ABSTRACT
X-rays are scattered mainly by the electron clouds of the atoms. Scattering arising by the nuclei of the atoms is negligible as the mass of the nucleus is three orders of magnitude larger than that of the electron. The location of hydrogen atoms from the difference Fourier map after the anisotropic refinement of the nonhydrogen atoms helps the unambiguous assignment of the atom types. For single crystal X-ray diffraction analysis, single crystals with dimensions of several tenths of a millimeter are needed. The method of isomorphous replacement is central to the X-ray analysis of protein crystals. In the intensity data collection on a single crystal, a mounted specimen should be rotated by the aid of the goniometer to bring each reflection into reflecting position. Depending on the type of the goniometer, the crystal can be rotated around one, two, or three axes.
