Circular dichroism (CD) analyses of protein-protein interactions

Circular dichroism (CD) spectroscopy is a useful technique for studying protein-protein interactions in solution. CD in the far ultraviolet region (178-260 nm) arises from the amides of the protein backbone and is sensitive to the conformation of the protein. Thus, CD can determine whether there are changes in the conformation of proteins when they interact. Changes in the conformation of the protein complexes as a function of temperature or added denaturants, compared to the individual proteins, can be used to determine binding constants. CD bands in the near ultraviolet (350-260 nm) and visible regions arise from aromatic amino acid side chains and prosthetic groups. There are often changes in these regions when proteins bind to each other. Because CD is a quantitative technique, these changes are directly proportional to the amount of the protein-protein complexes formed and thus also can be used to estimate binding constants.