How to Interpret a Circular Dichroism

The Circular Dichroism (CD) spectrum is an analytical method for studying the structure of molecules, especially biological macromolecules such as proteins and nucleic acids. It is based on the difference in absorption of left-handed and right-handed polarized light by molecules. Below is a basic guide on how to interpret a circular dichroism spectrum:

 

1.Understanding the Structure of the Graph

(1) The horizontal axis (X-axis) usually represents the wavelength (in nanometers, nm).

(2) The vertical axis (Y-axis) represents the difference in absorption, that is, the circular dichroism (in millidegrees, mdeg) or the difference in extinction coefficients at specific wavelengths.

 

2. Wavelength Range

Different types of samples require different wavelength ranges. For example, proteins are usually analyzed in the wavelength range of 190-250 nm, while nucleic acids are in the range of 260-320 nm.

 

3. Identification of Characteristic Peaks

(1) The CD spectrum of proteins often shows two characteristic peaks near 208 nm and 222 nm, which are related to the α-helix structure.

(2) The β-fold structure has a minimum value around 215 nm.

(3) The random curl structure is manifested as a weaker minimum value near 195 nm.

 

4. Analyzing the Secondary Structure of the Sample

(1) Based on the position and shape of the peaks, the proportion of the secondary structure of the sample (such as α-helix, β-fold, β-turn, and random curl) can be inferred.

(2) Professional software can help quantitatively analyze the proportion of these structures, such as DichroWeb, CDPro, K2D3, etc.