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. CD bands in the near ultraviolet (350-260 nm) and visible regions arise from aromatic and prosthetic groups. There are also changes in these regions when proteins bind to each other. Because CD is a quantitative technique, changes in CD spectra are directly proportional to the amount of the protein-protein complexes formed, and these changes can be used to estimate binding constants. Changes in the stability of the protein complexes as a function of temperature or added denaturants, compared to the isolated proteins, can also be used to determine binding constants.