Monday, July 08, 2013

When left is right and right is wrong.

If you smell spearmint and caraway seeds they smell different and you can easily pick one from the other. Amazingly the molecules responsible for the two aromas have the same structure, but they are mirror images of each other. This feature in chemistry is known as chirality or handedness.

Every amino acid, in every protein of your body has a left-handed twist to it. Infact every life form that ever existed on earth is left handed. If any one does find a right handed amino acid, you can be assured that it’s come from outer space. In 1849 Louis Pasteur was asked by a winery to investigate tartaric acid, a harmless waste from wine production. Grape seeds and yeast decomposes into tartaric acid and collects as crystals in the dregs of wine kegs. Tartaric acid born from yeast has a curious property, dissolve it in water and shine vertical slit of light through it, the beam will twist clockwise. Industrial, human made, tartaric acid has no such properties. Both the tartaric acids behave identically in reactions and the composition is the same. Louis Pasteur was determined to find out why. Pasteur examined the crystals of tartaric acid under a magnifying glass and found that crystals from yeast all twisted in one direction, while the industrial one had both right and left handed crystals. So Pasteur separated the left and right handed crystals with tweezers. He then dissolved them in water separately and passed a vertical slit of light. The yeast like crystals rotated the light clockwise while the mirror image rotated them anti clockwise. Pasteur had shown that there are two identical but mirror image types of tartaric acid. Chirality crops up in drug synthesis. When Ibuprofen is made, it typically consists of left and right handed versions (racemic mixture). Ideally, one of these will do its intended job and the other will harmlessly be eliminated from the body. But that is not what happened in 1950’s when a German drug company was marketing a remedy for morning sickness in pregnant women. The scientist could not separate the wrong handed form and the left-handed version created birth defects that included children born without legs or arms or their hands and legs stitched like turtle flippers to their trunks. The drug was thalidomide and became the most notorious drug of the 20th Century. Today though drug companies are paying a significant amount of attention to the different effects of chiral versions of the drugs they produce. Thalidomide though is making a comeback to treat diseases like leprosy and as an anti-cancer agent. By the way, the lovely aroma of lemons and oranges too is because of chirality.

1 comment:

  1. Aarina10:23 am

    Wow! Felt like I'm attending my Medicinal Chemistry lecture after all these years! Truly, enjoyed reading it!