Bulk-solvent correction in direct-methods phasing.

Abstract

It is shown that for crystals of large proteins at low diffraction resolution, with N approximately 10 000 independent non-H protein atoms and d(min) approximately 8 A, a simple bulk-solvent correction yields the Sayre equation in its classical form, F(h) = q summation operator(k)F(k)F(h - k). In the low-resolution protein case, the proportionality factor becomes q = 1/[(<rho(P) > - rho(S))V], where V is the unit-cell volume, rho(S) is the assumed constant electron density in the solvent regions of the crystal and <rho(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.