The term chiral (pronounced /ˈkaɪrəl/)
is used to describe an object that is non-superposable on its mirror image.
Human hands are perhaps the most
universally recognized example of chirality: The left hand is a
non-superposable mirror image of the right hand; no matter how the two
hands are oriented, it is impossible for all the major features of both
hands to coincide. This difference in symmetry becomes obvious if
someone attempts to shake the right hand of a person using his left
hand, or if a left-handed glove is placed on a right hand. The term chirality is derived from the Greek
word for hand, χειρ (cheir). It is a mathematical approach to the
concept of "handedness".
When used in
the context of chemistry, chirality usually refers to molecules. Two
mirror images of a molecule that cannot be superposed 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-"
A mixture of equal amounts of the two enantiomers is said to be a racemic mixture. Racemic mixtures
are not optically active even though their two enantiomer constituents
when separated are. Molecular chirality is of interest because of its
application to stereochemistry in inorganic
biologically active molecules are chiral, including the naturally
acids (the building
blocks of proteins),
In biological systems, most of these compounds are of the same
chirality: most amino acids are L and sugars are D. Typical
naturally occurring proteins, made of L amino acids, are known as left-handed proteins,
whereas D amino acids produce right-handed protein