MODULATION ANALYSIS OF THE ELECTRON-SPIN ECHO SIGNALS OF IN-VIVO OXIDIZED PRIMARY DONOR N-14 CHLOROPHYLL CENTERS IN BACTERIAL, P870 AND P960, AND PLANT PHOTOSYSTEM-I, P700, REACTION CENTERS

The primary donors of two structurally characterised photosynthetic ba cterial reaction centres, P870+ from Rhodobacter sphaeroides and P960 from Rhodopseudomonas viridis, and the spinach Photosystem I primary donor P700+ were studied by electron spin echo modulation. The values obtained for the in vivo isotropic hyperfine interaction, a(iso), were considerably smaller than literature reports from in vitro studies. T he results for P960+ and P870+ are in accord with data from other spec troscopic techniques and reveal that the electron is more localised in one half of the chlorophyll dimer with a ratio of 2: 1. Damping of th e modulations was most severe for P960+, which interacts with the non- haem iron of the reaction centre as well as a donor cytochrome, cyt c. The reduced hyperfine interactions obtained for P700+ indicate that P 700+ is either a chlorophyll dimer with the electron extensively local ised on one of the chlorophyll pair with a ratio of 3: 1 or 4: 1, depe nding on which of the published values of a(iso) for chlorophyll a+ is used, or a monomer.