Map-likelihood phasing

Journal Article · Sat Dec 01 04:00:00 EST 2001 · Acta Crystallographica. Section D: Biological Crystallography

DOI:https://doi.org/10.1107/S0907444901013749· OSTI ID:22347964

A map-likelihood function is described that can yield phase probabilities with very low model bias. The recently developed technique of maximum-likelihood density modification [Terwilliger (2000 ▶), Acta Cryst. D56, 965–972] allows a calculation of phase probabilities based on the likelihood of the electron-density map to be carried out separately from the calculation of any prior phase probabilities. Here, it is shown that phase-probability distributions calculated from the map-likelihood function alone can be highly accurate and that they show minimal bias towards the phases used to initiate the calculation. Map-likelihood phase probabilities depend upon expected characteristics of the electron-density map, such as a defined solvent region and expected electron-density distributions within the solvent region and the region occupied by a macromolecule. In the simplest case, map-likelihood phase-probability distributions are largely based on the flatness of the solvent region. Though map-likelihood phases can be calculated without prior phase information, they are greatly enhanced by high-quality starting phases. This leads to the technique of prime-and-switch phasing for removing model bias. In prime-and-switch phasing, biased phases such as those from a model are used to prime or initiate map-likelihood phasing, then final phases are obtained from map-likelihood phasing alone. Map-likelihood phasing can be applied in cases with solvent content as low as 30%. Potential applications of map-likelihood phasing include unbiased phase calculation from molecular-replacement models, iterative model building, unbiased electron-density maps for cases where 2F{sub o} − F{sub c} or σ{sub A}-weighted maps would currently be used, structure validation and ab initio phase determination from solvent masks, non-crystallographic symmetry or other knowledge about expected electron density.

• OSTI ID: 22347964
• Journal Information: Acta Crystallographica. Section D: Biological Crystallography, Journal Name: Acta Crystallographica. Section D: Biological Crystallography Journal Issue: Pt 12 Vol. 57; ISSN ABCRE6; ISSN 0907-4449
• Country of Publication: Denmark
• Language: English

Related Subjects

75 CONDENSED MATTER PHYSICS
SUPERCONDUCTIVITY AND SUPERFLUIDITY
CRYSTALS
DENSITY
DISTRIBUTION
ELECTRON DENSITY
ELECTRONS
MODIFICATIONS
POTENTIALS
PROBABILITY
RESPIRATORS
SOLVENTS
SWITCHES
SYMMETRY
VALIDATION
YIELDS