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Entropy maximization constrained by solvent flatness: a new method for macromolecular phase extension and map
S Xiang1, C W Carter, G Bricogne
1Department of Biochemistry and Biophysics, University of North Carolina at Chapel Hill, USA.
Acta Crystallographica. Section D, Biological Crystallography
|January 1, 1993
Summary
This study introduces an improved method for macromolecular modeling using maximum likelihood and exponential models, enhancing phase accuracy and electron density maps for crystal structure determination.
Area of Science:
- Crystallography
- Structural Biology
- Computational Chemistry
Background:
- Macromolecular crystallography relies on accurate phase information for electron density map generation.
- Conventional solvent flattening methods can introduce errors and limit crystallographic refinement.
- Maximum entropy methods offer a promising alternative for phase extension and refinement.
Purpose of the Study:
- To describe a practical procedure for exponential modeling to maximum likelihood of macromolecular data.
- To adapt and test the MICE program for improved phase determination and electron density map generation.
- To compare the performance of maximum entropy methods with conventional solvent flattening using experimental data.
Main Methods:
- Utilized the MICE computer program for exponential modeling and maximum likelihood estimation.
- Tested procedures with simulated data, generating perturbed amplitudes and phases with known molecular envelopes.
- Adapted MICE to incorporate solvent channel information through density averaging and flattening.
- Applied the method to experimental data of cytidine deaminase, comparing results with conventional solvent flattening and MIRAS phases.
Main Results:
- Exponential models can be constructed from as little as 16% of reflections with mean phase errors around 30 degrees.
- Maximum entropy-based electron density maps are quantitatively superior to conventionally solvent-flattened maps.
- The method provides better phase extrapolation than original MIRAS distributions, improving map quality.
- Conventional solvent flattening introduced substantial errors, potentially explaining limitations in crystallographic refinement.
Conclusions:
- The described procedure offers a robust and generally applicable method for macromolecular structure determination.
- Maximum entropy methods combined with solvent flattening provide a powerful, model-independent phase refinement strategy.
- This approach enhances the accuracy of electron density maps and aids in overcoming limitations of traditional methods.