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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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