Figure 3.

The T2Cu site as determined using femtosecond X-ray pulses. (a) 2F _o − F _c electron-density map of the SF-ROX AxNiR T2Cu site. Electron density is contoured at 2σ around the residues and ligands of the T2Cu site as a grey mesh. An oxygen diatom is ligated to the T2Cu in place of the normally observed water molecule. The O₂/superoxide is ligated by the catalytic residues Asp92_CAT and His249_CAT, which in turn are hydrogen-bonded to the linking water. (b) A simulated-annealing OMIT map showing the dioxygen ligand bound to the T2Cu site. A 2F _o − F _c electron-density map contoured at 2σ in grey mesh around the residues and Cu atom is shown superimposed with a ligand F _o − F _c electron-density map contoured at 5σ in green mesh around the refined dioxygen ligand. Atoms are coloured by element, with a different colour scheme used for the different chains. T2Cu, cyan sphere; water molecules, small red spheres; oxygen diatom, red sticks. Metal-coordinating bonds are shown as red dotted lines. Selected hydrogen bonds are shown as black dotted lines.