Why did Nature choose manganese to make oxygen?
This paper discusses the suitability of manganese for its function in catalysing the formation of molecular oxygen from water. Manganese is an abundant element. In terms of its inherent properties, Mn has a particularly rich redox chemistry compared with other d-block elements, with several oxidizing states accessible. The most stable-state Mn2+ behaves like a Group 2 element--it is mobile, weakly complexing, easily taken up by cells and redox-inactive in simple aqueous media. Only in the presence of suitable ligands does Mn2+ become oxidized, so it provides an uncomplicated building unit for the oxygen-evolving centre (OEC). The intermediate oxidation states Mn(III) and Mn(IV) are strongly complexed by O2(-) and form robust mixed-valence poly-oxo clusters in which the Mn(IV)/Mn(III) ratio can be elevated, one electron at a time, accumulating oxidizing potential and capacity. The OEC is a Mn4CaOx cluster that undergoes sequential oxidations by P680+ at potentials above 1V, ultimately to a super-oxidized level that includes one Mn(V) or a Mn(IV)-oxyl radical. The latter is powerfully oxidizing and provides the crucial 'power stroke' necessary to generate an O-O bond. This leaves a centre still rich in Mn(IV), ensuring a rapid follow-through to O2.
Armstrong
Fraser A
FA
Department of Chemistry, Inorganic Chemistry Laboratory, Oxford University, South Parks Road, Oxford OX1 3QR, UK. fraser.armstrong@chemistry.oxford.ac.uk
D002384:Catalysis; D008345:Manganese / Q000737:chemistry*; D008956:Models, Chemical; D010084:Oxidation-Reduction; D010100:Oxygen / Q000737:chemistry*; D045332:Photosystem II Protein Complex / Q000737:chemistry*; D014867:Water / Q000737:chemistry*
D045332:Photosystem II Protein Complex; D014867:Water; D008345:Manganese; D010100:Oxygen