With phases: how two wrongs can sometimes make a right.

Area of Science:

• Structural Biology
• Crystallography

Background:

• Weak experimental signals (isomorphous/anomalous differences) and partial molecular replacement (MR) phases often yield uninterpretable electron-density maps.
• Traditional methods like Patterson interpretation or direct methods may struggle with weak heavy-atom signals.

Purpose of the Study:

• To present a combined experimental and MR phasing strategy for crystal structure determination.
• To demonstrate the utility of MR phases for locating heavy-atom sites when experimental data is insufficient.

Main Methods:

• Utilizing carefully generated partial MR phases to identify heavy-atom positions.
• Integrating experimental phasing (heavy-atom derivatization) with MR phasing.
• Employing the partial MR model to assist in mask determination for solvent flattening.

Main Results:

• Successfully determined crystal structures by combining experimental and MR phasing approaches in two case studies.
• MR phases enabled reliable heavy-atom site location, bypassing limitations of Patterson or direct methods.
• The integrated approach simplified substructure determination and origin-choice reconciliation.

Conclusions:

• A hybrid approach combining experimental and molecular replacement phasing is crucial for solving challenging crystal structures.
• This integrated strategy enhances the reliability of heavy-atom substructure determination and electron-density map interpretation.