Interpreting of Circular Dichroism in Protein Structure Analysis

Circular Dichroism (CD) is a commonly used spectroscopic technique primarily employed to study the secondary structure of proteins and other macromolecules. CD spectroscopy detects the difference in absorption of left-handed and right-handed circularly polarized light as it passes through a protein sample, a difference caused by the chirality of the sample. Protein CD spectra are typically recorded in the wavelength range of 190-250 nm:

 

α-Helix

It has two negative peaks around 222 nm and 208 nm, and a positive peak around 190 nm.

 

β-Sheet

It has a negative peak around 218 nm, and a positive peak around 195 nm.

 

Random Coil

It has a negative peak around 200-205 nm.

 

β-Turns

It has a positive peak around 225 nm.

 

In order to accurately estimate the secondary structure content of proteins from CD spectra, it is usually necessary to use specialized software tools such as SELCON3, CONTIN, and CDPro etc. These software estimate the secondary structure proportions of proteins by comparing experimental data with known protein structure data in the database.

 

CD spectroscopy can not only be used to study static protein structures, but also to monitor the effects of pH, temperature, ionic strength, or chemical changes on protein structure. This is important for understanding protein folding, stability, interactions, and how they change as the environment changes. For more complex tertiary or higher-level structural information, it may be necessary to use X-ray crystallography, nuclear magnetic resonance (NMR), or other techniques in conjunction.