X-ray magnetic circular dichroism of Pseudomonas aeruginosa nickel(II) azurin. Funk, T., Kennepohl, P., Di Bilio, A., J., Wehbi, W., a., Young, A., T., Friedrich, S., Arenholz, E., Gray, H., B., & Cramer, S., P. J. Am. Chem. Soc., 126(18):5859-66, 5, 2004.
X-ray magnetic circular dichroism of Pseudomonas aeruginosa nickel(II) azurin. [pdf]Paper  X-ray magnetic circular dichroism of Pseudomonas aeruginosa nickel(II) azurin. [link]Website  abstract   bibtex   
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 L(3)-centroid energy is 853.1(1) eV, the branching ratio is 0.722(4), and the magnetic moment is 1.9(4) mu(B). Density functional theory (DFT) calculations on model NiAz structures establish that orbitals 3d(x2-y2) and 3d(z2) 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(x2-y2) having much greater ligand character.

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