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- Triplet Exciton Formation as a Novel Photoprotection Mechani...Triplet Exciton Formation as a Novel Photoprotection Mechanism in Chlorosomes of Chlorobium tepidum
Biophysical Journal, Volume 93, Issue 1, 1 July 2007, Pages 192-201
Hanyoup Kim, Hui Li, Julia A. Maresca, Donald A. Bryant and Sergei Savikhin
AbstractChlorosomes comprise thousands of bacteriochlorophylls (BChl , or ) in a closely packed structure surrounded by a lipid-protein envelope and additionally contain considerable amounts of carotenoids, quinones, and BChl . It has been suggested that carotenoids in chlorosomes provide photoprotection by rapidly quenching triplet excited states of BChl via a triplet-triplet energy transfer mechanism that prevents energy transfer to oxygen and the formation of harmful singlet oxygen. In this work we studied triplet energy transfer kinetics and photodegradation of chlorosomes isolated from wild-type and from genetically modified species with different types of carotenoids and from a carotenoid-free mutant. Supporting a photoprotective function of carotenoids, carotenoid-free chlorosomes photodegrade ∼3 times faster than wild-type chlorosomes. However, a significant fraction of the BChls forms a long-lived, triplet-like state that does not interact with carotenoids or with oxygen. We propose that these states are triplet excitons that form due to triplet-triplet interaction between the closely packed BChls. Numerical exciton simulations predict that the energy of these triplet excitons may fall below that of singlet oxygen and triplet carotenoids; this would prevent energy transfer from triplet BChl. Thus, the formation of triplet excitons in chlorosomes serves as an alternative photoprotection mechanism.
Abstract | Full Text | PDF (337 kb) - Pigment–pigment interactions and energy transfer in the ante...Pigment–pigment interactions and energy transfer in the antenna complex of the photosynthetic bacterium Rhodopseudomonasacidophila
Structure, Volume 4, Issue 4, 1 April 1996, Pages 449-462
Andy Freer, Steve Prince, Ken Sauer, Miroslav Papiz, Anna Hawthornthwaite Lawless, Gerry McDermott, Richard Cogdell and Neil W Isaacs
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Summary | Full Text | PDF (983 kb) - How Proteins Trigger Excitation Energy Transfer in the FMO C...How Proteins Trigger Excitation Energy Transfer in the FMO Complex of Green Sulfur Bacteria
Biophysical Journal, Volume 91, Issue 8, 15 October 2006, Pages 2778-2797
Julia Adolphs and Thomas Renger
AbstractA simple electrostatic method for the calculation of optical transition energies of pigments in protein environments is presented and applied to the Fenna-Matthews-Olson (FMO) complex of and . The method, for the first time, allows us to reach agreement between experimental optical spectra and calculations based on transition energies of pigments that are calculated in large part independently, rather than fitted to the spectra. In this way it becomes possible to understand the molecular mechanism allowing the protein to trigger excitation energy transfer reactions. The relative shift in excitation energies of the seven bacteriochlorophyll-a pigments of the FMO complex of and are obtained from calculations of electrochromic shifts due to charged amino acids, assuming a standard protonation pattern of the protein, and by taking into account the three different ligand types of the pigments. The calculations provide an explanation of some of the earlier results for the transition energies obtained from fits of optical spectra. In addition, those earlier fits are verified here by using a more advanced theory of optical spectra, a genetic algorithm, and excitonic couplings obtained from electrostatic calculations that take into account the influence of the dielectric protein environment. The two independent calculations of site energies strongly favor one of the two possible orientations of the FMO trimer relative to the photosynthetic membrane, which were identified by electron microscopic studies and linear dichroism experiments. Efficient transfer of excitation energy to the reaction center requires bacteriochlorophylls 3 and 4 to be the linker pigments. The temporal and spatial transfer of excitation energy through the FMO complex is calculated to proceed along two branches, with transfer times that differ by an order of magnitude.
Abstract | Full Text | PDF (965 kb) - …more
Copyright © 1997 The Biophysical Society. All rights reserved.
Biophysical Journal, Volume 72, Issue 1, 24-36, 1 January 1997
doi:10.1016/S0006-3495(97)78644-2
Research Article
Ultrafast absorption difference spectra of the Fenna-Matthews-Olson protein at 19 K: experiment and simulations
D.R. Buck, S. Savikhin and W.S. Struve
Ames Laboratory-USDOE, Iowa, USA.
Abstract
We describe simulations of absorption difference spectra in strongly coupled photosynthetic antennas. In the presence of large resonance couplings, distinctive features arise from excited-state absorption transitions between one- and two-exciton levels. We first outline the theory for the heterodimer and for the general N-pigment system, and we demonstrate the transition between the strong and weak coupling regimes. The theory is applied to Fenna-Matthews-Olson (FMO) bacteriochlorophyll a protein trimers from the green photosynthetic bacterium Prosthecochloris aestuarii and then compared with experimental low-temperature absorption difference spectra of FMO trimers from the green bacterium Chlorobium tepidum.
