title: 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.
authors.affiliation: Hauptman-Woodward Institute, 73 High Street, Buffalo, New York 14203, USA. guo@hwi.buffalo.edu
authors.forename: D Y
authors.initials: DY
authors.lastname: Guo
authors.forename: R H
authors.initials: RH
authors.lastname: Blessing
authors.forename: D A
authors.initials: DA
authors.lastname: Langs
mesh_terms: D043383:Adenosylhomocysteinase; D018360:Crystallography, X-Ray / Q000379:methods; D005583:Fourier Analysis; D005987:Glyceraldehyde-3-Phosphate Dehydrogenases / Q000737:chemistry*; D006867:Hydrolases / Q000737:chemistry*; D008962:Models, Theoretical; D011336:Probability; D011506:Proteins / Q000737:chemistry*; D012997:Solvents
chemical_list: D011506:Proteins; D012997:Solvents; D005987:Glyceraldehyde-3-Phosphate Dehydrogenases; D006867:Hydrolases; D043383:Adenosylhomocysteinase