Chirality
pronunciation: [kiyralitee]



Asymmetry resulting in an object not being superimposable upon its mirror image. chemical chirality is usually associated with a carbon atom having four different substituents. Two isomers which are mirror images of one another are called enantiomers; . Towards symmetrical environments, enantiomers behave identically, but towards other chiral molecules they are different. Thus they will have identical melting points, but may have different tastes and smells.

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Here is what Louis Pasteur (1822 - 1895), wrote to a friend in 1851:

" The molecular structures of the two tartaric acids are asymmetric, and on the other hand, they are rigorously the same, with the sole difference of showing asymmetry in opposite senses. Are the atoms of the right acid grouped on the spirals of a right-handed helix, or placed on the solid angles of an irregular tetrahedron, or disposed according to some particular asymmetric grouping or other? We cannot answer these questions.''

Perhaps you would understand this quotation better if you knew what Pasteur was investigating. when we shine light through a polarizer, most of the light is blocked off. The only light which gets through is light which is vibrating in the right direction. If we put another polarizer in the way, at right angles to the first, no light gets through at all. (If this is new information, stop here, and refer to polarized light, then come back here to continue reading.)

If there is a suitable chemical between the two filters, one which twists the light, the chemical will twist the light around so that some of it can get through the second filter. Tartaric acid in solution makes light rotate, or twist, but Louis Pasteur had a problem. The absolutely chemically identical racemic acid did nothing of the sort when it was dissolved and tested. It had no effect on the light at all. when he examined the tartaric acid, Pasteur found asymmetrical crystals, with all of the crystals having the same asymmetry. Racemic acid was different: half of the crystals looked just like tartaric acid, but the other half were the exact opposite. They were mirror images of the tartaric acid crystals.

Pasteur separated out the ''tartaric'' crystals from the racemic acid, and sure enough, a solution of these crystals rotated the light, just like tartaric acid, while the crystals left behind in his fossicking also rotated light, but in the opposite direction. So Pasteur shouted the French equivalent of ''Ringol'' and rushed off to write to an old friend about his marvellous discovery. Since that time, we have learned about many other chemicals which can turn up in two different forms: most of them are carban-containing, so we should not be surprised that the chemicals in our bodies are often of one handedness or the other.

Written by Peter Macinnis

©WebsterWorld Pty Ltd/contributors 2002


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