Circular dichroism is a technique that uses circular polarized light to gain information about secondary and tertiary structure. The pioneering work of Norma Greenfield and Gerald D. Fasman in the late 1960's made the use of CD a standard practice(3). There are many ways to get information about structure from CD spectra, but the most reliable method correlates CD spectra with X-ray structure determinations(4).
A number of conformational studies have been made using CD(5,6). These studies typically look at the different conformations the proteins take on in different environments (varying pH). More recently, CD has been used to monitor the folding and unfolding of proteins. A study done by J. M. Betton et al. used a phosphoglycerate kinase to observe the folding patterns of a two domain protein at equilibrium(7). By observing the assymetry of the CD curve at equilibrium, they concluded that the two domains refold independently from one another. Another study done with bovine growth hormone used CD in combination with absorbance spectra for disulfide bonds to determine the refolding in bGH is a multistep process(8). The researchers concluded that disulfide bonds are reformed after the protein has refolded into its secondary structure.
There are still other studies that were done to test the framework model of protein folding, which states that the formation of secondary structure preceeds that of tertiary structure(9-11). The study by D. N. Brems et al. makes a comparison of the kinetics of folding for bGH by using two techniques for detection, CD for secondary structure and near UV absorbance for tertiary (12). The kinetics observed for CD were faster than those for UV, indicating that secondary comes before tertiary.