X-ray Magnetic Circular Dichroism of Pseudomonas aeruginosa Nickel(II) Azurin

Abstract

We show that X-ray magnetic circular dichroism (XMCD) can be employed to probe the oxidation states and other electronic structural features of nickel active sites in proteins. As a calibration standard, we have measured XMCD and X-ray absorption (XAS) spectra for the nickel(II) derivative of Pseudomonas aeruginosa azurin (NiAz). Our analysis of these spectra confirms that the electronic ground state of NiAz is high-spin (S = 1); we also find that the L3-centroid energy is 853.1(1) eV, the branching ratio is 0.722(4), and the magnetic moment is 1.9(4) μ_B. Density functional theory (DFT) calculations on model NiAz structures establish that orbitals 3d_x^2-y^2 and 3d_z^2 are the two valence holes in the high-spin Ni(II) ground state, and in accord with the experimentally determined orbital magnetic moment, the DFT results also demonstrate that both holes are highly delocalized, with 3d_x^2_(-y)^2 having much greater ligand character.

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