Next: 7. Small Structures in Biology Up: Elementary X-Ray Diffraction for Biologists Previous: 5. The Resolution of the Structure
|Smaller molecules||Macro molecules|
|1.||Data collection||Diffract well to high resolution. Generally stable in air. Mount dry on fibre unless unstable. Several thousand reflections.||Diffract to lower resolution (some to higher resolution). Generally unstable to drying. Mount in capillary with mother liquor. Tens of thousands of reflections or more.|
|2.||Structure solution||Patterson, heavy atom or direct methods. One crystal only.||Isomorphous replacement. This involves many crystals. Heavy atom derivatives required.|
|3.||Refinement of structure||Full matrix least squares methods. Hydrogen atoms located.||Fitting of polypeptide or polynucleotide chain to electron density map. Constrained and/or restrained least squares methods. Hydrogen atoms not located. Most carbon atoms are also not well-defined except for high-resolution studies.|
See Figs. 3 and 4 (flow charts).
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