Receptors in your body are highly specific for shape of molecules.

I simple way to imagine this is to think of the two isomers of thalidomide as a left and right hand, they look almost identical (they are non-sumperimposable mirror images). Now think of your bodies receptors as a glove, and imagine that your body only has receptors the are right hand gloves so only the "right-hand" molecule will fit into it and not the "left-hand" molecule.

thanks that explains alot, but i have another question, do any of you know how you would go about synthesising only one of the isomers or isolating one from a racemix mixture, and whats the 'chiral stationary phase' you mentioned, sounds interesting.

Hmm can not seem to find anything that properly explains it,  would anyone explain or give me give me a link?

thanks that explains alot, but i have another question, do any of you know how you would go about synthesising only one of the isomers or isolating one from a racemix mixture, and whats the 'chiral stationary phase' you mentioned, sounds interesting.

Asymmetric synthesis, the synthesis of only one enantiomer of a compound, is no easy task.  Chemists take a variety of approaches in asymmetic synthesis.  Probably the easiest method is to begin the synthesis with a chiral starting material, usually isolated from a biological system.  Other times, chiral reagents, solvents, or catalysts are introduced in order to induce asymmetric synthesis.  For example, one can links an chiral reagent (known as an auxilliary) to the target compound, the sterics of that chiral reagent may inhibit the pathway leading to one enantiomer but will not affect the pathway leading to the desired enantiomer.  The best chiral catalysts are, of course, biological enzymes, but enzymes aren't useful to most chemists because they are very sensitive to conditions such as pH, solvent, ionic strenght, temp, etc., and may become denatured in the conditions used to run most organic reactions, they are relatively difficult to produce and isolate, and they are limmited to those discovered by biologists (since we do not yet know how to design enzymes de novo).

All these approaches require chiral reagents at one point or another, so isolating chiral materials becomes important as well.  By far the best way to isolate chiral materials is to isolate them from biological systems.  Organisms produce molecules with very high stereospecificity.  For example, all naturally-occuring sugars (with the exception of L-fucose) are in the D configuration and all amino acids (with the exception of glycine [achiral]) are in the L-configuration.

If the reagent needed cannot be isolated from a biological system, it must be separated from a racemic mixture.  Pasteur first did this by crystalizing tartaric acid and separating crystals of (+)-tartaric acid and (-)-tartaric acid.  However, because this is a very painstaking task, chemists will generally use other methods.  For example, while enantiomers may have the same physical properties, diastereomers will exhibit different physical properties. So, if the enantomers can be converted to diastereomers (often times by forming a diastereomeric salt in a reaction with a chiral acid/base), they can be separated on the basis of physical properties.  You could also separarate enantiomers through a method known as kinetic resolution, in which one enantiomer is selectively reacted away while the other remains unreacted.  Enantiomers can also be separated by chromatography (e.g. GC, HPLC) using a chrial stationary phase (usually a derivative of chiral amino acids or carbohydrates).  Since the stationary phase is itself chiral, enantiomers will have different affinities for the stationary phase and will therefore be separated by the chromatography.

under physiological conditions of the body the two isomeric forms of thalidomide are interconvertable

Yes, I believe this is true from what I have read.

This is probably a chemical myth then, that had the "correct" enentiomer been synthesised that a medical disaster may have been avoided. Maybe we should start a chemical "mythbusters" thread.