Electron Spin Polarisation in Quinone Reconstituted Photosynthetic Reaction Centers of RB. Sphaeroides R26.

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].