Mn Extended X-ray Absorption Fine Structure Research Articles

Structural Changes Correlated with Magnetic Spin State Isomorphism in the S2 State of the Mn4CaO5 Cluster in the Oxygen-Evolving Complex of Photosystem II.

The Mn4CaO5 cluster in Photosystem II catalyzes the four-electron redox reaction of water oxidation in natural photosynthesis. This catalytic reaction cycles through four intermediate states (Si, i = 0 to 4), involving changes in the redox state of the four Mn atoms in the cluster. Recent studies suggest the presence and importance of isomorphous structures within the same redox/intermediate S-state. It is highly likely that geometric and electronic structural flexibility play a role in the catalytic mechanism. Among the catalytic intermediates that have been identified experimentally thus far, there is clear evidence of such isomorphism in the S2 state, with a high-spin (5/2) (HS) and a low spin (1/2) (LS) form, identified and characterized by their distinct electron paramagnetic resonance (EPR spectroscopy) signals. We studied these two S2 isomers with Mn extended X-ray absorption fine structure (EXAFS) and absorption and emission spectroscopy (XANES/XES) to characterize the structural and electronic structural properties. The geometric and electronic structure of the HS and LS S2 states are different as determined using Mn EXAFS and XANES/XES, respectively. The Mn K-edge XANES and XES for the HS form are different from the LS and indicate a slightly lower positive charge on the Mn atoms compared to the LS form. Based on the EXAFS results which are clearly different, we propose possible structural differences between the two spin states. Such structural and magnetic redox-isomers if present at room temperature, will likely play a role in the mechanism for water-exchange/oxidation in photosynthesis.

Structure of Mn films grown on (111) and (001) fcc Ir determined by EXAFS and the multiple-scattering approach.

Mn films were grown on (001) and (111) fcc Ir lattices by molecular beam epitaxy, extended x-ray absorption fine structure (EXAFS) experiments were performed on these films in order to determine the Mn structures. As Mn can adopt different crystalline structures, it is first shown that the usual Fourier treatment of EXAFS oscillations is not an appropriate technique for this purpose. On the contrary, the application of multiple scattering theory of x-ray-absorption fine structure allows us to identify the structures. For relaxed (polycrystalline) thick Mn deposits, Mn crystallizes in the Mn\ensuremath{\alpha} structure. For unrelaxed (single-crystalline) thick Mn films, the Mn EXAFS oscillations are well simulated assuming a mixing of fcc and Mn\ensuremath{\alpha} structure. For thin Mn films in Mn/Ir (111) and (001) superlattices, this multiple scattering approach allows us to show that Mn is in a trigonal structure when it is grown on Ir(111) and a tetragonal structure when it is grown on Ir(001). Finally, the (\ensuremath{\surd}3\ifmmode\times\else\texttimes\fi{}\ensuremath{\surd}3)R30\ifmmode^\circ\else\textdegree\fi{} superstructure of Mn films grown on (111) Ir is shown to come from the epitaxy of a mixed fcc and Mn\ensuremath{\alpha} structures and not from the occurence of Mn Laves phases like ${\mathrm{Cu}}_{2}$Mg or ${\mathrm{Zn}}_{2}$Mg as assumed by several workers. \textcopyright{} 1996 The American Physical Society.