Issue Date: August 8, 2011
Model Cluster Yields Clues On Calcium’s Water-Splitting Role
A synthetic model of the manganese-calcium cluster at the heart of the oxygen-evolving complex, a component of the photosystem II protein complex that guides photosynthesis, suggests how calcium is critical to the cluster’s ability to oxidize water to form O2 in plants, algae, and cyanobacteria (Science, DOI: 10.1126/science.1206036). Scientists have tried to develop a model of the Mn4CaOn cluster to understand how it works and design artificial photosynthetic systems. The cluster is likely composed of a Mn3CaO4 cube bridged through an oxide or hydroxide to the fourth manganese atom. But synthesizing a cluster that accurately mimics the protein-bound form has proved difficult. In the latest effort at unraveling calcium’s role, a research group led by Theodor Agapie of California Institute of Technology used a rigid 1,3,5-triarylbenzene ligand festooned with pyridine and alcohol groups to make a Mn3CaO4 cluster with bond lengths that come close to those observed in the protein system. By comparing the Mn-Ca cluster to an all-Mn version they found that the Ca-containing cluster has a significantly lower reduction potential. The researchers propose that the redox-inactive Ca2+ allows positive charge to build up on the manganese atoms, enabling the Mn4+ oxidation state necessary to produce O2.
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