Low-resolution real-space envelopes: an approach to the ab initio macromolecular phase problem.

An ab initio approach to the phase problem in macromolecular x-ray crystallography is described. A random gas of hard-sphere point scatterers is allowed to condense under the constraint of the solvent fraction and the restraint of the observed Fourier amplitude data. Two applications to real macromolecular examples are discussed. This method produces an approximate outline of the bulk solvent regions and thus yields a low-resolution picture of the unit cell that can be extended to higher resolutions in special cases, such as through the use of molecular replacement or of noncrystallographic symmetry-based phase extension.