Circular dichroism spectroscopy

Anyone who has Polaroid sunglasses or a glare-reducing polarisation filter for a camera lens will be familiar with the phenomenon of plane polarisation of light.

Polarised light arises from the summation of left- and right-circularly polarised light (LCPL and RCPL). If these two components are of equal intensity and in phase, plane-polarised light results. If they're not in phase or are of different intensities, their interaction generates elliptically polarised light.

Molecules, such as proteins, which have chiral centres (eg the alpha-carbon atoms, disulfide bonds or aromatic side chains) absorb one of the circularly polarised light components slightly more than the other, resulting an elliptical polarisation of the transmitted light that's measured by the spectrometer. This optical activity is associated with peaks of the absorption spectrum and hence peptide bonds show circular dichroism (CD) activity in the region from 250nm to 178nm (the lowest wavelength that can be recorded on most CD spectrometers) and the aromatic side chains around 300nm to 260nm.

Some protein ligands or prosthetic groups show CD activity at higher wavelengths. Nucleic acids also have CD spectra at wavelengths where they absorb light.

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Contact Dr Amanda Nouwens for more information and bookings.

Applications

CD spectroscopy is the fastest way to check protein folding, and it takes 10 to 20 minutes to record a full spectrum.

You can use it to:

determine secondary structure of a protein. From the CD spectrum you can calculate regions of the protein that adopt alpha-helical, parallel and antiparallel beta-sheet, beta-turn, and 'random coil' secondary structures
determine if the structure of an expressed protein has folded correctly, especially if native protein is available for comparison
follow the unfolding of a protein (eg the denaturation and renaturation of the proteins)
determine changes in the environment of the aromatic amino acid side chains (another measure of folding or unfolding) as well as of interactions with other molecules, as measured around 280nm.

Equipment and location

Sample and set-up requirements

Protein concentration should be ~1mg/ml with little or no buffer/salts (as spectra must be recorded from 250nm to 180nm or lower). Below 200nm, oxygen buffers and salts (including NaCl and NaN3) absorb strongly and the signal-to-noise ratio therefore falls abruptly: this is partly offset by averaging multiple scans.

We house cuvettes with different pathlengths (1mm, 10mm) depending on sample requirements. Between ~35ul and 200ul of sample is needed depending on the cuvette used. You'll also need a buffer to use as a reference scan.

Optical activity in aromatic regions is around 10-fold weaker, so its measurement requires a higher protein concentration or longer pathlength (typically 1mm to 1cm).

Most of the protein sample can be recovered from cells if necessary.

Circular dichroism spectroscopy

Contact us

Applications

Equipment and location

Equipment

Location

Sample and set-up requirements

Charges and considerations

Charges

Considerations