title: Bacteriopheophytin triplet state in Rhodobacter sphaeroides reaction centers.
abstract: It is well established that photoexcitation of Rhodobacter sphaeroides reaction centers (RC) with reduced quinone acceptors results in the formation of a triplet state localized on the primary electron donor P with a significant yield. The energy of this long-lived and therefore potentially damaging excited state is then efficiently quenched by energy transfer to the RC spheroidenone carotenoid, with its subsequent decay to the ground state by intersystem crossing. In this contribution, we present a detailed transient absorption study of triplet states in a set of mutated RCs characterized by different efficiencies of triplet formation that correlate with lifetimes of the initial charge-separated state P(+)H A (-) . On a microsecond time scale, two types of triplet state were detected: in addition to the well-known spheroidenone triplet state with a lifetime of ~4 μs, in some RCs we discovered a bacteriopheophytin triplet state with a lifetime of ~40 μs. As expected, the yield of the carotenoid triplet increased approximately linearly with the lifetime of P(+)H A (-) , reaching the value of 42 % for one of the mutants. However, surprisingly, the yield of the bacteriopheophytin triplet was the highest in RCs with the shortest P(+)H A (-) lifetime and the smallest yield of carotenoid triplet. For these the estimated yield of bacteriopheophytin triplet was comparable with the yield of the carotenoid triplet, reaching a value of ~7 %. Possible mechanisms of formation of the bacteriopheophytin triplet state are discussed.
authors.affiliation: Faculty of Physics, Adam Mickiewicz University in Poznań, ul. Umultowska 85, 61-614, Poznan, Poland. rafal.bialek@gmail.com.
authors.forename: Rafał
authors.initials: R
authors.lastname: Białek
authors.affiliation: Faculty of Physics, Adam Mickiewicz University in Poznań, ul. Umultowska 85, 61-614, Poznan, Poland.
authors.forename: Gotard
authors.initials: G
authors.lastname: Burdziński
authors.affiliation: School of Biochemistry, Medical Sciences Building, University of Bristol, University Walk, Bristol, BS8 1TD, UK.
authors.forename: Michael R
authors.initials: MR
authors.lastname: Jones
authors.affiliation: Faculty of Physics, Adam Mickiewicz University in Poznań, ul. Umultowska 85, 61-614, Poznan, Poland.
authors.forename: Krzysztof
authors.initials: K
authors.lastname: Gibasiewicz
mesh_terms: D002338:Carotenoids / Q000737:chemistry* / Q000378:metabolism; D004735:Energy Transfer; D007700:Kinetics; D010100:Oxygen / Q000378:metabolism; D010674:Pheophytins / Q000737:chemistry* / Q000378:metabolism; D045322:Photosynthetic Reaction Center Complex Proteins / Q000737:chemistry / Q000378:metabolism; D012242:Rhodobacter sphaeroides / Q000737:chemistry* / Q000378:metabolism*
chemical_list: D010674:Pheophytins; D045322:Photosynthetic Reaction Center Complex Proteins; C104880:spheroidenone; C006678:bacteriopheophytin; D002338:Carotenoids; D010100:Oxygen
keywords: Bacteriopheophytin; Carotenoid; Rhodobacter sphaeroides; Spheroidenone; Triplet