Hydrogen bonds around the axial ligands. The structures around the axial ligand His68 (a) in the oxidized form 1 structure and (b) in the reduced form 1 structure. The amino-acid residues and the haem molecule are shown as stick models and water molecules are shown as spheres. The water molecule shows an alternate conformation in the reduced form 1 structure. Dashed lines indicate hydrogen bonds between the amino-acid residues and water molecules and between His68 and Phe63. The bond distances between the Nδ1 atom of His68 and the carbonyl O atom of Phe63, between the amide N atom of His68 and the O atom of water, and between the carbonyl O atom of Phe63 and the O atom of water (in Å) are indicated in the figures.