ONE-DIMENSIONAL AND 2-DIMENSIONAL ELECTRON-SPIN ECHO ENVELOPE MODULATION STUDY OF THE INTERMEDIATE ELECTRON-ACCEPTOR PHEOPHYTIN IN N-14-LABELED AND N-15-LABELED PHOTOSYSTEM-II

The intermediate electron acceptor of the photosystem II (PSII) reacti on center, which is a pheophytin-cr (Pheo) molecule, was studied by pu lsed EPR. Electron spin echo envelope modulation (ESEEM) experiments w ere carried out in N-14- and N-15-labeled PSII reaction centers. The a ssignment of the ESEEM frequencies was achieved by the application of three-pulse (stimulated echo) and four-pulse 2D-HYSCORE sequences. Com puter simulations of the N-14- and N-15-stimulated ESEEM spectra were performed, yielding a complete set of hyperfine and quadrupole couplin g constants of the four nitrogens of the macrocycle of the pheophytin. The nitrogen hyperfine couplings are characterized by considerable an isotropy while the isotropic part is either smaller or much larger tha n the nuclear Larmor frequency: The present results show that the spin density distribution on the Pheo(-) in PSII is highly asymmetric. In agreement with earlier theoretical predictions for Pheo(-) in vitro, t he ESEEM data show that in PSII the two unprotonated nitrogens bear co nsiderable spin density while the protonated nitrogens bear a small fr action of the spin density.