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
Water-Oxidation Chemistry of PSII
How do these small complexes compare with estimates of what the manganese cluster in PSII looks like? X-ray and EPR spectroscopies have helped to identify possible structures and oxidation states of the manganese atoms in the OEC. In particular, XAS has aided in assessing which additional atoms may be near manganese. XAS spectra of the S1 state of the Mn4 cluster in PSII indicate that it contains interatomic distances of 2.7 and 3.3 angstroms. In small molecule model complexes (for example, Figure 12c), which consist of a Mn2O2 core, the distance between manganese atoms is about 2.7 angstroms. Therefore, models that assume a distance of 2.7 angstroms for atoms in the Mn4 cluster in PSII are based on a similar core structure.
Using a value of 3.3 angstroms for the interatomic distance gives rise to several different structural models and forms the basis of considerable controversy in the field at this time. Assuming that the 3.3-angstrom distance arises from an additional manganese–manganese pair, Melvin Klein and Kenneth Sauer at the University of California at Berkeley have proposed the model of the Mn4 cluster in PSII shown in Figure 13.
In this model, two Mn2O2 cores are linked, separated by a distance of 3.3 angstroms on one side of the cluster. To attain the 3.3-angstrom distance, the Mn2O2 cores are connected by only one oxygen atom. The other atoms bridging the Mn2O2 cores are carboxylate groups derived from amino acids in the protein. Although some of the essential features of the Mn4 cluster of PSII are probably well represented by the model in Figure 13, it should be considered a "working" model to be adapted to accommodate new ideas and results.
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