A TIME-RESOLVED ELECTRON-NUCLEAR DOUBLE-RESONANCE STUDY OF THE PHOTOEXCITED TRIPLET-STATE OF P680 IN ISOLATED REACTION CENTERS OF PHOTOSYSTEM-II

A time-resolved ENDOR spectrum of the triplet state of the primary don or of photosystem II in isolated reaction centers has been obtained at cryogenic temperatures. Selective excitation of donor molecules, whic h have their z triplet axes parallel to the magnetic field, has permit ted the measurement of the A(zz) component of the hyperfine coupling t ensor of protons, in the reference frame of the zero field splitting t enser. For comparison, measurements have also been made on the photoex cited triplet state of monomeric chlorophyll a in vitro. The results i ndicate that in the temperature range 10-20 K the triplet state of the primary donor of photosystem II is largely located on a monomeric chl orophyll a molecule.