An electron paramagnetic resonance investigation of the electron transfer reactions in the chlorophyll d containing photosystem I of Acaryochloris marina

An unusual triplet population pathway in the Reaction Centre of the Chlorophyll-d binding Photosystem I of A. marina, as revealed by a combination of TR-EPR and ODMR spectroscopies

Photo-induced Chlorophyll (Chl) triplet states in the isolated Photosystem I (PSI) of Acaryochloris marina, that harbours Chl d as its main pigment, were investigated by Optically Detected Magnetic Resonance (ODMR) and Time-Resolved Electron Paramagnetic Resonance (TR-EPR), and as a function of pre-illumination of the sample under reducing redox poising. Fluorescence Detected Magnetic Resonance (FDMR) allowed resolving four Chl d triplet (3Chl d) populations (T1-T4) both in untreated and illuminated samples in the presence of ascorbate and N,N,N',N'-tetramethyl-p-phenylenediamine (TMPD). The FDMR signals increased following the pre-illumination treatment, particularly for the T3 and T4 populations, which are therefore sensitive to the redox state of PSI cofactors. Microwave-induced Triplet minus Singlet (TmS) spectra were detected in the |D|-|E| resonance window of the T3 and T4 triplets. These showed a broad singlet bleaching centred at 740 nm and also displayed complex spectral structure with several derivative-like features, indicating that both the T3 and T43Chl d populations are associated with the PSI reaction centre (RC) triplet, P3740. Parallel measurements by TR-EPR demonstrated that triplet signals observed under all conditions investigated are dominated by an electron spin polarisation (esp), which is typical of intersystem crossing, differently from what expected for recombination triplet states formed from a radical pair precursor. Moreover, stronger reductant conditions obtained by pre-illumination of the samples in the presence of dithionite and 5-methylphenazinium methyl sulfate (PMS) did not lead to a recombination triplet state esp, but rather to a decrease of the whole signal intensity. The energetics of A. marina PSI and the possible occurrence of distributions of cofactors redox properties are discussed in order to address the unexpected P3740 esp.

A unique photosystem I reaction center from a chlorophyll d-containing cyanobacterium Acaryochloris marina

Photosystem I (PSI) is a large protein supercomplex that catalyzes the light-dependent oxidation of plastocyanin (or cytochrome c₆ ) and the reduction of ferredoxin. This catalytic reaction is realized by a transmembrane electron transfer chain consisting of primary electron donor (a special chlorophyll (Chl) pair) and electron acceptors A₀ , A₁ , and three Fe₄ S₄ clusters, F_X , F_A , and F_B . Here we report the PSI structure from a Chl d-dominated cyanobacterium Acaryochloris marina at 3.3 Å resolution obtained by single-particle cryo-electron microscopy. The A. marina PSI exists as a trimer with three identical monomers. Surprisingly, the structure reveals a unique composition of electron transfer chain in which the primary electron acceptor A₀ is composed of two pheophytin a rather than Chl a found in any other well-known PSI structures. A novel subunit Psa27 is observed in the A. marina PSI structure. In addition, 77 Chls, 13 α-carotenes, two phylloquinones, three Fe-S clusters, two phosphatidyl glycerols, and one monogalactosyl-diglyceride were identified in each PSI monomer. Our results provide a structural basis for deciphering the mechanism of photosynthesis in a PSI complex with Chl d as the dominating pigments and absorbing far-red light.

Chlorophyll d and Acaryochloris marina: current status

The discovery of the chlorophyll d-containing cyanobacterium Acaryochloris marina in 1996 precipitated a shift in our understanding of oxygenic photosynthesis. The presence of the red-shifted chlorophyll d in the reaction centre of the photosystems of Acaryochloris has opened up new avenues of research on photosystem energetics and challenged the unique status of chlorophyll a in oxygenic photosynthesis. In this review, we detail the chemistry and role of chlorophyll d in photosynthesis and summarise the unique adaptations that have allowed the proliferation of Acaryochloris in diverse ecological niches around the world.

Phosphorescence study of chlorophyll d photophysics. Determination of the energy and lifetime of the photo-excited triplet state. Evidence of singlet oxygen photosensitization

Chlorophyll d (Chl d) is the major pigment in both photosystems (PSI and II) of the cyanobacterium Acaryochloris marina, whose pigment composition represents an interesting alternative in oxygenic photosynthesis. While abundant information is available relative to photophysical properties of Chl a , the understanding of Chl d photophysics is still incomplete. In this paper, we present for the first time a characterization of Chl d phosphorescence, which accompanies radiative deactivation of the photoexcited triplet state of this pigment. Reliable information was obtained on the energy and lifetime of the Chl d triplet state in frozen solutions at 77 K using diethyl ether and aqueous dispersions of Triton X100 as solvents. It is shown that triplet Chl d is effectively populated upon photoexcitation of pigment molecules and efficiently sensitizes singlet oxygen phosphorescence in aerobic solutions under ambient conditions. The data obtained are compared with the previous results of the phosphorescence studies of Chl a and Pheo a, and their possible biological implications are discussed.