Electronic Excitation Transfer from Carotenoid to Bacteriochlorophyll in the Purple Bacterium <i>Rhodopseudomonas acidophila</i>

Krueger, Brent P.; Scholes, Gregory D.; Jimenez, and Ralph; Fleming, Graham R.

doi:10.1021/jp973062t

Cited by 145 publications

(221 citation statements)

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“…Its lifetime is 60 fs, which is shorter than Spx in solution (100 fs). This value is characteristic for the Car S 2 state in the presence of EET to BChls (12,19), thus implying that Car 3 BChl excitation transfer in the LH1 complex of R. rubrum proceeds from the S 2 state.…”

Section: Resultsmentioning

confidence: 87%

An unusual pathway of excitation energy deactivation in carotenoids: Singlet-to-triplet conversion on an ultrafast timescale in a photosynthetic antenna

Gradinaru

Kennis

Papagiannakis

et al. 2001

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

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339

611

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Carotenoids are important biomolecules that are ubiquitous in nature and find widespread application in medicine. In photosynthesis, they have a large role in light harvesting (LH) and photoprotection. They exert their LH function by donating their excited singlet state to nearby (bacterio)chlorophyll molecules. In photosynthetic bacteria, the efficiency of this energy transfer process can be as low as 30%. Here, we present evidence that an unusual pathway of excited state relaxation in carotenoids underlies this poor LH function, by which carotenoid triplet states are generated directly from carotenoid singlet states. This pathway, operative on a femtosecond and picosecond timescale, involves an intermediate state, which we identify as a new, hitherto uncharacterized carotenoid singlet excited state. In LH complex-bound carotenoids, this state is the precursor on the reaction pathway to the triplet state, whereas in extracted carotenoids in solution, this state returns to the singlet ground state without forming any triplets. We discuss the possible identity of this excited state and argue that fission of the singlet state into a pair of triplet states on individual carotenoid molecules constitutes the mechanism by which the triplets are generated. This is, to our knowledge, the first ever direct observation of a singlet-to-triplet conversion process on an ultrafast timescale in a photosynthetic antenna. C arotenoids (Cars) serve a variety of functions in biological systems. In photosynthesis, they act as light-harvesting (LH) pigments by absorbing sunlight in the blue and green parts of the solar spectrum and transferring the excited state energy to nearby (bacterio)chlorophylls (BChl) (1, 2). The BChl molecules subsequently transfer this energy to a photochemical energyconverting protein known as the reaction center (RC), where the excited state energy is fixed by means of a series of electron transfer reactions (3). During these energy-and electrontransfer processes, which may take up to hundreds of picoseconds, the singlet excited and charge-separated states of BChl are subject to intersystem crossing to the triplet state, which occurs on a timescale of several nanoseconds. Although produced with a small probability, these BChl triplet states are potentially harmful to the organism because they can promote molecular oxygen to its singlet excited state, which is a highly reactive and damaging species. Cars can efficiently accept and safely dissipate BChl triplet and singlet oxygen states, and this photoprotective quality is utilized by essentially all photosynthetic organisms (1, 2).The first singlet excited state of Cars, S 1 , carries gerade symmetry (with respect to inversion) as does the ground state, S 0 , and is therefore dipole-forbidden. The second excited singlet state, S 2 , carries ungerade symmetry and is dipole-allowed (1). The specific strong Car absorption in the blue and green regions of the visible part of the electromagnetic spectrum is caused by the transition to this second excited state,...

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Section: Resultsmentioning

confidence: 87%

An unusual pathway of excitation energy deactivation in carotenoids: Singlet-to-triplet conversion on an ultrafast timescale in a photosynthetic antenna

Gradinaru

Kennis

Papagiannakis

et al. 2001

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

Self Cite

339

611

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“…Fl., Fluorescence. ET2, Time constant for Car S2 3 BChl EET [Ϸ50 fs (30)]. 21, Time constant for Car S2 3 S1 IC [Ϸ135 fs (30)].…”

Section: Resultsmentioning

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.

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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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“…retinal, the photoinduced cis-trans isomerization of which, initiates the vision process. [25][26][27][28] Polyenes also exhibit ultrafast photophysics for example in the lightharvesting and energy-transfer process by carotenoids in the photosynthetic reaction center 29,30 and the non-adiabatic transitions of isolated, longer polyenes such as alltrans-2,4,6,8-decatetraene. 31 Thus, fully understanding the intricate nature of the excited state dynamics of polyenes has far reaching implications in photochemistry, photophysics, and photobiology.…”

Section: Introductionmentioning

confidence: 99%

Between ethylene and polyenes - the non-adiabatic dynamics of cis-dienes

et al. 2012

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Perturbation theory (MS-MR-CASPT2) treatment of the electronic structure, we have simulated the non-adiabatic excited state dynamics of cyclopentadiene (CPD) and 1,2,3,4-tetramethyl-cyclopentadiene (Me 4 -CPD) following excitation to S 1 . It is observed that torsion around the carbon-carbon double bonds is essential in reaching a conical intersection seam connecting S 1 and S 0 . We identify two timescales; the induction time from excitation to the onset of population transfer back to S 0 (CPD: $25 fs, Me 4 -CPD: $71 fs) and the half-life of the subsequent population transfer (CPD: $28 fs, Me 4 -CPD: $48 fs). The longer timescales for Me 4 -CPD are a kinematic consequence of the inertia of the substituents impeding the essential out-of-plane motion that leads to the conical intersection seam. A bifurcation is observed on S 1 leading to population transfer being attributable, in a 5 : 2 ratio for CPD and 7 : 2 ratio for Me 4 -CPD, to two closely related conical intersections. Calculated time-resolved photoelectron spectra are in excellent agreement with experimental spectra validating the simulation results.

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Electronic Excitation Transfer from Carotenoid to Bacteriochlorophyll in the Purple Bacterium Rhodopseudomonas acidophila

Cited by 145 publications

References 50 publications

An unusual pathway of excitation energy deactivation in carotenoids: Singlet-to-triplet conversion on an ultrafast timescale in a photosynthetic antenna

An unusual pathway of excitation energy deactivation in carotenoids: Singlet-to-triplet conversion on an ultrafast timescale in a photosynthetic antenna

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

Between ethylene and polyenes - the non-adiabatic dynamics of cis-dienes

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Electronic Excitation Transfer from Carotenoid to Bacteriochlorophyll in the Purple Bacterium Rhodopseudomonas acidophila

Cited by 145 publications

References 50 publications

An unusual pathway of excitation energy deactivation in carotenoids: Singlet-to-triplet conversion on an ultrafast timescale in a photosynthetic antenna

An unusual pathway of excitation energy deactivation in carotenoids: Singlet-to-triplet conversion on an ultrafast timescale in a photosynthetic antenna

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

Between ethylene and polyenes - the non-adiabatic dynamics of cis-dienes

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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