Solvent effect in protein crystals. A neutron diffraction analysis of solvent and ion density

In protein crystallography, it has been customary to omit the low-order data in refinement procedures. These data contain, however, important information about the gross features of the unit cell content and particularly the scattering density of the solvent, i.e. solvent structure. In order to use the low-order Bragg reflections, a solvent evaluation procedure has been developed that permits the description of the low-order structure factors (F) as a combination of solvent and protein terms. This permits the use of all observed F values in a least-squares refinement, results in better refinement (lower R factor) and permits easier placement of water and ion locations. Coupled with the measurement of the crystal density by a density-gradient technique, the evaluation of the solvent scattering makes it possible to determine the amount of salt present in the solvent space. For myoglobin crystals grown from solutions containing close to 40% (w/w) ammonium sulfate only 13% (w/w) of salt is present in the solvent space. This is equivalent to seven (ionized) ammonium sulfate molecules, which is larger than the two to three sulfate ions observed in the crystal solvent space by X-ray diffraction.