Circular dichroism

Circular Dichroism

Circular dichroism (CD) is a spectroscopic technique used to study the structural and conformational properties of molecules. It is based on the differential absorption of left-handed (L) and right-handed (R) circularly polarized light by chiral molecules. CD spectroscopy provides valuable information about the secondary structure, folding, and interactions of proteins, nucleic acids, and other biomolecules.

History

The phenomenon of circular dichroism was first observed by Jean-Baptiste Biot in 1815, who noticed that certain crystals exhibited different colors when viewed through left and right circularly polarized light. However, it was not until the 1950s that CD spectroscopy became a widely used technique in the field of molecular biology.

Principle

Circular dichroism arises from the differential absorption of L and R circularly polarized light by chiral molecules. Chirality refers to the property of a molecule that cannot be superimposed onto its mirror image. Chiral molecules can exist in two enantiomeric forms, which are mirror images of each other.

When circularly polarized light passes through a chiral molecule, the electric field vector rotates either clockwise (R) or counterclockwise (L) as it propagates. The extent of rotation depends on the wavelength of light and the structural properties of the molecule. The difference in absorption between L and R circularly polarized light results in the observed CD signal.

Applications

CD spectroscopy has a wide range of applications in various fields, including biochemistry, biophysics, and pharmaceutical research. Some of the key applications of CD spectroscopy are:

1. Protein Structure: CD spectroscopy is commonly used to study the secondary structure of proteins, such as alpha-helices and beta-sheets. Changes in the CD spectrum can provide insights into protein folding, stability, and conformational changes.

2. Nucleic Acid Conformation: CD spectroscopy is also used to investigate the conformational properties of nucleic acids, such as DNA and RNA. It can provide information about the presence of secondary structures, such as double-stranded helices or single-stranded regions.

3. Ligand Binding: CD spectroscopy can be employed to study the binding interactions between biomolecules and ligands, such as small molecules or drugs. Changes in the CD spectrum upon ligand binding can indicate conformational changes or structural rearrangements.

4. Enzyme Activity: CD spectroscopy can be used to monitor changes in protein conformation and secondary structure during enzymatic reactions. It can provide insights into the mechanism of enzyme catalysis and the effects of inhibitors or cofactors.

Instrumentation

CD spectroscopy requires specialized instrumentation, typically consisting of a light source, a polarizer, a sample holder, and a detector. The light source emits circularly polarized light, which is then passed through a polarizer to select either L or R circular polarization. The sample is placed in a cuvette or a specialized cell holder, and the transmitted light is detected by a photodetector.

Modern CD spectrometers often incorporate additional features, such as temperature control, automated data acquisition, and spectral analysis software. These advancements have made CD spectroscopy more accessible and user-friendly for researchers.

Conclusion

Circular dichroism spectroscopy is a powerful technique for studying the structural and conformational properties of chiral molecules. It provides valuable insights into the secondary structure, folding, and interactions of proteins, nucleic acids, and other biomolecules. With its wide range of applications, CD spectroscopy continues to be an essential tool in various fields of research, contributing to our understanding of molecular biology and drug discovery.

See Also

• Protein structure
• Nucleic acid conformation
• Spectroscopy
• Chirality

References

1. Nakanishi, K. (1994). Circular Dichroism: Principles and Applications. VCH Publishers. 2. Berova, N., Nakanishi, K., & Woody, R. W. (2000). Circular Dichroism: Principles and Applications. Wiley-VCH. 3. Johnson, W. C. (1990). Circular Dichroism and the Conformational Analysis of Biomolecules. Springer.