title: Improvement of SAS triple invariant estimates for macromolecular direct-methods phasing.
abstract: Single-wavelength anomalous dispersion (SAS) data can in principle be phased by direct methods since a priori estimates of the three-phase structure invariants can be computed from these data. The mean phase error of the most reliable triple estimates for a small protein, however, is typically no better than 60 degrees, and does not bode well for applications to larger structures. A procedure is described that can substantially lower the error in these estimates and introduce a larger number of useful triple invariants into the phasing process. The mean phase error of the most reliable triples for a 2.5 A resolution data set from a Pt derivative of a 115-residue protein was reduced from 55 to 25 degrees by this method. It was also possible to identify a significant number of the poorest triple estimates, those with mean phase errors approaching 90 degrees, such that they could be reliably down-weighted or excluded from the phasing process.
authors.affiliation: Hauptman-Woodward Medical Research Institute, Inc., 73 High Street, Buffalo, NY 14203, USA. langs@hwi.buffalo.edu
authors.forename: D A
authors.initials: DA
authors.lastname: Langs
authors.forename: R H
authors.initials: RH
authors.lastname: Blessing
authors.forename: D
authors.initials: D
authors.lastname: Guo
mesh_terms: D018360:Crystallography, X-Ray / Q000379:methods; D046911:Macromolecular Substances; D011506:Proteins / Q000737:chemistry*; D012680:Sensitivity and Specificity
chemical_list: D046911:Macromolecular Substances; D011506:Proteins