IMPROVED PHASES, PHASE ERROR ESTIMATES AND ANOMALOUS SCATTERING MODELS FROM THE MULTIWAVELENGTH ANOMALOUS DIFFRACTION (MAD) OF NATIVE PROTEIN METAL CLUSTERS. Brian R. Crane and Elizabeth D. Getzoff, Department of Molecular Biology, The Scripps Research Institiute, La Jolla California, 92037

A strategy is presented for refining anomalous scattering models and calculating macromolecular phases from multiwavelength anomalous diffraction (MAD) of native protein metal clusters. This procedure, incorporated in the program MADPHSREF, refines an anomalous scattering model directly against Bijvoet and dispersive differences while making likelihood estimates of errors, applying stereochemical restraints, taking into account more than one type of anomalous scatterer, and partly compensating for inherent correlations between lack-of-closure expressions. Probabilistic rejection of aberrant observations, re-evaluated before each refinement cycle, improved refinement convergence and accuracy compared to other less flexible rejection criteria. MADPHSREF allows the facile combination of MAD phase information with phase information from other sources. For the sulfite reductase hemoprotein (SiRHP), relative weights for MAD and multiple isomorphous replacement (MIR) phases were determined by matching histograms of electron density. Accurate metal cluster geometries and the associated errors in atomic positions can be determined from refinement against anomalous differences using normal scattering phases from a refined structure. When applied to MAD data collected on SiRHP, these methods confirmed the 4Fe-4S cluster asymmetry initially observed in the refined 1.6 Å resolution structure and resulted in a MAD-phased, experimental, electron-density map that is of better quality than the combined MAD/MIR map originally used to solve the structure.