Abstract

Light excitation of the primary bacteriochlorophyll dimer, P, in photosynthetic reaction centers (RCs) of Rhodobacter (Rb.) sphaeroides R26 triggers a series of electron transfer events in the reaction center: Open image in new window in which I, QA and QB are a bacteriopheophytin, a primary and a secondary electron acceptor quinone, respectively. Each state is characterised by a lifetime k-1. The secondary radical pair P+·QA-·which is generated in this process has been studied extensively by time resolved EPR spectroscopy. The transient EPR spectrum of P·Q-·A shows a high degree of electron spin polarisation (ESP) due to magnetic interactions between the two spins. Simulation of experimental ESP spectra, using the spin-correlated radical pair model [1], have yielded valuable information about the mutual orientation, and magnetic and electronic interactions of both radicals. However, in most of these studies, the lifetime of the primary radical pair, P+·I -·, was assumed to be much shorter compared to the timescale of its magnetic interactions. Thus, the observed secondary radical pair was considered to be initially in a pure singlet state and any admixture of triplet character was neglected. We have investigated the effect of the intermediate radical pair P+·I-· on the ESP spectrum of P·QA-· in Zn-substituted RCs of Rb. sphaeroides R26 by prolongation of its lifetime by replacing the native ubiquinone-10 with different quinones [2]. To simulate the observed X-band ESP spectra we used the theoretical description of the transfer of electron spin-correlation from one radical pair to another presented by Hore [3]. Furthermore, the recently discovered effect magnetophotoselection on the laser flash-induced ESP spectra was taken into account [4].

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