X-Ray Crystallography

Note: These are incomplete and I have no intention of completing them.

X-Ray Chrystallography Table of Contents

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• Isomorphous replacement
  • Introduce (by diffusion) new heavy atoms into the crystal, all of which bind in the same place on the protein
  • Assumes that the new atom does not significantly perturb crystal structure
  • A single Isomorphous replacement limits each phase to 2 choices-- a second isomorphous replacement can be done to narrow it down
• Molecular replacement
  • Calculated from related structure
  • Introduces bias towards the related structure

• Refinement:
  • R value between 0.2 and 0.25 indicates that the chain tracing is most likely accurate but some side chains may be off
  • R value below 0.2 indicates that there are no major problems with the structure, although some side chain errors may remain
• Without refinement, positional errors can approach resolution/3.
  • A resolution of less than 2.5 Angstroms is largely correct
  • A resolution between 2.5 and 3.5 Angstroms will be approximately 80% correct if the sequence is known
  • A resolution of above 3.5 Angstroms indicates that errors may even exist in the chain trace

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