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 be en used to study the electronic structure of the blue copper site in p lastocyanin. Application of the orbital angular momentum sum rule lead s to an experimental (Cu 3d specific) orbital moment [L-z] of 0.07 +/- 0.02 (h) over bar/Cu atom. This compares to 0.054 (h) over bar, deriv ed from the EPR measurements and 0.059 (h) over bar/Cu atom from a SCF -X alpha-SW calculation. Application of the spin angular momentum sum rule leads to an experimental spin moment [S-z] of 0.18 +/- 0.02 (h) o ver bar/Cu atom, compared to 0.21 (h) over bar/Cu atom by SCF-X alpha- SW. XMCD sum rule analysis should be a useful probe of electronic stru cture for many other problems in inorganic and bioinorganic chemistry.