1,721,094 research outputs found
The fine tuning of carotenoid-chlorophyll interactions in light-harvesting complexes: An important requisite to guarantee efficient photoprotection via triplet-triplet energy transfer in the complex balance of the energy transfer processes
No full textElectronic coupling effects on photoinduced electron transfer in carotene-porphyrin-fullerene triads detected by time-resolved EPR
No full textPhotoinduced charge separation and recombination in a carotenoid-porphyrin-fullerene triad C-P-C60 (Bahr et al., 2000) have been followed by time-resolved electron paramagnetic resonance. The electron-transfer process has been characterized in a glass of 2-methyltetrahydrofuran and in the nematic phase of two uniaxial liquid crystals (E-7 and ZLI-1 167). In all the different media, the molecular triad undergoes two-step photoinduced electron transfer, with the generation of a long-lived charge-separated state (Ccenter dot+- P-C60center dot-) and charge recombination to the triplet state, localized in the Carotene moiety, mimicking different P-C-60(center dot-) 4 C aspects of the photosynthetic electron-transfer process. The magnetic interaction parameters have been evaluated by simulation of the spin-polarized radical pair spectrum. The weak exchange interaction parameter (J = +1.7 +/- 0.1 G) provides a direct measure of the dominant electronic Coupling matrix element V between the Ccenter dot+-P-C60(center dot-) radical pair state and the recombination triplet state C-3-P-C-60. Comparison of the estimated values of V for this triad and a structurally related triad differing only in the porphyrin bridge (octaalkylporphyrin vs tetraarylporphyrin) explains in terms of an electronic coupling effect the similar to 6-fold variation of the recombination rate induced by the modification of the porphyrin bridge as derived by kinetic experiments (Bahr et al., 2000)
FDMR of Chlorophyll triplets in Integrated Particles and Isolated Reaction Centers of Photosystem II. Identification of P680 Triplet.
No full textFluorescence detected magnetic resonance (FDMR) of various chlorophyll a triplets in PS II large particles was studied with the aim of identifying the nature of the different components underlying the complex line shape of the signals. The latter are deconvoluted into sums of Gaussians in different experimental conditions, and the charge recombination triplet is unequivocally identified as the source of two lines with negative signs at 722.5 and 992 MHz (\D\ = 0.0286 cm(-1), \E\ = 0.0043 cm(-1)). The origin of the other triplet signals from the integrated particles is discussed in terms of functional antenna components and products from alteration of the native environment of pigments. FDMR signals also arise from isolated reaction centre complexes and are all to be attributed to different triplet populations of the primary electron donor P680. The results are compared with those obtained by other authors on ADMR. The heterogeneity of P680 pigments in D1D2-cyt-b(559) complexes is shown to arise during preparation procedures
Unravelling electronic and structural requisites of triplet–triplet energy transfer by advanced electron paramagnetic resonance and density functional theory
No full textCharge Transfer in Model Bioinspired Carotene–Porphyrin Dyads
No full textWe present a computational study based on accurate DFT and TD-DFT methods on model bioinspired donor-acceptor dyads, formed by a carotenoid covalently linked to a tetraphenylporphyrin (TPP) at the ortho position of one of the TPP phenyl rings. Dyadic systems can be used in the construction of organic solar cells and development of efficient photocatalytic systems for the solar energy conversion, due to the unique advantages they offer in terms of synthetic feasibility. This study aims to describe the influence of chemical modifications on the absorption spectra, in particular on the lowest energy charge transfer bands. Effects of different metals of biological interest, i.e., Mg, Fe, Ni, and Zn, and of H2O and histidine molecules coordinated to the metals in different axial positions are rationalized
Structure and magnetic properties of aza-aromatic triplet states. The case of quinoxaline
No full textA recently developed quantum mechanical approach devoted to the study of structural and magnetic properties of organic free radicals was applied to the lowest triplet state of quinoxaline. From a quantitative point of view, hybrid Hartrec-Fock/Kohn-Sham methods (here B3LYP) confirm their reliability, whereas the Hartree-Fock method gives disappointing magnetic results. From an interpretative point of view, spin polarization effects are well represented by the semi-empirical McConnel relation between spin densities and isotropic hyperfine couplings
A distinctive pathway for Triplet-Triplet Energy Transfer photoprotection in Fucoxanthin Chlorophyll-binding Proteins from Cyclotella meneghiniana
No full textFucoxanthin Chlorophyll-binding Proteins (FCPs) are the major light-harvesting complexes of diatoms. In this work, FCPs isolated from Cyclotella meneghiniana have been studied by means of Optically Detected Magnetic Resonance (ODMR) and Time-Resolved Electron Paramagnetic Resonance (TR-EPR), with the aim to characterize the photoprotective mechanism based on triplet-triplet energy transfer (TTET). The spectroscopic properties of the chromophores carrying the triplet state have been interpreted on the basis of a delved analysis of the recently solved crystallographic structures of FCP. The results point toward a photoprotective role for two fucoxanthin molecules exposed to the exterior of the FCP monomers. This shows that FCP has adopted a structural strategy different from that of related light-harvesting complexes from plants and other microalgae, in which the photoprotective role is carried out by two highly conserved carotenoids in the interior of the complex
Pulsed EPR and ENDOR on the Peridinin Triplet State Involved in the Photoprotective Mechanism in Peridinin-Chlorophyll a-Proteins
No full textThe photoexcited triplet state of the carotenoid peridinin in the peridinin-chlorophyll a-protein (PCP) of the dinoflagellate Heterocapsa pygmaea has been investigated by pulsed electron paramagnetic resonance (EPR) and pulsed electron-nuclear double resonance (ENDOR) spectroscopies. The alpha- and beta-protons hyperfine couplings of the peridinin-conjugated chain have been derived from Davies and Mims ENDOR experiments. The spectroscopic results have been compared to those obtained for the main form of the PCP complex and for the high-salt PCP form from Amphidinium carterae. The EPR features of the peridinin triplet state are very similar in the antenna complexes belonging to the two different dinoflagellate species, proving that the triplet formation pathway and the triplet localization on one specific peridinin per subcluster are common features of different PCP antennas. No significant variation of the hyperfine couplings of the peridinin triplet state has been detected between the main form of the PCP complex from A. carterae and H. pygmaea. The spectroscopic results confirm the close relationship between the Amphidinium PCP and the corresponding Heterocapsa complex at least in terms of mutual arrangement of the chlorophyll a-peridinin pair involved in photoprotection and in terms of conformation of the peridinin-conjugated chain
Chlorophyll triplet quenching by fucoxanthin in the fucoxanthin–chlorophyll protein from the diatom Cyclotella meneghiniana
No full textIn this work we present an optically detected magnetic resonance (ODMR) study on the triplet states populated under illumination in the isolated fucoxanthin–chlorophyll light-harvesting complex from the diatom Cyclotella meneghiniana. Evidence for the quenching of chlorophyll triplet states by fucoxanthin is provided, showing that this carotenoid is able to perform the photoprotective role. For the first time, the magnetic parameters characterizing the fucoxanthin triplet state have been determined
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