23. Neutron Crystallography of Proteins

Abstract

This chapter discusses the neutron crystallography of proteins. The x-ray diffraction of single crystals provides the atomic structure of several hundred biological macromolecules, including many diverse proteins, viruses, t-RNA (tranfer ribonucleic acid) and DNA (deoxyribonucleic acid). These studies resulted in models that provide a basis for understanding enzyme catalytic mechanisms, ligand binding, and molecular function. Neutron diffraction provides an experimental method of directly locating hydrogen atoms, because of their relatively large neutron scattering lengths. The chapter present the current status of single-crystal neutron diffraction studies of proteins. The advantages and disadvantages of neutron diffraction is discussed. Neutron diffraction can locate light atoms such as hydrogen and deuterium, because they have scattering lengths similar to those of heavier atoms. The strategy of data collection is strongly dependent on the instrument used for that purpose. In x-ray protein crystallography, the multiple isomorphous replacement technique has been widely used to obtain the initial estimate of phases. The procedures used for structure refinement in x-ray crystallography can be classified as real-space, difference Fourier, and reciprocal-space techniques. These procedures differ in the function that is minimized. The results of hydrogen-exchange experiments and of methyl rotor analyses provide unique methods for correlating the dynamic properties of proteins with their three-dimensional structures. Such motions in proteins play an important role in their function and may affect processes such as ligand binding, enzyme catalysis, and electron transfer.