Structural movie of the changes observed in the water chain extending from the T1Cu to T2Cu sites of serial data sets ds1–ds40. All 2Fo − Fc density is contoured at 1σ. Dashed lines represent hydrogen bonds forming putative proton-transport pathways between water molecules. The waters labelled W9P and WG represent the partial-occupancy waters that are present when the proximal and gatekeeper conformations of Asp98, respectively, are observed. All residues, nitrite and nitric oxide are represented as sticks and water and Cu atoms are represented as as spheres; all are coloured by element.