Phase improvement in protein crystallography using a mixed electron density model

The proposed technique for phase improvement is based on the refinement of a so-called mixed electron density model. This model consists of two parts. The first is a partial stereochemically correct atomic model of the protein molecule related to the interpreted part of electron density of the unit cell. The second is an artificial atomic model describing the uninterpreted part of the residual electron density of the unit cell. The conventional free-atom crystallographic refinement of such a mixed model results in phase improvement. No attention is paid to the structural sense of the refined atomic positions of the mixed electron density model. This method of phase improvement has been applied for eye lens protein γ-crystallin IIIb at 2.7 Å resolution. A starting partial model of the molecule contained about 56% of the total number of residues. Phase refinement is done in two stages. Each stage leads to a significant increase in the quality of the electron density map, so that the partial atomic model of the protein molecule can be improved and expanded.