nonsuperimposability

Nonsuperimposability is the property of two three-dimensional objects that cannot be brought into exact coincidence by any rigid motion in space, meaning by translating and rotating one object relative to the other. If such a motion exists, the objects are congruent; if not, they are nonsuperimposable.

In chemistry and biochemistry, the term is commonly used in relation to chirality. A molecule is chiral if it is nonsuperimposable on its mirror image, and thus exists as a pair of enantiomers that are nonsuperimposable mirror images. The classic familiar example is the pair of human hands. In 1848, Louis Pasteur showed that enantiomeric forms of tartrate are nonsuperimposable and can be separated into distinct crystals.

Chirality is governed by molecular symmetry. Molecules that possess a mirror plane, a center of inversion, or an improper rotation axis (Sn) are achiral and are typically superimposable on their mirror image. Molecules lacking these symmetry elements are chiral. This property has important consequences in chemistry and biology because enantiomers can exhibit different physical properties and biological activities. For instance, they may interact differently with enzymes or receptors, and only one enantiomer may be biologically active in a given context. Racemic mixtures contain equal amounts of both enantiomers and are often less active or behave differently in biological systems.

See also: chirality, enantiomer, mirror image, stereochemistry, optical activity.