Two different conformations of the R1 side chain. A) The R1-I/III conformation. The structure of monomer I is shown as green sticks; the neighbouring azurin monomer is coloured blue. The black mesh is the refined 2mFo-DFc electron density contoured at 1 σ. The purple mesh is the positive difference electron density (mFo-DFc, contoured at 3.0 σ) that was observed before the spin label was added to the structural model during refinement. B) The R1-II/IV conformation. The figure is analog to panel A, but in this case the structure of monomer II is shown in green. C) Interaction topology diagram (Ligplot+, ) of the R1 side chain in monomer I (R1-I/III conformation) and its environment. Covalent bonds are indicated by solid lines, polar interactions by dashed lines and vdW interactions by red arcs. D) Same as panel C but for the R1 side chain in monomer II (R1-II/IV conformation).