Temperature Dependence of Electron Transfer to the M-Side Bacteriopheophytin in <i>Rhodobacter </i><i>capsulatus</i> Reaction Centers

Chuang, Jessica I.; Boxer, Steven G.; Holten, Dewey; Kirmaier, Christine

doi:10.1021/jp800082m

Cited by 31 publications

(79 citation statements)

References 118 publications

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“…This channel of electronic transport is much slower than that along the L-branch, 32 and the origin of this kinetic asymmetry is still debated. [33][34][35] We start with analyzing the general principles of electrontransfer energetics and what nonergodicity, caused by ultrafast primary charge separation, brings to the picture. This phenomenology serves to guide which parameters need to be used in the calculations of the reaction rates.…”

Section: Introductionmentioning

confidence: 99%

Energetics of Bacterial Photosynthesis

LeBard

Matyushov

2009

J. Phys. Chem. B

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We report the results of extensive numerical simulations and theoretical calculations of electronic transitions in the reaction center of Rhodobacter sphaeroides photosynthetic bacterium. The energetics and kinetics of five electronic transitions related to the kinetic scheme of primary charge separation have been analyzed and compared to experimental observations. Nonergodic formulation of the reaction kinetics is required for the calculation of the rates due to a severe breakdown of the system ergodicity on the time scale of primary charge separation, with the consequent inapplicability of the standard canonical prescription to calculate the activation barrier. Common to all reactions studied is a significant excess of the charge-transfer reorganization energy from the width of the energy gap fluctuations over that from the Stokes shift of the transition. This property of the hydrated proteins, breaking the linear response of the thermal bath, allows the reaction center to significantly reduce the reaction free energy of near-activationless electron hops and thus raise the overall energetic efficiency of the biological charge-transfer chain. The increase of the rate of primary charge separation with cooling is explained in terms of the temperature variation of induction solvation, which dominates the average donor-acceptor energy gap for all electronic transitions in the reaction center. It is also suggested that the experimentally observed break in the Arrhenius slope of the primary recombination rate, occurring near the temperature of the dynamical transition in proteins, can be traced back to a significant drop of the solvent reorganization energy close to that temperature.

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Section: Introductionmentioning

confidence: 99%

Energetics of Bacterial Photosynthesis

LeBard

Matyushov

2009

J. Phys. Chem. B

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“…sphaeroides is replaced with a BChl molecule, the so-called ␤-mutant RC (10), the rate of ET from P* to H A is decreased by a factor of 2. That study, together with studies of other mutants in which cofactors have been changed, indicated the importance of the energetics of the cofactors in influencing ET rates (11)(12)(13)(14).Recently, it was discovered that a magnesium chelatase (bchD) mutant of Rb. sphaeroides produces an RC that contains only Zn-BChl: no Mg-BChl or BPhe (we call this RC the Author contributions: S.L.…”

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confidence: 99%

Electron transfer in the Rhodobacter sphaeroides reaction center assembled with zinc bacteriochlorophyll

Lin

Jaschke²,

et al. 2009

Proc. Natl. Acad. Sci. U.S.A.

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The cofactor composition and electron-transfer kinetics of the reaction center (RC) from a magnesium chelatase (bchD) mutant of Rhodobacter sphaeroides were characterized. In this RC, the special pair (P) and accessory (B) bacteriochlorophyll (BChl) -binding sites contain Zn-BChl rather than BChl a. Spectroscopic measurements reveal that Zn-BChl also occupies the H sites that are normally occupied by bacteriopheophytin in wild type, and at least 1 of these Zn-BChl molecules is involved in electron transfer in intact Zn-RCs with an efficiency of >95% of the wild-type RC. The absorption spectrum of this Zn-containing RC in the near-infrared region associated with P and B is shifted from 865 to 855 nm and from 802 to 794 nm respectively, compared with wild type. The bands of P and B in the visible region are centered at 600 nm, similar to those of wild type, whereas the H-cofactors have a band at 560 nm, which is a spectral signature of monomeric Zn-BChl in organic solvent. The Zn-BChl H-cofactor spectral differences compared with the P and B positions in the visible region are proposed to be due to a difference in the 5th ligand coordinating the Zn. We suggest that this coordination is a key feature of protein-cofactor interactions, which significantly contributes to the redox midpoint potential of H and the formation of the charge-separated state, and provides a unifying explanation for the properties of the primary acceptor in photosystems I (PS1) and II (PS2). magnesium chelatase mutant ͉ photosynthetic bacterial reaction center ͉ photosystems I and II ͉ protein-cofactor interaction T he purple bacterial reaction center (RC) is a pigmentprotein complex that is capable of converting light energy to chemical energy with quantum yield approaching 1 (1-3). Electron transfer (ET) in this RC has been extensively studied; the structure and spectroscopic features of the complex are well known, the complex is very stable, and a large variety of mutants is available. This RC also serves as a model system for understanding protein-cofactor interactions and the role that protein plays in ET (4).The RC from Rhodobacter (Rb.) sphaeroides comprises 3 protein subunits, H, M, and L. As shown in Fig. 1, the RC complex binds 9 cofactors that form 2 potential ET chains (referred to as A and B) in a C2 symmetric arrangement. The ''special pair'' (P) is a dimer of bacteriochlorophyll (BChl) a molecules and is located on the periplasmic side of the cytoplasmic membrane. Two monomeric BChls (B A and B B , with the subscripts denoting which chain the cofactor belongs to) are present on either side of P. These are followed by 2 bacteriopheophytin (BPhe) molecules (H A and H B ). A nonheme iron and 2 quinones (Q A and Q B ) are near the cytoplasmic side of the RC (5, 6). When P is excited, an electron is transferred through the A branch cofactors, and then to Q B . In the WT RC, the times for ET from P* to H A to Q A to Q B are 3 ps, 200 ps, and 200 s, respectively. The transfer from P* to H A is thought to be via B A .The ET reactions P...

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“…The electron used only one of two possible branches. Much experimental work has been performed to solve this problem [9,10,27,28,37,38]. Generally, it is assumed that RCs have common features and the unidirectionality is caused mainly by asymmetry in the coupling integral or asymmetry in the free energy, or that both these asymmetries contribute to unidirectionality.…”

Section: Resultsmentioning

confidence: 99%

“…1). The involvement of the bacteriochlorophyll monomer may give rise to multiple pathways for electron transfer [7] and can partially determine the unidirectionality of charge separation along one branch [8,9]. The chain located on subunit M is inactive in ET, and the highly asymmetric functionality, however, can be decreased by amino acid mutations or cofactor modification.…”

Section: Introductionmentioning

confidence: 99%

Electronic pathway in reaction centers from Rhodobacter sphaeroides and Chloroflexus aurantiacus

Pudlak

Pinčák

2010

J Biol Phys

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The reaction centers (RC) of Chloroflexus aurantiacus and Rhodobacter sphaeroides H(M182)L mutant were investigated. Prediction for electron transfer (ET) at very low temperatures was also performed. To describe the kinetics of the C. aurantiacus RCs, the incoherent model of electron transfer was used. It was shown that the asymmetry in electronic coupling parameters must be included to explain the experiments. For the description of R. sphaeroides H(M182)L mutant RCs, the coherent and incoherent models of electron transfer were used. These two models are discussed with regard to the observed electron transfer kinetics. It seems likely that the electron transfer asymmetry in R. sphaeroides RCs is caused mainly by the asymmetry in the free energy levels of L-and M-side cofactors. In the case of C. aurantiacus RCs, the unidirectionality of the charge separation can be caused mainly by the difference in the electronic coupling parameters in two branches.

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Temperature Dependence of Electron Transfer to the M-Side Bacteriopheophytin in Rhodobacter capsulatus Reaction Centers

Cited by 31 publications

References 118 publications

Energetics of Bacterial Photosynthesis

Energetics of Bacterial Photosynthesis

Electron transfer in the Rhodobacter sphaeroides reaction center assembled with zinc bacteriochlorophyll

Electronic pathway in reaction centers from Rhodobacter sphaeroides and Chloroflexus aurantiacus

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