Analysis of Circular Dichroism Data

This chapter focuses on the analysis of circular dichroism (CD) data to determine thermodynamic parameters of folding, binding constants, and estimates of secondary structure. Proteins and polypeptides have CD bands in the far ultraviolet region that arise mainly from the amides of the protein backbone and are sensitive to their conformations. Proteins have CD bands in the near ultraviolet and visible regions, which arise from aromatic amino acids and prosthetic groups. CD can be used to determine the enthalpy, entropy and midpoints, and values of unfolding/refolding transitions of a protein if they are reversible as a function of temperature or denaturant. The change in CD as a function of ligand concentration has been used to study numerous systems. It is found that if two proteins bind to each other only when they are folded and the protein complex unfolds cooperatively and reversibly to give two unfolded monomers, it is easy to determine the binding constant by determining the thermodynamics of folding of the complex compared with the thermodynamics of folding of the monomers.