EPR characterisation of the triplet state in photosystem II reaction centers with singly reduced primary acceptor QA

Abstract

The triplet states of photosystem II core particles from spinach were studied using time-resolved cw EPR technique at different reduction states of the iron--quinone complex of the reaction center primary electron acceptor. With doubly reduced primary acceptor, the well-known photosystem II triplet state characterised by zero-field splitting parameters |D|=0.0286 cm(-1), |E|=0.0044 cm(-1) was detected. When the primary acceptor was singly reduced either chemically or photochemically, a triplet state of a different spectral shape was observed, bearing the same D and E values and characteristic spin polarization pattern arising from RC radical pair recombination. The latter triplet state was strongly temperature dependent disappearing at T=100 K, and had a much faster decay than the former one. Based on its properties, this triplet state was also ascribed to the photosystem II reaction center. A sequence of electron-transfer events in the reaction centers is proposed that explains the dependence of the triplet state properties on the reduction state of the iron--quinone primary acceptor complex.

Photoredox‐Switchable Resorcin[4]arene Cavitands: Radical Control of Molecular Gripping Machinery via Hydrogen Bonding

“…Control of the photoredox gripping machinery required identification of the hydrogen bonding (HB) signatures upon reduction to the SQ state and evaluation of its impact on the properties of the system. Studies of the reduced ubiquinones in the photosynthetic reaction center as well as systems that mimic their photochemistry attracted considerable attention over the past decade, and HB in the SQ state was analyzed by cyclic voltammetry and 1 H ENDOR spectroscopy . We therefore investigated model compounds 7 – 19 that represent single gripper walls (Figure ) by means of these methods in order to categorize the indicators of HB and analyze their influence on redox, magnetic, and photophysical properties in the SQ state.…”

Site-directed mutations at D1-His198 and D1-Thr179 of photosystem II in Synechocystis sp. PCC 6803: deciphering the spectral properties of the PSII reaction centre

“…The P680 triplet state has been monitored in the presence of singly reduced Q A by transient absorbance spectroscopy Schlodder et al 1998) and time-resolved electron paramagnetic resonance (EPR) (Feikema et al 2005). Under these conditions, the triplet decay is two or three orders of magnitude faster À -P680Q A Þ absorbance difference spectra of PSII core complexes from the wild-type (WT) and the (a) D1-His198Gln mutant and from the wild-type (WT) and the (b) D1-Thr179His mutant measured at 80 K. The spectra were normalized to K1 at the bleaching minimum to allow better comparison.…”

The wavelength of the incident light determines the primary charge separation pathway in Photosystem II

“…The spin polarized triplet state 3 P 680 is a useful probe to the physical environment and photochemistry of P 680 and the primary radical pair in PSII 36 . In intact PSII, 3 P 680 is not observable with EPR spectroscopy 37 . However, when Q A is either removed (in for example the D1/D2/Cyt b 559 preparation) 37 , 38 or double reduced either by chemical treatment 27 – 29 or extensive illumination under anaerobic conditions 39 , the spin polarized EPR signal from 3 P 680 , can be observed as a result of recombination of the P680 + Pheo − charge pair.…”