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Fig. 5. The dependence of the calculated copper–ligand atom distances, Cu–His-39 (□), Cu–Cys-89 (○), Cu–His-92 (D), and Cu–Met-97 (Ñ), on the spin density, r^Cu, of the copper atom. The corresponding spin densities on the Cys-89 sulfur atom were, r_S = 0.87 – r^Cu, whereas those of the other orbitals were fixed at the value given in Table 2. The total spin density was 1 in all cases. The vertical bars indicate the uncertainties of the distances. The vertical dashed lines at r_Cu = 0.40 and r_Cu = 0.50, corresponding to 0.85 < r_Cu/r_S < 1.35, are the outer bounds for the experimentally determined covalency of the blue copper site. The large fluctuation of the Cu–Met-97 distance for r_Cu > 0.50 and r_Cu < 0.38 is caused by an unrealistically large Cu–Met-97 distance (»6.5 Å) in one of the structures in a ensemble of 10 structures. The solid-line curves correspond to the average of all 10 structures, and the dashed curves correspond to an average of nine of the structures, where the diverging structure was removed.

Fig. 6. Single-exponential recovery of the longitudinal magnetization of the His-39/His-92 Hd² (A) and His-39/His-92 He¹ (B) signals observed in the SERF spectrum in Fig. 3. The solid lines correspond to three parameters single exponential fit to the data shown. Here the relaxation rates obtained from the fits are 0.309 ± 0.002 ms^–1 (A) and 3.3 ± 0.2 ms^–1 (B), respectively.