Fig. 4. T2Cu site in NO-bound/T1Cu-reduced Br^2DNiR: SRX v SF-ROX structures.

(A) SRX NO-bound structure (1.19 Å; orange) has two conformations of the active site: (B) one with NO present and (C) with three coordinated T2Cu site. (D) In the 1.3 Å XFEL FRIC structure (magenta), there are three alternative conformations of the T2Cu site: (E) one with NO, (F) second with water, and (G) third with three coordinated T2Cu site where NO has vacated the T2Cu site with alternative Ile²⁵² conformation of CD atom flipping down to fill the space. Water molecules are shown as red spheres, and T2Cu is shown as a blue sphere. Distances for possible hydrogen bonding are shown as black dashed lines; gray dashed lines represent unlikely bonding due to steric restraints, and coordination distances to T2Cu are shown as red dashed lines. OMIT F_o − F_c electron density maps around T2Cu ligands are contoured at 5σ level and colored green. Gray 2F_o − F_c electron density map is contoured at 1σ level.