Photosystem II is a membrane protein complex that uses light to catalyze water oxidation and quinone reduction. The oxygen-evolving complex (OEC) of photosystem II is the source of atmospheric oxygen and is also an important example of a metalloradical enzyme. Our long-term objectives are to develop an understanding at the molecular level of the structure and assembly of the active site of water oxidation, the mechanism of water oxidation, the electron-transfer properties of photosystem II and the damaging side reactions that occur in the process of water oxidation. In this project, a combined approach of biophysical studies of the OEC and bioinorganic studies of Mn model complexes will be used. There is an important synergism between these two approaches. The biophysical studies yield information on the enzyme's active site, whereas, the model systems are more easily characterized and aid in the interpretation of data from the natural system. Five specific aims will be pursued. (1) A new class of redox-active herbicides will be used to produce oxidized intermediates of the OEC in high yield for EPR and x-ray spectroscopic measurements. (2) A new application of near-infrared excitation resonance Raman spectroscopy will be used to study the structure and function of the Mn cluster in the OEC. (3) The first catalytic oxygen-evolving Mn model complex has been prepared; mechanistic and spectroscopic studies of this and related systems will be carried out. (4) The role of tyrosine Z as an electron/proton-transfer center will be characterized. (5) Paramagnetic cations will be substituted into the Ca(ll) site in the OEC in order to use EPR spectroscopy to study the Ca(ll) site. These proposed studies will lead to a clearer picture of the water-oxidation chemistry of photosystem II. They also have direct application to the structure and function of metal ion clusters and amino-acid radicals in other proteins and to the mechanisms of all enzymes that utilize oxygen or hydrogen peroxide as a substrate.