How to Determine Protein Secondary Structure Using Circular Dichroism
Circular Dichroism (CD) is one of the important methods for analyzing the optical activity of organic compounds. In the field of biochemistry, CD is widely used to study the structure and properties of biological macromolecules, especially proteins and nucleic acids. Determining the secondary structure of proteins is an important application of CD research.
I. Principle
The secondary structure of proteins mainly includes three forms: α-helix, β-sheet, and random coil. Their orientation and arrangement in space differ, leading to different absorption of left and right circularly polarized light, which is reflected in different CD spectra. By measuring and analyzing the CD spectrum of a protein, one can determine the types and contents of its secondary structures.
II. Method
In the measurement process, the protein sample is first dissolved in an appropriate buffer solution and then its CD spectrum is measured within a certain temperature and wavelength range (usually 190-250 nm). The original CD spectrum obtained is the differential absorption of circularly polarized light by the protein, which is related to the protein concentration and path length and needs to be converted into units to obtain the mean residue ellipticity per amino acid residue. Then, using appropriate algorithms (such as CONTIN, SELCON3, CDSSTR, etc.), the spectrum is analyzed to obtain the relative content of various secondary structures within the protein.
III. Precautions
When using CD for protein secondary structure analysis, the following points should be noted:
1. The purity of the protein sample should be high to avoid the influence of impurities on the spectrum.
2. The concentration of the protein should be moderate; too high or too low will affect the measurement results.
3. The buffer should not contain components that absorb UV light, such as surfactants, certain inorganic salts, etc.
4. Due to the measurement range and accuracy limitations of the CD instrument, the secondary structure of some proteins may not be accurately determined.
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