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

The spin-state of cytochrome b559 (cyt b559) 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 b559 into the high-spin (HS) form characterized by the g = 6.19 and g = 5.95 split signal. The amount of HS cyt b559 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 continuous illumination at 77 K and 200 K 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 S2-state EPR multi-line signal and release of Mn. In parallel, studies of model systems containing iron(III) protoporphyrin IX chloride (FePor), which is a good model compound for the cyt b559 prosthetic group, were performed by using optical and EPR spectroscopy. The interaction of FePor with imidazole (Im) in weakly polar solvent results in formation of bis-imidazole coordinated heme iron (FePorIm2) which mimics the bis-histidine axial ligation of cyt b559. The reaction of DDQ with the LS FePorIm2 complex leads to its conversion into the HS state (g┴ = 5.95, g║ = 2.00). It was shown that the spin conversion is due to donor-acceptor interaction of coordinated imidazole with this high-potential quinone causing the displacement of imidazole from the axial position. A similar mechanism of DDQ-induced spin change is assumed to be responsible for cyt b559 in PSII centers.