Application of the maximum entropy method in the macromolecular crystallography

Accurate structural refinement of a putative acylphosphatase from 1.3 A X-ray diffraction data was carried out using charge densities determined by the Maximum Entropy Method (MEM). The MEM charge density clearly revealed detailed features in the solvent region of the putative acylphosphatase crystalline structure, some of which have never been seen in conventional Fourier map. The structural model in solvent region was constructed as distributions of anisotropic water atoms. The omit-MEM maps and the difference-MEM maps were effective for revealing details of protein structure, such as multi-conformers in side-chains of amino acid residues, anisotropy of atoms, and hydrogen atoms. By model building using the MEM charge densities, the reliability factors, R1 and Rfree, in the SHELX refinements were improved to 9.6% and 10.0%, respectively. (author)