Chiral Circular Dichroism of Amino Acids

Amino acids, due to the existence of a chiral center on their α-carbon, exhibit optical activity. The two isomers of chiral amino acids are respectively L-type and D-type. Most of the amino acids in naturally occurring proteins are L-type. As these amino acids have optical activity, they interact with plane-polarized light, resulting in the rotation of polarized light.

 

Circular Dichroism (CD) is a spectroscopic technique used for studying the optical activity of biological macromolecules, such as proteins and nucleic acids. This technique mainly focuses on how molecules absorb left and right circularly polarized light.

 

For amino acids and proteins, CD spectra can provide us with the following information:

 

Protein Secondary Structure

CD spectra can help determine the relative amounts of α-helices, β-sheets, β-turns, and random coils.

 

Protein Structural Changes

Under changing conditions (such as temperature, pH, or the addition of a ligand), a protein's CD spectra may change, reflecting its structural changes.

 

Chirality Identification of Amino Acids

CD can be used to identify the relative amounts of L-type and D-type isomers in an amino acid sample.

 

Protein Stability

By measuring CD spectra at different temperatures, the melting temperature of a protein can be estimated, thus understanding its stability.

 

Protein-Ligand Interactions

The binding of certain small molecules or other biological macromolecules to a protein may cause changes in the protein's structure, which can be detected by CD spectra.

 

The advantage of circular dichroism is that it is very suitable for solving problems related to protein structure and dynamics. However, to obtain quantitative information, more complex analyses of the data are usually required, and other techniques, such as X-ray crystallography or nuclear magnetic resonance, may be needed to obtain more complete structural information.