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Electron density maps
85% of the macromolecular structures available from the Protein Data Bank (PDB) were determined by X-ray crystallography. The direct results of crystallographic experiments are electron density maps. Examining the correspondence between the electron density map and the published molecular model reveals the levels of uncertainty in the model.
Crystallography Produces Electron Density Maps
An X-ray crystallographic experiment produces an electron density map for the average unit cell of the protein crystal. The amino acid (or nucleotide) sequence of the crystallized polymer(s) is known in advance. The crystallographer fits the atoms of the known molecules into the electron density map, and refines the model and map to the limits of the resolution of the crystal (which is limited by the level of order or disorder in the crystal). The crystallographer then deposits a model of the asymmetric unit of the crystal in the PDB, along with the experimental diffraction data (amplitudes and widths of the X-ray reflection spots, or "structure factors") from which the electron density map can be reconstructed. Electron density maps are available for most PDB files from the Uppsala Electron Density Map Server.
Why Look At Electron Density Maps?
Examining the correspondence between the published model PDB file and the electron density map (EDM) provides much clearer insight into the uncertainties in the model than does merely examining the model itself (see also Quality assessment for molecular models). In addition to examining the entire map (2mFo-DFc) it is revealing to examine the difference map (mFo-DFc), which shows where the model fails to account for the map.
Visualizing Electron Density Maps
Crystallographers generally use "heavy duty" visualization and modeling software such as Coot or PyMOL, which require considerable practice to use effectively. Jmol first became capable of displaying electron density maps in January, 2010. Being able to display EDM's in Jmol opens the door to examining EDMs effectively in a web browser, with a user interface (yet to be developed) that requires no specialized software knowledge.
The ability to display electron density maps in Proteopedia is under development. Once it becomes possible, interactive maps in Jmol will be shown here. Until then, please see Electron Density Maps in Jmol.