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Figure 1
General strategy for neutron crystallographic refinement of biological macromolecules. The X-ray model serves as a starting point; H atoms are subsequently added and the model is optimized to obtain the best agreement with the observed neutron data. The initial positions of some H atoms can be predicted from the known geometries of certain functional groups. For others, particularly those in solvent molecules, initial positions can only be determined from inspecting the neutron scattering density map. The proportions of hydrogen and deuterium at labile sites must also be refined in cases where the crystal has been grown or soaked in D2O to enhance its neutron scattering properties. D atoms (neutron scattering length of 6.67 × 10−15 m) appear as strong positive peaks in neutron scattering density maps, thereby revealing the location of isotopically exchanged H atoms and enhancing the scattering power of water molecules, whilst H atoms themselves (neutron scattering length of −3.7 × 10−15 m) appear as negative troughs. For amino-acid side chains that can be ionized, the neutron scattering density maps must be inspected to determine whether or not a H or D atom is present and in what position.

Journal logoBIOLOGICAL
ISSN: 1399-0047
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