Circular Dichroism: Studies of Proteins

Abstract

Abstract Circular dichroism (CD) is the differential absorption of the left‐ and right‐circularly polarized components of plane‐polarized electromagnetic radiation. It can provide structural and dynamic information about biological macromolecules, particularly proteins. The CD spectra in the far‐UV (typically 180–240 nm) can give reliable quantitative estimates of the proportions of secondary structural features (helix, sheet, turn, etc.) present in proteins. The spectra in the near‐UV (260–320 nm) can be used to explore the environments of aromatic amino acid side‐chains and hence to give a measure of tertiary structure. Although CD cannot provide the high‐resolution structural data available from X‐ray crystallography or nuclear magnetic resonance, its convenience and applicability under a wide variety of experimental conditions make it the technique of choice in many applications, including exploring protein–ligand interactions, conformational changes and protein folding. Key concepts: Circular dichroism (CD) refers to the difference in absorption of the two components (left‐ and right‐circularly polarized) of plane‐polarized radiation. CD is observed when the absorbing molecule or group exhibits chirality (optical activity). Protein structures are chiral and so give rise to CD signals. CD can be used to explore protein structures under a wide range of experimental conditions. The relatively small CD signals observed from proteins mean that care must be taken to gather reliable experimental data. CD signals from proteins in the far‐UV and near‐UV can be used to explore their secondary and tertiary structures, respectively. CD is an ideal technique to monitor conformational changes in proteins which occur on binding to other molecules. CD can be used to assess the extents of protein unfolding (denaturation) and of refolding of denatured proteins.