The complex trapped in the active sites of the APD–4NC structure. (a) 4NC fitting the electron density. (b) 4NC binding the iron oxidized from the ferrous state to the ferric state. Hydrogen bonds and the 4NC–Fe3+ interaction are represented by dashed lines. Numbers indicate bond distances in Å. (c) The Fe-coordination sphere with overlaid ligands: intermediate, product and 4NC. The APD intermediate model is denoted with atom-indexed colours; the product ligand is in cyan and the APD–4NC model is in green.