What is Circular Dichroism (CD) Technique?

Circular Dichroism (CD) is a powerful technique for studying the secondary structure and conformational changes of biological molecules, such as proteins, nucleic acids, and carbohydrates. CD is based on the principle that when circularly polarized light is passed through a sample, the difference in absorption between left- and right-handed polarizations can reveal information about the secondary structure and conformation of the sample. CD is widely used in many fields, including biochemistry, biophysics, pharmacology, and structural biology.

Principles of Circular Dichroism:

Circular Dichroism occurs when circularly polarized light is passed through a sample and the difference in absorption between left- and right-handed polarizations is measured. The circularly polarized light interacts with the chiral centers of the molecule, causing a difference in absorption between the two polarizations. The magnitude and sign of the difference in absorption can be used to determine the secondary structure and conformation of the sample.

Sources of Circularly Polarized Light:

Circularly polarized light can be generated by a variety of sources, including quartz or mica plates, photoelastic modulators, and lasers. The most common source of circularly polarized light for CD is a quartz or mica plate, which converts linearly polarized light into circularly polarized light by introducing a phase difference between the two polarizations. The degree of circular polarization can be controlled by adjusting the thickness and orientation of the plate.

Detection of Circular Dichroism:

The difference in absorption between left- and right-handed polarizations is detected by a CD spectrophotometer, which measures the circular dichroism as a function of wavelength. CD spectrophotometers can be either single-wavelength or multi-wavelength instruments, and can operate in the UV or visible range.

Data Analysis:

The circular dichroism spectrum is analyzed to determine the secondary structure and conformation of the sample. The most common method of data analysis is the deconvolution method, where the experimental spectrum is compared to a reference set of spectra for known secondary structures, such as α-helices, β-sheets, and random coils. The secondary structure content is then calculated by fitting the experimental spectrum to a linear combination of the reference spectra. Other methods of data analysis, such as the continuum regression method and the neural network method, can also be used to determine the secondary structure content.

Applications of Circular Dichroism:

Circular Dichroism is widely used in many fields, including biochemistry, biophysics, pharmacology, and structural biology.

• In the field of biochemistry, CD is used to study the secondary structure and conformational changes of proteins, nucleic acids, and carbohydrates, as well as to determine the binding of ligands to proteins and nucleic acids.
• In biophysics, CD is used to study the folding and stability of proteins and nucleic acids, as well as to determine the thermodynamics of biomolecular interactions.
• On the other hand, in pharmacology, CD is used to study the binding of drugs to proteins and nucleic acids, as well as to determine the structure-activity relationships of drugs.
• In structural biology, CD is used to study the folding and stability of proteins and nucleic acids, as well as to determine the structure of membrane proteins and other difficult-to-study biomolecules.

Limitations of Circular Dichroism:

Circular Dichroism is a powerful technique but it also has some limitations. CD is most sensitive to changes in the secondary structure of the sample, so it may not be applicable to the study of tertiary and quaternary structures. CD is also sensitive to environmental factors, such as pH and temperature, which can affect the secondary structure and conformation of the sample.

Conclusion:

Circular dichroism (CD) is a powerful technique for studying the structure and conformational changes of biomolecules and materials. It provides information about the secondary structure, conformational changes, and ligand binding properties of molecules, and is widely used in many fields, including biochemistry, biophysics, pharmaceuticals, and materials science. Despite its limitations, CD is a valuable tool for understanding the properties and behavior of molecules and materials in a wide range of applications.