6. Structure determination of biological macromolecules using x-ray diffraction analysis

This chapter examines the structure determination of biological macromolecules using X-ray Diffraction Analysis (XRDA). XRDA of single crystals and of fibers has been the source of knowledge of the atomic structures of over a hundred biological macromolecules, including such important ones as hemoglobin, immunoglobulins, and DNA. It is found that to measure the full, three-dimensional diffraction pattern in ordered specimens it is necessary to rotate the specimen systematically so that diffraction from all Fourier components of interest occurs and is recorded. The simplest geometry for recording screenless photographs consists of rotating the crystal in the X-ray beam while recording the diffracted beams on a stationary film. It is found that usually, to facilitate the identification of the reflections, the crystal is oriented with a unit cell axis parallel to the rotation axis and perpendicular to the x-ray beam. A complete data set is obtained by recording small-angle oscillation photographs for all the necessary different orientations of the crystal around the rotation axis. The phase determination using isomorphous replacement is also elaborated.