Map self-validation: improved criteria to resolve the SIR or SAS phase ambiguity

Area of Science:

• Crystallography
• Structural Biology
• Biophysics

Background:

• Phase determination is crucial for solving macromolecular structures using X-ray diffraction.
• Existing methods for resolving phase ambiguity, such as SIR and SAS, have limitations.
• Correlation coefficient (CC) agreement has been used to assess phase accuracy.

Purpose of the Study:

• To enhance the selectivity and accuracy of crystallographic phase determination.
• To develop a novel scheme for resolving single isomorphous replacement (SIR) or single-wavelength anomalous scattering (SAS) phase ambiguity.
• To improve upon existing CC-based methods by utilizing complex-valued structure factor data.

Main Methods:

• The study proposes a new procedure based on the agreement between observed structure factors /F(h)/ and unbiased 'omit map' extrapolated values /X(h)/.
• The agreement is expressed using complex-valued F(h) and X(h) for improved selectivity.
• A weighted average of the two possible SIR or SAS phase choices is exploited, requiring six Fast Fourier Transforms (FFTs) per phase.

Main Results:

• The new method demonstrates significant improvement in selectivity compared to previous approaches.
• Trial calculations show encouraging results, even at resolutions as low as 4 Å.
• The use of complex-valued data enhances the ability to distinguish correct phases.

Conclusions:

• The proposed scheme offers a more robust and selective method for crystallographic phase determination.
• This approach has potential applications in solving challenging macromolecular structures.
• Further development and application of this method could advance structural biology research.