Locating heavy atoms by integrating direct methods and SIR techniques.

Area of Science:

• Crystallography
• Structural Biology
• X-ray Diffraction

Background:

• Estimating structure-factor moduli from diffraction data is crucial for solving crystal structures.
• Traditional methods often do not fully account for experimental errors in single isomorphous replacement (SIR) phasing.
• Accurate structure-factor moduli are essential for direct methods and subsequent structure determination.

Purpose of the Study:

• To develop and validate a novel approach for estimating structure-factor moduli in SIR experiments.
• To incorporate cumulative errors from measurements and lack of isomorphism into the estimation process.
• To improve the accuracy and reliability of crystallographic phasing and structure solution.

Main Methods:

• Direct methods were applied to the SIR case, calculating the joint probability distribution function P(E(H),E(p),E(d)).
• Cumulative errors from measurements and lack of isomorphism were explicitly included as a primitive random variable.
• Patterson techniques and tangent procedures were implemented in a computer program using the new estimates.

Main Results:

• The new approach provides estimated structure-factor moduli (|E(H)|) that depend on both diffraction data and experimental errors.
• Test structures demonstrated the efficiency of the new estimates.
• Automated heavy atom location was achieved using Patterson techniques with the improved |E(H)|(2) coefficients.

Conclusions:

• The developed method offers a more robust estimation of structure-factor moduli in SIR experiments.
• The new approach yields results highly competitive with traditional |E(H)|(2) estimates.
• This advancement has the potential to enhance automated crystallographic structure solution.