FEATURE ARTICLE
How Plants Produce Dioxygen
At its core, oxygen production comes down to the chemistry of a poorly understood manganese-containing complex in the membranes of plant chloroplasts
Veronika Szalai, Gary Brudvig
Making Models of the OEC
Because we know that the OEC contains four manganese atoms, it seems reasonable to assume that small compounds containing four manganese atoms could be synthesized in the laboratory. The aim would be to build molecules that convert water to O2 by behaving like the manganese cluster found in the protein PSII. Unfortunately, little progress toward this specific goal has been made. Although many complexes containing manganese exist, the vast majority of them neither oxidize water nor produce O2.
The first example of a complex that functionally mimics the manganese cluster in PSII was discovered more than two decades ago by Thomas Meyer and coworkers at the University of North Carolina at Chapel Hill; however, it does not contain manganese at all. It is a ruthenium complex containing a core of two ruthenium atoms, one oxygen atom and two water molecules (Figure 12a). The way in which this ruthenium complex evolves O2 has begun to be unraveled, but the mechanistic details of the final O2-formation step are still unknown. What is clear is that by removing electrons from the ruthenium atoms, the entire complex changes and eventually releases O2.
The key intermediate in this process is a ruthenium atom bonded to an oxygen atom through a double bond, a Ru=O species. The oxygen atom bound to ruthenium via a double bond is derived from one of the water molecules that was originally bound to the complex. Creation of a Ru=O species converts the unreactive oxygen atom in water to a very reactive oxygen atom capable of forming the O2 bond.
The second complex that has been reported to generate O2 is a manganese complex created by linking together two large molecules that each contain one manganese atom (Figure 12b). In the resulting species, the two manganese atoms are close to each other. Complexes like these have been proposed to contain Mn=O species that are similar to the Ru=O species described above.
The final manganese-containing complex shown to produce O2 catalytically has been developed in our laboratory. The core of our complex is similar to that found for the ruthenium complex, except that the water molecules are arranged differently. Current experiments suggest that our manganese model complex also uses a Mn=O species to produce O2.
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