title: TDSIR phasing: direct use of phase-invariant distributions in macromolecular crystallography.
abstract: A new strategy for employing three phase triples invariant estimates from Hauptman's single isomorphous replacement (SIR) and anomalous dispersion (SAS) joint probability distribution formulae is outlined which produces a single unique phase-invariant solution in the case where the positions of the heavy-atom scatterers is known. A similar but non-identical result is obtained for the phase invariants of a structure for which a molecular-replacement solution has been obtained. It is important to note that the values of the individual native/derivative phases can be determined directly from the probability distribution formulae without having to utilize the phase-invariant estimates in an active way. Elimination of the multisolution aspect of utilizing phase-invariant estimates should have important repercussions with regard to phasing macromolecular sets of derivatized data. Trial calculations based on experimentally measured 2.5 A data for three derivatives of cytochrome c550 are encouraging. The average of the three SIR maps resolves a number of structural ambiguities seen in the published multiple isomorphous replacement (MIR) map obtained from eight derivatives.
authors.affiliation: Medical Foundation of Buffalo, NY 14203, USA.
authors.forename: D A
authors.initials: DA
authors.lastname: Langs
authors.forename: D
authors.initials: D
authors.lastname: Guo
authors.forename: H A
authors.initials: HA
authors.lastname: Hauptman
mesh_terms: D018360:Crystallography, X-Ray; D003574:Cytochrome c Group / Q000737:chemistry; D006418:Heme / Q000737:chemistry; D015233:Models, Statistical; D012542:Scattering, Radiation
chemical_list: D003574:Cytochrome c Group; D006418:Heme; C031732:cytochrome C-550