title: Locating the anomalous scatterer substructures in halide and sulfur phasing.
abstract: Improved data quality now makes it feasible to exploit the weak anomalous signal derived only from the sulfurs inherent to the protein or in particular from halide ions incorporated by soaking. The latter technique requires the location of a high number of partially occupied halide sites. This number appears to be roughly proportional to the exposed protein surface. This paper explores the application of dual-space ab initio methods as implemented in the program SHELXD to the location of substructures of sulfur in SAD experiments, bromide in SAD and MAD experiments and iodide using SAD and SIRAS to determine the anomalous-atom substructure. Sets of atoms consistent with the Patterson function were generated as a starting point for the dual-space recycling procedure in SHELXD. The substructure is then expanded to the full structure by maximum-likelihood phasing with SHARP and density modification with the program DM. Success in the location of the substructures and subsequent phasing depends critically on the quality of the data and on the extent of the anomalous signal. This varies with each crystal and soak, but for the same crystal the significance of the anomalous signal was found to be highly sensitive to the redundancy of the intensity measurements, which in some cases made all the difference. This is illustrated by the determination of the previously unknown structure of repeat 11 of the human mannose-6-phosphate/insulin-like growth factor II receptor (Man6P/IGFII-receptor), with 310 amino acids in the asymmetric unit, which was phased by soaking the crystals in a cryoprotectant solution containing halide anions.
authors.affiliation: Lehrstuhl für Strukturchemie, Institut für Anorganische Chemie, Universität Göttingen, Tammannstrasse 4, 37077 Göttingen, Germany. uson@shelx.uni-ac.gwdg.de
authors.forename: Isabel
authors.initials: I
authors.lastname: Usón
authors.forename: Bernhard
authors.initials: B
authors.lastname: Schmidt
authors.forename: Rixa
authors.initials: R
authors.lastname: von Bülow
authors.forename: Susanne
authors.initials: S
authors.lastname: Grimme
authors.forename: Kurt
authors.initials: K
authors.lastname: von Figura
authors.forename: Miroslawa
authors.initials: M
authors.lastname: Dauter
authors.forename: Kanagalaghatta R
authors.initials: KR
authors.lastname: Rajashankar
authors.forename: Zbigniew
authors.initials: Z
authors.lastname: Dauter
authors.forename: George M
authors.initials: GM
authors.lastname: Sheldrick
mesh_terms: D000595:Amino Acid Sequence; D001665:Binding Sites; D001965:Bromides / Q000737:chemistry*; D018360:Crystallography, X-Ray / Q000379:methods*; D006801:Humans; D007454:Iodides / Q000737:chemistry*; D008958:Models, Molecular; D008969:Molecular Sequence Data; D011487:Protein Conformation; D017433:Protein Structure, Secondary; D011506:Proteins / Q000737:chemistry*; D017527:Receptor, IGF Type 2 / Q000737:chemistry*; D012542:Scattering, Radiation; D012984:Software; D013237:Stereoisomerism; D013455:Sulfur / Q000737:chemistry*
chemical_list: D001965:Bromides; D007454:Iodides; D011506:Proteins; D017527:Receptor, IGF Type 2; D013455:Sulfur