Structural detail of the trinuclear Copper centre. In each picture the electron density for the moiety is derived from omit Fourier syntheses computed with SigmaA weighted coefficients |Fo| - |Fc|; the moieties were not included in the structure factor calculations and five cycles of maximum likelihood refinement were computed using REFMAC prior to Fourier synthesis to minimise phase bias. Contour levels are 5 rms for both electron density maps. (a) In the holoCotA structure a dioxygen molecule is bound into the trinuclear centre. (b) In the apoCu(I) a hydroxyl group is bound to the trinuclear centre. Both pictures were made with PyMol .