[38] Low-resolution phasing

This chapter describes low-resolution phasing. Finding the solution of a phase problem is the major step between a set of diffraction amplitudes and the corresponding electron density map. For macromolecules near atomic resolution, this is usually accomplished by heavy-atom methods to obtain an electron density map interpretable in terms of an atomic model. Low-resolution amplitudes are traditionally neglected in this process. However, there are a number of cases where phasing the low-resolution data is important. These include (1) cases where the knowledge of a low-resolution envelope is needed to solve a high-resolution phase problem, which includes averaging and density modification methods, where the envelope is necessary, and molecular replacement methods, where the approximate knowledge of the position and orientation of the molecule is helpful, (2) large molecular complexes where a low-resolution image has valuable information in itself (for example, the ribosome), (3) crystals that do not diffract to high resolution, (4) molecules with a mixture of ordered and disordered regions—for example, the studies of the nucleic acid component in a virus, and (5) cases where the low-resolution cutoff causes large image distortions.