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The Astbury Centre for Structural Molecular Biology

Circular Dichroism


We are equipped with a Jasco J715 spectropolarimeter with a 6 cell changer, peltier temperature control, stopped flow capabilities and automated temperature ramping. The equipment is easy to use and potential users will be trained to carry out their own experiments.
Once trained, users may book time on the instrument on the booking sheets provided. The instrument is very heavily used and bookings may not be made more than one week in advance. In order to cover the running and maintenance costs of the instrument, users are charged at an hourly rate. Users must submit a grant number to Dr S.E. Radford BEFORE undertaking any experiments.
If you are a new user and would like to use the CD please contact Dr S.E. Radford (X 3170; email s.e.radford@leeds.ac.uk).

CD machine in lab

What is CD?

Circular Dichroism (CD) is observed when optically active matter absorbs left and right handed circularly polarized light slightly differently. It is measured with a CD spectropolarimeter, which is able to measure accurately in the far UV at wavelengths down to 190-170nm. In addition, the difference in the left and right handed absorbance is very small (usually in the range of 0.0001 absorbance units) corresponding to an ellipticity of a few 1/100th of a degree.
The CD spectrum is the wavelength dependency of the difference in absorption between the right and left-handed components.

The units of CD

The units used in CD spectroscopy often cause confusion!

The Jasco J715 gives its raw output in ellipticity, given the symbol bullet, and is measured in millidegrees (mdeg).

To compare your data with those of others, the ellipticity X is usually coverted to the Molar Ellipticity. This is given the symbol [bullet] (with units of degrees.cm squared. per decimole).

Thus: [bullet] = bullet / (10 x c x l)
where c is the Molar concentration of the sample (mole/L) and l is the pathlength in cm.

alternatively, for far UV CD measuremtns on proteins, the mean residue molar ellipticity is often used to reflect the fact that the peptide bond is the absorbing species in this case.
The mean residue molar ellipticity is given the symbol [bullet] MRW
and [X] MRW = [X]/(10 x cr x l), where cr is the mean residue molar concentration.

This means that cr = n x c, where n is the number of peptide bonds in the protein or peptide and,
of course, c = (1000 x n x cg)/M r, where cg is the macromolecule concentration in g/ml and Mr is the molecular weight of the species.

Fortunately, the CD spectropolarimeter will do many of these conversions for you. However, if you intend to carry out CD fitting, the different packages available often need their input in different units. Make sure your data is in the correct units!

What can CD do for you?

Useful CD links

CD fitting algorithms and other CD information.

Useful references

  1. Circular Dichroism and Linear Dichroism, A. Rodger and B. Norden (1997) Oxford University Press, Oxford, New York, Tokyo.
  2. Circular Dichroism and Optical Rotatory Dispersion of Proteins and Polypeptides, A.J. Alder, N.J. Greenfield and G.D. Fasman (1973) Methods Enzymology, 27, 675
  3. Computed Circular Dichroism Spectra for the Evaluation of Protein Conformation, N. Greenfield and G.D. Fasman (1996) Biochemistry, 8, 4108
  4. Protein Secondary Structure and Circular Dichroism: A Practical Guide, W.C. Johnson, (1990) Proteins, 7, 205