Strategies to enhance the excitation energy-transfer efficiency in a light-harvesting system using the intra-molecular charge transfer character of carotenoids

Fucoxanthin is a carotenoid that is mainly found in light-harvesting complexes from brown algae and diatoms. Due to the presence of a carbonyl group attached to polyene chains in polar environments, excitation produces an excited intra-molecular charge transfer. This intra-molecular charge transfer state plays a key role in the highly efficient (∼95%) energy-transfer from fucoxanthin to chlorophyll a in the light-harvesting complexes from brown algae. In purple bacterial light-harvesting systems the efficiency of excitation energy-transfer from carotenoids to bacteriochlorophylls depends on the extent of conjugation of the carotenoids. In this study we were successful, for the first time, in incorporating fucoxanthin into a light-harvesting complex 1 from the purple photosynthetic bacterium, Rhodospirillum rubrum G9+ (a carotenoidless strain). Femtosecond pump-probe spectroscopy was applied to this reconstituted light-harvesting complex in order to determine the efficiency of excitation energy-transfer from fucoxanthin to bacteriochlorophyll a when they are bound to the light-harvesting 1 apo-proteins

Excited-state dynamics of all the mono-cis and the major di-cis isomers of β-apo-8′-carotenal as revealed by femtosecond time-resolved transient absorption spectroscopy

Cis isomers of carotenoids play important roles in light harvesting and photoprotection in photosynthetic bacteria, such as the reaction center in purple bacteria and the photosynthetic apparatus in cyanobacteria. Carotenoids containing carbonyl groups are involved in efficient energy transfer to chlorophyll in light-harvesting complexes, and their intramolecular charge–transfer (ICT) excited states are known to be important for this process. Previous studies, using ultrafast laser spectroscopy, have focused on the central-cis isomer of carbonyl-containing carotenoids, revealing that the ICT excited state is stabilized in polar environments. However, the relationship between the cis isomer structure and the ICT excited state has remained unresolved. In this study, we performed steady-state absorption and femtosecond time-resolved absorption spectroscopy on nine geometric isomers (7-cis, 9-cis, 13-cis, 15-cis, 13′-cis, 9,13′-cis, 9,13-cis, 13,13′-cis, and all-trans) of β-apo-8′-carotenal, whose structures are well-defined, and discovered correlations between the decay rate constant of the S1 excited state and the S0−S1 energy gap, as well as between the position of the cis-bend and the degree of stabilization of the ICT excited state. Our results demonstrate that the ICT excited state is stabilized in polar environments in cis isomers of carbonyl-containing carotenoids and suggest that the position of the cis-bend plays an important role in the stabilization of the excited state

Intramolecular charge-transfer enhances energy transfer efficiency in carotenoid-reconstituted light-harvesting 1 complex of purple photosynthetic bacteria

In bacterial photosynthesis, the excitation energy transfer (EET) from carotenoids to bacteriochlorophyll a has a significant impact on the overall efficiency of the primary photosynthetic process. This efficiency can be enhanced when the involved carotenoid has intramolecular charge-transfer (ICT) character, as found in light-harvesting systems of marine alga and diatoms. Here, we provide insights into the significance of ICT excited states following the incorporation of a higher plant carotenoid, β-apo-8′-carotenal, into the carotenoidless light-harvesting 1 (LH1) complex of the purple photosynthetic bacterium Rhodospirillum rubrum strain G9+. β-apo-8′-carotenal generates the ICT excited state in the reconstituted LH1 complex, achieving an efficiency of EET of up to 79%, which exceeds that found in the wild-type LH1 complex

Singlet and triplet excited states dynamics of photosynthetic pigment chlorophyll a investigated by sub-nanosecond pump-probe spectroscopy

Singlet and triplet excited states dynamics of the photosynthetic pigment chlorophyll a in various solvents have been investigated by sub-ns pump-probe spectroscopic measurements with a sub-ns time resolution and a temporal window up to 400 μs. The singlet and triplet lifetimes of chlorophyll a were determined respectively to be 5.2–7.0 ns and 1.2–12 μs, depending on solvents. On a basis of a global analysis of time-resolved spectroscopic data, we estimated an yield of the intersystem crossing from the singlet to triplet excited states chlorophyll a to be about 30% depending not on surrounding environments. The value is much lower than the previously reported values

Strong coherent coupling of vibronic oscillations in spheroidene

AbstractThe four-wave mixing signals from spheroidene have been systematically investigated at various coherent periods τ. The pronounced vibronic oscillations of the fundamental modes in the ground state are clearly observed in the transient grating configuration (τ=0). Interestingly, it was found that the overtones and coupled modes become very strong compared to the fundamental modes when the signal is measured in the virtual photon-echo configuration (τ<0). The experimental results have been analyzed using a Brownian oscillator model. The intensity change with respect to the coherent period is qualitatively reproduced by the calculations. We discuss a plausible model that allows a quantitative agreement between experiment and calculation

Ultrafast relaxation kinetics of the dark S1 state in all-trans-β-carotene explored by one- and two-photon pump–probe spectroscopy

Femtosecond one-and two-photon pump-probe dispersive spectroscopic measurements have been applied to the investigation of the vibrational relaxation kinetics of the dark S-1 (2(1)A(g)(-)) state in beta-carotene, combining a higher sensitive detection system with tunable visible and infrared excitation pulses. The two-photon excitation measurements enable the preferential detection of the dark S-1 state. The tunable infrared excitation pulses allowed selective excitation to a different vibrational level of S-1. The S-1 dynamics at early delay times depend strongly on excitation energy. A dependence of the initial S-1 dynamics on excitation energy is discussed in term of the vibrational relaxation of S-1. (C) 2009 Elsevier B. V. All rights reserve

Comparison of transient grating signals from spheroidene in an organic solvent and in pigment-protein complexes from Rhodobacter sphaeroides 2.4.1

The concurrent dynamics of the electronic excitation and vibronic oscillations of spheroidene have been investigated by means of the transient grating (TG) spectroscopy. The third-order optical responses of spheroidene in an organic solvent, in the LH2 light-harvesting antenna complexes, and in chromatophores have been compared in order to investigate the influence of the environment surrounding this photosynthetic pigment. Vibronic coherent oscillations with a period of several tens of femtosecond have been clearly observed superimposed on a slowly varying background, which reflects the electronic dynamics. The dynamics of the coherent oscillations have been analyzed by means of the wavelet analysis. Within our experimental accuracy, the decay times of the C=C and C-C stretching modes and C-CH3 rocking mode of each specimen are very close. The experimental results have also been analyzed using a Brownian oscillator model. For these numerical calculations, the spectral density for the underdamped modes has been determined from the Raman spectrum of spheroidene. It was found that the low-frequency modes that reflect the influence of the protein environment can be approximated by the overdamped Brownian oscillator. The experimentally observed linear absorption spectra as well as the third-order optical responses, i.e., TG curves, are reproduced very well by these calculations. The close agreement between the experiments and calculations indicates that the Feynman-diagrammatic approach can be applied to express not only the internal conversion but also the intermolecular excitation energy-transfer processes. The vibronic decay rates of spheroidene in LH2 complexes and chromatophores are evaluated to be about 20% larger than in the organic solven