November 12, 2007

The Chiral Fallacy

diagrammatic representation of chiral moleculesDo you wear your left gloves on your right hand? I know you don't. It is a fact that though the gloves are almost identical they are not interchangeable. One is the mirror image (copy) of the other. So, wearing your left gloves on your right hand is wrong (isn't left always wrong?).

When a molecule docks on another, to bind, it is said that the molecule has an affinity for it. It may (or may not) elicit a response (efficacy), as a result of this combination. But for this molecule in question; be it a hormone, an enzyme or a medicine, to combine with its consort (receptor/substrate), the molecule needs to embark on the substrate first. In a three dimensional world (leaving aside time dimension for a moment), the molecule (ligand/enzyme) has to use at least 3 points (bonds) to anchor. As shown in the figure, the pyramidal looking molecules display mirror image symmetry. Think of the black balls as indicating depth; the other three balls rest on the substrate molecule like a tripod, in a condition called 3-point attachment.

Now imagine that you are holding the black ball and placing the 'planar tripod' on three imaginary holes (of complementary colors), which correspond spatially to the three projections. No problem. You try to repeat the same thing with the other pyramid, you can't get your jigsaw done! This occurs due to the presence of an asymmetric carbon atom.

This is because though the molecules have the same molecular structures their orientations are different. This is known as isomerism, a phenomenon akin to the gloves analogy. Isomerism has immense importance in human physiology, biochemistry and pharmacology.
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