Circular Dichroism
Also known as: CD spectroscopy, circular dichroism, CD analysis
Circular Dichroism (CD) spectroscopy measures the differential absorption of left- and right-circularly polarized light by optically active molecules, particularly proteins and nucleic acids. Introduced by Greenfield and Fasman in 1969, CD is a rapid, non-destructive technique for characterizing secondary structure (alpha-helix, beta-sheet), monitoring protein folding transitions, and assessing conformational changes in response to pH, temperature, or ligand binding.
Key highlights
- Rapid and non-destructive; spectra acquired in seconds to minutes
- Requires micrograms of protein; no isotopic labeling or crystallization needed
- Directly probes secondary structure without extensive interpretation
- Excellent for monitoring real-time kinetics of folding or binding transitions
Intuition
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How it works
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When to use it
Use CD spectroscopy to rapidly assess protein secondary structure content, monitor thermal denaturation or pH-induced folding changes, and detect conformational changes upon ligand binding. CD is ideal for sample-limited studies and high-throughput screening. Assumes the sample is soluble, non-aggregated, and optically clear in the measurement wavelength range. Works best for proteins > 5 kDa and nucleic acids > 20 base pairs.
Strengths & limitations
- Rapid and non-destructive; spectra acquired in seconds to minutes
- Requires micrograms of protein; no isotopic labeling or crystallization needed
- Directly probes secondary structure without extensive interpretation
- Excellent for monitoring real-time kinetics of folding or binding transitions
- Low sensitivity to tertiary structure details; cannot resolve individual residues or side-chain interactions
- Far-UV measurements (below 200 nm) require degassed samples and special cells; atmospheric absorption limits wavelength range
- Spectral overlap between different secondary structures limits quantitative accuracy; best estimates are ±5% error
- Aggregation, scattering, or light absorption by other components in the sample degrade signal
Common pitfalls
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Applications
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Frequently asked
What wavelengths are used in protein CD spectroscopy?
Far-UV CD (190-240 nm) detects peptide bond chromophore transitions and is most sensitive to secondary structure. Near-UV CD (250-350 nm) probes aromatic side chains (tryptophan, tyrosine) and provides information about tertiary structure and hydrophobic environment.
How accurate is secondary structure estimation from CD?
Typical accuracy is ± 5% for the major secondary structures (helix, sheet). Accuracy improves with good spectral quality, appropriate deconvolution algorithm, and high-quality reference datasets. Minor structures (turns, coils) are estimated less accurately.
Can CD detect protein-protein interactions?
Indirectly, yes. If a ligand or binding partner causes conformational change in the protein, CD will detect the shift in secondary structure. However, weak interactions that don't perturb secondary structure may not be detected; surface plasmon resonance or isothermal titration calorimetry are better for binding affinity determination.
Sources
- 1.Greenfield, N. J., & Fasman, G. D. (1969). Computed circular dichroism spectra for protein secondary structures. Biochemistry, 8(10), 4108-4116.
- 2.Yang, J. T., Wu, C. S., & Martinez, H. M. (1986). Calculation of protein conformation from circular dichroism. Methods in Enzymology, 130, 208-269.
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Cite this page
ScholarGate. (2026, June 3). Circular Dichroism. ScholarGate. https://scholargate.app/spectroscopy/circular-dichroism