Experimental methods for measuring accurate high-amplitude phases and their importance in isomorphous replacement...

Area of Science:

• Crystallography
• Structural Biology
• Biophysics

Background:

• Conventional experimental phasing methods struggle with low-resolution reflections, leading to phase ambiguity.
• Strong, low-resolution reflections are vital for determining the unit cell's large-scale matter distribution.
• These reflections are critical for initial phasing and model building in structure determination.

Purpose of the Study:

• To highlight the pivotal importance of a limited number of strong/low-resolution reflection phases.
• To describe a procedure for deriving these crucial phases.
• To improve the success rate of initial phasing and model-building efforts.

Main Methods:

• Comparison of map correlation coefficients after density modification.
• Utilizing a marginal ;starting' Multiple Isomorphous Replacement (MIR) data set from rhombohedral insulin crystals.
• Employing an ;expanded' data set combining accurate three-beam diffraction phases with the ;starting' MIR data.

Main Results:

• A small number of high-amplitude/low-resolution reflections disproportionately contribute to initial structure generation.
• Density modification of an ;expanded' data set showed significant improvement compared to the ;starting' data set.
• Accurate phases from a limited number of low-resolution reflections enhance initial structural models.

Conclusions:

• Low-resolution reflection phases are essential for successful initial phasing and model building.
• A procedure to derive these phases can overcome challenges in structure determination.
• Combining accurate low-resolution phases with experimental phasing methods can accelerate the structure-solution pathway for novel proteins.