Figure 1.

Electron density of the haem group. F _o − F _c haem OMIT maps are shown as grey and pink meshes contoured at the 3.0σ and 7.0σ levels, respectively, in the oxidized form 1 structure (a), in the oxidized form 2 structure (b), in the reduced form 1 structure (c) and in the reduced form 2 structure (d). The porphyrin-ring numbers and the positions of the 6-propionate and 7-propionate groups are indicated in the figures.