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Examples of levo drugs include levothyroxine, levocitrizine, and levodopa. Is there any specific reason why the receptors in our body exhibit this stereoisomerism and hold a high preference for the levo isomer form of any drugs?

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  • $\begingroup$ I can't answer this definitively, but I believe it's because the amino acids that make up the normal binding partner for the receptors are levo. So the parts fit together better and easier than trying to me dextro versions. It would be interesting to know if the HTS and Fragment libraries used in modern drug discovery even include D- rotated molecules for this reason. $\endgroup$ – Nathan Sep 30 '15 at 14:46
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This is purely coincidental. The term levo simply means the direction that the pure enantiomer of the compound rotates plane polarized light at a specific wavelength and has no direct bearing on the interactions with biological systems. A quick search of the Dictionary of Drugs database gave 6845 compounds with optical rotation >0 (dextro-compounds) and 8406 compounds with optical rotation <0 (levo-compounds). So there are plenty of chiral drugs that rotate plane polarized light each direction.

The inclusion of 'levo' (or 'dextro' for that matter) into the name of the compound was done to distinguish enantiomers of chiral compounds. This naming convention as fallen out of favor due to the improved ability of chemists to know the absolute stereochemistry (e.g., R/S) and distinguish enantiomers based on their structure.

Dextroamphetamine (Adderall) and dextromethorphan (Robitussin) are a couple of examples of drugs with 'dextro' in their names.

With respect to the stereochemical nature of HTS and fragment libraries, most compounds are achiral (do not rotate plane polarized light). I'm not aware of any library biased toward optical rotation, and I'm not aware of any chemist making any selection of compound based solely on an optical rotation measurement. In fact, chiral compounds are often studied first as the racemate. If there is activity, the individual enantiomers are tested.

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