CIRCULAR DICHROISM SPECTROSCOPY AND THE VACUUM ULTRAVIOLET REGION

Circular dichroism (CD) is a special kind of electronic absorption spectroscopy that uses circularly polarized light rather than isotropic light. CD is a particularly useful technique because it is sensitive to the conformation of a molecule, and because it can be applied to randomly oriented molecules in solution and in the gas phase. For a molecule to exhibit a CD spectrum it must be asymmetric, that is, it must contain no plane or center of symmetry. Since most biological molecules are asymmetric, CD spectroscopy has been used extensively to investigate their conformation. It has also been used to investigate the electronic properties of chromophores when simple asymmetric derivatives of known conformation are available. Most readers will be familiar with plane-polarized light. With this type of polarization the direction of the electric field vector is constant to within a sign, while the magnitude of the electric vector is modulated. Circularly polarized light is the antithesis of plane-polarized light. A snapshot of circularly polarized light is depicted in Figure 1 where we see that the magnitude of the electric vector is