Solving crystal structures from two-wavelength X-ray powder diffraction data - breaking the phase ambiguity in the...

Area of Science:

• Crystallography
• Structural Biology
• Materials Science

Background:

• Protein crystallography traditionally relies on techniques like molecular replacement or heavy-atom methods for phase determination.
• Ab initio structure solution from powder diffraction data remains challenging, particularly for noncentrosymmetric structures.

Purpose of the Study:

• To introduce and validate direct methods for resolving phase ambiguities in powder diffraction.
• To demonstrate the feasibility of ab initio structure solution for noncentrosymmetric crystals using two-wavelength anomalous diffraction.

Main Methods:

• Simulated two-wavelength anomalous powder diffraction data using the known structure of leotidine hydrobromide.
• Utilized X-ray wavelengths at 0.920 Å and 1.500 Å with Bromine (Br) as the anomalous scatterer.
• Developed a method to partition overlapping diffraction peaks and extract structure-factor amplitudes.

Main Results:

• Accurate localization of the Br substructure was achieved using data from the 1.500 Å wavelength.
• Unique phases for centric reflections and phase doublets for noncentric reflections were obtained.
• Direct methods successfully broke phase ambiguities, leading to an interpretable electron-density map and complete structure determination after Fourier iterations.

Conclusions:

• Direct methods are effective for breaking phase ambiguities in powder diffraction data.
• Two-wavelength anomalous diffraction combined with direct methods enables ab initio structure solution of noncentrosymmetric crystals.
• This approach offers a promising pathway for solving complex crystal structures directly from powder data.