| Cytochrome b559 (Cyt b559), beta-carotene (Car), and chlorophyll (Chl) cofactors participate in the secondary electron-transfer pathways in photosystem II (PS II), which are believed to protect PS II from photodamage under conditions in which the dominant electron-transfer pathway, responsible for water oxidation and plastoquinone reduction, is inhibited. Among these cofactors, Cyt b559, the terminal electron donor in PS II, is preferentially photooxidized under conditions in which the dominant electron-transfer pathway is blocked. When Cyt b559 is preoxidized, the photooxidation of intermediate electron donors is observed. These include several of the 11 Car and 35 Chl molecules present per PS II, which are oxidized to their cation radicals Car•+ and Chl•+. In this thesis, the discovery of the secondary electron donors, their structures and electron-transfer properties, and progress in the characterization of the secondary electron-transfer pathways are introduced in Chapter one. In Chapter two, the Car in the D2 subunit of PS II, midway between Cyt b 559 and the primary oxidant, is characterized as the initial electron donor in the secondary electron transfer pathway by near-IR and EPR experiments, and its near-IR spectrum is determined. In Chapter three, a peak at 750 nm in the near-IR spectrum of PS II is assigned to a neutral radical of Car (Car •) based on its similarity to a Car• peak in the model system MCM-41, and further supported by its increase with increasing pH in PS II samples. In Chapter four, a comparison of the nearIR spectra of spinach BBY PS II samples, spinach core PS II samples, and Synechocystis core PS II samples indicate that there are species differences between spinach and Synechocystis PS II that do not vary depending on the type of spinach PS II preparation. However, there is a difference in the lifetime of the cation radicals between the different spinach PS II preparations, indicating that the presence of difference subunits affects the environment of some of the radicals located on interfaces. In this thesis, the spectroscopic signatures from different secondary electron donors have been further characterized. |
