Determination of the Magnetic Axes of Cobalt(II) and Nickel(II) Azurins from ¹H NMR Data:  Influence of the Metal and Axial Ligands on the Origin of Magnetic Anisotropy in Blue Copper Proteins^†

The orientation and the axial, Δχ_ax, and rhombic, Δχ_rh, components of the magnetic susceptibility tensor anisotropy for the cobalt(II) and nickel(II) derivatives of azurin from Pseudomonas aeruginosa have been determined from ¹H NMR data. For both derivatives, the axial geometry of the system determines the orientation of the χ-tensor, whose z-axis forms an angle of 18.6 and 20.1 degrees with the Cu−OGly45 axial bond in the cobalt(II) and nickel(II) derivatives, respectively. For protons close to this axis, large negative pseudocontact shifts are observed, while those close to the NNS plane of the equatorial ligands experience lower and positive pseudocontact shifts for the same distance. Dipolar shifts are larger in the cobalt derivative, not only because of the larger spin number but also due to its intrinsically higher anisotropy. The contact contribution to the hyperfine shifts for the coordinated residues has been evaluated and analyzed in terms of unpaired spin delocalization mechanisms and geometry considerations. The results are extended to other blue copper proteins whose cobalt derivatives have been studied by ¹H NMR. The electronic structure and its implications in the redox properties of the native copper proteins are also commented.