Bulk-solvent correction in direct-methods phasing

It is shown that for crystals of large proteins at low diffraction resolution, with N ≃ 10 000 independent non-H protein atoms and d_min ≃ 8 Å, a simple bulk-solvent correction yields the Sayre equation in its classical form, . In the low-resolution protein case, the proportionality factor becomes , where V is the unit-cell volume, ρ_S is the assumed constant electron density in the solvent regions of the crystal and 〈ρ_P〉 is the average electron density in the protein regions. The classical form of the tangent formula follows from the bulk-solvent-corrected Sayre equation and its validity at low resolution is verified in empirical calculations.