Energy Transfer, Charge Separation and Pigment Arrangement in the Reaction Center of Photosystem II

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Energy transfer and charge separation in the isolated Photosystem II reaction center at room temperature were studied with transient absorption difference spectroscopy upon selective excitation of the reaction center pigments. The measurements were performed with two dye lasers, which had a spectral bandwidth of less than 1 nm, and with an instrument response function of 5 or 18 ps depending on the type of experiment. Small changes with time constants of 0.6 ns and 120 ps are attributed to damaged reaction centers. Selective excitation of the long-wavelength pigments, presumably P680 and the pheophytins, led to charge separation in 3 ps. Selective excitation of the short-wavelength pigments, presumably accessory chlorophylls, led to charge separation in 30 ps with the same quantum efficiency. This excludes equilibration of the excited state between accessory chlorophyll and P680 in less than 30 ps. The overlap of the fluorescence spectrum of accessory chlorophyll with the absorption of P680 is very good and the slow energy transfer is attributed to an about 30 Å center-to-center distance, which makes the histidines 118 in helix II of the D1 and D2 proteins likely binding sites of the chlorophylls nearest to the long-wavelength pigments, P680 and pheophytin. Reevaluation of the literature in the light of these data suggests that P680 is a dimer with nearly (anti) parallel QY-transition moments of the constituent monomers, making an angle with their connecting axis close to the magic angle, and that the geometry of P680 and the pheophytins is not C2-symmetrical around an axis perpendicular to the membrane.



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