Circular Dichroism: Studies of Proteins

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.

Keywords: protein folding; protein unfolding; conformational changes; secondary structure; tertiary structure

Figure 1.

Origin of the CD effect. The left‐ (L) and right‐ (R) circularly polarized components of plane‐polarized radiation: (a) the two components have the same amplitude and when combined generate plane‐polarized radiation and (b) the components are of different magnitude and the resultant (dashed line) is elliptically polarized.

Figure 2.

The far‐UV CD spectra associated with various types of secondary structure in proteins. Red, α helix; blue, antiparallel β sheet; green, type I β turn and orange, irregular structure.

Figure 3.

The near‐UV CD spectrum of type II dehydroquinase from Streptomyces coelicolor. Signals arising from Trp, Tyr and Phe are in the approximate spectral regions 290–305 nm, 275–282 nm and 255–270 nm, respectively.

Figure 4.

The near‐UV CD spectrum of the molybdate‐sensing protein ModE from E. coli. The black and red lines indicate spectra recorded in the absence and presence of 1 mM sodium molybdate, respectively.

Figure 5.

The far‐ and near‐UV CD spectra of the Cɛ3 domain of immunoglobulin E. The main graph shows the far‐UV CD spectrum and the inset shows (in red) the near‐UV CD spectrum. The very small CD signal in the near‐UV CD is indicative of the lack of stable tertiary structure.

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Further Reading

Fasman GD (ed.) (1996) Circular Dichroism and the Conformational Analysis of Biomolecules. New York: Plenum Press.

Kelly SM and Price NC (1997) The application of circular dichroism to studies of protein folding and unfolding. Biochimica et Biophysica Acta 1338: 161–185.

Strickland EH (1974) Aromatic contributions to circular dichroism spectra of proteins. Critical Reviews in Biochemistry 2: 113–175.

Woody RW (1995) Circular dichroism. Methods in Enzymology 246: 34–71.

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Kelly, Sharon M, and Price, Nicholas C(Dec 2009) Circular Dichroism: Studies of Proteins. In: eLS. John Wiley & Sons Ltd, Chichester. http://www.els.net [doi: 10.1002/9780470015902.a0003043.pub2]