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Photosynthetic regulation: the pigments of nature (16) | Dr Jasper van Thor
Recent discoveries have shown that the photosynthetic antennae and reaction centres of oxygenic photosynthesis can be replaced, depending on environmental conditions, for pigments that better match the changed illumination conditions. One of these pigments is chlorophyll-f, a far-red absorbing molecule. The low site-energy and expected exciton trapping behaviour gives rise to questions of bioenergetics: The dissertation will focus on the physical mechanisms of energy transfer and yield of charge separation when far-red pigments are included in the antennae.
Dr Jasper van Thor
Abstract: Acaryochloris marina is a unique cyanobacterium that contains chlorophyll (Chl) d as a major pigment. Because Chl d has smaller excitation energy than Chl a used in ordinary photosynthetic organisms, the energetics of the photosystems of A. marina have been the subject of interest. It was previously shown that the redox potentials (E m's) of the redox-active pheophytin a (Pheo) and the primary plastoquinone electron acceptor (QA) in photosystem II (PSII) of A. marina are higher than those in Chl a-containing PSII, to compensate for the smaller excitation energy of Chl d (Allakhverdiev et al., Proc Natl Acad Sci USA 107: 3924-3929, 2010; ibid. 108: 8054-8058, 2011). To clarify the mechanisms of these E m increases, in this study, we have investigated the molecular interactions of Pheo and QA in PSII core complexes from A. marina using Fourier transform infrared (FTIR) spectroscopy. Light-induced FTIR difference spectra upon single reduction of Pheo and QA showed that spectral features in the regions of the keto and ester C=O stretches and the chlorin ring vibrations of Pheo and in the CO/CC stretching region of the Q A (-) semiquinone anion in A. marina are significantly different from those of the corresponding spectra in Chl a-containing cyanobacteria. These observations indicate that the molecular interactions, including the hydrogen bond interactions at the C=O groups, of these cofactors are modified in their binding sites of PSII proteins. From these results, along with the sequence information of the D1 and D2 proteins, it is suggested that A. marina tunes the E m's of Pheo and QA by altering nearby hydrogen bond networks to modify the structures of the binding pockets of these cofactors.
Pub.: 07 Jan '15, Pinned: 25 Jan '17
Abstract: Cyanobacteria are unique among bacteria in performing oxygenic photosynthesis, often together with nitrogen fixation and, thus, are major primary producers in many ecosystems. The cyanobacterium, Leptolyngbya sp. strain JSC-1, exhibits an extensive photoacclimative response to growth in far-red light that includes the synthesis of chlorophylls d and f. During far-red acclimation, transcript levels increase more than twofold for ~900 genes and decrease by more than half for ~2000 genes. Core subunits of photosystem I, photosystem II, and phycobilisomes are replaced by proteins encoded in a 21-gene cluster that includes a knotless red/far-red phytochrome and two response regulators. This acclimative response enhances light harvesting for wavelengths complementary to the growth light (λ = 700 to 750 nanometers) and enhances oxygen evolution in far-red light.
Pub.: 13 Sep '14, Pinned: 25 Jan '17
Abstract: Chlorophylls are magnesium-tetrapyrrole molecules that play essential roles in photosynthesis. All chlorophylls have similar five-membered ring structures, with variations in the side chains and/or reduction states. Formyl group substitutions on the side chains of chlorophyll a result in the different absorption properties of chlorophyll b, chlorophyll d, and chlorophyll f. These formyl substitution derivatives exhibit different spectral shifts according to the formyl substitution position. Not only does the presence of various types of chlorophylls allow the photosynthetic organism to harvest sunlight at different wavelengths to enhance light energy input, but the pigment composition of oxygenic photosynthetic organisms also reflects the spectral properties on the surface of the Earth. Two major environmental influencing factors are light and oxygen levels, which may play central roles in the regulatory pathways leading to the different chlorophylls. I review the biochemical processes of chlorophyll biosynthesis and their regulatory mechanisms.
Pub.: 19 Mar '14, Pinned: 25 Jan '17
Abstract: Chlorophyll f is a photosynthetic pigment that was discovered in 2010. In this study, we present investigations on spectral and dynamic characteristics of singlet-excited and triplet states of Chl f with the application of ultrafast time-resolved absorption and fluorescence spectroscopies. The pigment was studied at room temperature in two organic solvents: pyridine and diethyl ether that have different characters of coordination of the chlorophyll magnesium (Mg) atom (hexa- and penta-coordination, respectively). Cryogenic measurements (77 K) were performed in 2-methyltetrahydrofuran (hexa-coordination). The singlet-excited state lifetime was measured to be 5.6 ns at room temperature regardless of Mg coordination and 8.1 ns at 77 K. The fluorescence quantum yield of Chl f was also determined in pyridine to be 0.16. The triplet state lifetime was studied in detail in pyridine at room temperature, and the inherent lifetime was estimated to ~150 μs. Selective measurements at 77 K demonstrated that the metastability of the triplet state greatly enhances, and its lifetime increases by a factor of more than three.
Pub.: 20 Feb '14, Pinned: 25 Jan '17
Abstract: A Chl f-containing filamentous cyanobacterium was purified from stromatolites and named as Halomicronema hongdechloris gen., sp. nov. after its phylogenetic classification and the morphological characteristics. Hongdechloris contains four main carotenoids and two chlorophylls, a and f. The ratio of Chl f to Chl a is reversibly changed from 1:8 under red light to an undetectable level of Chl f under white-light culture conditions. Phycobiliproteins were induced under white light growth conditions. A fluorescence emission peak of 748 nm was identified as due to Chl f. The results suggest that Chl f is a red-light inducible chlorophyll.
Pub.: 17 Jul '12, Pinned: 25 Jan '17