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Chlorophyll triplet states in thylakoid membranes of Acaryochloris marina. Evidence for a triplet state sitting on the photosystem I primary donor populated by intersystem crossing

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Abstract

Photo-induced triplet states in the thylakoid membranes isolated from the cyanobacterium Acaryocholoris marina, that harbours Chlorophyll (Chl) d as its main chromophore, have been investigated by Optically Detected Magnetic Resonance (ODMR) and time-resolved Electron Paramagnetic Resonance (TR-EPR). Thylakoids were subjected to treatments aimed at poising the redox state of the terminal electron transfer acceptors and donors of Photosystem II (PSII) and Photosystem I (PSI), respectively. Under ambient redox conditions, four Chl d triplet populations were detectable, identifiable by their characteristic zero field splitting parameters, after deconvolution of the Fluorescence Detected Magnetic Resonance (FDMR) spectra. Illumination in the presence of the redox mediator N,N,N′,N′-Tetramethyl-p-phenylenediamine (TMPD) and sodium ascorbate at room temperature led to a redistribution of the triplet populations, with T3 (|D|= 0.0245 cm−1, |E|= 0.0042 cm−1) becoming dominant and increasing in intensity with respect to untreated samples. A second triplet population (T4, |D|= 0.0248 cm−1, |E|= 0.0040 cm−1) having an intensity ratio of about 1:4 with respect to T3 was also detectable after illumination in the presence of TMPD and ascorbate. The microwave-induced Triplet-minus-Singlet spectrum acquired at the maximum of the |D|–|E| transition (610 MHz) displays a broad minimum at 740 nm, accompanied by a set of complex spectral features that overall resemble, despite showing further fine spectral structure, the previously reported Triplet-minus-Singlet spectrum attributed to the recombination triplet of PSI reaction centre, \({}^{3}P_{740}^{{}}\) [Schenderlein M, Çetin M, Barber J, et al. Spectroscopic studies of the chlorophyll d containing photosystem I from the cyanobacterium Acaryochloris marina. Biochim Biophys Acta 1777:1400–1408]. However, TR-EPR experiments indicate that this triplet displays an eaeaea electron spin polarisation pattern which is characteristic of triplet sublevels populated by intersystem crossing rather than recombination, for which an aeeaae polarisation pattern is expected instead. It is proposed that the observed triplet, which leads to the bleaching of the P740 singlet state, sits on the PSI reaction centre.

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Data sets generated during the current study are available from the corresponding author on reasonable request.

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Acknowledgements

SS and APC acknowledge support from Regione Lombardia through the project “Enhancing Photosynthesis” (DSS 16652 30/11/2021) on behalf of the winners of the “Lombardia è Ricerca – 2020” award. AAP acknowledge support from by Russian Science Foundation Grant RSF 21-74-10085. AA and DC acknowledge financial support from the University of Padova (P-DiSC-2019). MB acknowledges support from P-DiSC#02BIRD2020-UNIPD.

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"S.S. and D.C. designed the research and wrote the bulk of the paper; S.S., A.A. and M.B. performed ODMR and TR-EPR spectroscopic investigations and analysed the data; A.A.P., A.P.C. and S.S. maintained the strains, developed the biochemical purification and performed preliminary spectroscopic investigations of the purified material; All authors reviewed the paper and contributed to its final redaction"

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Correspondence to Stefano Santabarbara or Donatella Carbonera.

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Santabarbara, S., Agostini, A., Petrova, A.A. et al. Chlorophyll triplet states in thylakoid membranes of Acaryochloris marina. Evidence for a triplet state sitting on the photosystem I primary donor populated by intersystem crossing. Photosynth Res 159, 133–152 (2024). https://doi.org/10.1007/s11120-023-01023-z

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  • DOI: https://doi.org/10.1007/s11120-023-01023-z

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