The structure and function of the reaction center of Photosystem II (PSII) will be studied by cw and pulsed EPR spectroscopy. (I) Electron Spin Echo Envelope Modulation (ESEEM) experiments will be performed on the triplet state of the primary electron donor of PSII (3P680) and the acceptor-side non-heme iron in its low-spin Fe3(S=1/2) form. The binding of NO, an inhibitor of the photosynthetic electron transfer of PSII, will be studied by ESEEM. The effect of the NO on the properties of the 3P680 will be monitored.
(II) The spin-lattice relaxation of 3P680 with the non-heme iron in the high-(S=2) and low- (S=o) spin state will be studied; the detection of a dipolar coupling will allow the estimation of the Fe3-P680 distance. We will study the relaxation enhancement effect of Dy2+ on the acceptor-side radicals of PSII, the plastoquinone QA and the pheophytin Pheo. This will allow an estimate of the location of the radicals relative to the surface of the thylakoid membrane. The proposed research will result in original information about (a) the identity and the environment of certain paramagnetic centers in PSII i.e. 1H, 14N hyperfine interactions, hydrogend bonds, (b) the structural organisations of PSII i.e. distances between paramagnetic centers, ligand identification, (c) functional aspects of Photosystem II i.e. properties of the 3P680, the inhibotory effect of NO on the electron transport.