The primary donors of two structurally characterised photosynthetic bacterial 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, aiso, were considerably smaller than literature reports from in vitro studies. The results for P960+and P870+are in accord with data from other spectroscopic techniques and reveal that the electron is more localised in one half of the chlorophyll dimer with a ratio of 2:1. Damping of the 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 P700+is either a chlorophyll dimer with the electron extensively localised on one of the chlorophyll pair with a ratio of 3:1 or 4:1, depending on which of the published values of aisofor chlorophyll a+is used, or a monomer. © 1993.
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