Chirality

When used in the context of chemistry, chirality usually refers to molecules. Two mirror images of a molecule that cannot be superimposed onto each other are referred to as enantiomers or optical isomers. Because the difference between right and left hands is universally known and easy to observe, many pairs of enantiomers are designated as "right-" and "left-handed." A mixture of equal amounts of the two enantiomers is said to be a racemic mixture. Molecular chirality is of interest because of its application to stereochemistry in inorganic chemistry, organic chemistry, physical chemistry, biochemistry, and supramolecular chemistry.

The symmetry of a molecule (or any other object) determines whether it is chiral. A molecule is achiral (not chiral) if and only if it has an axis of improper rotation; that is, an n-fold rotation (rotation by 360°/n) followed by a reflection in the plane perpendicular to this axis that maps the molecule onto itself. (See chirality (mathematics).) A simplified rule applies to tetrahedrally-bonded carbon, as shown in the illustration: if all four substituents are different, the molecule is chiral. A chiral molecule is not necessarily asymmetric, that is, devoid of any symmetry elements, as it can have, for example, rotational symmetry.

In drugs

Many chiral drugs must be made with high enantiomeric purity due to potential side-effects of the other enantiomer. (The other enantiomer may also merely be inactive.)

* Thalidomide: Thalidomide is racemic. One enantiomer is effective against morning sickness, whereas the other is teratogenic. In this case, administering just one of the enantiomers to a pregnant patient does not help, as the two enantiomers are readily interconverted in vivo. Thus, if a person is given either enantiomer, both the D and L isomers will eventually be present in the patient's serum.
* Ethambutol: Whereas one enantiomer is used to treat tuberculosis, the other causes blindness.
* Naproxen: One enantiomer is used to treat arthritis pain, but the other causes liver poisoning with no analgesic effect.
* Steroid receptor sites also show stereoisomer specificity.
* Penicillin's activity is stereodependent. The antibiotic must mimic the D-alanine chains that occur in the cell walls of bacteria in order to react with and subsequently inhibit bacterial transpeptidase enzyme.
* Only L-propranolol is a powerful adrenoceptor antagonist, whereas D-propranolol is not. However, both have local anesthetic effect.
* The L-isomer of Methorphan, levomethorphan is a potent opioid analgesic, while the D-isomer, dextromethorphan is a dissociative cough suppressant.
* S(-) isomer of carvedilol, a drug that interacts with adrenoceptors, is 100 times more potent as beta receptor blocker than R(+) isomer. However, both the isomers are approximately equipotent as alpha receptor blockers.
* The D-isomers of amphetamine and methamphetamine are strong CNS stimulants, while the L-isomers of both drugs lack appreciable CNS(central nervous system) stimulant effects, but instead stimulate the peripheral nervous system. For this reason, the Levo-isomer of methamphetamine is available as an OTC nasal inhaler in some countries, while the Dextro-isomer is banned from medical use in all but a few countries in the world, and highly regulated in those countries who do allow it to be used medically.

wIKIPEDIA