Abstract

Low temperature (77–90 K) measurements of absorption spectral changes induced by red light illumination in isolated photosystem II (PSII) reaction centers (RCs, D1/D2/Cyt b559 complex) with different external acceptors and in PSII core complexes have shown that two different electron donors can alternatively function in PSII: chlorophyll (Chl) dimer P 680 absorbing at 684 nm and Chl monomer Chl D1 absorbing at 674 nm. Under physiological conditions (278 K) transient absorption difference spectroscopy with 20-fs resolution was applied to study primary charge separation in spinach PSII core complexes excited at 710 nm. It was shown that the initial electron transfer reaction takes place with a time constant of ∼0.9 ps. This kinetics was ascribed to charge separation between P 680 ∗ and Chl D1 absorbing at 670 nm accompanied by the formation of the primary charge-separated state P 680 + Chl DI - , as indicated by 0.9-ps transient bleaching at 670 nm. The subsequent electron transfer from Chl D 1 - occurred within 13–14 ps and was accompanied by relaxation of the 670-nm band, bleaching of the Pheo D1 Q x absorption band at 545 nm, and development of the anion-radical band of Pheo D 1 - at 450–460 nm, the latter two attributable to formation of the secondary radical pair P 680 + Pheo D 1 - . The 14-ps relaxation of the 670-nm band was previously assigned to the Chl D1 absorption in isolated PSII RCs [Shelaev, Gostev, Nadtochenko, Shkuropatov, Zabelin, Mamedov, Semenov, Sarkisov and Shuvalov, Photosynth. Res. 98 (2008) 95–103]. We suggest that the longer wavelength position of P 680 (near 680 nm) as a primary electron donor and the shorter wavelength position of Chl D1 (near 670 nm) as a primary acceptor within the Q y transitions in RC allow an effective competition with an energy transfer and stabilization of separated charges. Although an alternative mechanism of charge separation with Chl D 1 ∗ as the primary electron donor and Pheo D1 as the primary acceptor cannot be ruled out, the 20-fs excitation at the far-red tail of the PSII core complex absorption spectrum at 710 nm appears to induce a transition to a low-energy state P 680 ∗ with charge-transfer character (probably P D 1 δ + P D 2 δ - ) which results in an effective electron transfer from P 680 ∗ (the primary electron donor) to Chl D1 as the intermediary acceptor.

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