Calcium Manganese Oxides as Oxygen Evolution Catalysts: O2 Formation Pathways Indicated by 18O-Labelling Studies

Oxygen evolution catalysed by calcium manganese and manganese-only oxides was studied in O-18-enriched water. Using membrane-inlet mass spectrometry, we monitored the formation of the different O-2 isotopologues O-16(2), (OO)-O-16-O-18 and O-18(2) in such reactions simultaneously with good time resolution. From the analysis of the data, we conclude that entirely different pathways of dioxygen formation catalysis exist for reactions involving hydrogen peroxide (H2O2), hydrogen persulfate (HSO5-) or single-electron oxidants such as Ce-IV and [Ru-III(bipy)(3)](3+). Like the studied oxide catalysts, the active sites of manganese catalase and the oxygen-evolving complex (OEC) of photosystem II (PSII) consist of moxido manganese or mu-oxido calcium manganese sites. The studied processes show very similar O-18-labelling behaviour to the natural enzymes and are therefore interesting model systems for in vivo oxygen formation by manganese metalloenzymes such as PSII.