Direct Phaing of Protein Crystals with High Solvent Content

Determining the phases of a diffraction pattern is crucial since the diffraction pattern of a protein crystal yields only the magnitude of the Fourier transform of the electron density. In order to invert the diffraction pattern to get the protein structure, the phase problem must be solved. An iterative transform method is proposed for solving the phase problem in protein crystallography. In each iteration, a weighted average electron-density map is constructed to define an estimated protein mask. Density modifications are then imposed through the histogram matching technique in the protein region, and the hybrid input–output algorithm in the solvent region. Starting from random initial phases, after thousands of iterations the calculated protein mask evolves into the correct shape and the phases converge to the correct values with an average error of 30◦ ∼ 40◦ for high-resolution data for several protein crystals with high solvent content. With the use of non-crystallographic symmetry and other density constraints, the method could potentially be extended to phase protein crystals with less than 50% solvent fraction. The new phasing algorithm can supplement and enhance the traditional refinement tools.