Chlorophyll triplet states in thylakoid membranes of Acaryochloris marina. Evidence for a triplet state sitting on the photosystem I primary donor populated by intersystem crossingстатья
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Аннотация: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, identifable by their characteristic zero feld 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 detectableafter 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 complexspectral features that overall resemble, despite showing further fne spectral structure, the previously reported Triplet-minusSinglet spectrum attributed to the recombination triplet of PSI reaction centre, 3P740 [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.
