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Mixed exciton-charge-transfer states in photosystem II: Stark spectroscopy on site-directed mutants.

ABSTRACT: We investigated the electronic structure of the photosystem II reaction center (PSII RC) in relation to the light-induced charge separation process using Stark spectroscopy on a series of site-directed PSII RC mutants from the cyanobacterium Synechocystis sp. PCC 6803. The site-directed mutations modify the protein environment of the cofactors involved in charge separation (P(D1), P(D2), Chl(D1), and Phe(D1)). The results demonstrate that at least two different exciton states are mixed with charge-transfer (CT) states, yielding exciton states with CT character: (P(D2)(?)(+)P(D1)(?)(-)Chl(D1)) (673 nm) and (Chl(D1)(?)(+)Phe(D1)(?)(-)) (681 nm) (where the subscript indicates the wavelength of the electronic transition). Moreover, the CT state P(D2)(+)P(D1)(-) acquires excited-state character due to its mixing with an exciton state, producing (P(D2)(+)P(D1)(-))(?) (684 nm). We conclude that the states that initiate charge separation are mixed exciton-CT states, and that the degree of mixing between exciton and CT states determines the efficiency of charge separation. In addition, the results reveal that the pigment-protein interactions fine-tune the energy of the exciton and CT states, and hence the mixing between these states. This mixing ultimately controls the selection and efficiency of a specific charge separation pathway, and highlights the capacity of the protein environment to control the functionality of the PSII RC complex.

SUBMITTER: Romero E

PROVIDER: S-EPMC3400784 | biostudies-other | 2012 Jul

REPOSITORIES: biostudies-other

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Mixed exciton-charge-transfer states in photosystem II: Stark spectroscopy on site-directed mutants.

Romero Elisabet E Diner Bruce A BA Nixon Peter J PJ Coleman Wiliam J WJ Dekker Jan P JP van Grondelle Rienk R

Biophysical journal 20120717 2

We investigated the electronic structure of the photosystem II reaction center (PSII RC) in relation to the light-induced charge separation process using Stark spectroscopy on a series of site-directed PSII RC mutants from the cyanobacterium Synechocystis sp. PCC 6803. The site-directed mutations modify the protein environment of the cofactors involved in charge separation (P(D1), P(D2), Chl(D1), and Phe(D1)). The results demonstrate that at least two different exciton states are mixed with char ...[more]

PMID: 22853895

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Project description:Secondary electron transfer in photosystem II (PSII), which occurs when water oxidation is inhibited, involves redox-active carotenoids (Car), as well as chlorophylls (Chl), and cytochrome b 559 (Cyt b 559), and is believed to play a role in photoprotection. CarD2 may be the initial point of secondary electron transfer because it is the closest cofactor to both P680, the initial oxidant, and to Cyt b 559, the terminal secondary electron donor within PSII. In order to characterize the role of CarD2 and to determine the effects of perturbing CarD2 on both the electron-transfer events and on the identity of the redox-active cofactors, it is necessary to vary the properties of CarD2 selectively without affecting the ten other Car per PSII. To this end, site-directed mutations around the binding pocket of CarD2 (D2-G47W, D2-G47F, and D2-T50F) have been generated in Synechocystis sp. PCC 6803. Characterization by near-IR and EPR spectroscopy provides the first experimental evidence that CarD2 is one of the redox-active carotenoids in PSII. There is a specific perturbation of the Car(∙+) near-IR spectrum in all three mutated PSII samples, allowing the assignment of the spectral signature of Car D2 (∙+) ; Car D2 (∙+) exhibits a near-IR peak at 980 nm and is the predominant secondary donor oxidized in a charge separation at low temperature in ferricyanide-treated wild-type PSII. The yield of secondary donor radicals is substantially decreased in PSII complexes isolated from each mutant. In addition, the kinetics of radical formation are altered in the mutated PSII samples. These results are consistent with oxidation of CarD2 being the initial step in secondary electron transfer. Furthermore, normal light levels during mutant cell growth perturb the shape of the Chl(∙+) near-IR absorption peak and generate a dark-stable radical observable in the EPR spectra, indicating a higher susceptibility to photodamage further linking the secondary electron-transfer pathway to photoprotection.

Dataset Information

Mixed exciton-charge-transfer states in photosystem II: Stark spectroscopy on site-directed mutants.

Publications

Mixed exciton-charge-transfer states in photosystem II: Stark spectroscopy on site-directed mutants.

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