Circular Dichroism
Circular Dichroism (CD) spectroscopy analyses chirality in molecules through their optical activity. This technique measures the difference in the absorption of left and right circularly polarized light in optically active substances
Advantages
- Molecules of any size can be studied
- Measurements can be performed on small amounts of material in physiological buffers
- Experiments are quick and easy to perform
- It has a unique sensitivity to asymmetry in the structure of the molecule
Principles
Electromagnetic waves contain electric and magnetic field components that oscillate perpendicularly in the direction of a light beam’s propagation. The directionality of these components defines the waves’ polarization. In unpolarized light, the electric and magnetic fields oscillate in all directions. For CD spectroscopy, circularly polarized light is used, meaning that its plane of polarization is made to rotate either clockwise (right circular polarization) or anti-clockwise (left circular polarization) while propagating.
To obtain a CD spectrum, the sample is placed in a cuvette (path depending on the concentration). The sample is, firstly irradiated with left rotating polarized light. A second irradiation is performed with right polarized light. Now, due to the intrinsic asymmetry of chiral molecules, they will interact with circularly polarized light differently according to the direction of rotation. There is going to be a tendency to absorb more for one of the rotation directions. The difference between absorption of left and right circularly polarized light is the CD data.
Applications
- Protein secondary structure
- Protein tertiary structure
- Nucleic acids higher-order structure
- Thermal stability
References:
Kelly, Sharon M., et al. “How to Study Proteins by Circular Dichroism.” Biochimica et Biophysica Acta -Proteins and Proteomics, vol. 1751, no. 2, 2005, 119–39