Anomalous signal indicators in protein crystallography

Area of Science:

• Crystallography
• Structural Biology
• Computational Chemistry

Background:

• Single-wavelength anomalous diffraction (SAD) phasing is crucial for solving protein structures.
• Accurate assessment of SAD phasing feasibility and substructure solution is vital for structural biologists.
• Simulating experimental errors in X-ray data is essential for method development.

Purpose of the Study:

• To develop a Monte Carlo procedure for simulating X-ray diffraction data with realistic errors.
• To evaluate the utility of simulated data for estimating SAD phasing outcomes and figures of merit.
• To assess the feasibility of solving protein structures using the SAD method based on simulated data.

Main Methods:

• A Monte Carlo procedure was employed to generate random structure factors.
• Simulated errors were incorporated to mimic a protein X-ray data set with heavy-atom content.
• The simulation was used to estimate Bijvoet ratios, figures of merit, and correlation coefficients.

Main Results:

• The simulated data set effectively estimates Bijvoet ratios and figures of merit for SAD phasing.
• The simulation provides an estimate of the correlation between Bijvoet amplitude differences and heavy-atom model structure-factor amplitudes.
• The procedure allows for the estimation of significant Bijvoet intensity differences, termed 'measurability', as a quality indicator.

Conclusions:

• The developed Monte Carlo procedure is a valuable tool for gauging the feasibility of SAD phasing and substructure solution.
• The simulation aids in understanding the quality of anomalous data through the 'measurability' metric.
• This method can guide experimental design and data interpretation in protein crystallography.