TheoryThe crystal structure of a protein is often discussed in terms of its unit cell. The unit cell is a box containing one or more motifs, a spatial arrangement of atoms. The units cells are tiled in three-dimensional space to describe the crystal. The unit cell is given by its lattice parameters, the length of the cell edges and the angles between them, while the positions of the atoms inside the unit cell are described by the set of atomic positions measured from a lattice point.

To display the crystal cell of a PDB structure:

• Read a PDB file into ICM. For instruction see the section entitled Finding a PDB Structure.
• Select the whole object. You can do this by double clicking on the name of the structure in the ICM Workspace (a selection is highlighted blue in the ICM Workspace and green crosses in the graphical display} or you can use the right-click button and drag it over the whole structure in the graphical display.
• Select the menu Tools/Xray/Crystallograhic Cell and a data entry box will be displayed.
• Click OK

The crystallographic cell will be displayed as a box as shown below.