PARENT SESSION
Posters P6A Type II reaction centres: Excited state dynamics and donor side. Abstracts (313-346)

EPR and absorption spectroscopy studies of the primary donor cation radical in heterodimer mutant reaction centers of Blastochloris viridis. Shipra Prakash^*,1, Els Schulten¹, A Alia¹, Peter Gast², Jan Raap¹, Johan Lugtenburg¹, Huub de Groot¹, Joerg Matysik¹, ¹ Leiden Institute of Chemistry, Leiden, The Netherlands² Department of Biophysics, Leiden, The Netherlands

ABSTRACT- The reaction centers of the purple bacterium Rhodobacter Sphaeroides(WT) were selectively ¹³C labelled in all the Bacteriochorophyll (BChl) and Bacteriopheophytin (BPhe) cofactors. The ¹³C solid state photo-CIDNP MAS NMR spectra were recorded to study the electronic structure of the primary electron donor and the acceptor at the atomic scale. The data clearly show that the two cofactors forming the special pair are already different in the electronic ground state. One of the two BChl molecules appear to be special, whereas the second show chemical shifts similar to the accessory BChl cofactors. The origin of this difference will be discussed. It is aimed to investigate how the electronic structure of the ground state of the primary donor P is tuned to facilitate the emission of an electron in the photochemically excited state. Comparison of dark NMR spectra with photo-CIDNP provides a detailed insight into the process of electron emission from the two BChl molecules of the special pair, its electronic ground state and the effects of the protein environment. There is strong evidence that both the electronic and the spatial structure for the rings of the two BChls of the primary donor P and photochemically active BPhe are essentially conserved during the photochemistry and its associated reorganization process. The photo-CIDNP spectra also show signals of an additional BChl molecule which is tentatively assigned to an accessory BChl molecule. This assignment would imply that the accessory BChl carries electron spin during the photochemical process.

KEY WORDS: bacterial reaction center, special pair, photo-CIDNP MAS NMR, Rhodobacter Sphaeroides (WT)