P680 (PD1PD2) and ChlD1 as alternative electron donors in photosystem II core complexes and isolated reaction centers

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 P 680 ∗ and ChlD1 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 PheoD1 Qx 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 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 Chl D 1 ∗ 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 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 ChlD1 as the intermediary acceptor.