Singlet Excited States and the Light‐Harvesting Function of Carotenoids in Bacterial Photosynthesis

Koyama, Yasushi; Kuki, Michitaka; Andersson, Per Ola; Gillbro, Tomas

doi:10.1111/j.1751-1097.1996.tb03021.x

Cited by 196 publications

(215 citation statements)

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“…The time constants obtained at 540 and 550 nm for both spinach and Arabidopsis, where the largest differences in amplitude were observed, yielded lifetimes ranging from Ϸ7 to 11 ps. Within the experimental uncertainty, this value is identical to the intrinsic S 1 lifetime of carotenoids with 11 conjugated double bonds (23,24). Measurements of the S 1 lifetime of Zea in a variety of solvents produced a value of Ϸ9 ps (8,25), whereas recently a value of 11 ps was obtained by using a reconstituted LHCII protein with Zea as the sole carotenoid species (26).…”

Section: Discussionmentioning

confidence: 61%

Evidence for direct carotenoid involvement in the regulation of photosynthetic light harvesting

Holt

et al. 2003

Proc. Natl. Acad. Sci. U.S.A.

200

177

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Nonphotochemical quenching (NPQ) refers to a process that regulates photosynthetic light harvesting in plants as a response to changes in incident light intensity. By dissipating excess excitation energy of chlorophyll molecules as heat, NPQ balances the input and utilization of light energy in photosynthesis and protects the plant against photooxidative damage. To understand the physical mechanism of NPQ, we have performed femtosecond transient absorption experiments on intact thylakoid membranes isolated from spinach and transgenic Arabidopsis thaliana plants. These plants have well defined quenching capabilities and distinct contents of xanthophyll (Xan) cycle carotenoids. The kinetics probed in the spectral region of the S 1 3 S n transition of Xans (530 -580 nm) were found to be significantly different under the quenched and unquenched conditions, corresponding to maximum and no NPQ, respectively. The lifetime and the spectral characteristics indicate that the kinetic difference originated from the involvement of the S 1 state of a specific Xan, zeaxanthin, in the quenched case.G reen plants live with a continual paradox: They have evolved to both use and dissipate solar energy with high efficiency. Highly reactive, photooxidative intermediates are inevitable byproducts of photosynthesis. An excess photon flux can exacerbate the damage caused by these intermediates, leading to problems ranging from reversible decreases in photosynthetic efficiency, to, in the worst case, death of the plant. Nonphotochemical quenching (NPQ) is a process that thermally dissipates the absorbed light energy in photosystem (PS) II that exceeds a plant's capacity for CO 2 fixation, minimizing the deleterious effects of high light. Although NPQ has been phenomenologically documented for years, a fundamental understanding of its physical mechanism remains elusive (1-3).Feedback deexcitation or energy-dependent quenching (qE) (2, 3) is the major, rapidly reversible component of NPQ in a variety of plants, including spinach and Arabidopsis thaliana (4,5), and is the focus of this study. qE is characterized by a light-induced absorbance change at 535 nm (⌬A 535 ) (6) and the shortening of specific components of chlorophyll (Chl) fluorescence lifetimes, the exact numerical value for the shortened lifetime depending on the specific form of the photosynthetic system and the measurement conditions. For isolated thylakoid systems with closed reaction centers, a Chl lifetime component is reduced from Ϸ2.0 to Ϸ0.4 ns (4). It requires the buildup of a pH gradient (⌬pH) under high light conditions (2, 3), which triggers the enzymatic conversion of carotenoids, violaxanthin (Vio) to zeaxanthin (Zea), by means of the xanthophyll (Xan) cycle (Fig. 1a).Currently two hypotheses concerning the mechanism of qE exist, one in which the effect of Zea is solely structural (termed indirect quenching) and the other in which Zea acts as an energy acceptor for excitation transfer from the first Chl singlet excited state (termed direct quenching). The direct ...

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Section: Discussionmentioning

confidence: 61%

Evidence for direct carotenoid involvement in the regulation of photosynthetic light harvesting

Holt

et al. 2003

Proc. Natl. Acad. Sci. U.S.A.

200

177

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“…4,[9][10][11] Systematic extension of the optical studies carried out on model polyenes has provided a catalog of the energies of the 2 1 A g -and 1 1 B u + states in several homologous series of synthetic and naturally occurring carotenoids. [12][13][14] Transition energies shift to lower energy with increasing conjugation and often are expressed by E(0-0) ) A + B/N, where N is the number of conjugated double bonds in the polyene. For the 1 1 A g -f 1 1 B u + transitions, this empirical approximation finds support from simple models of ππ* transitions in linear systems, both in the particle in a box model (for which A ≈ 0) and in extensions that include alternation in bond lengths along the carbon chain.…”

Section: Introductionmentioning

confidence: 99%

Optical Spectroscopy of Long Polyenes

Christensen¹,

Faksh²,

Meyers³

et al. 2004

J. Phys. Chem. A

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We have synthesized a homologous series of soluble, linearly conjugated oligomers and related polymers using molybdenum alkylidene catalysts. We have developed HPLC procedures to isolate the oligomers according to their chain lengths and have obtained the absorption spectra of the purified oligomers in room temperature solutions and in 77 K glasses. The oligomer absorption spectra are structured and remarkably similar to those of simple polyenes with comparable numbers of conjugated double bonds (N). Furthermore, the electronic origins of the low-energy, strongly allowed 1 1 A g -f 1 1 B u + transitions follow the E(0-0) ) A + B/N behavior previously noted in simple polyenes and carotenoids. Extrapolation of data for oligomers with N ) 3-15 suggests E(0-0) ≈ 14 000 cm -1 (λ ≈ 700 nm) in the long polyene limit. The oligomer spectra exhibit modest red shifts on cooling, suggesting minimal conformational disorder in the room temperature samples. In contrast, the absorption spectrum of the longest soluble polymer (N > 100) in this series undergoes a significant red shift and sharpening upon cooling from 300 to 77 K. This indicates that the room temperature polymer is disordered due to relatively low thermal barriers for torsional motion about carbon-carbon single bonds. Unlike the longer oligomers, the low-temperature absorption of the polymer shows well-defined vibronic structure. The polymerization reactions lead to a distribution of conjugation lengths in the unpurified polymer sample. However, the vibronically resolved, red-shifted absorption at low temperature (λ(0-0) ) 630 nm) indicates that this distribution is dominated by species with very long conjugation lengths. The resolution of the low-temperature spectrum argues that the absorption is due to the superposition of almost identical 1 1 A g -f 1 1 B u + spectra and that all conjugated segments in this sample absorb near the asymptotic limit (1/N ≈ 0).

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“…Despite intensive investigation of the underlying mechanisms (8,10,12,(15)(16)(17)(18)(19)(20), many aspects of the electronic energy transfer (EET) from the Cars are still poorly understood. The electronic states of Cars often are described by analogy to polyenes ( Only the S 0 7 S 2 transition is optically dipole-allowed.…”

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confidence: 99%

“…In the light-harvesting complexes of purple bacteria, such as light-harvesting complex 2 (LH2) of Rhodobacter sphaeroides or Rhodopseudomonas acidophila, carotenoids (Cars) play important roles as light-harvesting and photoprotective pigments, as they do in the chloroplasts of higher plants (9,10). In Rb.…”

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confidence: 99%

Femtosecond dynamics of the forbidden carotenoid S ₁ state in light-harvesting complexes of purple bacteria observed after two-photon excitation

Walla

Linden

Hsu

et al. 2000

Proc. Natl. Acad. Sci. U.S.A.

134

178

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Time-resolved excited-state absorption intensities after direct twophoton excitation of the carotenoid S 1 state are reported for light-harvesting complexes of purple bacteria. Direct excitation of the carotenoid S 1 state enables the measurement of subsequent dynamics on a fs time scale without interference from higher excited states, such as the optically allowed S 2 state or the recently discovered dark state situated between S1 and S2. The lifetimes of the carotenoid S 1 states in the B800-B850 complex and B800-B820 complex of Rhodopseudomonas acidophila are 7 ؎ 0.5 ps and 6 ؎ 0.5 ps, respectively, and in the light-harvesting complex 2 of Rhodobacter sphaeroides Ϸ1.9 ؎ 0.5 ps. These results explain the differences in the carotenoid to bacteriochlorophyll energy transfer efficiency after S 2 excitation. In Rps. acidophila the carotenoid S1 to bacteriochlorophyll energy transfer is found to be quite inefficient ( ET1 <28%) whereas in Rb. sphaeroides this energy transfer is very efficient ( ET1 Ϸ80%). The results are rationalized by calculations of the ensemble averaged time constants. We find that the Car S 1 3 B800 electronic energy transfer (EET) pathway (Ϸ85%) dominates over Car S 1 3 B850 EET (Ϸ15%) in Rb. sphaeroides, whereas in Rps. acidophila the Car S 1 3 B850 EET (Ϸ60%) is more efficient than the Car S 1 3 B800 EET (Ϸ40%). The individual electronic couplings for the Car S1 3 BChl energy transfer are estimated to be approximately 5-26 cm ؊1 . A major contribution to the difference between the energy transfer efficiencies can be explained by different Car S 1 energy gaps in the two species. P lants and some bacteria have achieved remarkably high efficiencies of solar light conversion in photosynthesis. Recently, high-resolution crystal structures of some peripheral light-harvesting complexes of photosynthetic purple bacteria (1-3) have become available, enabling us to study the basic principles of the highly efficient collection of solar energy (4-8).In the light-harvesting complexes of purple bacteria, such as light-harvesting complex 2 (LH2) of Rhodobacter sphaeroides or Rhodopseudomonas acidophila, carotenoids (Cars) play important roles as light-harvesting and photoprotective pigments, as they do in the chloroplasts of higher plants (9, 10). In Rb. sphaeroides Ͼ95% of the photons absorbed by the Cars are transferred as excitation energy to the reaction center; in the strain 7050 of Rps. acidophila, which is the strain used in this report, the corresponding number is Ϸ70% (9, 11, 12). In the subunits of the B800-B850 complex of Rps. acidophila there is one Car (rhodopin glucoside) molecule per two bacteriochlorophyll (BChl) molecules in the B850 ring and one BChl in the B800 ring (1). Nine of these subunits comprise the LH2 ring, B800-B850 complex, of Rps. acidophila. Spectroscopic and biochemical reports suggest that the structures of the B800-B820 complex of Rps. acidophila and LH2 of Rb. sphaeroides (with the Car spheroidene) are similar (13,14).Despite intensive investigation of the underlying me...

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Singlet Excited States and the Light‐Harvesting Function of Carotenoids in Bacterial Photosynthesis

Cited by 196 publications

References 71 publications

Evidence for direct carotenoid involvement in the regulation of photosynthetic light harvesting

Evidence for direct carotenoid involvement in the regulation of photosynthetic light harvesting

Optical Spectroscopy of Long Polyenes

Femtosecond dynamics of the forbidden carotenoid S ₁ state in light-harvesting complexes of purple bacteria observed after two-photon excitation

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Singlet Excited States and the Light‐Harvesting Function of Carotenoids in Bacterial Photosynthesis

Cited by 196 publications

References 71 publications

Evidence for direct carotenoid involvement in the regulation of photosynthetic light harvesting

Evidence for direct carotenoid involvement in the regulation of photosynthetic light harvesting

Optical Spectroscopy of Long Polyenes

Femtosecond dynamics of the forbidden carotenoid S 1 state in light-harvesting complexes of purple bacteria observed after two-photon excitation

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Femtosecond dynamics of the forbidden carotenoid S ₁ state in light-harvesting complexes of purple bacteria observed after two-photon excitation