CD is an absorption spectroscopy method based on the differential absorption of left and right circularly polarized light. Optically active chiral molecules will preferentially absorb one direction of the circularly polarized light. The difference in absorption of the left and right circularly polarized light can be measured and quantified. UV CD is used to determine aspects of protein secondary structure.
In fact, alpha-helical and beta-sheet secondary structures of proteins have specific CD spectra in the 180-260 nm wavelength range, that are very different from unfolded proteins. Using well developed algorithms it is possible to deconvolute the CD spectra of a given protein to estimate the percentage of each secondary structure elements. By following the disappearance of secondary structure while increasing temperature it is possible to measure the thermal stability of the protein and to assess its relative stability in different buffers. CD can also be used to study how proteins change structure and/or stability when forming protein-nanoparticle complexes.
The technique is quite sensitive and requires limited amounts of material. It is essential that the buffer system (or other chemical compounds added to the protein sample) uses non chiral molecules, nor has strong absorption in the 190-300 nm wavelength.