/optical isomerism?

Chirality (or optical isomerism) is a physical property of a molecule which has a non-superimposable mirror image. In other words, a molecule which, when reflected cannot be rotated in any way to make the original molecule. 

The simplest example of this is human hands, they are mirror images but cannot be superimposed upon one another. Another example is the letter R. You can prove this to yourself by looking at your hands side by side with both palms facing towards you and try to superimpose them. You'll soon see that no matter how much you rotate them, they will not fit on top of one another.

The most common optical isomers are 'asymmetric' carbons, those with 4 different groups bonded to them (e.g. (NH2)HC(OH)(COOH) ), however it can be found in coordination complexes and even larger organic molecules. Optical isomers are indistinguishable in their chemical properties as they take part in the same reactions with achiral reagents, however if polarised light is passed through a solution of a chiral molecule, it will be rotated slightly. Passing polarised light through this molecule's optical isomer will rotate the light the same amount in the opposite direction.

Another key difference of two optical isomers is how they interact with biological systems, most notably enzymes (which are themselves chiral). Often, only one optical isomer will take part in reactions catalysed by enzymes and the other will not react at all (due to the importance of shape in enzyme active sites). A perfect example of this is the molecule carvone, one isomer of which smells like spearmint and the other of caraway seeds. The difference in smell is due to the different interaction with receptors in the nose. 

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