Jevgenij Chmeliov

Economic photoprotection in photosystem II that retains a complete light-harvesting system with slow energy traps

The light-harvesting antenna of higher plant photosystem II has an intrinsic capability for self-defence against intense sunlight. The thermal dissipation of excess energy can be measured as the non-photochemical quenching of chlorophyll fluorescence. It has recently been proposed that the transition between the light-harvesting and self-defensive modes is associated with a reorganization of light-harvesting complexes. Here we show that despite structural changes, the photosystem II cross-section does not decrease. Our study reveals that the efficiency of energy trapping by the non-photochemical quencher(s) is lower than the efficiency of energy capture by the reaction centres. Consequently, the photoprotective mechanism works effectively for closed rather than open centres. This type of defence preserves the exceptional efficiency of electron transport in a broad range of light intensities, simultaneously ensuring high photosynthetic productivity and, under hazardous light conditions, sufficient photoprotection for both the reaction centre and the light-harvesting pigments of the antenna.

Excitation Migration, Quenching, and Regulation of Photosynthetic Light Harvesting in Photosystem II

Excitation energy transfer and quenching in LHCII aggregates is considered in terms of a coarse-grained model. The model assumes that the excitation energy transfer within a pigment-protein complex is much faster than the intercomplex excitation energy transfer, whereas the quenching ability is attributed to a specific pigment-protein complex responsible for the nonphotochemical quenching (NPQ). It is demonstrated that the pump-probe experimental data obtained at low excitation intensities for LHCII aggregates under NPQ conditions can be equally well explained at two limiting cases, either describing the excitation kinetics in the migration-limited or in the trap-limited regime. Thus, it is concluded that low excitation conditions do not allow one to unambiguously define the relationship between the mean times of excitation migration and trapping. However, this could be achieved by using high excitation conditions when exciton-exciton annihilation is dominant. In this case it was found that in the trap-limited regime the excitation kinetics in the aggregate should be almost insensitive to the excitation density, meaning that singlet-singlet annihilation has little effect on the NPQ decay kinetics, whereas in the migration-limited case there is a clear intensity dependence. In order to account for the random distribution of the NPQ-traps within the LHCII aggregates, excitation diffusion in a continuous medium with random static traps was considered. This description demonstrates a very good correspondence to the experimental fluorescence kinetics assuming a lamellar (quasi-3D) structure of the antenna characterized by the dimension d=2.4 and therefore justifying the diffusion-limited approach on which the model is based. Using the coarse-grained model to describe the aggregate we estimate one NPQ-trap per 100 monomeric LHCII complexes. Finally we discuss the origin of the traps responsible for excitation quenching under NPQ conditions.

Exciton Dynamics in Semiconducting Carbon Nanotubes

We report a femtosecond transient absorption spectroscopic study on the (6, 5) single-walled carbon nanotubes and the (7, 5) inner tubes of a dominant double-walled carbon nanotube species. We found that the dynamics of exciton relaxation probed at the first transition-allowed state (E(11)) of a given tube type exhibits a markedly slower decay when the second transition-allowed state (E(22)) is excited than that measured by exciting its first transition-allowed state (E(11)). A linear intensity dependence of the maximal amplitude of the transient absorption signal is found for the E(22) excitation, whereas the corresponding amplitude scales linearly with the square root of the E(11) excitation intensity. Theoretical modeling of these experimental findings was performed by developing a continuum model and a stochastic model with explicit consideration of the annihilation of coherent excitons. Our detailed numerical simulations show that both models can reproduce reasonably well the initial portion of decay kinetics measured upon the E(22) and E(11) excitation of the chosen tube species, but the stochastic model gives qualitatively better agreement with the intensity dependence observed experimentally than those obtained with the continuum model.

Modeling of exciton quenching in photosystem II

Excitation energy transfer and trapping by the artificially postulated traps in photosystem II (PSII) were modeled in terms of a coarse-grained model. The model is based on the assumption that the excitation energy transfer within a pigment-protein complex is much faster than the intercomplex excitation energy transfer. As a result, the excitation energy transfer and trapping rates by the reaction center (RC) were rescaled by the relevant entropic factors and an additional trapping rate for a specific pigment-protein complex responsible for the non-photochemical quenching (NPQ) had to be included into the theoretical framework. For the analysis, dimeric models of PSII were considered. The efficiency of the excitation quenching by the NPQ traps, depending on their positioning and on the trapping rate, was analyzed. It was concluded that the highest efficiency of the NPQ quencher could be achieved when they are localized in the major light-harvesting complexes, LHCII, and the excitation relaxation in this state is fast, of the order of picoseconds and even faster. The origin of the state responsible for NPQ is discussed.

Light harvesting in a fluctuating antenna.

One of the major players in oxygenic photosynthesis, photosystem II (PSII), exhibits complex multiexponential fluorescence decay kinetics that for decades has been ascribed to reversible charge separation taking place in the reaction center (RC). However, in this description the protein dynamics is not taken into consideration. The intrinsic dynamic disorder of the light-harvesting proteins along with their fluctuating dislocations within the antenna inevitably result in varying connectivity between pigment-protein complexes and therefore can also lead to nonexponential excitation decay kinetics. On the basis of this presumption, we propose a simple conceptual model describing excitation diffusion in a continuous medium and accounting for possible variations of the excitation transfer rates. Recently observed fluorescence kinetics of PSII of different sizes are perfectly reproduced with only two adjustable parameters instead of the many decay times and amplitudes required in standard analysis procedures; no charge recombination in the RC is required. The model is also able to provide valuable information about the structural and functional organization of the photosynthetic antenna and in a straightforward way solves various contradictions currently existing in the literature.

The nature of self-regulation in photosynthetic light-harvesting antenna

The photosynthetic apparatus of green plants is well known for its extremely high efficiency that allows them to operate under dim light conditions. On the other hand, intense sunlight may result in overexcitation of the light-harvesting antenna and the formation of reactive compounds capable of ‘burning out’ the whole photosynthetic unit. Non-photochemical quenching is a self-regulatory mechanism utilized by green plants on a molecular level that allows them to safely dissipate the detrimental excess excitation energy as heat. Although it is believed to take place in the plants major light-harvesting complexes (LHC) II, there is still no consensus regarding its molecular nature. To get more insight into its physical origin, we performed high-resolution time-resolved fluorescence measurements of LHCII trimers and their aggregates across a wide temperature range. Based on simulations of the excitation energy transfer in the LHCII aggregate, we associate the red-emitting state, having fluorescence maximum at ∼700 nm, with the partial mixing of excitonic and chlorophyll–chlorophyll charge transfer states. On the other hand, the quenched state has a totally different nature and is related to the incoherent excitation transfer to the short-lived carotenoid excited states. Our results also show that the required level of photoprotection in vivo can be achieved by a very subtle change in the number of LHCIIs switched to the quenched state.

Exciton Band Structure in Bacterial Peripheral Light-Harvesting Complexes

The variability of the exciton spectra of bacteriochlorophyll molecules in light-harvesting (LH) complexes of photosynthetic bacteria ensures the excitation energy funneling trend toward the reaction center. The decisive shift of the energies is achieved due to exciton spectra formation caused by the resonance interaction between the pigments. The possibility to resolve the upper Davydov sub-band corresponding to the B850 ring and, thus, to estimate the exciton bandwidth by analyzing the temperature dependence of the steady-state absorption spectra of the LH2 complexes is demonstrated. For this purpose a self-modeling curve resolution approach was applied for analysis of the temperature dependence of the absorption spectra of LH2 complexes from the photosynthetic bacteria Rhodobacter (Rba.) sphaeroides and Rhodoblastus (Rbl.) acidophilus. Estimations of the intradimer resonance interaction values as follows directly from obtained estimations of the exciton bandwidths at room temperature give 385 and 397 cm(-1) for the LH2 complexes from the photosynthetic bacteria Rba. sphaeroides and Rhl. acidophilus, respectively. At 4 K the corresponding couplings are slightly higher (391 and 435 cm(-1), respectively). The retained exciton bandwidth at physiological conditions supports the decisive role of the exciton coherence determining light absorption in bacterial light-harvesting antenna complexes.

Fluorescence blinking of single major light-harvesting complexes

Recent time-resolved studies have revealed the switching behavior of single photosynthetic light-harvesting complexes. In this work, we suggest a conceptual diffusion-controlled model, which is able to describe essential protein dynamics underlying this switching phenomenon. The calculated blinking statistics is compared with the experimental results measured under various experimental conditions and not only reproduces the power-law behavior at intermediate times, but also follows the experimentally observed deviations from such behavior on a shorter timescale. We find that even under ordinary light- harvesting conditions, some antenna complexes are quenched and their fraction noticeably increases in a more acid environment. As a result, the lability of the protein scaffold allows the coexistence of light-harvesting and excitation- quenching states and therefore gives rise to regulatory switching known as non- photochemical quenching.

Excitons in the LH3 complexes from purple bacteria.

The noncovalently bound and structurally identical bacteriochlorophyll a chromophores in the peripheral light-harvesting complexes LH2 (B800-850) and LH3 (B800-820) from photosynthetic purple bacteria ensure the variability of the exciton spectra in the near-infrared (820-850 nm) wavelength region. As a result, the spectroscopic properties of the antenna complexes, such as positions of the maxima in the exciton absorption spectra, give rise to very efficient excitation transfer toward the reaction center. In this work, we investigated the possible molecular origin of the excitonically coupled B820 bacteriochlorophylls in LH3 using femtosecond transient absorption spectroscopy, deconvolution of steady-state absorption spectra, and modeling of the electrostatic intermolecular interactions using a charge density coupling approach. Compared to LH2, the upper excitonic level is red-shifted from 755 to 790 nm and is associated with an approximate 2-fold decrease of B820 intrapigment coupling. The absorption properties of LH3 cannot be reproduced by only changing the B850 site energy but also require a different scaling factor to be used to calculate interpigment couplings and a change of histidine protonation state. Several protonation patterns for distinct amino acid groups are presented, giving values of 162-173 cm(-1) at 100 K for the intradimer resonance interaction in the B820 ring.

Dynamic feedback of the photosystem II reaction centre on photoprotection in plants

Photosystem II of higher plants is protected against light damage by thermal dissipation of excess excitation energy, a process that can be monitored through non-photochemical quenching of chlorophyll fluorescence. When the light intensity is lowered, non-photochemical quenching largely disappears on a time scale ranging from tens of seconds to many minutes. With the use of picosecond fluorescence spectroscopy, we demonstrate that one of the underlying mechanisms is only functional when the reaction centre of photosystem II is closed, that is when electron transfer is blocked and the risk of photodamage is high. This is accompanied by the appearance of a long-wavelength fluorescence band. As soon as the reaction centre reopens, this quenching, together with the long-wavelength fluorescence, disappears instantaneously. This allows plants to maintain a high level of photosynthetic efficiency even in dangerous high-light conditions.Photosystem II is protected against light damage by thermal dissipation of excess energy. Picosecond fluorescence spectroscopy uncovers a mechanism that is only functional when the reaction centre is closed, that is, when the risk of photodamage is high.