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

Abstract

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 -benzoquinone (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 (⩽0 V). It was also demonstrated that DDQ treatment damages the oxygen evolving complex, leading to inhibition of oxygen evolution, decrease of the S 2 -state EPR multiline signal and release of Mn 2+ . 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 bis -imidazole coordinated heme iron (Fe III Por Im 2 ) which mimic the bis -histidine axial ligation of Cyt b 559 . The reaction of DDQ with the LS Fe III Por Im 2 complex leads to its transformation into the HS state ( g ⊥ =5.95, g ∥ =2.00). It was shown that the spin conversion occurs due to the donor–acceptor interaction of coordinated imidazole with this high-potential quinone causing the displacement of imidazole from the axial position. The similar mechanism of DDQ-induced spin change is assumed to be valid for the native membrane Cyt b 559 in PSII centers.