Molecular Chirality

Definition

A chiral molecule is a type of molecule that has a non-superposable mirror image. The feature that is most often the cause of chirality in molecules is the presence of an asymmetric carbon atom.

Enantiomers

Two mirror images of a chiral molecule are called enantiomers or optical isomers.

Types of Enantiomers (D- and L-)

Pairs of enantiomers are often designated as "right-", "left-handed" or, if they have no bias, "achiral". As polarized light passes through a chiral molecule, the plane of polarization, when viewed along the axis toward the source, will be rotated clockwise (to the right) or anticlockwise (to the left). A right handed rotation is dextrorotary (d); that to the left is levorotary (l). The d- and l-isomers are the same compound but are called enantiomers.

Racemic Mixtures (Equal Mixtures of D- and L-)

An equimolar mixture of the two optical isomers, which is called a racemic mixture, will produce no net rotation of polarized light as it passes through.

Applications

It is studied for its application to stereochemistry in

• Inorganic Chemistry
• Organic Chemistry
• Physical Chemistry
• Biochemistry
• Supramolecular Chemistry (Nanochemistry)

More recent developments in chiral chemistry include the development of chiral inorganic nanoparticles that may have the similar tetrahedral geometry as chiral centers associated with sp3 carbon atoms traditionally associated with chiral compounds, but at larger scale. Helical and other symmetries of chiral nanomaterials were also obtained.

Manifestations of chirality

• Flavor: the artificial sweetener aspartame has two enantiomers. L-aspartame tastes sweet whereas D-aspartame is tasteless.
• Odor: R-(–)-carvone smells like spearmint whereas S-(+)-carvone smells like caraway.
• Drug effectiveness: the antidepressant drug citalopram is sold as a racemic mixture. However, studies have shown that only the (S)-(+) enantiomer (escitalopram) is responsible for the drug's beneficial effects.
• Drug safety: D‑penicillamine is used in chelation therapy and for the treatment of rheumatoid arthritis whereas L‑penicillamine is toxic as it inhibits the action of pyridoxine, an essential B vitamin.