Primary Reactions in Bacterial Photosynthesis: Dynamics of Reaction Centers with Modified Electron Acceptors

Abstract

The photoinduced electron transfer in the bacterial reaction center from Rb. sphaeroides R26.1 has been studied extensively in the last years. It is now well established, that the electron transfer steps along the active branch predominantly follow a sequential mechanism, in which anions of the monomeric BChl a (_A ^-) BPhe a (H_A ^-) and primary ubiquinone (Q_A ^-) are formed with time constants of 3 ps, 0.9 ps and 200 ps, respectively. Strong evidence for the reaction via B_A came from experiments on pigment modified samples, where the state BA^- is in equilibrium to an energetically increased state P⁺H_A ^- and is populated to a much higher extent than in native RCs [1,2]. We report experiments on RCs where 3-acetyl-Phe a and 3-vinyl-BPhe a are introduced at the BPhe a-sites H_A/B [3]. These two pigments are especially interesting, since they both represent a structural link between Phe a used as electron acceptor in plant photosystems and the bacterial BPhe a. The bacteriochlorin type pigment 3-vinyl-BPhe a differs from BPhe a only by its substituent at the C3-position (vinyl instead of acetyl), which results in similar absorption characteristics of the two pigments, with only small shifts of the main bands in the Q_y- and the Q_x-region of 3-vinyl-BPhe a to higher frequencies. The 3-acetyl-Phe a differs from BPhe a in the hydrogenation state of its ring II macrocycle and as a chlorin type pigment shows spectral features more similar to Phe a. The absorption spectra of the corresponding RCs are shown in Fig. 1.