title: Globbic approximation in low-resolution direct-methods phasing.
abstract: Probabilistic direct-methods phasing theory, originally based on a uniform atomic distribution hypothesis, is shown to be adaptable to a non-uniform bulk-solvent-compensated globbic approximation for protein crystals at low resolution. The effective number n(g) of non-H protein atoms per polyatomic glob increases with decreasing resolution; low-resolution phases depend on the positions of only N(g) = N(a)/n(g) globs rather than N(a) atoms. Test calculations were performed with measured structure-factor data and the refined structural parameters from a protein crystal with approximately 10 000 non-H protein atoms per molecule and approximately 60% solvent volume. Low-resolution data sets with d(min) ranging from 15 to 5 A gave n(g) = ad(min) + b, with a = 1.0 A(-1) and b = -1.9 for the test case. Results of tangent-formula phase-estimation trials emphasize that completeness of the low-resolution data is critically important for probabilistic phasing.
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: D003460:Crystallization; D018360:Crystallography, X-Ray / Q000379:methods*; D008432:Mathematical Computing; D011487:Protein Conformation; D017510:Protein Folding; D011506:Proteins / Q000737:chemistry*; D012997:Solvents
chemical_list: D011506:Proteins; D012997:Solvents