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Concept
Raman optical activity
Summary
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.
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 Fribourg, and Lutz Hecht with Laurence Barron at the University of Glasgow.
The basic principle of Raman optical activity is that there is interference between light waves scattered by the polarizability 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.
Raman optical activity can be observed in a number of forms, depending on the polarization of the incident and the scattered
light. For instance, in the scattered circular polarization (SCP) experiment, the incident light
is linearly polarized and differences in circular polarization of the scattered light are measured.
In the dual circular polarization (DCP), both the incident and the scattered light are circularly
polarized, either in phase (DCPI ) or out of phase (DCPII ).
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).
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Circular dichroism
Circular dichroism (CD) is dichroism involving circularly polarized light, i.e., the differential absorption of left- and right-handed light. Left-hand circular (LHC) and right-hand circular (RHC) polarized light represent two possible spin angular momentum states for a photon, and so circular dichroism is also referred to as dichroism for spin angular momentum. This phenomenon was discovered by Jean-Baptiste Biot, Augustin Fresnel, and Aimé Cotton in the first half of the 19th century.