Primary Electron Transfer Reactions in Membrane-Bound Open and Closed Reaction Centers from Purple Bacterium Rhodobacter sphaeroides

Abstract

Most ultrafast transient absorption studies of primary electron transfer in reaction centers from purple bacteria have been performed in complexes isolated from their natural lipid membrane environment using detergent. In this contribution we present near-UV-vis transient absorption studies of reaction centers embedded in their natural membrane environment. The evolution of absorption spectra recorded with subpicosecond resolution and reecting primarily electron transfer reactions has been compared to data obtained previously for isolated reaction centers. We conclude that the overall spectral evolution in both types of samples is similar, and the environment of the reaction center protein has only a minor eect on the primary electron transfer reactions. The dierences between the two samples are explained in terms of dierent energetic levels (and their dierent temporal evolution) of the two initial charge separated states P + B − A and P + H − A , with P being the primary electron donor and BA and HA the two consecutive electron acceptors. Additionally, in the electric eld generated by P + H − A , BA in membrane-bound reaction centers undergoes a stronger electrochromic shift than in isolated reaction centers.

Analysis of the Kinetics of P+HA– Recombination in Membrane-Embedded Wild-Type and Mutant Rhodobacter sphaeroides Reaction Centers between 298 and 77 K Indicates That the Adjacent Negatively Charged QA Ubiquinone Modulates the Free Energy of P+HA– and May Influence the Rate of the Protein Dielectric Response

“…36,37 The natural lipid bilayer environment of RCs may in principle exert somewhat different effects on ET reactions than the artificial environment of the detergent micelle required for isolation of RCs, and small differences in the primary ET reactions between isolated and membrane-bound RCs have been reported. 36,43 In order to properly refer the results of the present work on membrane-embedded RCs to a previous study of the temperature dependence of P + H A − recombination performed on isolated RCs, 32 Figure 2A compares transient absorption kinetics at 690 nm recorded over 100 ns for either isolated or membrane-bound WT RCs in which Q A had been prereduced. As explained in previous reports, 17,31 P + H A − recombination is observed at this wavelength, the decay of the initial absorption increase being mostly due to the decay of the ΔA(H A − − H A ) transient absorption signal, with a possible small admixture of the ΔA(P + − P) signal.…”

Putative Hydrogen Bond to Tyrosine M208 in Photosynthetic Reaction Centers from Rhodobacter capsulatus Significantly Slows Primary Charge Separation

“…This residue is Tyr M208 in Blastochloris viridis and Rhodobacter capsulatus and Tyr M210 in Rhodobacter sphaeroides. Both theoretical work − and experiments on mutants − have led to consensus that this conserved residue is a significant contributor to extremely rapid and efficient initial A-side charge separation. Interestingly, there are no potential hydrogen bond partners available to Tyr M208, which is unique among the roughly 28 (depending on species) tyrosines in the RC.…”