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

Abstract

The orientation and the axial, Deltachiax, and rhombic, Deltachirh, 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 chi-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.

Active Site Comparison of CoII Blue and Green Nitrite Reductases

“…[52] More sizable changes upon the addition of nitrite could be anticipated at even lower pH values, however the T1Cu I /T2Co II NiR derivatives were not stable under these conditions. [27] which has a classic T1 copper site, and Co II rusticyanin (RST), [29] which possesses a distorted center (see Table 3). [27] ).…”

“…[27] which has a classic T1 copper site, and Co II rusticyanin (RST), [29] which possesses a distorted center (see Table 3). [27] ). [27] ), but the C e H 3 proton resonance of this ligand will not display any angular dependence.…”

“…[27] ). [27] ), but the C e H 3 proton resonance of this ligand will not display any angular dependence.…”

“…[27] q) on the dihedral angles must prevail. This suggests that a s-type spin delocalization mechanism dominates in this case.…”

Paramagnetic Tools in Protein NMR

“…The exchange of a lanthanide ion can also be done for Mg 2 þ , which, like Ca 2 þ , is a group 2 alkaline earth metal [56]. Diamagnetic transition metals have also been exchanged for paramagnetic ones, for instance to obtain PCSs to monitor protein domain motions after introduction of Co 2 þ in the Zn 2 þ binding site in protein PA0128 [57], and Co 2 þ and Ni 2 þ were exchanged for Cu 2 þ in the blue copper protein azurin to obtain contact and pseudocontact shifts of the metal coordinating residues [41,58].…”