Principles of Protein Structure Assignment 1996

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A Short Primer on Circular Dichroism

CD is a variant of absorption spectroscopy which measures the difference in absorption of left and right polarised light (in the ultraviolet) by a medium. Left polarized light will be absorbed more strongly than right polarized light (or vice-versa) if the absorbing bond is asymmetric, i.e. if it is chiral. In the far UV (180 - 250 nm) the CD of a protein is primarily that of the amide chromophores along the backbone which result from bonding between the component amino acids. Although the peptide bond is planar and hence symmetric, there is usually an asymmetric alpha-carbon on either side, hence the peptide bond transitions interact to give a CD signal which is very sensitive to secondary structure.

Here are the far UV CD spectra characteristic of alpha-helix, beta-sheet and random coil structures.

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Last updated 2nd Apr '96