Rates of primary electron transfer in photosynthetic reaction centres and their mechanistic implications

Fleming, Graham R.; Martin, Jean−Louis; Breton, Jacques

doi:10.1038/333190a0

Cited by 440 publications

(338 citation statements)

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“…Our results demonstrate that at all temperatures a major part of the excited state dynamics of the PSII RC upon red excitation is determined by a fast and a slower decaying component, each with its own DAS. Both components accelerate with temperature, which is in contrast to the situation in the bacterial RC where the rate associated with radical pair formation decreases with temperature (30).…”

Section: Discussioncontrasting

confidence: 39%

“…Based on this result and on the work in ref. 29, it was suggested that the rate of primary charge separation increases upon lowering the temperature, similar to what has been observed for the rate of charge separation in the purple bacterial RC (30). However, it is not clear whether or not the 1.9-ps time constant at 1.4 K and the 3-or 20-ps processes at RT can be attributed to the same physical process.…”

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confidence: 74%

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Charge separation in the reaction center of photosystem II studied as a function of temperature

Groot

Mourik

Eijckelhoff

et al. 1997

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

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In photosystem II of green plants the key photosynthetic reaction consists of the transfer of an electron from the primary donor called P680 to a nearby pheophytin molecule. We analyzed the temperature dependence of this reaction by subpicosecond transient absorption spectroscopy over the temperature range 20-240 K using isolated photosystem II reaction centers from spinach. After excitation in the red edge of the Q y absorption band, the decay of the excited state can conveniently be described by two kinetic components that both accelerate with temperature. This temperature behavior differs remarkably from that observed in purple bacterial reaction centers. We attribute the first component, which accelerates from 2.6 ps at 20 K to 0.4 ps at 240 K, to charge separation after direct excitation of P680, and explain its temperature dependence by an intermediate that lies in energy above the singlet-excited P680 and that possibly has charge-transfer character. The second component accelerates from 120 ps at 20 K to 18 ps at 240 K and is attributed to charge separation after direct excitation of the ''trap'' state near-degenerate with P680 and subsequent slow energy transfer from this trap state to P680. We suggest that the slow energy transfer from the trap state to P680 plays an important role in the kinetics of radical pair formation at room temperature.The photochemical reaction center (RC) of photosystem II (PSII) (the D1-D2 cyt.b559 complex) is the smallest unit in PSII that shows photochemical activity. The RC contains six chlorophyll (Chl) a and two pheophytin (Pheo) a molecules (1-4) that all have their lowest electronic transition around 675 nm, as well as two ␤-carotenes. The D1 and D2 polypeptides are homologous to the L and M subunits of bacterial RCs, suggesting an arrangement of the core pigments in the RC of PSII similar to that in the bacterial RC (5). The two additional Chl molecules are probably located near the periphery of the D1-D2 complex.The characterization of energy transfer and charge separation in the PSII RC is less well established than is the case for the bacterial RC (for a review, see ref. 6). This may be due to the fact that the first PSII RC was isolated only in 1987 (7) and that there is no structure available. However, the excited state kinetics of the PSII RC are also inherently more complicated than those of bacterial RCs. The primary electron donor in PSII, called P680, is isoenergetic with some of the other pigments in the RC (8-13) and as a consequence forms only a very shallow trap for excitations.After excitation of P680 an electron is transferred to the photoactive Pheo molecule (6). In isolated PSII RC preparations, further electron transport cannot occur, because the quinone acceptors are lost during the isolation procedure. A straightforward interpretation of the results from pump-probe measurements in terms of pigment to pigment energy transfer and charge separation rates has proven difficult, mainly due to the congested absorption spectrum of the PSII RC. At r...

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Section: Discussioncontrasting

confidence: 39%

mentioning

confidence: 74%

Charge separation in the reaction center of photosystem II studied as a function of temperature

Groot

Mourik

Eijckelhoff

et al. 1997

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

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“…viridis at low temperature that, with regard to the two-step model, k2 -70 kl [35]. To calculate this ratio, values for a number of spectral parameters of the three states in eqn 1 had to be assumed.…”

Section: Mechanism Of the Initial Electron Transfer Reactionmentioning

confidence: 99%

Subpicosecond characterization of the optical properties of the primary electron donor and the mechanism of the initial electron transfer in Rhodobacter capsulatus reaction centers

Kirmaier

Holten

1988

FEBS Letters

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The optical difference spectrum of the excited primary electron donor, P*, in Rb. capsulatus reaction centers has been measured 600 fs after a 350‐fs flash at 870 nm. The spectrum is characterized by bleaching in the ground state absorption bands at 855 and 600 nm, and a weak featureless transient absorption in between. The lack of significant (if any) bleaching at 800 nm indicates that P does not contribute appreciable oscillator strength to the 800‐nm ground state absorption band. The conversion of P* to P+BPH− L is accompanied by isosbestic points in the transient difference spectra at 765 and 798 nm. The existence of the longer‐wavelength isosbestic point, occurring essentially at the near‐infrared absorbance maximum of the accessory BChls, provides compelling evidence that if state P*BChl− L forms, its transient concentration is exceedingly small. At certain wavelengths in the near‐infrared the absorption changes develop somewhat more slowly than the rate at which P* decays, a finding that may reflect a contribution from readjustments in the pigment‐protein complex in response to electron transfer.

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“…This picture is based on earlier transient absorption data, where two time constants, x 3 ps and ~200 ps were reported [ 7-91. However, the theoretical description of this direct electron transfer meets a number of difficulties [ 9,[13][14][15] : The large distance between P and H,, as well as the high speed of the forward reaction and the slow recombination hardly fit into the frame of conventional ET theory. Superexchange-type electron transfer via a virtually populated radical pair involving the accessory bacteriochlorophyll B, and indirect electron transfer via a short-lived intermediate state P+Bi have been discussed as a solution to the problem [ From the excited state P*, which exists for 3.5 ps, an electron is transferred to the bacteriochlorophyll B,.…”

Section: Introductionmentioning

confidence: 99%

Detailed studies of the subpicosecond kinetics in the primary electron transfer of reaction centers of Rhodopseudomonas viridis

Dressler

Umlauf

Schmidt

et al. 1991

Chemical Physics Letters

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The primary, light-induced charge separation in reaction centers of Rhodopseudomonas widis is investigated with femtosecond time resolution. The absorption changes after direct excitation of the primary donor P at 955 nm are investigated in the time range from 100 fs to 600 ps. The experimental data, taken at various probing wavelengths, reveal one subpicosecond and two picosecond time constants: 0.65 k 0.2 ps, 3.5 k 0.4 ps, and 200f 20 ps. The previously undetected 0.65 ps kinetics can be observed clearly in the spectral range of the QX and Qu transitions of the monomeric bacteriochlorophylls. The experimental data support the idea that the accessory bacteriochlorophyll B, participates in the electron-transfer process.

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Rates of primary electron transfer in photosynthetic reaction centres and their mechanistic implications

Cited by 440 publications

References 25 publications

Charge separation in the reaction center of photosystem II studied as a function of temperature

Charge separation in the reaction center of photosystem II studied as a function of temperature

Subpicosecond characterization of the optical properties of the primary electron donor and the mechanism of the initial electron transfer in Rhodobacter capsulatus reaction centers

Detailed studies of the subpicosecond kinetics in the primary electron transfer of reaction centers of Rhodopseudomonas viridis

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