Spectroscopy and electron transfer dynamics of the bacterial photosynthetic reaction center

Friesner, Richard A.; Won, Youjip

doi:10.1016/s0005-2728(89)80062-3

Cited by 131 publications

(73 citation statements)

References 103 publications

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“…The 5-nm red shift in peak location comparing 295 to 77 K data is identical to the shift observed in the temperature dependence of fluorescence from low-energy Chls of PSI in S. 6803 (Wittmershaus et al 1992) and similar to temperature-dependent red shifts observed for P700 (Iwaki et al 1992) and reaction centers from purple bacteria (Friesner and Won 1989). These spectral shifts indicate that a change in energy level is occurring, probably as the result of a liquid-to-solid phase transition in the sample upon freezing (Wittmershaus et al 1992).…”

Section: Observations On Anisotropy Resultssupporting

confidence: 81%

Resolution of low-energy chlorophylls in Photosystem I of Synechocystis sp. PCC 6803 at 77 and 295 K through fluorescence excitation anisotropy

et al. 1994

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Fluorescence excitation spectra of highly anisotropic emission from Photosystem I (PS I) were measured at 295 and 77 K on a PS II-less mutant of the cyanobacterium Synechocystis sp. PCC 6803 (S. 6803). When PS I was excited with light at wavelengths greater than 715 nm, fluorescence observed at 745 nm was highly polarized with anisotropies of 0.32 and 0.20 at 77 and 295 K, respectively. Upon excitation at shorter wavelengths, the 745-nm fluorescence had low anisotropy. The highly anisotropic emission observed at both 77 and 295 K is interpreted as evidence for low-energy chlorophylls (Chls) in cyanobacteria at room temperature. This indicates that low-energy Chls, defined as Chls with first excited singlet-state energy levels below or near that of the reaction center, P700, are not artifacts of low-temperature measurements.If the low-energy Chls are a distinct subset of Chls and a simple two-pool model describes the excitation transfer network adequately, one can take advantage of the low-energy Chls' high anisotropy to approximate their fluorescence excitation spectra. Maxima at 703 and 708 nm were calculated from 295 and 77 K data, respectively. Upper limits for the number of low-energy Chls per P700 in PS I from S. 6803 were calculated to be 8 (295 K) and 11 (77 K).

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Section: Observations On Anisotropy Resultssupporting

confidence: 81%

Resolution of low-energy chlorophylls in Photosystem I of Synechocystis sp. PCC 6803 at 77 and 295 K through fluorescence excitation anisotropy

et al. 1994

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“…However, the P* decay contains little information about how an electron is transferred from P* to HA. Several mechanisms have been proposed for the initial electron transfer in bacterial RCs (22)(23)(24)(25)(26)(27)(28)(29). A major concern is the role of BA.…”

Section: Resultsmentioning

confidence: 99%

Mechanism of the initial charge separation in bacterial photosynthetic reaction centers.

Chan

DiMagno

Chen

et al. 1991

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

134

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The initial electron tranfer in reaction centers from Rhodobacter sphaeroides R26 was studied by a subplcosecond transient pump-probe technique. The measured kinetics at various wavelengths were analyzed and compared with several mechanisms for electron tra er. An unambiguous determination of the initial electron transfer mechanism in reaction centers cannot be made by studying the anion absorption region (6404690 nm), due to the spectral congestion in this region. However, correlations between the stimulated emission decay of the excited state of the special pair, P*, at 926 nm and bleaching of the bacteriopheophytin Qx absorption at 545 mu suggest that the electron transfer at 283 K is dominated by a two-step sequential mechanism, whereas one-step superexchange and the two-step sequential mechanism have about equal contributions at 22 K.The highly efficient primary charge separation in photosynthesis occurs in a membrane-bound protein complex called the reaction center (RC). The x-ray crystal structure of RCs from Rhodopseudomonas (Rps.) viridis and Rhodobacter (Rb.) sphaeroides has been determined and reveals a pseudo C2 symmetry (1-3). In the RC, two strongly interacting bacteriochlorophylls form the special pair, denoted P. Along each side of the C2 axis there is a monomeric bacteriochlorophyll (BA and BB) adjacent to the special pair, followed by a bacteriopheophytin (HA and HB) and then by a quinone molecule (QA and QB). There is also a single non-heme iron on the C2 axis between the two quinones.Electron transfer could occur along either branch related by the C2 symmetry, yet it occurs only down one branch (branch A) from P* to HA in about 3 ps at room temperature. One unresolved question concerns the role of BA in the initial charge separation process between P* and HA. Early picosecond and femtosecond measurements on RCs from Rb. sphaeroides and Rps. viridis at room temperature and at 10 K (4-9) did not detect any intermediate state involving B-, and the decay of P* and the bleaching of HA had the same time constant. These observations indicated that the initial electron transfer from P*BAHA to P+BAH-occurred either by a single-step superexchange mechanism or by a two-step hopping process with the second step much faster than the first step. The one-step superexchange mechanism has been supported by measurements by Kirmaier and Holten (10,11). Using an electric field to perturb the electron transfer, Lockhart et al. (12) concluded that (at 77 K) the two-step mechanism was unlikely. The same conclusion was reached by Ogrodnik et al. (13) (19) suggests that for reasonable parameter values, the electron transfer is dominated by the two-step sequential mechanism at room temperature, while the contribution of the one-step superexchange mechanism becomes important at low temperature.In this paper, we address the mechanism of the initial electron transfer by using kinetic data for Rb. sphaeroides R26 RCs obtained at several wavelengths where the various species involved absorb. The analysis of our da...

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“…The individual Qy bands are usually named after the chromophores that are thought to provide the major contribution-i.e., B (BL and BM), for Rhodobacter sphaeroides around 800 nm, and H (HL and HM) around 750 nm, and the exciton bands of P (P-around 890 nm at cryogenic temperatures; P+ around 810 nm). In addition to interchromophore interactions, the spectrum presumably is influenced by charge transfer transitions (3,4) and by electrostatic interactions with the protein solvent (5)(6)(7). Various calculations of exciton interactions have been published, but the degree of excitonic coupling, the environmental influences, and the possible contribution of charge transfer transitions to the spectrum are still subject to debate (5)(6)(7).…”

mentioning

confidence: 99%

“…In addition to interchromophore interactions, the spectrum presumably is influenced by charge transfer transitions (3,4) and by electrostatic interactions with the protein solvent (5)(6)(7). Various calculations of exciton interactions have been published, but the degree of excitonic coupling, the environmental influences, and the possible contribution of charge transfer transitions to the spectrum are still subject to debate (5)(6)(7). An alteration of the electronic state is bound to perturb any interaction and, viewing the reaction center as an interacting hexamer, modify all bands.…”

mentioning

confidence: 99%

Femtosecond spectral evolution of the excited state of bacterial reaction centers at 10 K.

Vos

Lambry

Robles

et al. 1992

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

110

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The femtosecond spectral evolution of reaction centers of Rhodobacter sphaeroides R-26 was studied at 10 K. Transient spectra in the near infrared region, obtained with 45-fs pulses (pump pulses centered at 870 um and continuum probe pulses), were analyzed with associated kinetics at specific wavelengths. The reaction center of photosynthetic purple bacteria contains an ensemble of six chromophores, which are bound to two protein subunits L and M (1, 2). Two of the chromophores are bacteriopheophytins (HL and HM); the other four are the bacteriochlorophylls PL, PM, BL, and BM. It is generally accepted that the former two are strongly coupled; they are referred to as the special pair P. Furthermore, not only PL and PM, but all chromophores display at least some degree of interaction. The ground state absorption spectrum in principle may consist of bands that each contain contributions of all six chromophores. The individual Qy bands are usually named after the chromophores that are thought to provide the major contribution-i.e., B (BL and BM), for Rhodobacter sphaeroides around 800 nm, and H (HL and HM) around 750 nm, and the exciton bands of P (P-around 890 nm at cryogenic temperatures; P+ around 810 nm). In addition to interchromophore interactions, the spectrum presumably is influenced by charge transfer transitions (3, 4) and by electrostatic interactions with the protein solvent (5-7). Various calculations of exciton interactions have been published, but the degree of excitonic coupling, the environmental influences, and the possible contribution of charge transfer transitions to the spectrum are still subject to debate (5-7). An alteration of the electronic state is bound to perturb any interaction and, viewing the reaction center as an interacting hexamer, modify all bands. For example, compared to that of the singlet ground state, the triplet spectrum displays considerable changes in the B bands and also minor perturbations of the H bands, apart from the very strong changes in the P bands (8). These features have been explained in terms of excitonic coupling (9). The assessment of interpigment and pigment-protein interactions may prove essential to understand the nature ofthe extremely fast electron transport in the reaction center. Therefore, of high functional interest is the spectrum of the lowest singlet excited state of the reaction center pigment system P*, which is the precursor of electron transport. The kinetics of this state are most easily probed by its stimulated emission. In active R. sphaeroides reaction centers, it decays initially with a time constant of -3 ps at room temperature (10-12) and 1.2 ps at 10 K (13, 14). The charge pair P+HjL emerges with about the same time constant, as probed by the appearance of an electrochromic shift of the B band and the bleaching of the HL band (10,11,13).Recent data indicate that the spectrum of the reaction center also evolves on a time scale faster than the electron transfer from P to HL. At room temperature, a kinetic component with a time co...

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Spectroscopy and electron transfer dynamics of the bacterial photosynthetic reaction center

Cited by 131 publications

References 103 publications

Resolution of low-energy chlorophylls in Photosystem I of Synechocystis sp. PCC 6803 at 77 and 295 K through fluorescence excitation anisotropy

Resolution of low-energy chlorophylls in Photosystem I of Synechocystis sp. PCC 6803 at 77 and 295 K through fluorescence excitation anisotropy

Mechanism of the initial charge separation in bacterial photosynthetic reaction centers.

Femtosecond spectral evolution of the excited state of bacterial reaction centers at 10 K.

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