Photosynthesis Research 59: 95–104, 1999. © 1999 Kluwer Academic Publishers. Printed in the Netherlands. 95 Regular paper Functional LH1 antenna complexes influence electron transfer in bacterial photosynthetic reaction centers Ronald W. Visschers 1,∗ , Simone I.E. Vulto 1 , Michael R. Jones 3 , Rienk van Grondelle 2 & Ruud Kraayenhof 1 1 Institute of Molecular Biological Sciences, Department of Structural Biology, BioCentrum Amsterdam, Vrije Uni- versiteit, De Boelelaan 1087, 1081 HV Amsterdam, The Netherlands; 2 Institute of Molecular Biological Sciences, Department of Biophysics, BioCentrum Amsterdam, Vrije Universiteit, De Boelelaan 1081, 1081 HV Amsterdam, The Netherlands; 3 Krebs Institute for Biomolecular Research and Robert Hill Institute for Photosynthesis, De- partment of Molecular Biology and Biotechnology, University of Sheffield, Western Bank, Sheffield, S10 2UH, UK; ∗ Author for correspondence (fax: +31-20-444-7157; e-mail: ronaldv@bio.vu.nl) Received 28 July 1998; accepted in revised form 5 November 1998 Key words: bacteriochlorophyll a, electron transfer, light harvesting, photosynthesis, Rhodobacter sphaeroides, reaction center Abstract The effect of the light harvesting 1 (LH1) antenna complex on the driving force for light-driven electron transfer in the Rhodobacter sphaeroides reaction center has been examined. Equilibrium redox titrations show that the presence of the LH1 antenna complex influences the free energy change for the primary electron transfer reaction through an effect on the reduction potential of the primary donor. A lowering of the redox potential of the primary donor due to the presence of the core antenna is consistently observed in a series of reaction center mutants in which the reduction potential of the primary donor was varied over a 130 mV range. Estimates of the magnitude of the change in driving force for charge separation from time-resolved delayed fluorescence measurements in the mutant reaction centers suggest that the mutations exert their effect on the driving force largely through an influence on the redox properties of the primary donor. The results demonstrate that the energetics of light-driven electron transfer in reaction centers are sensitive to the environment of the complex, and provide indirect evidence that the kinetics of electron transfer are modulated by the presence of the LH1 antenna complexes that surround the reaction center in the natural membrane. Abbreviations: DAD – 2,3,5,6-tetramethylphenylenediamine; H L – primary electron acceptor in the reaction center; LH1 – light harvesting 1 antenna complex; P – primary donor of electrons in the reaction center; PES – phenazine- ethosulfate; PMS – phenazine-methosulfate; Q A – primary acceptor quinone; Rb. – Rhodobacter; RC – reaction center; RC-only – reaction center-only strain; RCLH1 – reaction center plus light harvesting complex 1 strain; TMPD – N,N,N ′ ,N ′ -tetramethylphenylene diamine Introduction Photosynthetic reaction centers (RCs) of plants and bacteria efficiently convert excited state energy into a charge separated state across the photosynthetic mem- brane. The RC excited state that drives charge separa- tion arises as a result of either direct excitation of RC pigments or energy transfer from the antenna pigments that surround the RC (van Grondelle et al. 1994). The atomic structure of the RCs from two purple non- sulphur bacteria shows that the complex contains two linear, nearly symmetrical branches of redox cofactors that span the photosynthetic membrane (Deisenhofer et al. 1985; Allen et al. 1987; Ermler et al. 1994).