The Link to X-Ray Protein Crystallography

A. Crystal Production

(wet lab)

  1. Production
  2. Purification
  3. Crystallisation



B. Data Collection

(x-ray lab)

  1. Mounting
  2. Shooting
  3. Detection


C. Structure Solution

(computer lab)

  1. Indexing
  2. Integration
  3. Scaling
  4. Phasing
  5. Building
  6. Refinement
  7. Validation
  8. Publication

Phase determination

The phase problem is notorious in x-ray crystallography. The x-ray detector can only record intensities but not phases of the electromagnetic waves. Each reflection on the diffraction pattern or structure factor corresponds to a wave consisting of an amplitude and a phase. The amplitude is easily calculated by taking the square root of the intensity, but the phase is lost during the data collection. However, the phases contain vital information for the determination of the electron density distribution in the crystal. For easier representation a structure factor can be represented as a vector or complex number Fhkl with the amplitude |Fhkl| and the phase φhkl. The representation is called an Argand diagramme. There are different ways to find initial phases that can subsequently be refined.


  1. Direct methods (work only for small molecules)
  2. Molecular replacement (MR)
  3. Isomorphous replacement (SIR, MIR)
  4. Anomalous dispersion (SAD, MAD)
  5. Combination of methods (SIRAS, MIRAS)

Mission / Organisations / Trivia / Links / Florian Fisch / 5 June 2009