SAD phasing by combination of direct methods with the SOLVE/RESOLVE procedure.

Area of Science:

• Structural Biology
• Crystallography
• Biophysics

Background:

• Single-wavelength anomalous diffraction (SAD) phasing is crucial for determining protein structures.
• A key challenge in SAD phasing is resolving intrinsic phase ambiguity.
• Current methods often rely solely on heavy atom-derived phase information (Sim distribution).

Purpose of the Study:

• To improve initial phase accuracy in SAD phasing.
• To integrate direct-methods phase relationships (Cochran distribution) with experimental SAD data.
• To enhance the OASIS direct-methods procedure and combine it with SOLVE/RESOLVE.

Main Methods:

• Utilized both Sim (heavy atom) and Cochran (direct-methods) distributions for initial phasing.
• Improved the OASIS direct-methods phasing procedure.
• Integrated the enhanced OASIS with the SOLVE/RESOLVE software package.

Main Results:

• Experimental SAD data from three known proteins were used for validation.
• Phases derived using combined Sim and Cochran information were more accurate.
• The RESOLVE program, initialized with experimental, Sim, and direct-methods phases, yielded superior results.

Conclusions:

• Combining heavy atom and direct-methods phase information significantly improves initial SAD phasing.
• The enhanced OASIS procedure integrated with SOLVE/RESOLVE offers a more robust phasing solution.
• Accurate initial phases are critical for successful protein structure determination via SAD.