Circular dichroism spectroscopy for the study of protein-ligand interactions

Circular dichroism (CD) is the difference in absorption of left and right circularly polarized light, usually by a solution containing the molecules of interest. A signal is only measured for chiral molecules such as proteins. A CD spectrum provides information about the bonds and structures responsible for this chirality. When a small molecule (or ligand) binds to a protein, it acquires an induced CD (ICD) spectrum through chiral perturbation to its structure or electron rearrangements. The wavelengths of this ICD are determined by the ligand's own absorption spectrum, and the intensity of the ICD spectrum is determined by the strength and geometry of its interaction with the protein. Thus, ICD can be used to probe the binding of ligands to proteins. This chapter outlines protein CD and ICD, together with some of the issues relating to experimental design and implementation.