Neutron Diffraction

Abstract Neutron diffraction, a technique analogous to X‐ray diffraction, is well‐suited to the study of biological materials. By differentiating between hydrogen and its heavier isotope, deuterium, neutron diffraction is able to provide detailed information about the hydrogen atoms within biological macromolecules. It allows biologists to study solvation effects, protonation–deprotonation equilibria, hydrogen bonding and other types of chemistry, all of which are invisible to X‐ray techniques. Recent years have seen the construction of new neutron research facilities, notably the Spallation Neutron Source in the USA, the World's brightest neutron source. Even with such high‐flux sources, the intensity of neutrons hitting the sample is several orders of magnitude lower than a typical X‐ray experiment. However, the nonionizing nature of neutrons means that beam damage to the specimen is low, even over extended exposure times. Key Concepts: Neutron diffraction is closely related to the complementary technique of X‐ray crystallography, a method that led to a revolution in biological sciences. Hydrogen plays a central role in many biological processes, but is invisible to X‐rays. Neutrons are able to distinguish between hydrogen and deuterium, so selective deuteration of a single molecule or group can be used to locate it within a complex macromolecular assembly. The neutron scattering contrast of samples can be controlled by changing the deuterium content of the solvent (water–heavy water exchange). Neutrons have the ability to discriminate between nitrogen, carbon and oxygen, which can be difficult with X‐rays. Neutron experiments are carried out at large scale, national or international neutron research centres, because production of the neutrons requires a nuclear reactor or a spallation source. Animal behaviourists must participate in conservation planning to protect the future of biodiversity. Lipid bilayers provide the fundamental architecture of biological membranes.