Anion radicals of pheophytin and chlorophyll a: their role in the primary charge separations of plant photosynthesis

Fujita, Ichiro; Davis, M. S.; Fajer, J.

doi:10.1021/ja00487a079

Cited by 205 publications

(113 citation statements)

References 2 publications

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“…The optical difference spectrum for Ao, presented here, closely resembles the chl a-/chl difference spectrum generated electrolytically in dimethylformamide by Fujita et al [17]. However, only a minor absorbance increase occurred at 640 nm.…”

Section: Resultssupporting

confidence: 85%

“…The changes seen in the 480-550 nm region resemble a difference spectrum of pheophytin a-/pheophytin [17] but their rapid appearance upon 215 K illumination was not accompanied by large absorbance changes at 410 and 670 nm as would be expected for such a transition. Moreover, the signals did not increase upon further illumination and bore no relationship to the rate of formaWe are able to confirm that 'Al' has two distinct components.…”

Section: Resultsmentioning

confidence: 69%

“…It seems more probable that chl a-, with in vitro reduction potential of about -0.9 V, will act as the reductant for the iron-sulphur centres (E,,, of X > -700 mV) rather than pheophytin (Em= -0.64 V). Upon excitation, P700* is energetically capable of reducing a chlorophyll acceptor [17]. The chemical nature of A1 remains elusive.…”

Section: Resultsmentioning

confidence: 99%

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Optical difference spectrum of the electron acceptor A_o in photosystem I

Mansfield

Evans

1985

FEBS Letters

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Section: Resultssupporting

confidence: 85%

Section: Resultsmentioning

confidence: 69%

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Optical difference spectrum of the electron acceptor A_o in photosystem I

Mansfield

Evans

1985

FEBS Letters

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“…The spectral shape and characteristic peaks at 428, 450, 517, and 547 nm are practically the same as those previously reported for PS II complexes (18,(33)(34)(35)(36). The two bleaches at 517 and 547 nm arise from the Qx bands of Phe a, the bleach at 428 nm is the Soret band (18,33), and the 450 nm peak is due to the Phe a Ϫ absorption (33,37). Our result hence represents the lightinduced difference spectra due to the redox reaction of photoactive Phe a (38)(39)(40)(41)(42), which is in the D1 branch and denoted Phe a D1 , in the PS II core complexes.…”

Section: Resultssupporting

confidence: 80%

Spectroelectrochemical determination of the redox potential of pheophytina, the primary electron acceptor in photosystem II

Kato

Sugiura

Oda

et al. 2009

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

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Thin-layer cell spectroelectrochemistry, featuring rigorous potential control and rapid redox equilibration within the cell, was used to measure the redox potential Em(Phe a/Phe a ؊ ) of pheophytin (Phe) a, the primary electron acceptor in an oxygen-evolving photosystem (PS) II core complex from a thermophilic cyanobacterium Thermosynechococcus elongatus. Interferences from dissolved O 2 and water reductions were minimized by airtight sealing of the sample cell added with dithionite and mercury plating on the gold minigrid working electrode surface, respectively. The result obtained at a physiological pH of 6.5 was E m(Phe a/Phe a ؊ ) ‫؍‬ ؊505 ؎ 6 mV vs. SHE, which is by Ϸ100 mV more positive than the values measured Ϸ30 years ago at nonphysiological pH and widely accepted thereafter in the field of photosynthesis research. Using the P680* ؊ Phe a free energy difference, as estimated from kinetic analyses by previous authors, the present result would locate the E m(P680/P680 ؉ ) value, which is one of the key parameters but still resists direct measurements, at approximately ؉1,210 mV. In view of these pieces of information, a renewed diagram is proposed for the energetics in PS II.charge separation ͉ photosynthesis ͉ spectroelectrochemistry ͉ water oxidation I n the photosynthetic primary process of higher plants, algae, and cyanobacteria, photosystem (PS) I and PS II cooperate in series to convert photon energy into chemical energy through light-induced charge separation and subsequent electron transfers. PS II appears to catalyze water oxidation at a pentanuclear Mn 4 Ca cluster that accumulates oxidizing equivalents (see recent reviews in refs. 1-5). It is generally supposed that the water oxidation is triggered by charge separation between the primary electron donor, P680, and the primary electron acceptor, pheophytin (Phe) a. The initial radical pair P680 ϩ Phe a Ϫ formed by the charge separation, drives forward electron transfer from Phe a Ϫ to the first plastoquinone Q A , and hole transfer from P680 ϩ to the Mn 4 Ca cluster through a redox-active tyrosine residue denoted Y Z , thus preventing charge recombination. Recent X-ray crystallography clarified the arrangement of protein subunits and cofactors in PS II with 2.9-3.7-Å resolution (6-9), visualizing the electron transfer pathway.To date, the nature of P680 remains controversial. Available experimental data and theoretical analyses suggest that P680 is assignable to the pigment cluster of the four chlorophyll a molecules (denoted P D1 , P D2 , Chl D1 , and Chl D2 ) in the PS II reaction center (1, 5). Further uncertainty surrounds the value of the redox potential E m (P680/P680 ϩ ). Although the E m (P680/ P680 ϩ ) value is an essential parameter to draw a whole picture of the PS II energetics, its direct measurement has never been attained (10) because water, present as dominant (solvent) molecules in most sample solutions, is oxidized first heavily and tends to mask the subsequent oxidation of the target entity P680 at a higher potential. The E m ...

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“…Using the value of 32 mM_' . cm-' for the difference extinction coefficient at 685 nm (taken from the long-wavelength band of Ph in vitro [ 17]), . At 6 K, this signal was also seen at low microwave power (0.1 mW), but almost unobservable at 50 mW due to power saturation ( fig.2).…”

Section: Illum~ationmentioning

confidence: 99%

EPR properties of an intermediary electron acceptor (pheophytin) in photosystem‐II reaction centers at cryogenic temperatures

1980

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Anion radicals of pheophytin and chlorophyll a: their role in the primary charge separations of plant photosynthesis

Cited by 205 publications

References 2 publications

Optical difference spectrum of the electron acceptor A_o in photosystem I

Optical difference spectrum of the electron acceptor A_o in photosystem I

Spectroelectrochemical determination of the redox potential of pheophytina, the primary electron acceptor in photosystem II

EPR properties of an intermediary electron acceptor (pheophytin) in photosystem‐II reaction centers at cryogenic temperatures

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Anion radicals of pheophytin and chlorophyll a: their role in the primary charge separations of plant photosynthesis

Cited by 205 publications

References 2 publications

Optical difference spectrum of the electron acceptor Ao in photosystem I

Optical difference spectrum of the electron acceptor Ao in photosystem I

Spectroelectrochemical determination of the redox potential of pheophytina, the primary electron acceptor in photosystem II

EPR properties of an intermediary electron acceptor (pheophytin) in photosystem‐II reaction centers at cryogenic temperatures

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Optical difference spectrum of the electron acceptor A_o in photosystem I

Optical difference spectrum of the electron acceptor A_o in photosystem I