Neutron diffraction experiments for protein

Kurihara, Kazuo; Okazaki, Nobuo; Kuroki, Ryota

Crystallographic analysis using neutron diffraction allows identification and position determination of light atoms like hydrogen. This method has been used for three-dimensional structure determination of organic compounds as well as macromolecules like protein. Hydrogen atoms in proteins, as well as those in solvent molecules, play significant roles in many naturally occurring processes, such as catalytic function and molecular recognition. In the field of neutron crystallography novel diffractometers and techniques for preparation and crystallization of target samples has been developed to complement the low flux of neutron sources to permit higher measurement performance. In Japan, single-crystal diffractometers named BIX-3 and BIX-4 were constructed with Neutron Imaging Plates as a detector. These diffractometers have contributed to the investigation of hydrogen-related molecular structures; for example, determination of hydrogen atom positions which are difficult to predict based on X-ray structure data, precise configuration of hydrogen bonds, and the orientation degree of freedom of hydration water molecules. On the other hand, a complementary application of neutron diffraction with X-ray diffraction has also been developed. Using a joint structure refinement method with X-ray diffraction data, elucidation of an enzymatic reaction mechanism and observation of a particular atomic configuration including hydrogen atoms were successfully achieved in neutron crystallographic studies of drug-discovery-target proteins. The information obtained from these neutron analyses has been consolidated into a database called Hydrogen and Hydration Database for Biomolecules which permits the analysis of key statistical information. In Japan as well as overseas, efforts to acquire higher measurement performance are now in progress to further investigate mechanisms involving hydrogen atoms, and to increase the application of neutron crystallographic studies.