Unveiling large charge transfer character of PSII in an iron-deficient cyanobacterial membrane: A Stark fluorescence spectroscopy study

In this work, we applied Stark fluorescence spectroscopy to an iron-stressed cyanobacterial membrane to reveal key insights about the electronic structures and excited state dynamics of the two important pigment-protein complexes, IsiA and PSII, both of which prevail simultaneously within the membrane during iron deficiency and whose fluorescence spectra are highly overlapped and hence often hardly resolved by conventional fluorescence spectroscopy. Thanks to the ability of Stark fluorescence spectroscopy, the fluorescence signatures of the two complexes could be plausibly recognized and disentangled. The systematic analysis of the SF spectra, carried out by employing standard Liptay formalism with a realistic spectral deconvolution protocol, revealed that the IsiA in an intact membrane retains almost identical excited state electronic structures and dynamics as compared to the isolated IsiA we reported in our earlier study. Moreover, the analysis uncovered that the excited state of the PSII subunit of the intact membrane possesses a significantly large CT character. The observed notably large magnitude of the excited state CT character may signify the supplementary role of PSII in regulative energy dissipation during iron deficiency.

Vibronic Exciton-Phonon States in Stack-Engineered van der Waals Heterojunction Photodiodes

Stack engineering, an atomic-scale metamaterial strategy, enables the design of optical and electronic properties in van der Waals heterostructure devices. Here we reveal the optoelectronic effects of stacking-induced strong coupling between atomic motion and interlayer excitons in WSe₂/MoSe₂ heterojunction photodiodes. To do so, we introduce the photocurrent spectroscopy of a stack-engineered photodiode as a sensitive technique for probing interlayer excitons, enabling access to vibronic states typically found only in molecule-like systems. The vibronic states in our stack are manifest as a palisade of pronounced periodic sidebands in the photocurrent spectrum in frequency windows close to the interlayer exciton resonances and can be shifted "on demand" through the application of a perpendicular electric field via a source-drain bias voltage. The observation of multiple well-resolved sidebands as well as their ability to be shifted by applied voltages vividly demonstrates the emergence of interlayer exciton vibronic structure in a stack-engineered optoelectronic device.

A kaleidoscope of photosynthetic antenna proteins and their emerging roles

Photosynthetic light-harvesting antennae are pigment-binding proteins that perform one of the most fundamental tasks on Earth, capturing light and transferring energy that enables life in our biosphere. Adaptation to different light environments led to the evolution of an astonishing diversity of light-harvesting systems. At the same time, several strategies have been developed to optimize the light energy input into photosynthetic membranes in response to fluctuating conditions. The basic feature of these prompt responses is the dynamic nature of antenna complexes, whose function readily adapts to the light available. High-resolution microscopy and spectroscopic studies on membrane dynamics demonstrate the crosstalk between antennae and other thylakoid membrane components. With the increased understanding of light-harvesting mechanisms and their regulation, efforts are focusing on the development of sustainable processes for effective conversion of sunlight into functional bio-products. The major challenge in this approach lies in the application of fundamental discoveries in light-harvesting systems for the improvement of plant or algal photosynthesis. Here, we underline some of the latest fundamental discoveries on the molecular mechanisms and regulation of light harvesting that can potentially be exploited for the optimization of photosynthesis.

Dynamics of diverse coherences in primary charge separation of bacterial reaction center at 77 K revealed by wavelet analysis

To uncover the mechanism behind the high photo-electronic conversion efficiency in natural photosynthetic complexes it is essential to trace the dynamics of electronic and vibrational quantum coherences. Here we apply wavelet analysis to two-dimensional electronic spectroscopy data for three purple bacterial reaction centers with mutations that produce drastically different rates of primary charge separation. From the frequency distribution and dynamic evolution features of the quantum beating, electronic coherence with a dephasing lifetime of ~50 fs, vibronic coherence with a lifetime of ~150 fs and vibrational/vibronic coherences with a lifetime of 450 fs are distinguished. We find that they are responsible for, or couple to, different specific steps during the primary charge separation process, i.e., intradimer charge transfer inside the special bacteriochlorophyll pair followed by its relaxation and stabilization of the charge-transfer state. The results enlighten our understanding of how quantum coherences participate in, and contribute to, a biological electron transfer reaction.

Dual Singlet Excited-State Quenching Mechanisms in an Artificial Caroteno-Phthalocyanine Light Harvesting Antenna

Under excess illumination, photosystem II of plants dissipates excess energy through the quenching of chlorophyll fluorescence in the light harvesting antenna. Various models involving chlorophyll quenching by carotenoids have been proposed, including (i) direct energy transfer from chlorophyll to the low-lying optically forbidden carotenoid S₁ state, (ii) formation of a collective quenched chlorophyll–carotenoid S₁ excitonic state, (iii) chlorophyll–carotenoid charge separation and recombination, and (iv) chlorophyll–chlorophyll charge separation and recombination. In previous work, the first three processes were mimicked in model systems: in a Zn-phthalocyanine–carotenoid dyad with an amide linker, direct energy transfer was observed by femtosecond transient absorption spectroscopy, whereas in a Zn-phthalocyanine–carotenoid dyad with an amine linker excitonic quenching was demonstrated. Here, we present a transient absorption spectroscopic study on a Zn-phthalocyanine–carotenoid dyad with a phenylene linker. We observe that two quenching phases of the phthalocyanine excited state exist at 77 and 213 ps in addition to an unquenched phase at 2.7 ns. Within our instrument response of ∼100 fs, carotenoid S₁ features rise which point at an excitonic quenching mechanism. Strikingly, we observe an additional rise of carotenoid S₁ features at 3.6 ps, which shows that a direct energy transfer mechanism in an inverted kinetics regime is also in effect. We assign the 77 ps decay component to excitonic quenching and the 3.6 ps/213 ps rise and decay components to direct energy transfer. Our results indicate that dual quenching mechanisms may be active in the same molecular system, in addition to an unquenched fraction. Computational chemistry results indicate the presence of multiple conformers where one of the dihedral angles of the phenylene linker assumes distinct values. We propose that the parallel quenching pathways and the unquenched fraction result from such conformational subpopulations. Our results suggest that it is possible to switch between different regimes of quenching and nonquenching through a conformational change on the same molecule, offering insights into potential mechanisms used in biological photosynthesis to adapt to light intensity changes on fast time scales.

Dynamic Stark Effect in Two-Dimensional Spectroscopy Revealing Modulation of Ultrafast Charge Separation in Bacterial Reaction Centers by an Inherent Electric Field

Despite extensive study, mysteries remain regarding the highly efficient ultrafast charge separation processes in photosynthetic reaction centers (RCs). In this work, transient Stark signals were found to be present in ultrafast two-dimensional electronic spectra recorded for purple bacterial RCs at 77 K. These arose from the electric field that is inherent to the intradimer charge-transfer intermediate of the bacteriochlorophyll pair (P), P_A⁺P_B^-. By comparing three mutated RCs, a correlation was found between the efficient formation of P_A⁺P_B^- and a fast charge separation rate. Importantly, the energy level of P* was changed due to the Stark shift, influencing the driving force for P* → P⁺B_A^- electron transfer and hence its rate. Furthermore, the orientation and amplitude of the inherent electric field varied in different ways upon different mutation, leading to contrasting changes in the rates. This mechanism of modulation provides a solution to a long-lasting inconsistency between experimental observations and activation energy theory.

Absence of far-red emission band in aggregated core antenna complexes

Reported herein is a Stark fluorescence spectroscopy study performed on photosystem II core antenna complexes CP43 and CP47 in their native and aggregated states. The systematic mathematical modeling of the Stark fluorescence spectra with the aid of conventional Liptay formalism revealed that induction of aggregation in both the core antenna complexes via detergent removal results in a single quenched species characterized by a remarkably broad and inhomogenously broadened emission lineshape peaking around 700 nm. The quenched species possesses a fairly large magnitude of charge-transfer character. From the analogy with the results from aggregated peripheral antenna complexes, the quenched species is thought to originate from the enhanced chlorophyll-chlorophyll interaction due to aggregation. However, in contrast, aggregation of both core antenna complexes did not produce a far-red emission band at ∼730 nm, which was identified in most of the aggregated peripheral antenna complexes. The 730-nm emission band of the aggregated peripheral antenna complexes was attributed to the enhanced chlorophyll-carotenoid (lutein1) interaction in the terminal emitter locus. Therefore, it is very likely that the no occurrence of the far-red band in the aggregated core antenna complexes is directly related to the absence of lutein1 in their structures. The absence of the far-red band also suggests the possibility that aggregation-induced conformational change of the core antenna complexes does not yield a chlorophyll-carotenoid interaction associated energy dissipation channel.

Complete mapping of energy transfer pathways in the plant light-harvesting complex Lhca4

The Lhca4 antenna complex of plant Photosystem I (PSI) is characterized by extremely red-shifted and broadened absorption and emission bands from its low-energy chlorophylls (Chls). The mixing of a charge-transfer (CT) state with the excited state manifold causing these so-called red forms results in highly complicated multi-component excited energy transfer (EET) kinetics within the complex. The two-dimensional electronic spectroscopy experiments presented here reveal that EET towards the CT state occurs on three timescales: fast from the red Chls (within 1 ps), slower (5-7 ps) from the stromal side Chls, and very slow (100-200 ps) from a newly discovered 690 nm luminal trap. The excellent agreement between the experimental data with the previously presented Redfield-Förster exciton model of Lhca4 strongly supports the equilibration scheme of the bulk excitations with the dynamically localized CT on the stromal side. Thus, a complete picture of the energy transfer pathways leading to the population of the CT final trap within the whole Lhca4 complex is presented. In view of the environmental sensitivity of the CT contribution to the Lhca4 energy landscape, we speculate that one role of the CT states is to regulate the EET from the peripheral antenna to the PSI core.

Unraveling the Excited-State Dynamics and Light-Harvesting Functions of Xanthophylls in Light-Harvesting Complex II Using Femtosecond Stimulated Raman Spectroscopy

Photosynthesis in plants starts with the capture of photons by light-harvesting complexes (LHCs). Structural biology and spectroscopy approaches have led to a map of the architecture and energy transfer pathways between LHC pigments. Still, controversies remain regarding the role of specific carotenoids in light-harvesting and photoprotection, obligating the need for high-resolution techniques capable of identifying excited-state signatures and molecular identities of the various pigments in photosynthetic systems. Here we demonstrate the successful application of femtosecond stimulated Raman spectroscopy (FSRS) to a multichromophoric biological complex, trimers of LHCII. We demonstrate the application of global and target analysis (GTA) to FSRS data and utilize it to quantify excitation migration in LHCII trimers. This powerful combination of techniques allows us to obtain valuable insights into structural, electronic, and dynamic information from the carotenoids of LHCII trimers. We report spectral and dynamical information on ground- and excited-state vibrational modes of the different pigments, resolving the vibrational relaxation of the carotenoids and the pathways of energy transfer to chlorophylls. The lifetimes and spectral characteristics obtained for the S1 state confirm that lutein 2 has a distorted conformation in LHCII and that the lutein 2 S1 state does not transfer to chlorophylls, while lutein 1 is the only carotenoid whose S1 state plays a significant energy-harvesting role. No appreciable energy transfer takes place from lutein 1 to lutein 2, contradicting recent proposals regarding the functions of the various carotenoids (Son et al. Chem.2019, 5 (3), 575–584). Also, our results demonstrate that FSRS can be used in combination with GTA to simultaneously study the electronic and vibrational landscapes in LHCs and pave the way for in-depth studies of photoprotective conformations in photosynthetic systems.

Quieting a noisy antenna reproduces photosynthetic light-harvesting spectra

Photosynthesis achieves near unity light-harvesting quantum efficiency yet it remains unknown whether there exists a fundamental organizing principle giving rise to robust light harvesting in the presence of dynamic light conditions and noisy physiological environments. Here, we present a noise-canceling network model that relates noisy physiological conditions, power conversion efficiency, and the resulting absorption spectra of photosynthetic organisms. Using light conditions in full solar exposure, light filtered by oxygenic phototrophs, and light filtered under seawater, we derived optimal absorption characteristics for efficient solar power conversion. We show how light-harvesting antennae can be tuned to maximize power conversion efficiency by minimizing excitation noise, thus providing a unified theoretical basis for the observed wavelength dependence of absorption in green plants, purple bacteria, and green sulfur bacteria.

Charge transfer states in phycobilisomes

Phycobilisomes (PBs) absorb light and supply downstream photosynthetic processes with excitation energy in many cyanobacteria and algae. In response to a sudden increase in light intensity, excess excitation energy is photoprotectively dissipated in PBs by means of the orange carotenoid protein (OCP)-related mechanism or via a light-activated intrinsic decay channel. Recently, we have identified that both mechanisms are associated with far-red emission states. Here, we investigate the far-red states involved with the light-induced intrinsic mechanism by exploring the energy landscape and electro-optical properties of the pigments in PBs. While Stark spectroscopy showed that the far-red states in PBs exhibit a strong charge-transfer (CT) character at cryogenic temperatures, single molecule spectroscopy revealed that CT states should also be present at room temperature. Owing to the strong environmental sensitivity of CT states, the knowledge gained from this study may contribute to the design of a new generation of fluorescence markers.

Stark fluorescence spectroscopy on peridinin-chlorophyll-protein complex of dinoflagellate, Amphidinium carterae

Because of their peculiar but intriguing photophysical properties, peridinin-chlorophyll-protein complexes (PCPs), the peripheral light-harvesting antenna complexes of photosynthetic dinoflagellates have been unique targets of multidimensional theoretical and experimental investigations over the last few decades. The major light-harvesting chlorophyll a (Chl a) pigments of PCP are hypothesized to be spectroscopically heterogeneous. To study the spectral heterogeneity in terms of electrostatic parameters, we, in this study, implemented Stark fluorescence spectroscopy on PCP isolated from the dinoflagellate Amphidinium carterae. The comprehensive theoretical modeling of the Stark fluorescence spectrum with the help of the conventional Liptay formalism revealed the simultaneous presence of three emission bands in the fluorescence spectrum of PCP recorded upon excitation of peridinin. The three emission bands are found to possess different sets of electrostatic parameters with essentially increasing magnitude of charge-transfer character from the blue to redder ones. The different magnitudes of electrostatic parameters give good support to the earlier proposition that the spectral heterogeneity in PCP results from emissive Chl a clusters anchored at a different sites and domains within the protein network.

Strong plasmonic fluorescence enhancement of individual plant light-harvesting complexes

Plasmonic coupling of metallic nanoparticles and adjacent pigments can dramatically increase the brightness of the pigments due to the enhanced local electric field. Here, we demonstrate that the fluorescence brightness of a single plant light-harvesting complex (LHCII) can be significantly enhanced when coupled to a gold nanorod (AuNR). The AuNRs utilized in this study were prepared via chemical reactions, and the hybrid system was constructed using a simple and economical spin-assisted layer-by-layer technique. Enhancement of fluorescence brightness of up to 240-fold was observed, accompanied by a 109-fold decrease in the average (amplitude-weighted) fluorescence lifetime from approximately 3.5 ns down to 32 ps, corresponding to an excitation enhancement of 63-fold and emission enhancement of up to 3.8-fold. This large enhancement is due to the strong spectral overlap of the longitudinal localized surface plasmon resonance of the utilized AuNRs and the absorption or emission bands of LHCII. This study provides an inexpensive strategy to explore the fluorescence dynamics of weakly emitting photosynthetic light-harvesting complexes at the single molecule level.

Both electronic and vibrational coherences are involved in primary electron transfer in bacterial reaction center

Understanding the mechanism behind the near-unity efficiency of primary electron transfer in reaction centers is essential for designing performance-enhanced artificial solar conversion systems to fulfill mankind’s growing demands for energy. One of the most important challenges is distinguishing electronic and vibrational coherence and establishing their respective roles during charge separation. In this work we apply two-dimensional electronic spectroscopy to three structurally-modified reaction centers from the purple bacterium Rhodobacter sphaeroides with different primary electron transfer rates. By comparing dynamics and quantum beats, we reveal that an electronic coherence with dephasing lifetime of ~190 fs connects the initial excited state, P*, and the charge-transfer intermediate \documentclass[12pt]{minimal} \usepackage{amsmath} \usepackage{wasysym} \usepackage{amsfonts} \usepackage{amssymb} \usepackage{amsbsy} \usepackage{mathrsfs} \usepackage{upgreek} \setlength{\oddsidemargin}{-69pt} \begin{document}$${\mathrm{P}}_{\mathrm{A}}^ + {\mathrm{P}}_{\mathrm{B}}^ -$$\end{document}PA+PB-; this \documentclass[12pt]{minimal} \usepackage{amsmath} \usepackage{wasysym} \usepackage{amsfonts} \usepackage{amssymb} \usepackage{amsbsy} \usepackage{mathrsfs} \usepackage{upgreek} \setlength{\oddsidemargin}{-69pt} \begin{document}$${\mathrm{P}}^ \ast \to {\mathrm{P}}_{\mathrm{A}}^ + {\mathrm{P}}_{\mathrm{B}}^ -$$\end{document}P*→PA+PB- step is associated with a long-lived quasi-resonant vibrational coherence; and another vibrational coherence is associated with stabilizing the primary photoproduct, \documentclass[12pt]{minimal} \usepackage{amsmath} \usepackage{wasysym} \usepackage{amsfonts} \usepackage{amssymb} \usepackage{amsbsy} \usepackage{mathrsfs} \usepackage{upgreek} \setlength{\oddsidemargin}{-69pt} \begin{document}$${\mathrm{P}}^ + {\mathrm{B}}_{\mathrm{A}}^ -$$\end{document}P+BA-. The results show that both electronic and vibrational coherences are involved in primary electron transfer process and they correlate with the super-high efficiency.

Distinguishing electronic and vibrational coherences helps to clarify the near-unity efficiency of primary electron transfer in reaction centres. Here, the authors report their respective correlation with the electron transfer rate by comparing the 2D electronic spectra of three mutant reaction centres.

The future of quantum biology

Biological systems are dynamical, constantly exchanging energy and matter with the environment in order to maintain the non-equilibrium state synonymous with living. Developments in observational techniques have allowed us to study biological dynamics on increasingly small scales. Such studies have revealed evidence of quantum mechanical effects, which cannot be accounted for by classical physics, in a range of biological processes. Quantum biology is the study of such processes, and here we provide an outline of the current state of the field, as well as insights into future directions.

Oxygenic Photoreactivity in Photosystem II Studied by Rotating Ring Disk Electrochemistry

Protein film photoelectrochemistry has previously been used to monitor the activity of photosystem II, the water-plastoquinone photooxidoreductase, but the mechanistic information attainable from a three-electrode setup has remained limited. Here we introduce the four-electrode rotating ring disk electrode technique for quantifying light-driven reaction kinetics and mechanistic pathways in real time at the enzyme-electrode interface. This setup allows us to study photochemical H₂O oxidation in photosystem II and to gain an in-depth understanding of pathways that generate reactive oxygen species. The results show that photosystem II reacts with O₂ through two main pathways that both involve a superoxide intermediate to produce H₂O₂. The first pathway involves the established chlorophyll triplet-mediated formation of singlet oxygen, which is followed by its reduction to superoxide at the electrode surface. The second pathway is specific for the enzyme/electrode interface: an exposed antenna chlorophyll is sufficiently close to the electrode for rapid injection of an electron to form a highly reducing chlorophyll anion, which reacts with O₂ in solution to produce O₂^•-. Incomplete H₂O oxidation does not significantly contribute to reactive oxygen formation in our conditions. The rotating ring disk electrode technique allows the chemical reactivity of photosystem II to be studied electrochemically and opens several avenues for future investigation.

Uphill energy transfer in photosystem I from Chlamydomonas reinhardtii. Time-resolved fluorescence measurements at 77 K

Energetic properties of chlorophylls in photosynthetic complexes are strongly modulated by their interaction with the protein matrix and by inter-pigment coupling. This spectral tuning is especially striking in photosystem I (PSI) complexes that contain low-energy chlorophylls emitting above 700 nm. Such low-energy chlorophylls have been observed in cyanobacterial PSI, algal and plant PSI-LHCI complexes, and individual light-harvesting complex I (LHCI) proteins. However, there has been no direct evidence of their presence in algal PSI core complexes lacking LHCI. In order to determine the lowest-energy states of chlorophylls and their dynamics in algal PSI antenna systems, we performed time-resolved fluorescence measurements at 77 K for PSI core and PSI-LHCI complexes isolated from the green alga Chlamydomonas reinhardtii. The pool of low-energy chlorophylls observed in PSI cores is generally smaller and less red-shifted than that observed in PSI-LHCI complexes. Excitation energy equilibration between bulk and low-energy chlorophylls in the PSI-LHCI complexes at 77 K leads to population of excited states that are less red-shifted (by ~ 12 nm) than at room temperature. On the other hand, analysis of the detection wavelength dependence of the effective trapping time of bulk excitations in the PSI core at 77 K provided evidence for an energy threshold at ~ 675 nm, above which trapping slows down. Based on these observations, we postulate that excitation energy transfer from bulk to low-energy chlorophylls and from bulk to reaction center chlorophylls are thermally activated uphill processes that likely occur via higher excitonic states of energy accepting chlorophylls.

Spectrally decomposed dark-to-light transitions in Synechocystis sp. PCC 6803

Photosynthetic activity and respiration share the thylakoid membrane in cyanobacteria. We present a series of spectrally resolved fluorescence experiments where whole cells of the cyanobacterium Synechocystis sp. PCC 6803 and mutants thereof underwent a dark-to-light transition after different dark-adaptation (DA) periods. Two mutants were used: (i) a PSI-lacking mutant (ΔPSI) and (ii) M55, a mutant without NAD(P)H dehydrogenase type-1 (NDH-1). For comparison, measurements of the wild-type were also carried out. We recorded spectrally resolved fluorescence traces over several minutes with 100 ms time resolution. The excitation light was at 590 nm so as to specifically excite the phycobilisomes. In ΔPSI, DA time has no influence, and in dichlorophenyl-dimethylurea (DCMU)-treated samples we identify three main fluorescent components: PB-PSII complexes with closed (saturated) RCs, a quenched or open PB-PSII complex, and a PB-PSII 'not fully closed.' For the PSI-containing organisms without DCMU, we conclude that mainly three species contribute to the signal: a PB-PSII-PSI megacomplex with closed PSII RCs and (i) slow PB → PSI energy transfer, or (ii) fast PB → PSI energy transfer and (iii) complexes with open (photochemically quenched) PSII RCs. Furthermore, their time profiles reveal an adaptive response that we identify as a state transition. Our results suggest that deceleration of the PB → PSI energy transfer rate is the molecular mechanism underlying a state 2 to state 1 transition.

Dynamics of the mixed exciton and charge-transfer states in light-harvesting complex Lhca4: Hierarchical equation approach

We model the energy transfer dynamics in the Lhca4 peripheral antenna of photosystem I from higher plants. Equilibration between the bulk exciton levels of the antenna and the red-shifted charge-transfer (CT) states is described using the numerically inexpensive Redfield-Förster approach and exact hierarchical equation (HEOM) method. We propose a compartmentalization scheme allowing a quantitatively correct description of the dynamics with the Redfield-Förster theory, including the exciton-type relaxation within strongly coupled compartments and hopping-type migration between them. The Redfield-Förster method gives the kinetics close to the HEOM solution when treating the CT state as dynamically localized. We also demonstrate that the excited states strongly coupled with the CT should be considered as localized as well.

Mechanistic Regimes of Vibronic Transport in a Heterodimer and the Design Principle of Incoherent Vibronic Transport in Phycobiliproteins

Following the observation of coherent oscillations in nonlinear spectra of photosynthetic pigment protein complexes, in particular, phycobilliproteins such as PC645, coherent vibronic transport has been suggested as a design principle for novel light-harvesting materials. Vibronic transport between energetically remote pigments is coherent when the presence of a vibration resonant with the electronic energy gap supports transient delocalization between the electronic excited states. We establish the mechanism of vibronic transport for a model heterodimer across a wide range of molecular parameter values. The resulting mechanistic map demonstrates that the molecular parameters of phycobiliproteins in fact support incoherent vibronic transport. This result points to an important design principle: Incoherent vibronic transport is more efficient than a coherent mechanism when energetic disorder exceeds the coupling between the donor and vibrationally excited acceptor states. Finally, our results suggest that the role of coherent vibronic transport in pigment protein complexes should be reevaluated.

Switching an Individual Phycobilisome Off and On

Photosynthetic organisms have found various smart ways to cope with unexpected changes in light conditions. In many cyanobacteria, the lethal effects of a sudden increase in light intensity are mitigated mainly by the interaction between phycobilisomes (PBs) and the orange carotenoid protein (OCP). The latter senses high light intensities by means of photoactivation and triggers thermal energy dissipation from the PBs. Due to the brightness of their emission, PBs can be characterized at the level of individual complexes. Here, energy dissipation from individual PBs was reversibly switched on and off using only light and OCP. We reveal the presence of quasistable intermediate states during the binding and unbinding of OCP to PB, with a spectroscopic signature indicative of transient decoupling of some of the PB rods during docking of OCP. Real-time control of emission from individual PBs has the potential to contribute to the development of new super-resolution imaging techniques.

Vibronic Coherence in the Charge Separation Process of the Rhodobacter sphaeroides Reaction Center

Two-dimensional electronic spectroscopy was applied to a variant of the reaction center (RC) of purple bacterium Rhodobacter sphaeroides lacking the primary acceptor ubiquinone in order to understand the ultrafast separation and transfer of charge between the bacteriochlorin cofactors. For the first time, characteristic 2D spectra were obtained for the participating excited and charge-transfer states, and the electron-transfer cascade (including two different channels, the P* and B* channels) was fully mapped. By analyzing quantum beats using 2D frequency maps, excited-state vibrational modes at 153 and 33 cm-1 were identified. We speculate that these modes couple to the charge separation (CS) process and collectively optimize the CS and are responsible for the superhigh efficiency.

Photoinduced processes in nucleic acids and proteins: concluding remarks

Light induced charge and energy transport in nucleic acids and proteins is the basis of fundamental biological processes such as photosynthesis, vision, DNA-photostability, DNA-photodamage and photosensing. This article summarises the concluding remarks given at the Faraday Discussions meeting on this topic. The specific themes arising from the meeting that are discussed herein include charge transfer in nucleic acids and proteins, excited state dynamics and topology, vibrations and dynamic disorder, proteins and charge transfer states, nanobiophotonics coupled to biomedical applications and photosynthesis.

Photoprotection through ultrafast charge recombination in photochemical reaction centres under oxidizing conditions

Engineering natural photosynthesis to address predicted shortfalls in food and energy supply requires a detailed understanding of its molecular basis and the intrinsic photoprotective mechanisms that operate under fluctuating environmental conditions. Long-lived triplet or singlet excited electronic states have the potential to cause photodamage, particularly in the presence of oxygen, and so a variety of mechanisms exist to prevent formation of such states or safely dissipate their energy. Here, we report a dramatic difference in spectral evolution in fully reduced and partially oxidized Rhodobacter sphaeroides reaction centres (RCs) following excitation of the monomeric bacteriochlorophyll (BChl) cofactors at 805 nm. Three types of preparation were studied, including RCs purified as protein/lipid nanodiscs using the copolymer styrene maleic acid. In fully reduced RCs such excitation produces membrane-spanning charge separation. In preparations of partially oxidized RCs the spectroscopic signature of this charge separation is replaced by that of an energy dissipation process, including in the majority sub-population of reduced RCs. This process, which appears to take place on both cofactor branches, involves formation of a BChl⁺/bacteriopheophytin^- radical pair that dissipates energy via recombination to a vibrationally hot ground state. The possible physiological role of this dissipative process under mildly oxidizing conditions is considered.This article is part of the themed issue 'Enhancing photosynthesis in crop plants: targets for improvement'.

Phycocyanin: One Complex, Two States, Two Functions

Solar energy captured by pigments embedded in light-harvesting complexes can be transferred to neighboring pigments, dissipated, or emitted as fluorescence. Only when it reaches a reaction center is the excitation energy stabilized in the form of a charge separation and converted into chemical energy. Well-directed and regulated energy transfer within the network of pigments is therefore of crucial importance for the success of the photosynthetic processes. Using single-molecule spectroscopy, we show that phycocyanin can dynamically switch between two spectrally distinct states originating from two different conformations. Unexpectedly, one of the two states has a red-shifted emission spectrum. This state is not involved in energy dissipation; instead, we propose that it is involved in direct energy transfer to photosystem I. Finally, our findings suggest that the function of linker proteins in phycobilisomes is to stabilize one state or the other, thus controlling the light-harvesting functions of phycocyanin.

Energy transfer and trapping in Synechococcus WH 7803

Excitation energy transfer (EET) and trapping in Synechococcus WH 7803 whole cells and isolated photosystem I (PSI) complexes have been studied by time-resolved emission spectroscopy at room temperature (RT) and at 77 K. With the help of global and target analysis, the pathways of EET and the charge separation dynamics have been identified. Energy absorbed in the phycobilisome (PB) rods by the abundant phycoerythrin (PE) is funneled to phycocyanin (PC645) and from there to the core that contains allophycocyanin (APC660 and APC680). Intra-PB EET rates have been estimated to range from 11 to 68/ns. It was estimated that at RT, the terminal emitter of the phycobilisome, APC680, transfers its energy at a rate of 90/ns to PSI and at a rate of 50/ns to PSII. At 77 K, the redshifted Chl a states in the PSI core were heterogeneous, with maximum emission at 697 and 707 nm. In 72% of the PSI complexes, the bulk Chl a in equilibrium with F697 decayed with a main trapping lifetime of 39 ps.

A functional compartmental model of the Synechocystis PCC 6803 phycobilisome

In the light-harvesting antenna of the Synechocystis PCC 6803 phycobilisome (PB), the core consists of three cylinders, each composed of four disks, whereas each of the six rods consists of up to three hexamers (Arteni et al., Biochim Biophys Acta 1787(4):272-279, 2009). The rods and core contain phycocyanin and allophycocyanin pigments, respectively. Together these pigments absorb light between 400 and 650 nm. Time-resolved difference absorption spectra from wild-type PB and rod mutants have been measured in different quenching and annihilation conditions. Based upon a global analysis of these data and of published time-resolved emission spectra, a functional compartmental model of the phycobilisome is proposed. The model describes all experiments with a common set of parameters. Three annihilation time constants are estimated, 3, 25, and 147 ps, which represent, respectively, intradisk, interdisk/intracylinder, and intercylinder annihilation. The species-associated difference absorption and emission spectra of two phycocyanin and two allophycocyanin pigments are consistently estimated, as well as all the excitation energy transfer rates. Thus, the wild-type PB containing 396 pigments can be described by a functional compartmental model of 22 compartments. When the interhexamer equilibration within a rod is not taken into account, this can be further simplified to ten compartments, which is the minimal model. In this model, the slowest excitation energy transfer rates are between the core cylinders (time constants 115-145 ps), and between the rods and the core (time constants 68-115 ps).

Exciton-vibrational resonance and dynamics of charge separation in the photosystem II reaction center

The dynamics of charge separation in the photosystem II reaction center (PSII-RC) in the presence of intramolecular vibrations with their frequency matching the energy gap between the exciton state acting as the primary electron donor and the first charge-transfer (CT) state are investigated. A reduced PSII-RC 4-state model explicitly including a CT state is analyzed within Redfield relaxation theory in the multidimensional exciton-vibrational (vibronic) basis. This model is used to study coherent energy/electron transfers and their spectral signatures obtained by two-dimensional electronic spectroscopy (2DES). Modeling of the time-resolved 2D frequency maps obtained by wavelet analysis reveals the origins of the coherences which produce the observed oscillating features in 2DES and allows comparing the lifetimes of the coherences. The results suggest faster excitonic decoherence as compared with longer-lived vibronic oscillations. The emerging picture of the dynamics unravels the role of resonant vibrations in sustaining the effective energy conversion in the PSII-RC. We demonstrate that the mixing of the exciton and CT states promoted by a resonant vibrational quantum allows faster penetration of excitation energy into the CT with subsequent dynamic localization at the bottom of the CT potential induced by the remaining non-resonant nuclear modes. The degree of vibration-assisted mixing and, correspondingly, the rate of primary charge separation, increases significantly in the case of electron-vibrational resonance. The observed features illustrate the principles of quantum design of the photosynthetic unit. These principles are connected with the phenomenon of coherent mixing within vibronic eigenstates, increasing the effectiveness of charge separation not only upon coherent and impulsive laser excitation utilized in the 2DES experiment, but also under natural conditions under non-coherent non-impulsive solar light illumination.

Robust light harvesting by a noisy antenna

Photosynthetic light harvesting can be very efficient in solar energy conversion while taking place in a highly disordered and noisy physiological environment. This efficiency is achieved by the ultrafast speed of the primary photosynthetic processes, which is enabled by a delicate interplay of quantum effects, thermodynamics and environmental noise. The primary processes take place in light-harvesting antennas built from pigments bound to a fluctuating protein scaffold. Here, we employ ultrafast single-molecule spectroscopy to follow fluctuations of the femtosecond energy transfer times in individual LH2 antenna complexes of purple bacteria. By combining single molecule results with ensemble spectroscopy through a unified theoretical description of both, we show how the protein fluctuations alter the excitation energy transfer dynamics. We find that from the thirteen orders of magnitude of possible timescales from picoseconds to minutes, the relevant fluctuations occur predominantly on a biological timescale of seconds, i.e. in the domain of slow protein motion. The measured spectra and dynamics can be explained by the protein modulating pigment excitation energies only. Moreover, we find that the small spread of pigment mean energies allows for excitation delocalization between the coupled pigments to survive. These unique features provide fast energy transport even in the presence of disorder. We conclude that this is the mechanism that enables LH2 to operate as a robust light-harvester, in spite of its intrinsically noisy biological environment.

Evidence for coherent mixing of excited and charge-transfer states in the major plant light-harvesting antenna, LHCII

LHCII, the major light harvesting antenna from plants, plays a dual role in photosynthesis. In low light it is a light-harvester, while in high light it is a quencher that protects the organism from photodamage. The switching mechanism between these two orthogonal conditions is mediated by protein dynamic disorder and photoprotective energy dissipation. The latter in particular is thought to occur in part via spectroscopically 'dark' states. We searched for such states in LHCII trimers from spinach, at both room temperature and at 77 K. Using 2D electronic spectroscopy, we explored coherent interactions between chlorophylls absorbing on the low-energy side of LHCII, which is the region that is responsible for both light-harvesting and photoprotection. 2D beating frequency maps allow us to identify four frequencies with strong excitonic character. In particular, our results show the presence of a low-lying state that is coupled to a low-energy excitonic state. We assign this to a mixed excitonic-charge transfer state involving the state with charge separation within the Chl a603-b609 heterodimer, borrowing some dipole strength from the Chl a602-a603 excited states. Such a state may play a role in photoprotection, in conjunction with specific and environmentally controlled realizations of protein dynamic disorder. Our identification and assignment of the coherences observed in the 2D frequency maps suggests that the structure of exciton states as well as a mixing of the excited and charge-transfer states is affected by coupling of these states to resonant vibrations in LHCII.

On the performance of a photosystem II reaction centre-based photocell

The photosystem II reaction centre is the photosynthetic complex responsible for oxygen production on Earth. Its water splitting function is particularly favoured by the formation of a stable charge separated state via a pathway that starts at an accessory chlorophyll. Here we envision a photovoltaic device that places one of these complexes between electrodes and investigate how the mean current and its fluctuations depend on the microscopic interactions underlying charge separation in the pathway considered. Our results indicate that coupling to well resolved vibrational modes does not necessarily offer an advantage in terms of power output but can lead to photo-currents with suppressed noise levels characterizing a multi-step ordered transport process. Besides giving insight into the suitability of these complexes for molecular-scale photovoltaics, our work suggests a new possible biological function for the vibrational environment of photosynthetic reaction centres, namely, to reduce the intrinsic current noise for regulatory processes.

The effectiveness of styrene-maleic acid (SMA) copolymers for solubilisation of integral membrane proteins from SMA-accessible and SMA-resistant membranes

Solubilisation of biological lipid bilayer membranes for analysis of their protein complement has traditionally been carried out using detergents, but there is increasing interest in the use of amphiphilic copolymers such as styrene maleic acid (SMA) for the solubilisation, purification and characterisation of integral membrane proteins in the form of protein/lipid nanodiscs. Here we survey the effectiveness of various commercially-available formulations of the SMA copolymer in solubilising Rhodobacter sphaeroides reaction centres (RCs) from photosynthetic membranes. We find that formulations of SMA with a 2:1 or 3:1 ratio of styrene to maleic acid are almost as effective as detergent in solubilising RCs, with the best solubilisation by short chain variants (<30kDa weight average molecular weight). The effectiveness of 10kDa 2:1 and 3:1 formulations of SMA to solubilise RCs gradually declined when genetically-encoded coiled-coil bundles were used to artificially tether normally monomeric RCs into dimeric, trimeric and tetrameric multimers. The ability of SMA to solubilise reaction centre-light harvesting 1 (RC-LH1) complexes from densely packed and highly ordered photosynthetic membranes was uniformly low, but could be increased through a variety of treatments to increase the lipid:protein ratio. However, proteins isolated from such membranes comprised clusters of complexes in small membrane patches rather than individual proteins. We conclude that short-chain 2:1 and 3:1 formulations of SMA are the most effective in solubilising integral membrane proteins, but that solubilisation efficiencies are strongly influenced by the size of the target protein and the density of packing of proteins in the membrane.

Limits on Natural Photosynthesis

Photosynthesis in nature does not use the far infrared part of the solar spectrum (λ > 900 nm), comprising about 30% of the incoming solar energy. By simple thermodynamic arguments it is explained that this is due to the unavoidable back reactions during the night. It follows that λ ≈ 900 nm provides a natural limit on artificial photosynthesis. The same limit holds for a two-tandem Si solar cell.

Excitonic and Vibrational Coherence in the Excitation Relaxation Process of Two LH1 Complexes as Revealed by Two-Dimensional Electronic Spectroscopy

Ultrafast excitation relaxation within a manifold exciton state and long-lived vibrational coherence are two universal characteristics of photosynthetic antenna complexes. In this work, we studied the two-dimensional electronic spectra of two core light-harvesting (LH1) complexes of Thermochromatium (Tch.) tepidum, native Ca²⁺-LH1 and modified Ba²⁺-LH1. The role of the vibrational coherence in the exciton relaxation was revealed by comparing the two LH1 with similar structures but different electronic properties and by the evolution of the exciton and vibrational coherence as a function of temperature.

Comparison of excitation energy transfer in cyanobacterial photosystem I in solution and immobilized on conducting glass

Excitation energy transfer in monomeric and trimeric forms of photosystem I (PSI) from the cyanobacterium Synechocystis sp. PCC 6803 in solution or immobilized on FTO conducting glass was compared using time-resolved fluorescence. Deposition of PSI on glass preserves bi-exponential excitation decay of ~4-7 and ~21-25 ps lifetimes characteristic of PSI in solution. The faster phase was assigned in part to photochemical quenching (charge separation) of excited bulk chlorophylls and in part to energy transfer from bulk to low-energy (red) chlorophylls. The slower phase was assigned to photochemical quenching of the excitation equilibrated over bulk and red chlorophylls. The main differences between dissolved and immobilized PSI (iPSI) are: (1) the average excitation decay in iPSI is about 11 ps, which is faster by a few ps than for PSI in solution due to significantly faster excitation quenching of bulk chlorophylls by charge separation (~10 ps instead of ~15 ps) accompanied by slightly weaker coupling of bulk and red chlorophylls; (2) the number of red chlorophylls in monomeric PSI increases twice-from 3 in solution to 6 after immobilization-as a result of interaction with neighboring monomers and conducting glass; despite the increased number of red chlorophylls, the excitation decay accelerates in iPSI; (3) the number of red chlorophylls in trimeric PSI is 4 (per monomer) and remains unchanged after immobilization; (4) in all the samples under study, the free energy gap between mean red (emission at ~710 nm) and mean bulk (emission at ~686 nm) emitting states of chlorophylls was estimated at a similar level of 17-27 meV. All these observations indicate that despite slight modifications, dried PSI complexes adsorbed on the FTO surface remain fully functional in terms of excitation energy transfer and primary charge separation that is particularly important in the view of photovoltaic applications of this photosystem.

Polarization-controlled optimal scatter suppression in transient absorption spectroscopy

Ultrafast transient absorption spectroscopy is a powerful technique to study fast photo-induced processes, such as electron, proton and energy transfer, isomerization and molecular dynamics, in a diverse range of samples, including solid state materials and proteins. Many such experiments suffer from signal distortion by scattered excitation light, in particular close to the excitation (pump) frequency. Scattered light can be effectively suppressed by a polarizer oriented perpendicular to the excitation polarization and positioned behind the sample in the optical path of the probe beam. However, this introduces anisotropic polarization contributions into the recorded signal. We present an approach based on setting specific polarizations of the pump and probe pulses, combined with a polarizer behind the sample. Together, this controls the signal-to-scatter ratio (SSR), while maintaining isotropic signal. We present SSR for the full range of polarizations and analytically derive the optimal configuration at angles of 40.5° between probe and pump and of 66.9° between polarizer and pump polarizations. This improves SSR by (or compared to polarizer parallel to probe). The calculations are validated by transient absorption experiments on the common fluorescent dye Rhodamine B. This approach provides a simple method to considerably improve the SSR in transient absorption spectroscopy.

Achieving Exciton Delocalization in Quantum Dot Aggregates Using Organic Linker Molecules

The design of new complex structures containing semiconductor quantum dots offers a means to create a variety of new meso-solids and molecules. The control of the coupling properties between the dots, accompanied by the energetic tunability of the dots themselves, paves the way toward the application and use of novel quantum properties. Here we present our approach to alteration of interdot coupling using organic linking molecules in a system of covalently bonded, aggregated quantum dots. We used ultrafast transient absorption measurements to identify marks of exciton delocalization over nearest neighbors to some extent. In linking molecules incorporating a benzene ring, the delocalized electron cloud displayed a profound influence over the interdot effects, leading the way to easy coupling control in quantum-based devices, under ambient conditions.

Estimation of damped oscillation associated spectra from ultrafast transient absorption spectra

When exciting a complex molecular system with a short optical pulse, all chromophores present in the system can be excited. The resulting superposition of electronically and vibrationally excited states evolves in time, which is monitored with transient absorption spectroscopy. We present a methodology to resolve simultaneously the contributions of the different electronically and vibrationally excited states from the complete data. The evolution of the excited states is described with a superposition of damped oscillations. The amplitude of a damped oscillation cos(ω_nt)exp(-γ_nt) as a function of the detection wavelength constitutes a damped oscillation associated spectrum DOAS_n(λ) with an accompanying phase characteristic φ_n(λ). In a case study, the cryptophyte photosynthetic antenna complex PC612 which contains eight bilin chromophores was excited by a broadband optical pulse. Difference absorption spectra from 525 to 715 nm were measured until 1 ns. The population dynamics is described by four lifetimes, with interchromophore equilibration in 0.8 and 7.5 ps. We have resolved 24 DOAS with frequencies between 130 and 1649 cm^-1 and with damping rates between 0.9 and 12 ps^-1. In addition, 11 more DOAS with faster damping rates were necessary to describe the "coherent artefact." The DOAS contains both ground and excited state features. Their interpretation is aided by DOAS analysis of simulated transient absorption signals resulting from stimulated emission and ground state bleach.

A method to decompose spectral changes in Synechocystis PCC 6803 during light-induced state transitions

Cyanobacteria have developed responses to maintain the balance between the energy absorbed and the energy used in different pigment-protein complexes. One of the relatively rapid (a few minutes) responses is activated when the cells are exposed to high light intensities. This mechanism thermally dissipates excitation energy at the level of the phycobilisome (PB) antenna before it reaches the reaction center. When exposed to low intensities of light that modify the redox state of the plastoquinone pool, the so-called state transitions redistribute energy between photosystem I and II. Experimental techniques to investigate the underlying mechanisms of these responses, such as pulse-amplitude modulated fluorometry, are based on spectrally integrated signals. Previously, a spectrally resolved fluorometry method has been introduced to preserve spectral information. The analysis method introduced in this work allows to interpret SRF data in terms of species-associated spectra of open/closed reaction centers (RCs), (un)quenched PB and state 1 versus state 2. Thus, spectral differences in the time-dependent fluorescence signature of photosynthetic organisms under varying light conditions can be traced and assigned to functional emitting species leading to a number of interpretations of their molecular origins. In particular, we present evidence that state 1 and state 2 correspond to different states of the PB-PSII-PSI megacomplex.

Competing charge transfer pathways at the photosystem II-electrode interface

The integration of the water-oxidation enzyme, photosystem II (PSII), into electrodes allows the electrons extracted from water-oxidation to be harnessed for enzyme characterization and driving novel endergonic reactions. However, PSII continues to underperform in integrated photoelectrochemical systems despite extensive optimization efforts. Here, we performed protein-film photoelectrochemistry on spinach and Thermosynechococcus elongatus PSII, and identified a competing charge transfer pathway at the enzyme-electrode interface that short-circuits the known water-oxidation pathway: photo-induced O₂ reduction occurring at the chlorophyll pigments. This undesirable pathway is promoted by the embedment of PSII in an electron-conducting matrix, a common strategy of enzyme immobilization. Anaerobicity helps to recover the PSII photoresponses, and unmasked the onset potentials relating to the Q_A/Q_B charge transfer process. These findings have imparted a fuller understanding of the charge transfer pathways within PSII and at photosystem-electrode interfaces, which will lead to more rational design of pigment-containing photoelectrodes in general.

Kinetic isotope effect of proton-coupled electron transfer in a hydrogen bonded phenol-pyrrolidino[60]fullerene

Proton-coupled electron transfer (PCET) plays a central role in photosynthesis and potentially in solar-to-fuel systems. We report a spectroscopy study on a phenol-pyrrolidino[60]fullerene. Quenching of the singlet excited state from 1 ns to 250 ps is assigned to PCET. A H/D exchange study reveals a kinetic isotope effect (KIE) of 3.0, consistent with a concerted PCET mechanism.

Identification and characterization of multiple emissive species in aggregated minor antenna complexes

Aggregation induced conformational change of light harvesting antenna complexes is believed to constitute one of the pathways through which photosynthetic organisms can safely dissipate the surplus of energy while exposed to saturating light. In this study, Stark fluorescence (SF) spectroscopy is applied to minor antenna complexes (CP24, CP26 and CP29) both in their light-harvesting and energy-dissipating states to trace and characterize different species generated upon energy dissipation through aggregation (in-vitro) induced conformational change. SF spectroscopy could identify three spectral species in the dissipative state of CP24, two in CP26 and only one in CP29. The comprehensive analysis of the SF spectra yielded different sets of molecular parameters for the multiple spectral species identified in CP24 or CP26, indicating the involvement of different pigments in their formation. Interestingly, a species giving emission around the 730nm spectral region is found to form in both CP24 and CP26 following transition to the energy dissipative state, but not in CP29. The SF analyses revealed that the far red species has exceptionally large charge transfer (CT) character in the excited state. Moreover, the far red species was found to be formed invariably in both Zeaxanthin (Z)- and Violaxathin (V)-enriched CP24 and CP26 antennas with identical CT character but with larger emission yield in Z-enriched ones. This suggests that the carotenoid Z is not directly involved but only confers an allosteric effect on the formation of the far red species. Similar far red species with remarkably large CT character were also observed in the dissipative state of the major light harvesting antenna (LHCII) of plants [Wahadoszamen et al. PCCP, 2012], the fucoxanthin-chlorophyll protein (FCP) of brown algae [Wahadoszamen et al. BBA, 2014] and cyanobacterial IsiA [Wahadoszamen et al. BBA, 2015], thus pointing to identical sites and pigments active in the formation of the far red quenching species in different organisms.

How exciton-vibrational coherences control charge separation in the photosystem II reaction center

In photosynthesis absorbed sun light produces collective excitations (excitons) that form a coherent superposition of electronic and vibrational states of the individual pigments. Two-dimensional (2D) electronic spectroscopy allows a visualization of how these coherences are involved in the primary processes of energy and charge transfer. Based on quantitative modeling we identify the exciton-vibrational coherences observed in 2D photon echo of the photosystem II reaction center (PSII-RC). We find that the vibrations resonant with the exciton splittings can modify the delocalization of the exciton states and produce additional states, thus promoting directed energy transfer and allowing a switch between the two charge separation pathways. We conclude that the coincidence of the frequencies of the most intense vibrations with the splittings within the manifold of exciton and charge-transfer states in the PSII-RC is not occurring by chance, but reflects a fundamental principle of how energy conversion in photosynthesis was optimized.