Definitions

optical activity

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.

Ability of a substance to rotate the plane of polarization of a beam of light passed through it, either as crystals or in solution. Clockwise rotation as one faces the light source is “positive,” or dextrorotary; counterclockwise rotation “negative,” or levorotary. Louis Pasteur was the first to recognize that molecules with optical activity are stereoisomers (see isomerism). Optical isomers occur in pairs that are nonsuperimposable mirror images of one another. They have the same physical properties except for their effect on polarized light; in chemical properties they differ only in their interactions with other stereoisomers (see asymmetric synthesis).

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Raman optical activity (ROA) is a vibrational spectroscopic technique that is reliant on the difference in intensity of Raman scattered right and left circularly polarised light due to molecular chirality.

History of Raman optical activity

The field began with the doctoral work of Laurence D. Barron with Peter Atkins at the University of Oxford and was later further developed by Barron with David Buckingham at the University of Cambridge.

More developments, including important contributions to the development of practical Raman optical activity instruments, were made by Werner Hug of the University of Friburg, and Lutz Hecht with Laurence Barron at the University of Glasgow.

Theory of Raman optical activity

The basic principle of Raman optical activity is that there is interference between light waves scattered by the polarisability and optical activity tensors of a chiral molecule, which leads to a difference between the intensities of the right- and left-handed circularly polarised scattered beams. The spectrum of intensity differences recorded over a range of wavenumbers reveals information about chiral centres in the sample molecule.

Biological Raman optical activity spectroscopy

Due to its sensitivity to chirality, Raman optical activity is a useful probe of biomolecular structure and behaviour in aqueous solution. It has been used to study protein, nucleic acid, carbohydrate and virus structures. Though the method does not reveal information to the atomic resolution of crystallographic approaches, it is able to examine structure and behaviour in biologically more realistic conditions (compare the dynamic solution structure examined by Raman optical activity to the static crystal structure).

Related spectroscopic methods

Raman optical activity spectroscopy is related to Raman spectroscopy and circular dichroism.

Raman optical activity instruments

A simple introduction to Raman optical activity instruments can be found on Laurence Barron's site Much of the existing work in the field has utilised custom-made instruments, though commercial instruments are now available.

External links

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