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Next: 7. Small Structures in Biology Up: Elementary X-Ray Diffraction for Biologists Previous: 5. The Resolution of the Structure

6. Differences in Techniques for Small and Large Molecules


    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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Next: 7. Small Structures in Biology Up: Elementary X-Ray Diffraction for Biologists Previous: 5. The Resolution of the Structure

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