Mechanism Of Excitation Energy Transfer Research Articles

Remembering Jan Amesz (1934-2001): a great gentleman, a major discoverer, and an internationally renowned biophysicist of both oxygenic and anoxygenic photosynthesisa.

We present here the research contributions of Jan Amesz (1934-2001) on deciphering the details of the early physico-chemical steps in oxygenic photosynthesis in plants, algae and cyanobacteria, as well as in anoxygenic photosynthesis in purple, green, and heliobacteria. His research included light absorption and the mechanism of excitation energy transfer, primary photochemistry, and electron transfer steps until the reduction of pyridine nucleotides. Among his many discoveries, we emphasize his 1961 proof, with L. N. M. Duysens, of the "series scheme" of oxygenic photosynthesis, through antagonistic effects of Light I and II on the redox state of cytochrome f. Further, we highlight the following research on oxygenic photosynthesis: the experimental direct proof that plastoquinone and plastocyanin function at their respective places in the Z-scheme. In addition, Amesz's major contributions were in unraveling the mechanism of excitation energy transfer and electron transport steps in anoxygenic photosynthetic bacteria (purple, green and heliobacteria). Before we present his research, focusing on his key discoveries, we provide a glimpse of his personal life. We end this Tribute with reminiscences from three of his former doctoral students (Sigi Neerken; Hjalmar Pernentier, and Frank Kleinherenbrink) and from several scientists (Suleyman Allakhverdiev; Robert Blankenship; Richard Cogdell) including two of the authors (G. Garab and A. Stirbet) of this Tribute.

Exploring optoelectronic properties and excitation energy transfer mechanism in hybrid thin films of poly (N,N′-bis-4-butylphenyl- N,N′-bisphenyl)benzidine/fluorol 7GA

Exploring optoelectronic properties and excitation energy transfer mechanism in hybrid thin films of poly (N,N′-bis-4-butylphenyl- N,N′-bisphenyl)benzidine/fluorol 7GA

Carotenoid-Mediated Long-Range Energy Transfer in the Light Harvesting-Reaction Center Complex from Photosynthetic Bacterium Roseiflexus castenholzii.

The light harvesting-reaction center complex (LH-RC) of Roseiflexus castenholzii binds bacteriochlorophylls a (BChls a), B800 and B880, absorbing around 800 and 880 nm, respectively. We comparatively investigated the interband excitation energy transfer (EET) dynamics of the wild-type LH-RC (wt-LH-RC) of Rfl. castenholzii and its carotenoid (Car)-less mutant (m-LH-RC) and found that Car can boost the B800 → B880 EET rate from (2.43 ps)-1 to (1.75 ps)-1, accounting for 38% acceleration of the EET process. Interestingly, photoexcitation of wt-LH-RC at 800 nm induced pronounced excitation dynamics of Car despite the insufficient photon energy for direct Car excitation, a phenomenon which is attributed to the BChl-Car exciplex 1[B800(↑↑)···Car(↓↓)]*. Such an exciplex is suggested to play an essential role in promoting the B800 → B880 EET process, as corroborated by the recently reported cryo-EM structures of wt-LH-RC and m-LH-RC. The mechanism of Car-mediated EET will be helpful to deepen the understanding of the role of Car in bacterial photosynthesis.

Redshifting and Blueshifting of β82 Chromophores in the Phycocyanin Hexamer of Porphyridium purpureum Phycobilisomes Due to Linker Proteins.

Phycobilisomes in cyanobacteria and red algae are large protein complexes that absorb light and transfer energy for use in photosynthesis. The light energy absorbed by chromophores binding to phycobiliproteins in the peripheral rods can be funneled to the core through chromophores at very high efficiency. The molecular mechanism of excitation energy transfer within a phycobilisome is an example of a higher and unique function in a living organism. However, the mechanism underlying the high efficiency remains unclear. Thus, this study was carried out as a step to resolve this mechanism theoretically. The three-dimensional structure of phycobilisomes containing the linker proteins of the red alga Porphyridium purpureum was determined by cryoelectron microscopy at 2.82 Å resolution in 2020. Using these data, the absorption wavelength of each β82 chromophore in the phycocyanin hexamer located next to the core was calculated using quantum chemical treatment, considering the electric effect from its surrounding phycocyanin proteins and two linker proteins. In addition to unaffected chromophores, chromophores that were redshifted and blueshifted under the electrical influence of the two linker proteins were found. Namely, the chromophore serving as the energy sink in the rod was determined.

Mechanism of Ir(ppy)3 Guest Exciton Formation with the Exciplex-Forming TCTA:TPBI Cohost within a Phosphorescent Organic Light-Emitting Diode Environment.

Cohosts based on hole transporting and electron transporting materials often act as exciplexes in the form of intermolecular charge transfer complexes. Indeed, exciplex-forming cohosts have been widely developed as the host materials for efficient phosphorescent organic light-emitting diodes (OLEDs). In host–guest systems of OLEDs, the guest can be excited by two competing mechanisms, namely, excitation energy transfer (EET) and charge transfer (CT). Experimentally, it has been reported that the EET mechanism is dominant and the excitons are primarily formed in the host first and then transferred to the guest in phosphorescent OLEDs based on exciplex-forming cohosts. With this, exciplex-forming cohosts are widely employed for avoiding the formation of trapped charge carriers in the phosphorescent guest. However, theoretical studies are still lacking toward elucidating the relative importance between EET and CT processes in exciting the guest molecules in such systems. Here, we obtain the kinetics of guest excitation processes in a few trimer model systems consisting of an exciplex-forming cohost pair and a phosphorescent guest. We adopt the Förster resonance energy transfer (FRET) rate constants for the electronic transitions between excited states toward solving kinetic master equations. The input parameters for calculating the FRET rate constants are obtained from density functional theory (DFT) and time-dependent DFT. The results show that while the EET mechanism is important, the CT mechanism may still play a significant role in guest excitations. In fact, the relative importance of CT over EET depends strongly on the location of the guest molecule relative to the cohost pair. This is understandable as both the coupling for EET and the interaction energy for CT are strongly influenced by the geometric constraints. Understanding the energy transfer pathways from the exciplex state of cohost to the emissive state of guest may provide insights for improving exciplex-forming materials adopted in OLEDs.

1,4-bis[β-(2-benzoxazoly1) vinyl] benzene (BBVB) laser dye and sodium salt of meso-tetrakis (4-sulfonatophenyl) porphyrin (TPPS); spectroscopic investigation and DFT, NBO and TD-DFT calculations

Excitation energy transfer (ET) from 1,4-bis[β-(2-benzoxazoly1) vinyl] benzene as a donor to sodium salt of meso-tetrakis (4-sulfonatophenyl) porphyrin (TPPS) as an acceptor was investigated in the absence and presence of metallic silver nanoparticles (Ag NPs). The emission intensities and Stern –Volmer constants (KSV) of the ET processes were calculated in methanol (MeOH) and 40% ethylene glycol (EG) as 5.87 × 104 M−1 and 6.27 × 104 M−1, respectively. KSV value in the presence of Ag NPs was 1.028 × 105 M−1 in methanol, much higher than in absence of NPs. This is attributed to the role of Ag NPs in increasing the strength of the donor–acceptor interactions. These results denote the use of Ag NPs for measuring the proximity of large biomolecules and ET-based assays. Also, the critical ET distance Ro was calculated as 36 Å. This magnitude is higher than the common value of 10 Å for the collisional ET mechanism. This high Ro value indicates the underlying mechanism as long-range dipole–dipole interaction. The BBVB, TPPS, and BBVB/TPPS molecular structures (MSs) were optimized using the DFT/ B3LYP/6-311G (d) method for the quantum calculations. The calculated electronic UV–Vis absorption spectra for BBVB and BBVB/TPPS MSs in MeOH were examined by the time-dependent density functional theory (TD-DFT) at CAM-B3LYP/6-311G++(3d,3p) level. Interestingly, all theoretical results of BBVB and BBVB/TPPS molecules were highly matched with the experimental results. Furthermore, the Natural Bond Orbital (NBO) analyses for BBVB and BBVB/TPPS MSs were obtained using the B3LYP/6-311G(d) level of theory.

Structural insight into the mechanism of energy transfer in cyanobacterial phycobilisomes

Phycobilisomes (PBS) are the major light-harvesting machineries for photosynthesis in cyanobacteria and red algae and they have a hierarchical structure of a core and peripheral rods, with both consisting of phycobiliproteins and linker proteins. Here we report the cryo-EM structures of PBS from two cyanobacterial species, Anabaena 7120 and Synechococcus 7002. Both PBS are hemidiscoidal in shape and share a common triangular core structure. While the Anabaena PBS has two additional hexamers in the core linked by the 4th linker domain of ApcE (LCM). The PBS structures predict that, compared with the PBS from red algae, the cyanobacterial PBS could have more direct routes for energy transfer to ApcD. Structure-based systematic mutagenesis analysis of the chromophore environment of ApcD and ApcF subunits reveals that aromatic residues are critical to excitation energy transfer (EET). The structures also suggest that the linker protein could actively participate in the process of EET in both rods and the cores. These results provide insights into the organization of chromophores and the mechanisms of EET within cyanobacterial PBS.

Energy Transfer Dynamics in Light-Harvesting Complex 2 Variants Containing Oxidized B800 Bacteriochlorophyll a.

Excitation energy transfer (EET) in light-harvesting proteins is vital for photosynthetic activities. The pigment compositions and their organizations in these proteins are responsible for the EET functions. Thus, changing the pigment compositions in light-harvesting proteins contributes to a better understanding of EET mechanisms. In this study, we investigated the EET dynamics of two light-harvesting complex 2 (LH2) variants, in which nine B800 bacteriochlorophyll (BChl) a pigments were entirely or half converted to 3-acetyl chlorophyll (AcChl) a. The AcChl a pigments showed a Qy band, which was blue-shifted by 107 nm from B800 BChl a in the two variants. EET from AcChl a to B850 BChl a was observed in both fully oxidized and half-oxidized LH2 variants, but the EET rates were lower than that from B800 to B850 BChl a. EET from AcChl a to the co-present B800 was barely detected in the half-oxidized LH2. The preferential EET from AcChl a to B850 instead of B800 was rationalized by little spectral overlap of AcChl a with B800 BChl a and the pigment geometry in the protein. The EET rate from B800 to B850 BChl a in the half-oxidized LH2 was analogous to that in native LH2, indicating that partial oxidation of B800 did not disturb the EET channel from the residual B800 to B850.

Real-Time Path Integral Simulation of Exciton-Vibration Dynamics in Light-Harvesting Bacteriochlorophyll Aggregates.

The mechanism of excitation energy transfer in photoexcited bacteriochlorophyll (BChl) aggregates poses intriguing questions, which have important implications for the observed efficiency of photosynthesis. We investigate this process through fully quantum mechanical calculations of exciton-vibration dynamics in chains and rings of BChl a molecules, with parameters characterizing the B850 ring of the LH2 complex of photosynthetic bacteria. The calculations are performed using the modular path integral methodology, which allows the exact treatment of 50 intramolecular vibrations on each pigment using parameters obtained from spectroscopic Huang-Rhys factors with computational effort that scales linearly with aggregate length. Our results indicate that the interplay between electronic and vibrational time scales leads to the rapid suppression but not the overdamping of electronic coherence, which facilitates the spreading of excitation energy throughout the aggregate.

Fabrication and Characterization of Anthracene Doped P-terphenyl Thin Films By Spin Coating Technique; Investigation of Fluorescence Properties.

Thin films of p-terphenyl luminophors doped by varying amounts of anthracene were prepared by using spin coating technique. The morphological, structural, and photophysical investigation of thin films of p-terphenyl as a function of anthracene concentration is studied by using scanning electron microscopy (SEM), X-ray diffraction (XRD), fluorescence spectroscopy and fluorescence microscopy. The results of XRD and SEM studies indicated that the doped p-terphenyl thin film is homogeneous as compared with a bare p-terphenyl thin film. The fluorescence spectroscopy results indicate complete quenching of p-terphenyl fluorescence and simultaneous sensitization of blue anthracene like emission towards the red side of the spectrum with maximum intensity at 410 nm. The blue intense emission of anthracene seen in fluorescence microscopy images is in agreement with observed fluorescence spectral results. A suitable mechanism of excitation energy transfer (EET) from p-terphenyl to anthracene molecules is proposed and discussed on the basis of energy level diagram. The efficient EET is believed to occur by the orientation of phenyl rings of p-terphenyl in excited state. As the concentration of doped anthracene increases, the fluorescence intensity of doped p-terphenyl and Full Width at Half Maximum (FWHM) found to be increased. The p-terphenyl film containing 0.65 moles of anthracene is of FWHM as low as 28.51 nm. Such narrow band blue emitting doped luminophors are of demand in light emitting diodes (OLED) and scintillation applications.

Electronic excitation energy transfer studies in binary mixtures of novel optoelectronically active 1,3,4-oxadiazoles and coumarin derivatives

The mechanisms of excitation energy transfer are studied using systems consisting of blue light emitting novel thiophene substituted 1,3,4-oxadiazole derivatives as donors and green light emitting laser dye coumarin-334 (C-334) as an acceptor in ethanol and poly(methyl methacrylate) [PMMA] media from steady-state and time-resolved methods. Experimental observations like bimolecular quenching constant (kq) greater than translation diffusion rate parameter (kd), critical transfer distance (R0) greater than diffusion length (Dl), donor-acceptor interaction strength (α) greater than donor-donor interaction strength (β) and biexponential decay of the donor in the presence of the acceptor confirms that the overall energy transfer from donor to acceptor is due to Förster resonance energy transfer (FRET). Furthermore, enhancement of energy transfer efficiency and fluorescence intensity in polymer media suggests that these donor-acceptor doped PMMA matrixes may have potential applications such as luminescent solar concentrator (LSC), energy transfer dye lasers (ETDL) and chemical sensors.

Benchmark ab initio calculations on intermolecular structures and the exciton character of poly(p-phenylenevinylene) dimers.

Benchmark ab initio calculations have been performed for poly(p-phenylenevinylene) (PPV) dimers, a paradigmatic material for studying excitation energy transfer mechanisms. Second-order Møller-Plesset perturbation theory was utilized with the scaled opposite spin approach (SOS-MP2) and correlation consistent basis sets to determine the geometric properties and interaction energies in the ground state. Vertical excitations and optimized structures for the S1 state were computed using the SOS second-order algebraic diagrammatic construction method. For the ground state properties, extrapolation to the complete basis set (CBS) limit and correction for the basis set superposition error (BSSE) were performed. While all results computed with different basis sets and considering BSSE correction or not agreed at the CBS limit, a strong bias was observed either using augmented basis sets or BSSE corrections, proving that these approaches are not advisable for calculating intermolecular distances and interaction energies with smaller basis sets. The lower states for vertical excitations were largely local excitons where the hole/electron pair was confined to single chains. For higher excited states, interchain charge transfer (CT) states were also observed. Geometry optimization of the S1 state led to significant reductions in the intermolecular distances and energetic stabilization, with Stokes shifts between 1.4 eV and 0.9 eV (with increasing chain length), and significant CT values between 0.5e and 0.4e.

Structural basis for energy harvesting and dissipation in a diatom PSII-FCPII supercomplex.

Light-harvesting antenna systems in photosynthetic organisms harvest solar energy and transfer it to the photosynthetic reaction centres to initiate charge-separation and electron-transfer reactions. Diatoms are one of the important groups of oxyphototrophs and possess fucoxanthin chlorophyll a/c-binding proteins (FCPs) as light harvesters. The organization and association pattern of FCP with the photosystem II (PSII) core are unknown. Here we solved the structure of PSII-FCPII supercomplexes isolated from a diatom, Chaetoceros gracilis, by single-particle cryoelectron microscopy. The PSII-FCPII forms a homodimer. In each monomer, two FCP homotetramers and three FCP monomers are associated with one PSII core. The structure reveals a highly complicated protein-pigment network that is different from the green-type light-harvesting apparatus. Comparing these two systems allows the identification of energy transfer and quenching pathways. These findings provide structural insights into not only excitation-energy transfer mechanisms in the diatom PSII-FCPII, but also changes of light harvesters between the red- and green-lineage oxyphototrophs during evolution.

Snapshot transient absorption spectroscopy: toward in vivo investigations of nonphotochemical quenching mechanisms

Although the importance of nonphotochemical quenching (NPQ) on photosynthetic biomass production and crop yields is well established, the in vivo operation of the individual mechanisms contributing to overall NPQ is still a matter of controversy. In order to investigate the timescale and activation dynamics of specific quenching mechanisms, we have developed a technique called snapshot transient absorption (TA) spectroscopy, which can monitor molecular species involved in the quenching response with a time resolution of 30s. Using intact thylakoid membrane samples, we show how conventional TA kinetic and spectral analyses enable the determination of the appropriate wavelength and time delay for snapshot TA experiments. As an example, we show how the chlorophyll-carotenoid charge transfer and excitation energy transfer mechanisms can be monitored based on signals corresponding to the carotenoid (Car) radical cation and Car S1 excited state absorption, respectively. The use of snapshot TA spectroscopy together with the previously reported fluorescence lifetime snapshot technique (Sylak-Glassman et al. in Photosynth Res 127:69-76, 2016) provides valuable information such as the concurrent appearance of specific quenching species and overall quenching of excited Chl. Furthermore, we show that the snapshot TA technique can be successfully applied to completely intact photosynthetic organisms such as live cells of Nannochloropsis. This demonstrates that the snapshot TA technique is a valuable method for tracking the dynamics of intact samples that evolve over time, such as the photosynthetic system in response to high-light exposure.

Ultrafast Excitation Energy Dynamics in a Diatom Photosystem I-Antenna Complex: A Femtosecond Fluorescence Upconversion Study.

Fucoxanthin chlorophyll (Chl) a/ c-binding proteins (FCPs) are unique light-harvesting antennas in diatoms. Recent time-resolved fluorescence analysis of photosystem I with FCP associated (PSI-FCPI) has mainly shown excitation energy transfer among Chls a from FCPI to PSI in tens of picoseconds. However, it remains unclear how each pigment, especially carotenoids and Chl c, in the FCPI is functionally related to the energy transfer in a femtosecond time range. Here, we reveal ultrafast excitation energy transfer mechanism in the PSI-FCPI preparations isolated from a diatom, Chaetoceros gracilis, by means of femtosecond time-resolved fluorescence spectroscopy with an upconversion system. Compared with the fluorescence lifetime components of PSI core-like complexes, the energy transfer of Chl c → Chl a in the FCPI was observed within hundreds of femtoseconds, and the energy in the FCPI was transferred to PSI in ∼2 ps. The comparative fluorescence analyses provide physical insights into the energy transfer machinery within FCPI and from FCPI to PSI.

Low-Energy Chlorophylls in Fucoxanthin Chlorophyll a/ c-Binding Protein Conduct Excitation Energy Transfer to Photosystem I in Diatoms.

Photosynthetic organisms handle solar energy precisely to achieve efficient photochemical reactions. Because there are a wide variety of light-harvesting antennas in oxyphototrophs, the excitation energy transfer mechanisms are thought to differ significantly. In this study, we compared excitation energy dynamics between photosystem I (PSI) cores and a complex between PSI and fucoxanthin chlorophyll (Chl) a/ c-binding protein I (PSI-FCPI) isolated from a diatom, Chaetoceros gracilis, by means of picosecond time-resolved fluorescence analyses. Time-resolved spectra measured at 77 K clearly show that low-energy Chls in the FCPI transfer not only most of the excitation energy to the reaction center Chls in the PSI cores but also the remaining energy to carotenoids for quenching. Under room-temperature conditions, the energy in the low-energy Chls is rapidly equilibrated on Chls in the PSI cores by uphill energy transfer within a few tens of picoseconds. These findings provide solid evidence that the low-energy Chls in the FCPI contribute to the photochemical reactions in PSI.

Temperature Dependence of Chlorophyll Triplet Quenching in Two Photosynthetic Light-Harvesting Complexes from Higher Plants and Dinoflagellates.

Chlorophyll (Chl) triplet states generated in photosynthetic light-harvesting complexes (LHCs) can be quenched by carotenoids to prevent the formation of reactive singlet oxygen. Although this quenching occurs with an efficiency close to 100% at physiological temperatures, the Chl triplets are often observed at low temperatures. This might be due to the intrinsic temperature dependence of the Dexter mechanism of excitation energy transfer, which governs triplet quenching, or by temperature-induced conformational changes. Here, we report about the temperature dependence of Chl triplet quenching in two LHCs. We show that both the effects contribute significantly. In LHC II of higher plants, the core Chls are quenched with a high efficiency independent of temperature. A different subpopulation of Chls, which increases with lowering temperature, is not quenched at all. This is probably caused by the conformational changes which detach these Chls from the energy-transfer chain. In a membrane-intrinsic LHC of dinoflagellates, similarly two subpopulations of Chls were observed. In addition, another part of Chl triplets is quenched by carotenoids with a rate which decreases with temperature. This allowed us to study the temperature dependence of Dexter energy transfer. Finally, a part of Chls was quenched by triplet-triplet annihilation, a phenomenon which was not observed for LHCs before.

Selective immobilization of bacterial light-harvesting proteins and their photoelectric responses

Abstract

Electronic Couplings in (Bio-) Chemical Processes.

During the last two decades, 2D optical techniques have been extended to the visible range, targeting electronic transitions. Since the report of the very first 2D electronic measurement (Hybl et al. in J Chem Phys 115:6606-6622, [2001]), two-dimensional electronic spectroscopy (2DES) has allowed fundamentally new insights into the structure and dynamics of condensed-phase systems (Ginsberg et al. in Acc Chem Res 42:1352-1363, 2009; Jonas in Annu Rev Phys Chem 54:425-463, 2003), producing experiments that measure correlations among electronic states of an absorbing species within complex systems. 2DES is used to investigate photo-physical phenomena involving electronic or vibrational couplings in multi-chromophoric systems [energy transfer in photosynthesis is one great example of how 2DES can disentangle various energy transfer pathways (Brixner et al. in Nature 625-628, 2005; Engel et al. in Nature 446:782-786, 2007; Collini et al. in Nature 463:644-647, 2010)], but also ultrafast photochemical processes in which the tracked molecules change permanently or are heterogeneous (Ruetzel et al. in Proc Natl Acad Sci 111:4764-4769, 2014; Consani et al. in Science 339:1586-1589, 2013). We divide this chapter according to some of the major areas that have been established thanks to 2DES in the following fields: heterogeneity of systems, excitation energy transfer mechanisms, photo-induced coherent oscillations associated with electronic and vibrational couplings, and complex chemical reactions (Fig.1). Fig.1 Main fields impacted by two-dimensional electronic spectroscopy (2DES) in condensed phase. The major discoveries of each field will be described in different paragraphs.

Excitation transfer and trapping kinetics in plant photosystem I probed by two-dimensional electronic spectroscopy.

Photosystem I is a robust and highly efficient biological solar engine. Its capacity to utilize virtually every absorbed photon's energy in a photochemical reaction generates great interest in the kinetics and mechanisms of excitation energy transfer and charge separation. In this work, we have employed room-temperature coherent two-dimensional electronic spectroscopy and time-resolved fluorescence spectroscopy to follow exciton equilibration and excitation trapping in intact Photosystem I complexes as well as core complexes isolated from Pisum sativum. We performed two-dimensional electronic spectroscopy measurements with low excitation pulse energies to record excited-state kinetics free from singlet-singlet annihilation. Global lifetime analysis resolved energy transfer and trapping lifetimes closely matches the time-correlated single-photon counting data. Exciton energy equilibration in the core antenna occurred on a timescale of 0.5ps. We further observed spectral equilibration component in the core complex with a 3-4ps lifetime between the bulk Chl states and a state absorbing at 700nm. Trapping in the core complex occurred with a 20ps lifetime, which in the supercomplex split into two lifetimes, 16ps and 67-75ps. The experimental data could be modelled with two alternative models resulting in equally good fits-a transfer-to-trap-limited model and a trap-limited model. However, the former model is only possible if the 3-4ps component is ascribed to equilibration with a "red" core antenna pool absorbing at 700nm. Conversely, if these low-energy states are identified with the P700 reaction centre, the transfer-to-trap-model is ruled out in favour of a trap-limited model.