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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) - Energy Transfers in the B808–866 Antenna from the Green Bact...Energy Transfers in the B808–866 Antenna from the Green Bacterium Chloroflexus aurantiacus
Biophysical Journal, Volume 74, Issue 4, 1 April 1998, Pages 2069-2075
Vladimir I. Novoderezhkin, Alexandra S. Taisova, Zoya G. Fetisova, Robert E. Blankenship, Sergei Savikhin, Daniel R. Buck and Walter S. Struve
AbstractEnergy transfers within the B808–866 BChl antenna in chlorosome-membrane complexes from the green photosynthetic bacterium were studied in two-color pump-probe experiments at room temperature. The steady-state spectroscopy and protein sequence of the B808–866 complex are reminiscent of well-studied LH2 antennas from purple bacteria. B808→B866 energy transfers occur with ∼2ps kinetics; this is slower by a factor of ∼2 than B800→B850 energy transfers in LH2 complexes from or . Anisotropy studies show no evidence for intra-B808 energy transfers before the B808→B866 step; intra-B866 processes are reflected in 350–550fs anisotropy decays. Two-color anisotropies under 808nm excitation suggest the presence of a B808→B866 channel arising either from direct laser excitation of upper B866 exciton components that overlap the B808 absorption band or from excitation of B866 vibronic bands in nontotally symmetric modes.
Abstract | Full Text | PDF (200 kb) - Excitation Energy Transfer Dynamics and Excited-State Struct...Excitation Energy Transfer Dynamics and Excited-State Structure in Chlorosomes of Chlorobium phaeobacteroides
Biophysical Journal, Volume 84, Issue 2, 1 February 2003, Pages 1161-1179
Jakub Pšenčík, Ying-Zhong Ma, Juan B. Arellano, Jan Hála and Tomas Gillbro
AbstractThe excited-state relaxation within bacteriochlorophyll (BChl) and in chlorosomes of has been studied by femtosecond transient absorption spectroscopy at room temperature. Singlet-singlet annihilation was observed to strongly influence both the isotropic and anisotropic decays. Pump intensities in the order of 10 photons×pulse×cm were required to obtain annihilation-free conditions. The most important consequence of applied very low excitation doses is an observation of a subpicosecond process within the BChl manifold (∼200–500 fs), manifesting itself as a rise in the red part of the Q absorption band of the BChl aggregates. The subsequent decay of the kinetics measured in the BChl region and the corresponding rise in the baseplate BChl is not single-exponential, and at least two components are necessary to fit the data, corresponding to several BChl →BChl transfer steps. Under annihilation-free conditions, the anisotropic kinetics show a generally slow decay within the BChl band (10–20ps) whereas it decays more rapidly in the BChl region (∼1ps). Analysis of the experimental data gives a detailed picture of the overall time evolution of the energy relaxation and energy transfer processes within the chlorosome. The results are interpreted within an exciton model based on the proposed structure.
Abstract | Full Text | PDF (336 kb) - …more
Copyright © 1996 The Biophysical Society. All rights reserved.
Biophysical Journal, Volume 71, Issue 2, 995-1010, 1 August 1996
doi:10.1016/S0006-3495(96)79301-3
Research Article
Excitation energy transfer in chlorosomes of green bacteria: theoretical and experimental studies
Z. Fetisova, A. Freiberg, K. Mauring, V. Novoderezhkin, A. Taisova and K. Timpmann
Abstract
A theory of excitation energy transfer within the chlorosomal antennae of green bacteria has been developed for an exciton model of aggregation of bacteriochlorophyll (BChl) c (d or e). This model of six exciton-coupled BChl chains with low packing density, approximating that in vivo, and interchain distances of approximately 2 nm was generated to yield the key spectral features found in natural antennae, i.e., the exciton level structure revealed by spectral hole burning experiments and polarization of all the levels parallel to the long axis of the chlorosome. With picosecond fluorescence spectroscopy it was demonstrated that the theory explains the antenna-size-dependent kinetics of fluorescence decay in chlorosomal antenna, measured for intact cells of different cultures of the green bacterium C. aurantiacus, with different chlorosomal antenna size determined by electron microscopic examination of the ultrathin sections of the cells. The data suggest a possible mechanism of excitation energy transfer within the chlorosome that implies the formation of a cylindrical exciton, delocalized over a tubular aggregate of BChl c chains, and Forster-type transfer of such a cylindrical exciton between the nearest tubular BChl c aggregates as well as to BChl a of the baseplate.
