Article ID Journal Published Year Pages File Type
29610 Journal of Photochemistry and Photobiology B: Biology 2011 7 Pages PDF
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 P680 absorbing at 684 nm and Chl monomer ChlD1 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 P680∗ and ChlD1 absorbing at 670 nm accompanied by the formation of the primary charge-separated state P680+ChlDI-, as indicated by 0.9-ps transient bleaching at 670 nm. The subsequent electron transfer from ChlD1- occurred within 13–14 ps and was accompanied by relaxation of the 670-nm band, bleaching of the PheoD1 Qx absorption band at 545 nm, and development of the anion-radical band of PheoD1- at 450–460 nm, the latter two attributable to formation of the secondary radical pair P680+PheoD1-. The 14-ps relaxation of the 670-nm band was previously assigned to the ChlD1 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 P680 (near 680 nm) as a primary electron donor and the shorter wavelength position of ChlD1 (near 670 nm) as a primary acceptor within the Qy   transitions in RC allow an effective competition with an energy transfer and stabilization of separated charges. Although an alternative mechanism of charge separation with ChlD1∗ as the primary electron donor and PheoD1 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 P680∗ with charge-transfer character (probably PD1δ+PD2δ-) which results in an effective electron transfer from P680∗ (the primary electron donor) to ChlD1 as the intermediary acceptor.

Research highlights► 20-fs excitation of the PSII core complex at 710 nm induces a transition to a low-energy state of P680∗ with charge-transfer character (PD1δ+PD2δ-). ► Charge separation between P680∗ and ChlD1 absorbing at 670 nm occurs within ∼0.9 ps. ► Electron transfer from ChlD1- to PheoD1 occurs within 13–14 ps.

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