Theory and methods of data analysis.

This chapter will describe the basic theory of X-ray diffraction from crystals and how the anomalous scattering of atoms within a structure can provide information which may be used to determine the phases of the structure factors associated with the diffracted X-rays. In addition, the procedures and software used for the analysis of diffraction data and structure determination are described.

• X-ray diffraction.
  • Classical X-ray scattering.
  • Anomalous scattering.
    • Classical description of dispersion.
    • Absorption and the complex refractive index.
  • Diffraction from a crystal.
  • Effects of anomalous scattering on the structure factor equation.
  • Phase problem.
• Determination of the heavy atom positions.
  • Patterson methods.
  • Difference Fourier techniques for locating heavy atoms.
• Direct methods applied to the solution of the heavy atom structure.
  • Underlying theory.
  • MULTAN software package.
  • The convergence method.
  • The magic integer method.
  • Figures of merit.
  • Application to the solution of the heavy atom structure.
• Multiple wavelength Anomalous Diffraction.
  • Classical approach.
    • Treatment of errors in the classical approach to MAD phasing.
  • Algebraic approach.
• Analysis of diffraction image data collected using the rotation method.
  • The rotation method.
  • Orientation refinement and reflection integration.
    • Orientational refinement.
    • Integrated intensities by profile fitting.
    • Lorentz-Polarisation corrections.
  • Image scaling and merging.
  • Scaling of data from different wavelengths.
  • Calculation of phases.
  • Phase improvement and density modification.
  • Quality of electron density maps.
  • Model refinement.
• Evaluation of anomalous scattering factors, and .