Map-likelihood phasing
- Bioscience Division, Mail Stop M888, Los Alamos National Laboratory, Los Alamos, NM 87545 (United States)
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, Vol. 57, Issue Pt 12; Other Information: PMCID: PMC2745887; PMID: 11717488; PUBLISHER-ID: gr2165; OAI: oai:pubmedcentral.nih.gov:2745887; Copyright (c) International Union of Crystallography 2001; This is an open-access article distributed under the terms described at http://journals.iucr.org/services/termsofuse.html.; Country of input: International Atomic Energy Agency (IAEA); ISSN 0907-4449
- Country of Publication:
- Denmark
- Language:
- English
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