Circular dichroism and its application to the study of biomolecules.

Circular dichroism (CD) is an excellent method for the study of the conformations adopted by proteins and nucleic acids in solution. Although not able to provide the beautifully detailed residue-specific information available from nuclearmagnetic resonance (NMR) and X-ray crystallography, CD measurements have two major advantages: they can be made on small amounts of material in physiological buffers and they provide one of the best methods for monitoring any structural alterations that might result from changes in environmental conditions, such as pH, temperature, and ionic strength. This chapter describes the important basic steps involved in obtaining reliable CD spectra: careful instrument and sample preparation, the selection of appropriate parameters for data collection, and methods for subsequent data processing. The principal features of protein and nucleic acid CD spectra are then described, and the main applications of CD are discussed. These include: methods for analyzing CD data to estimate the secondary structure composition of proteins, methods for following the unfolding of proteins as a function of temperature or added chemical denaturants, the study of the effects of mutations on protein structure and stability, and methods for studying macromolecule-ligand and macromolecule-macromolecule interactions.