optical activity

optical activity the ability of asymmetric compounds to rotate the orientation of planar polarized light. Such compounds and their mirror images are know as enantiomers, or optical isomers. Although differing in geometric arrangement, enantiomers possess identical chemical and physical properties. Since each type of enantiomer affects polarized light differently, optical activity can be used to identify which enantiomer is present in a sample and its purity. Certain molecular groups, known as chromophores, possess high optical activity due to mobile electrons that interact with light and are responsible for the color of certain objects (e.g. chlorophyll chromophore). Optical activity is measured by two methods: optical rotation, which observes a sample's effect on the velocities of right and left circularly polarized light beams; and circular dichroism, which observes a sample's absorption of right and left polarized light. See also polarization of light .

optical activity The ability of some compounds to rotate the plane of polarized light because of the asymmetry of the molecule. If the plane of light is rotated to the right, the substance is dextrorotatory and is designated by the prefix (+); if laevorotatory (rotated to the left), the prefix is (−). A mixture of the two forms is optically inactive and is termed racemic.

Sucrose is dextrorotatory but is hydrolysed to glucose (dextrorotatory) and fructose, which is more strongly laevorotatory so hydrolysis changes optical activity from (+) to (−); hence, the mixture of glucose and fructose is termed invert sugar.

(The obsolete notation for (+) was d‐ and for (−) was l‐; this is distinct from D‐ and L‐, which are used to designate stereo‐isomerism; see D‐.)