X-ray Magnetic Circular Dichroism Sum Rule Analysis of the Blue Copper Site in Plastocyanin. A Probe of Orbital and Spin Angular Momentum

Cu L edge X-ray magnetic circular dichroism (XMCD) spectroscopy has been used to study the electronic structure of the blue copper site in plastocyanin. Application of the orbital angular momentum sum rule leads to an experimental (Cu 3d specific) orbital moment 〈Lz 〉 of 0.07 ± 0.02 ℏ/Cu atom. This compares to 0.054 ℏ derived from the EPR measurements and 0.059 ℏ/Cu atom from a SCF-Xα-SW calculation. Application of the spin angular momentum sum rule leads to an experimental spin moment 〈S z 〉 of 0.18 ± 0.02 ℏ/Cu atom, compared to 0.21 ℏ/Cu atom by SCF-Xα-SW. XMCD sum rule analysis should be a useful probe of electronic structure for many other problems in inorganic and bioinorganic chemistry.