Spin conversion of cytochrome b559 in photosystem II induced by exogenous high potential quinone

The spin-state of cytochrome b(559) (Cyt b(559)) was studied in photosystem II (PSII) membrane fragments by low-temperature EPR spectroscopy. Treatment of the membranes with 2,3-dichloro-5,6-dicyano-p-belizoquinone (DDQ) converts the native low-spin (LS) form of Cyt b(559) to the high-spin (HS) form characterized with the g = 6.19 and g = 5.95 split signal. The HS Cyt b(559) was pH dependent with the amplitude increasing toward more acidic pH values (pH 5.5-8.5). The HS state was not photochemically active upon 77 and 200 K continuous illumination under our conditions and was characterized by a low reduction potential (less than or equal to 0 V). It was also demonstrated that DDQ treatment damages the oxygen evolving complex, leading to inhibition of oxygen evolution, decrease of the S-state EPR multiline signal and release of Mn2+. In parallel, studies of model systems containing iron(III) protoporphyrin IX chloride (Fe(III)Por), which is a good model compound for the Cyt b(559) prosthetic group, were performed by using optical and EPR spectroscopy. The interaction of Fe(III)Por with imidazole (Im) in weakly polar solvent results in formation of bisimidazole coordinated heme iron (Fe(III)Por Im,) which mimic the bis-histidine axial ligation of Cyt b(559). The reaction of DDQ with the LS Fe(III)Por Im, complex leads to its transformation into the HS state (g-, = 5.95, g(parallel to) = 2.00). It was shown that the spin conversion occurs due to the donor-acceptor interaction of coordinated imidazole with this highpotential quinone causing the displacement of imidazole from the axial position. The similar mechanism of DDQinduced spin change is assumed to be valid for the native membrane Cyt b(559) in PSII centers. (C) 2003 Elsevier B.V. All rights reserved.