Polarized X-ray absorption spectroscopy of single-crystal Mn(V) complexes relevant to the oxygen-evolving complex of photosystem II.
High-valent Mn-oxo species have been suggested to have a catalytically important role in the water splitting reaction which occurs in the Photosystem II membrane protein. In this study, five- and six-coordinate mononuclear Mn(V) compounds were investigated by polarized X-ray absorption spectroscopy in order to understand the electronic structure and spectroscopic characteristics of high-valent Mn species. Single crystals of the Mn(V)-nitrido and Mn(V)-oxo compounds were aligned along selected molecular vectors with respect to the X-ray polarization vector using X-ray diffraction. The local electronic structure of the metal site was then studied by measuring the polarization dependence of X-ray absorption near-edge spectroscopy (XANES) pre-edge spectra (1s to 3d transition) and comparing with the results of density functional theory (DFT) calculations. The Mn(V)-nitrido compound, in which the manganese is coordinated in a tetragonally distorted octahedral environment, showed a single dominant pre-edge peak along the MnN axis that can be assigned to a strong 3d(z(2))-4p(z) mixing mechanism. In the square pyramidal Mn(V)-oxo system, on the other hand, an additional peak was observed at 1 eV below the main pre-edge peak. This component was interpreted as a 1s to 3d(xz,yz) transition with 4px,y mixing, due to the displacement of the Mn atom out of the equatorial plane. The XANES results have been correlated to DFT calculations, and the spectra have been simulated using a TD (time-dependent)-DFT approach. The relevance of these results to understanding the mechanism of the photosynthetic water oxidation is discussed.
Yano
Junko
J
Physical Biosciences Division, Lawrence Berkeley National Laboratory, Berkeley, California 94720, USA. jyano@lbl.gov
Robblee
John
J
Pushkar
Yulia
Y
Marcus
Matthew A
MA
Bendix
Jesper
J
Workman
José M
JM
Collins
Terrence J
TJ
Solomon
Edward I
EI
George
Serena Debeer
SD
Yachandra
Vittal K
VK
D003460:Crystallization; D004583:Electrons; D017895:Manganese Compounds / Q000737:chemistry*; D008958:Models, Molecular; D015394:Molecular Structure; D017672:Nitrogen Compounds / Q000737:chemistry; D010100:Oxygen / Q000737:chemistry* / Q000378:metabolism*; D045332:Photosystem II Protein Complex / Q000737:chemistry* / Q000378:metabolism*; D013053:Spectrophotometry; D014965:X-Rays
D017895:Manganese Compounds; D017672:Nitrogen Compounds; D045332:Photosystem II Protein Complex; D010100:Oxygen