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Ab initio phase determination for viruses with high symmetry: a feasibility study
J Tsao1, M S Chapman, M G Rossmann
1Department of Biological Sciences, Purdue University, West Lafayette, Indiana 47907.
Summary
Determining spherical virus structures from a shell model is feasible. Careful consideration of phasing, radius, data completeness, and model positioning is crucial for accurate electron density map generation.
Area of Science:
- Structural biology
- Crystallography
- Virology
Background:
- Spherical viruses possess high internal symmetry, posing challenges for complete structure determination.
- Initial models based on spherical shells are often used as a starting point.
Purpose of the Study:
- To investigate conditions enabling complete structure determination of spherical viruses using an initial spherical shell model.
- To identify potential problems and their solutions in this structure determination process.
Main Methods:
- Analysis of phase creation and centric symmetry issues.
- Evaluation of the impact of average model shell radius selection.
- Assessment of errors in structure amplitude measurements and data completeness.
- Examination of initial spherical-shell model positioning within the unit cell.
Main Results:
- Centric phases can be resolved by averaging electron density maps if internal molecular symmetry differs from crystallographic symmetry.
- Incorrect radii can lead to incorrect solutions (Babinet opposite) or failure to converge.
- Positioning errors of 1.6 Å can cause phasing errors at resolutions >20 Å.
Conclusions:
- Complete structure determination of spherical viruses from an initial shell model is possible under specific conditions.
- Accurate model radius, precise positioning, complete data, and correct phasing are critical for successful structure solution.
- Understanding these parameters is essential for reliable viral structure elucidation.