CD spectroscopy is a technique that measures the difference in absorption of left- and right-circularly polarized light by a molecule that exhibits chirality, such as proteins, nucleic acids, and other biomolecules. The basic principles of CD spectroscopy include:
- Chirality: Molecules that are not superimposable on their mirror images are chiral molecules. Proteins and nucleic acids are chiral molecules that exhibit optical activity, meaning that they can rotate the plane of polarized light.
- Circularly polarized light: CD spectroscopy uses circularly polarized light, which is produced by passing linearly polarized light through a quarter-wave plate. The resulting light has a clockwise (right-circularly polarized) or counterclockwise (left-circularly polarized) rotation.
- Dichroism: Proteins and nucleic acids absorb circularly polarized light differently depending on their secondary structure. This differential absorption is known as dichroism. The magnitude and sign of the dichroism signal are indicative of the amount and type of secondary structure present in the molecule.
- CD spectrum: The CD spectrum is a plot of the difference in absorbance between left- and right-circularly polarized light as a function of wavelength. The CD spectrum is characteristic of the secondary structure of the molecule and can be used to determine the relative amounts of alpha helices, beta sheets, and random coils.
CD spectroscopy can be used to study a wide range of biological molecules, including proteins, nucleic acids, lipids, and carbohydrates. CD spectroscopy is often used in conjunction with other techniques, such as X-ray crystallography and nuclear magnetic resonance (NMR) spectroscopy, to provide complementary information about protein and nucleic acid structure and function.