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Although the global demand for energy is steadily increasing, most of the current sources of energy are either nonrenewable, nonsustainable, or contribute toward greenhouse gases in the environment.  In addition to wind power and biofuels, solar energy is a renewable and clean alternative energy source, but current methods used to convert it into transportable fuels are costly and inefficient.  However, solar energy is used efficiently in Nature via the process of photosynthesis in plants, cyanobacteria, and algae.


Photosynthesis is a biochemical process by which green plants convert solar energy to carbohydrates.  In this process, water is oxidized and dioxygen is formed via the photo-induced reaction:

2H2O → O2 + 4e- + 4H+

The photosynthetic splitting of water (i.e. oxygen evolution) is the source of nearly all of the O2 in the atmosphere, and takes place in the oxygen evolving complex (OEC), which is located in the multisubunit membrane protein complex Photosystem II (PSII).  The OEC is a cluster of four Mn atoms and one Ca atom (Mn4CaO5), and has been shown via x-ray spectroscopy to be linked by mono- and di-μ-oxo or hydroxo bridges.  The OEC cycles through five different oxidation states, known as Si states (i = 0 - 4), coupling the one-electron photochemistry of the reaction center with the four-electron chemistry of water oxidation.  Although the structure and mechanism of Mn4CaO5 has been extensively studied by various methods, the precise molecular details remain elusive.


The critical questions that remain to be answered involve how the Mn4CaO5 cluster changes structurally and electronically as the OEC proceeds through the S-state Kok cycle.  In our laboratory, we investigate the oxidation state, electronic structure, geometry, and associated cofactors of the catalytic Mn cluster in PSII using X-ray and electron paramagnetic resonance (EPR) spectroscopy.  Manganese K-edge x-ray spectroscopy, K-beta x-ray emission spectroscopy (XES), and extended x-ray absorption fine structure (EXAFS) studies have determined not only the oxidation states and structural features, but also the changes that occur in the oxidation state of the Mn cluster and in its structural organization during the accumulation of oxidizing equivalents leading to O2 formation.  By obtaining the structure of Mn4CaO5 and understanding the mechanism of photosynthetic water oxidation, we hope to provide helpful guidelines toward the design and utilization of biomimetic catalysts for water splitting and efficient energy-consuming applications.


It is clear that Mn complexes play crucial roles in the metabolism of O2 in many biological systems.  In the Kok cycle, O2 is not released until the transition from S4 to S0, whereas the release of protons occurs along the cycle starting from S0.  There is no doubt that this process is critical for keeping the oxidation potential of the OEC low enough so that subsequent oxidation events can occur.  Although the formation of the O-O bond during photosynthesis has been extensively investigated via a number of methods, the mechanism of this process is still under debate.

Previous work has suggested that high-valent Mn-oxo species play a catalytically important role in the water splitting reaction leading to the formation of O2, and many inorganic transition metal complexes with varying ligands have been synthesized to model these types of intermediates.  X-ray spectroscopic studies of some of these complexes will help to identify the reactive intermediates in the Kok cycle, providing a useful tool toward understanding the O-O bond formation in both natural and artificial systems.

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