Restrained structure-factor least-squares refinement of protein structures using a vector processing computer

A least-squares refinement program RESTRAIN has been developed, which is capable of refining macromolecular structures using structure amplitudes, phases from isomorphous replacement or anomalous scattering and pseudo-energy restraints. In addition to positional parameters and isotropic temperature factors, anisotropic mean-square displacements may be refined either as individual atomic U tensors or as TLS tensors applied to groups of atoms. Anharmonic effects may be handled by coupling together occupancies to enable the electron density of an atomic group to be distributed over more than one subsite. A novel way of restraining groups of atoms to be planar has been developed that does not require dummy atoms and does not restrain the plane to lie in its current orientation.