Why did Nature choose manganese to make oxygen?

Abstract

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 Mn 2+ 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 Mn 2+ 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 O 2− 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 Mn 4 CaO x cluster that undergoes sequential oxidations by P680 + at potentials above 1 V, 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 O 2 .