Chooch

Chooch derives experimental anomalous-scattering factors f' and f'' from X-ray fluorescence spectra to support wavelength selection and phasing (MAD and SAD) in macromolecular crystallography.

Key Features:

• Fortran implementation: Implemented as a Fortran program that processes X-ray fluorescence spectra to extract anomalous-scattering information.
• Derivation of f'': Calculates experimental values of f'', the imaginary component of the anomalous-scattering factor, near the absorption edge where tabular data are often inadequate.
• Scaling with theoretical f'': Leverages known theoretical values of f'' away from the absorption edge to scale the experimental fluorescence spectrum for accurate determination near the edge.
• Kramers–Kronig relation: Applies the Kramers–Kronig relation to compute the real part f' from the derived f'' values.
• Optimization for crystallographic experiments: Provides magnitudes of f' and f'' at selected wavelengths to support wavelength selection and heavy-atom refinement for phasing programs.

Scientific Applications:

• Wavelength selection for MAD and SAD: Optimizes wavelength choice for Multi-wavelength Anomalous Dispersion (MAD) and Single-wavelength Anomalous Diffraction (SAD) experiments in macromolecular crystallography.
• Heavy-atom refinement and phasing: Supplies experimentally derived f' and f'' values used in heavy-atom refinement and phasing with crystallographic programs.
• Phase determination and model building: Improves phase information necessary for constructing accurate macromolecular models.

Methodology:

Implemented in Fortran; the program scales an experimental fluorescence spectrum using theoretical f'' values away from the absorption edge to derive f'' near the edge, then applies the Kramers–Kronig relation to calculate f'.