Ab initio phase determination and phase extension using non-crystallographic symmetry

Area of Science:

• Structural Biology
• Crystallography
• Biophysics

Background:

• Accurate phase information is crucial for interpreting electron density maps in crystallography.
• Non-crystallographic symmetry (NCS) provides a powerful constraint for phase determination.

Purpose of the Study:

• To explore the application of Non-crystallographic symmetry (NCS) for improving and determining crystallographic phases.
• To demonstrate ab initio phase determination using NCS, leading to interpretable electron density maps.

Main Methods:

• Cyclical averaging of electron density maps to refine existing phases.
• Stepwise phase extension to higher resolutions using NCS constraints.
• Ab initio phase initiation from low-resolution data or models.

Main Results:

• Successfully improved phase accuracy through iterative electron density modification.
• Demonstrated successful phase extension to higher resolutions where only amplitudes were available.
• Showcased ab initio phasing from low-resolution models and electron microscopy images.

Conclusions:

• Non-crystallographic symmetry (NCS) is a versatile tool for ab initio phase determination in crystallography.
• The described methods enable the generation of interpretable electron density maps, even with limited initial phase information.
• Successful virus structure determinations highlight the practical utility of NCS-based phasing strategies.