Catalytic Oxygen Evolution by a Bioinorganic Model of the Photosystem II Oxygen-Evolving Complex

Bioinorganic chemistry is a discipline that few undergraduates experience within a laboratory setting. This article details a series of experiments suitable for advanced undergraduates that will introduce them to the preparation and characterization of a bioinorganic model complex. Techniques and concepts emphasized include inorganic synthesis, UVandNdash;visible spectroscopy, titrations, catalytic mechanisms, oxidationandNdash;reduction reactions, and kinetics. The experiments are relevant to photosynthetic water oxidation, a process catalyzed by photosystem II (PSII) at an active site that contains a tetrameric μ-oxo-bridged manganese (Mn₄) cluster. Bioinorganic models of the Mn₄ cluster are invaluable in understanding the mechanism of water oxidation by PSII and possibly in developing artificial water-oxidation catalysts. In this laboratory, students explore the chemistry of a dimeric μ-oxo-bridged manganese complex that is a functional model for the Mn₄ cluster. They synthesize the complex [(terpy)(H₂O)Mn^III(O)₂Mn^IV(OH₂)(terpy)]^3- (terpy = 2,2':6',2"-terpyridine) and characterize the oxidation of terpy by one-electron using UV–visible spectroscopy. The terpy complex catalytically oxidizes H₂O to O₂ when peroxymonosulfate (oxone, HSO₅^-) is used as the primary oxidant. Students will measure the rate of O₂ production and calculate the deuterium kinetic isotope effect when D₂O is used in place of H₂O as a substrate.