November 19, 2007

Chiral Delight

arrangement of rings to exemplify chiral structuresThe word Chiral comes from the Greek word meaning hand ('Cheiro' the great palmist also got his name from the Greek word). Like those molecules described in Chiral Fallacy, many of our biomolecules in physiology remain in either of these two states. If you were to shine a plane-polarized light through a liquid containing this molecule, the molecules would rotate the beam of light either in a right or left direction.

Just a DIY. Take a polarizer (You may get one from a camera shop or you may get one from your mobile phone LCD display!). Now hold one in a fixed position while you rotate the other over it and watch the translucency change. This occurs due to obliteration of the wave functions of light, as the electromagnetic wave gets chopped off by the two consecutive polarizers. If you place a bottle of water between these polarizers, the light will remain where it was. Replace water with dextrose, a sugar, you will find that the beam now rotates to the right!

Substances which rotate plane-polarized light to the right are termed (+) whereas those rotating towards the left are termed (-). The Cahn-Ingold-Prelog system is commonly used to describe chiral molecules as R (rectus) or S (sinister). An equimolar mixture of both will not rotate the beam. Such a mixture is called racemic mixture.

The thyroid gland, for example, secretes the L (levo) isomer of thyroxine. The D (dextro) enantiomer has little activity but has cholesterol lowering activity instead. Chiral drugs like levocetirizine (used in allergy), S-amlodipine (a calcium channel blocker cardiovascular drug used in hypertension), esomeprazole (a drug for peptic ulcer) or levofloxacin (an antibiotic) are some popular examples.

New drug developments will target this phenomenon of optical isomerism and exploit eutomers (active enantiomers are called eutomers and inactive counterparts are distomers) for more specific effect and less adverse reactions.
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