Unravelling the fluorescence kinetics of light-harvesting proteins with simulated measurements

The plant light-harvesting pigment-protein complex LHCII is the major antenna sub-unit of PSII and is generally (though not universally) accepted to play a role in photoprotective energy dissipation under high light conditions, a process known Non-Photochemical Quenching (NPQ). The underlying mechanisms of energy trapping and dissipation within LHCII are still debated. Various models have been proposed for the underlying molecular detail of NPQ, but they are often based on different interpretations of very similar transient absorption measurements of isolated complexes. Here we present a simulated measurement of the fluorescence decay kinetics of quenched LHCII aggregates to determine whether this relatively simple measurement can discriminate between different potential NPQ mechanisms. We simulate not just the underlying physics (excitation, energy migration, quenching and singlet-singlet annihilation) but also the signal detection and typical experimental data analysis. Comparing this to a selection of published fluorescence decay kinetics we find that: (1) Different proposed quenching mechanisms produce noticeably different fluorescence kinetics even at low (annihilation free) excitation density, though the degree of difference is dependent on pulse width. (2) Measured decay kinetics are consistent with most LHCII trimers becoming relatively slow excitation quenchers. A small sub-population of very fast quenchers produces kinetics which do not resemble any observed measurement. (3) It is necessary to consider at least two distinct quenching mechanisms in order to accurately reproduce experimental kinetics, supporting the idea that NPQ is not a simple binary switch.

Can red-emitting state be responsible for fluorescence quenching in LHCII aggregates?

Non-photochemical quenching (NPQ) is responsible for protecting the light-harvesting apparatus of plants from damage at high light conditions. Although it is agreed that the major part of NPQ, an energy-dependent quenching (qE), originates in the light-harvesting antenna, its exact mechanism is still debated. In our earlier work (Chmeliov et al. in Nat Plants 2:16045, 2016), we have analyzed the time-resolved fluorescence (TRF) from the trimers and aggregates of the major light-harvesting complexes of plants (LHCII) over a broad temperature range and came to a conclusion that three distinct states are required to describe the experimental data: two of them correspond to the emission bands centered at ~680 and ~700 nm, and the third state is responsible for the excitation quenching. This was opposite to earlier suggestions of a two-state model, where the red-shifted fluorescence and excitation quenching were assumed to be related. To examine such possibility, in the current work we repeat our analysis of the TRF data in terms of the two-state model. We find that even though it can reasonably describe the aggregate fluorescence, it fails to do so for the LHCII trimers. We conclude that the red-emitting state cannot be responsible for fluorescence quenching in the LHCII aggregates and reaffirm that the three-state model is the simplest possible description of the experimental data.

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 plant's 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.

Excitation migration in fluctuating light-harvesting antenna systems

Complex multi-exponential fluorescence decay kinetics observed in various photosynthetic systems like photosystem II (PSII) have often been explained by the reversible quenching mechanism of the charge separation taking place in the reaction center (RC) of PSII. However, this description does not account for the intrinsic dynamic disorder of the light-harvesting proteins as well as their fluctuating dislocations within the antenna, which also facilitate the repair of RCs, state transitions, and the process of non-photochemical quenching. Since dynamic fluctuations result in varying connectivity between pigment-protein complexes, they can also lead to non-exponential excitation decay kinetics. Based on this presumption, we have recently proposed a simple conceptual model describing excitation diffusion in a continuous medium and accounting for possible variations of the excitation transfer pathways. In the current work, this model is further developed and then applied to describe fluorescence kinetics originating from very diverse antenna systems, ranging from PSII of various sizes to LHCII aggregates and even the entire thylakoid membrane. In all cases, complex multi-exponential fluorescence kinetics are perfectly reproduced on the entire relevant time scale without assuming any radical pair equilibration at the side of the excitation quencher, but using just a few parameters reflecting the mean excitation energy transfer rate as well as the overall average organization of the photosynthetic antenna.

Dynamic quenching in single photosystem II supercomplexes

Time-resolved fluorescence measurements of single PSII supercomplexes to investigate blinking and dynamic quenching in the context of non-photochemical quenching (NPQ).

Optimal Energy Transfer in Light-Harvesting Systems

Photosynthesis is one of the most essential biological processes in which specialized pigment-protein complexes absorb solar photons, and with a remarkably high efficiency, guide the photo-induced excitation energy toward the reaction center to subsequently trigger its conversion to chemical energy. In this work, we review the principles of optimal energy transfer in various natural and artificial light harvesting systems. We begin by presenting the guiding principles for optimizing the energy transfer efficiency in systems connected to dissipative environments, with particular attention paid to the potential role of quantum coherence in light harvesting systems. We will comment briefly on photo-protective mechanisms in natural systems that ensure optimal functionality under varying ambient conditions. For completeness, we will also present an overview of the charge separation and electron transfer pathways in reaction centers. Finally, recent theoretical and experimental progress on excitation energy transfer, charge separation, and charge transport in artificial light harvesting systems is delineated, with organic solar cells taken as prime examples.

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.

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.

Modeling the NMR signatures associated with the functional conformational switch in the major light-harvesting antenna of photosystem II in higher plants

The major photosystem II antenna complex, LHCII, possesses an intrinsic conformational switch linked to the formation of a photoprotective, excitation-quenching state. Recent solid state NMR experiments revealed that aggregation-induced quenching in (13)C-enriched LHCII from C. reinhardtii is associated with changes to the chemical shifts of three specific (13)C atoms in the Chla conjugated macrocycle. We performed DFT-based NMR calculations on the strongly-quenched crystal structure of LHCII (taken from spinach). We demonstrate that specific Chla-xanthophyll interactions in the quenched structure lead to changes in the Chla(13)C chemical shifts that are qualitatively similar to those observed by solid state NMR. We propose that these NMR changes are due to modulations in Chla-xanthophyll associations that occur due to a quenching-associated functional conformation change in the lutein and neoxanthin domains of LHCII. The combination of solid-state NMR and theoretical modeling is therefore a powerful tool for assessing functional conformational switching in the photosystem II antenna.

Hierarchical coarse-graining model for photosystem II including electron and excitation-energy transfer processes

We propose a hierarchical reduction scheme to cope with coupled rate equations that describe the dynamics of multi-time-scale photosynthetic reactions. To numerically solve nonlinear dynamical equations containing a wide temporal range of rate constants, we first study a prototypical three-variable model. Using a separation of the time scale of rate constants combined with identified slow variables as (quasi-)conserved quantities in the fast process, we achieve a coarse-graining of the dynamical equations reduced to those at a slower time scale. By iteratively employing this reduction method, the coarse-graining of broadly multi-scale dynamical equations can be performed in a hierarchical manner. We then apply this scheme to the reaction dynamics analysis of a simplified model for an illuminated photosystem II, which involves many processes of electron and excitation-energy transfers with a wide range of rate constants. We thus confirm a good agreement between the coarse-grained and fully (finely) integrated results for the population dynamics.

Fluorescence lifetime snapshots reveal two rapidly reversible mechanisms of photoprotection in live cells of Chlamydomonas reinhardtii

Photosynthetic organisms avoid photodamage to photosystem II (PSII) in variable light conditions via a suite of photoprotective mechanisms called nonphotochemical quenching (NPQ), in which excess absorbed light is dissipated harmlessly. To quantify the contributions of different quenching mechanisms to NPQ, we have devised a technique to measure the changes in chlorophyll fluorescence lifetime as photosynthetic organisms adapt to varying light conditions. We applied this technique to measure the fluorescence lifetimes responsible for the predominant, rapidly reversible component of NPQ, qE, in living cells of Chlamydomonas reinhardtii. Application of high light to dark-adapted cells of C. reinhardtii led to an increase in the amplitudes of 65 ps and 305 ps chlorophyll fluorescence lifetime components that was reversed after the high light was turned off. Removal of the pH gradient across the thylakoid membrane linked the changes in the amplitudes of the two components to qE quenching. The rise times of the amplitudes of the two components were significantly different, suggesting that the changes are due to two different qE mechanisms. We tentatively suggest that the changes in the 65 ps component are due to charge-transfer quenching in the minor light-harvesting complexes and that the changes in the 305 ps component are due to aggregated light-harvesting complex II trimers that have detached from PSII. We anticipate that this technique will be useful for resolving the various mechanisms of NPQ and for quantifying the timescales associated with these mechanisms.

Intra- and inter-monomeric transfers in the light harvesting LHCII complex: the Redfield-Förster picture

We further develop the model of energy transfer in the LHCII trimer based on a quantitative fit of the linear spectra (including absorption (OD), linear dichroism (LD), circular dichroism (CD), and fluorescence (FL)) and transient absorption (TA) kinetics upon 650 nm and 662 nm excitation. The spectral shapes and relaxation/migration rates have been calculated using the combined Redfield-Förster approach capable of correctly describing fast relaxation within strongly coupled chlorophyll (Chl) a and b clusters and slow migration between them. Within each monomeric subunit of the trimeric complex there is fast (sub-ps) conversion from Chl's b to Chl's a at the stromal side accompanied by slow (>10 ps) equilibration between the stromal- and lumenal-side Chl a clusters in combination with slow (>13 ps) population of Chl's a from the 'bottleneck' Chl a604 site. The connection between monomeric subunits is determined by exciton coupling between the stromal-side Chl's b from the two adjacent subunits (Chl b601'-608-609 cluster) making a simultaneous fast (sub-ps) population of the Chl's a possible from both subunits. Final equilibration occurs via slow (>20 ps) migration between the Chl a clusters located on different monomeric subunits. This migration includes up-hill transfers from the red-most Chl a610-611-612 clusters located at the peripheral side in each subunit to the Chl a602-603 dimers located at the inner side of the trimeric LHCII complex.

Excitation energy transfer and trapping in higher plant Photosystem II complexes with different antenna sizes

We performed picosecond fluorescence measurements on well-defined Photosystem II (PSII) supercomplexes from Arabidopsis with largely varying antenna sizes. The average excited-state lifetime ranged from 109 ps for PSII core to 158 ps for the largest C(2)S(2)M(2) complex in 0.01% α-DM. Excitation energy transfer and trapping were investigated by coarse-grained modeling of the fluorescence kinetics. The results reveal a large drop in free energy upon charge separation (>700 cm(-1)) and a slow relaxation of the radical pair to an irreversible state (∼150 ps). Somewhat unexpectedly, we had to reduce the energy-transfer and charge-separation rates in complexes with decreasing size to obtain optimal fits. This strongly suggests that the antenna system is important for plant PSII integrity and functionality, which is supported by biochemical results. Furthermore, we used the coarse-grained model to investigate several aspects of PSII functioning. The excitation trapping time appears to be independent of the presence/absence of most of the individual contacts between light-harvesting complexes in PSII supercomplexes, demonstrating the robustness of the light-harvesting process. We conclude that the efficiency of the nonphotochemical quenching process is hardly dependent on the exact location of a quencher within the supercomplexes.

The photoprotective molecular switch in the photosystem II antenna

We have reviewed the current state of multidisciplinary knowledge of the photoprotective mechanism in the photosystem II antenna underlying non-photochemical chlorophyll fluorescence quenching (NPQ). The physiological need for photoprotection of photosystem II and the concept of feed-back control of excess light energy are described. The outline of the major component of nonphotochemical quenching, qE, is suggested to comprise four key elements: trigger (ΔpH), site (antenna), mechanics (antenna dynamics) and quencher(s). The current understanding of the identity and role of these qE components is presented. Existing opinions on the involvement of protons, different LHCII antenna complexes, the PsbS protein and different xanthophylls are reviewed. The evidence for LHCII aggregation and macrostructural reorganization of photosystem II and their role in qE are also discussed. The models describing the qE locus in LHCII complexes, the pigments involved and the evidence for structural dynamics within single monomeric antenna complexes are reviewed. We suggest how PsbS and xanthophylls may exert control over qE by controlling the affinity of LHCII complexes for protons with reference to the concepts of hydrophobicity, allostery and hysteresis. Finally, the physics of the proposed chlorophyll-chlorophyll and chlorophyll-xanthophyll mechanisms of energy quenching is explained and discussed. This article is part of a Special Issue entitled: Photosystem II.