Excitation Energy Transfer Processes Research Articles

Specificity of Photochemical Energy Conversion in Photosystem I from the Green Microalga <i>Chlorella ohadii</i>

Primary excitation energy transfer and charge separation reactions in photosystem I (PSI) from the extremophile desert green alga Chlorella ohadii, grown in low light, were studied using broadband femtosecond pump-probe spectroscopy in the spectral range from 400 to 850 nm and in the time range of 50 fs–500 ps. Photochemical reactions were induced by the excitation into the blue and red edges of the chlorophyll Qy absorption band, and compared with similar processes in PSI from the cyanobacterium Synechocystis sp. PCC 6803. When PSI from C. ohadii was excited at a wavelength of 660 nm, the processes of energy redistribution in the light-harvesting antenna of the complex were observed in a time interval of up to 25 ps, while the formation of a stable ion-radical pair P700+A1− was kinetically heterogeneous with characteristic times of 25 and 120 ps. With an alternative variant of excitation into the red edge of the Qy band at a wavelength of 715 nm, in half of the complexes, primary charge separation reactions were observed in the time range of 7 ps. In the rest of the complexes, the formation of the ion-radical pair P700+A1− was limited by energy transfer and occurred with a characteristic time of 70 ps. Similar photochemical reactions in PSI from Synechocystis 6803 were significantly faster: upon excitation at a wavelength of 680 nm, in ~30% of the complexes, the formation of primary ion-radical pairs occurred with a time of 3 ps. Upon excitation at 720 nm, kinetically unresolvable ultrafast primary charge separation was observed in half of the complexes, and the subsequent formation of a P700+A1− ion-radical pair was observed at 25 ps. The photodynamics of PSI from C. ohadii had a noticeable similarity with the processes of excitation energy transfer and charge separation in PSI from the microalga Chlamydomonas reinhardtii; however, in the PSI from C. ohadii slower components in the energy transfer dynamics were also observed.

Vibrational Relaxation Completes the Excitation Energy Transfer and Localization of Vibronic Excitons in Allophycocyanin α84-β84.

Phycobilisomes are light-harvesting complexes that play a key role in photosynthesis in cyanobacteria, which generate more than 40% of the world's oxygen. The near-unity excitation energy transfer efficiency from phycobilisomes to photosystems highlights its importance in understanding efficient energy transfer processes. Spectroscopic studies have shown that the 280 fs rapid excitonic downhill energy transfer within the α84-β84 chromophore dimer in allophycocyanin (APC), a subunit of phycobilisomes, is crucial to this efficiency. However, the role of strong chromophore-protein interactions and vibrational relaxation requires further exploration to fully explain this efficient downhill energy transfer. A theory is required that adequately describes exciton dynamics in an intermediate region while also incorporating vibrational relaxation mediated by protein bath modes. In this work, we incorporate vibrational relaxation into modified Redfield theory by introducing coupling fluctuation. We holistically simulate the rapid excitation energy transfer process of the α84-β84 chromophore dimer in APC and successfully model the recently observed rapid energy capture. We find that vibrational relaxation dictates capture of excitons by the localized state of the β84 chromophore. The calculated rate shows excellent agreement with previous ultrafast spectroscopic experiments. Our results show that the inclusion of vibrational relaxation is essential for systems that utilize vibronic coupling to enhance energy transfer and capture. Consequently, incorporating vibrational relaxation into Modified Redfield theory shows promise for accurately describing the excitation energy transfer process in other photosynthetic systems.

Reduced density matrix dynamics in multistate harmonic models via time-convolution and time-convolutionless quantum master equations with quantum-mechanical and semiclassical kernels.

In this work, we explore the electronic reduced density matrix (RDM) dynamics using time-convolution (TC) and time-convolutionless (TCL) quantum master equations (QMEs) that are based on perturbative electronic couplings within the framework of multistate harmonic (MSH) models. The MSH model Hamiltonian consistently incorporates the electronic-vibrational correlations between all pairs of states by satisfying the pairwise reorganization energies directly obtained from all-atom simulations, representing the globally heterogeneous environments that couple to the multiple states differently. We derive the exact quantum-mechanical and a hierarchy of semiclassical approximate expressions for the kernels in TC and TCL QMEs that project the full RDM for general shifted harmonic systems, including the MSH model. These QMEs are applied to simulate RDM dynamics of photoinduced charge transfer (PICT) in organic photovoltaic carotenoid-porphyrin-fullerene triad solvated in tetrahydrofuran solution and the excitation energy transfer (EET) dynamics in photosynthetic Fenna-Matthews-Olson complexes from C. tepidum and P. aestuarii. Our results show that while both TC and TCL QMEs capture similar phenomena in PICT and EET processes, TC QME generally provides more accurate results than TCL QME, particularly in the initial oscillation of EET population dynamics. This study highlights the effectiveness of the TC and TCL QMEs in modeling RDM dynamics of nonadiabatic processes, offering insights for realistic condensed phase systems.

Preparation and Characterization of Novel α-Naphthol Luminophors for Optoelectronic Devices.

Luminophors of α-Naphthol doped with varying concentrations of anthracene and perylene were prepared by conventional solid state reaction method to explore its fluorescence characteristics. The fluorescence spectra of bicomponent and tricomponent luminophors reveals the fluorescence excitation energy transfer (FRET) process. The prepared bi and tri component luminophors were excited at 290nm which corresponds to host excitation wavelength. The weak violet fluorescence of host, α-Naphthol, gets quenched and give anthracene like emission in bicomponent luminophor systems. Further doping with second host, perylene, in anthracene containing α-Naphthol luminophors, red shifted emission was observed at 605nm. The structural parameters, thermal stability and electrical properties of doped luminophors were studied by using X-ray diffraction, TGA-DSC and Cyclic Voltammetry respectively. The particle size of the prepared luminophors was confirmed by scanning electron microscopy (SEM).

Photoinduced Nonadiabatic Dynamics of a Single-Walled Carbon Nanotube-Porphyrin Complex.

Single-walled carbon nanotubes (SWCNTs) have gained a lot of attention in the past few decades due to their promising optoelectronic properties. In addition, SWCNTs can form complexes that have good chemical stability and transport properties with other optical functional materials through noncovalent interactions. Elucidating the detailed mechanism of these complexes is of great significance for improving their optoelectronic properties. Nevertheless, simulating the photoinduced dynamics of these complexes accurately is rather challenging since they usually contain hundreds of atoms. To save computational efforts, most of the previous works have ignored the excitonic effects by employing nonadiabatic carrier (electron and hole) dynamics simulations. To properly consider the influence of excitonic effects on the photoinduced ultrafast processes of the SWCNT-tetraphenyl porphyrin (H2TPP) complex and to further improve the computational efficiency, we developed the nonadiabatic molecular dynamics (NAMD) method based on the extended tight binding-based simplified Tamm-Dancoff approximation (sTDA-xTB), which is applied to study the ultrafast photoinduced dynamics of the noncovalent SWCNT-porphyrin complex. In combination with statically electronic structure calculations, the present work successfully reveals the detailed microscopic mechanism of the ultrafast excitation energy transfer process of the complex. Upon local excitation on the H2TPP molecule, an ultrafast energy transfer process occurs from H2TPP (SWCNT-H2TPP*) to SWCNT (SWCNT*-H2TPP) within 10 fs. Then, two slower processes corresponding to the energy transfer from H2TPP to SWCNT and hole transfer from H2TPP to SWCNT take place in the 1 ps time scale. The sTDA-xTB-based electronic structure calculation and NAMD simulation results not only match the previous experimental observations from static and transient spectra but also provide more insights into the detailed information on the complex's photoinduced dynamics. Therefore, the sTDA-xTB-based NAMD method is a powerful theoretical tool for studying the ultrafast photoinduced dynamics in large extended systems with a large number of electronically excited states, which could be helpful for the subsequent design of SWCNT-based functional materials.

Structural and spectroscopic characterization of the peridinin-chlorophyll a-protein (PCP) complex from Heterocapsa pygmaea (HPPCP)

Light harvesting proteins are optimized to efficiently collect and transfer light energy for photosynthesis. In eukaryotic dinoflagellates these complexes utilize chlorophylls and a special carotenoid, peridinin, and arrange them for efficient excitation energy transfer. At the same time, the carotenoids protect the system by quenching harmful chlorophyll triplet states. Here we use advanced spectroscopic techniques and X-ray structure analysis to investigate excitation energy transfer processes in the major soluble antenna, the peridinin chlorophyll a protein (PCP) from the free living dinoflagellate Heterocapsa pygmaea. We determined the 3D-structure of this complex at high resolution (1.2 Å). For better comparison, we improved the reference structure of this protein from Amphidinium carterae to a resolution of 1.15 Å. We then used fs and ns time-resolved absorption spectroscopy to study the mechanisms of light harvesting, but also of the photoprotective quenching of the chlorophyll triplet state. The photoprotection site was further characterized by Electron Spin Echo Envelope Modulation (ESEEM) spectroscopy to yield information on water molecules involved in triplet-triplet energy transfer.Similar to other PCP complexes, excitation energy transfer from peridinin to chlorophyll is found to be very efficient, with transfer times in the range of 1.6–2.1 ps. One of the four carotenoids, the peridinin 614, is well positioned to quench the chlorophyll triplet state with high efficiency and transfer times in the range of tens of picoseconds. Our structural and dynamic data further support, that the intrinsic water molecule coordinating the chlorophyll Mg ion plays an essential role in photoprotection.

Simulation of the Pump-Probe Spectra and Excitation Energy Relaxation of the B850 Band of the LH2 Complex in Purple Bacteria.

Ultrafast spectroscopic techniques have been vital in studying excitation energy transfer (EET) in photosynthetic light harvesting complexes. In this paper, we simulate the pump-probe spectra of the B850 band of the light harvesting complex 2 (LH2) of purple bacteria, by using the hierarchical equation of motion method and the optical response function approach. The ground state bleach, stimulated emission, and excited state absorption components of the pump-probe spectra are analyzed in detail. The laser pulse-induced population dynamics are also simulated to help understand the main features of the pump-probe spectra and the EET process. It is shown that the excitation energy relaxation is an ultrafast process with multiple time scales. The first 40 fs of the pump-probe spectra is dominated by the relaxation of the k = ±1 states to both the k = 0 and higher energy states. Dynamics on a longer time scale around 200 fs reflects the relaxation of higher energy states to the k = 0 state.

Coarse-Grained Approach to Simulate Signatures of Excitation Energy Transfer in Two-Dimensional Electronic Spectroscopy of Large Molecular Systems.

Two-dimensional electronic spectroscopy (2DES) has proven to be a highly effective technique in studying the properties of excited states and the process of excitation energy transfer in complex molecular assemblies, particularly in biological light-harvesting systems. However, the accurate simulation of 2DES for large systems still poses a challenge because of the heavy computational demands it entails. In an effort to overcome this limitation, we devised a coarse-grained 2DES method. This method encompasses the treatment of the entire system by dividing it into distinct weakly coupled segments, which are assumed to communicate predominantly through incoherent exciton transfer. We first demonstrate the efficiency of this method through simulation on a model dimer system, which demonstrates a marked improvement in calculation efficiency, with results that exhibit good concordance with reference spectra calculated with less approximate methods. Additionally, the application of this method to the light-harvesting antenna 2 (LH2) complex of purple bacteria showcases its advantages, accuracy, and limitations. Furthermore, simulating the anisotropy decay in LH2 induced by energy transfer and its comparison with experiments confirm that the method is capable of accurately describing dynamical processes in a biologically relevant system. This method presented lends itself to an extension that accounts for the effect of intrasegment relaxation processes on the 2DES spectra, which for computational efficiency are ignored in the implementation reported here. It is envisioned that the method will be employed in the future to accurately and efficiently calculate 2D spectra of more extensive systems, such as photosynthetic supercomplexes.

Sub-100-fs energy transfer in coenzyme NADH is a coherent process assisted by a charge-transfer state

Excitation energy transfer (EET) is a key photoinduced process in biological chromophoric assemblies. Here we investigate the factors which can drive EET into efficient ultrafast sub-ps regimes. We demonstrate how a coherent transport of electronic population could facilitate this in water solvated NADH coenzyme and uncover the role of an intermediate dark charge-transfer state. High temporal resolution ultrafast optical spectroscopy gives a 54±11 fs time constant for the EET process. Nonadiabatic quantum dynamical simulations computed through the time-evolution of multidimensional wavepackets suggest that the population transfer is mediated by photoexcited molecular vibrations due to strong coupling between the electronic states. The polar aqueous solvent environment leads to the active participation of a dark charge transfer state, accelerating the vibronically coherent EET process in favorably stacked conformers and solvent cavities. Our work demonstrates how the interplay of structural and environmental factors leads to diverse pathways for the EET process in flexible heterodimers and provides general insights relevant for coherent EET processes in stacked multichromophoric aggregates like DNA strands.

Enhancement of vibrationally assisted energy transfer by proximity to exceptional points, probed by fluorescence-detected vibrational spectroscopy

Emulation of energy transfer processes in natural systems on quantum platforms can further our understanding of complex dynamics in nature. One notable example is the demonstration of vibrationally assisted energy transfer (VAET) on a trapped-ion quantum emulator, which offers insights for the energetics of light harvesting. In this work, we expand the study of VAET simulation with trapped ions to a non-Hermitian quantum system comprising a PT-symmetric chromophore dimer weakly coupled to a vibrational mode. We first characterize exceptional points (EPs) and non-Hermitian features of the excitation energy transfer processes in the absence of the vibration, finding a degenerate pair of second-order EPs. Exploring the non-Hermitian dynamics of the whole system including vibrations, we find that energy transfer accompanied by absorption of phonons from a vibrational mode can be significantly enhanced near such a degenerate EP. Our calculations reveal a unique spectral feature accompanying the coalescing of eigenstates and eigenenergies that provides a unique approach to probe the degenerate EP by fluorescence-detected vibrational spectroscopy. Enhancement of the VAET process near the EP is found to be due to maximal favorability of phonon absorption at the degenerate EP, enabling multiple simultaneous excitations. Our work on improving VAET processes in non-Hermitian quantum systems paves the way for leveraging non-Hermiticity in quantum dynamics related to excitation energy transfer. Published by the American Physical Society 2024

High-Resolution Frequency-Domain Spectroscopic and Modeling Studies of Photosystem I (PSI), PSI Mutants and PSI Supercomplexes.

Photosystem I (PSI) is one of the two main pigment-protein complexes where the primary steps of oxygenic photosynthesis take place. This review describes low-temperature frequency-domain experiments (absorption, emission, circular dichroism, resonant and non-resonant hole-burned spectra) and modeling efforts reported for PSI in recent years. In particular, we focus on the spectral hole-burning studies, which are not as common in photosynthesis research as the time-domain spectroscopies. Experimental and modeling data obtained for trimeric cyanobacterial Photosystem I (PSI3), PSI3 mutants, and PSI3-IsiA18 supercomplexes are analyzed to provide a more comprehensive understanding of their excitonic structure and excitation energy transfer (EET) processes. Detailed information on the excitonic structure of photosynthetic complexes is essential to determine the structure-function relationship. We will focus on the so-called "red antenna states" of cyanobacterial PSI, as these states play an important role in photochemical processes and EET pathways. The high-resolution data and modeling studies presented here provide additional information on the energetics of the lowest energy states and their chlorophyll (Chl) compositions, as well as the EET pathways and how they are altered by mutations. We present evidence that the low-energy traps observed in PSI are excitonically coupled states with significant charge-transfer (CT) character. The analysis presented for various optical spectra of PSI3 and PSI3-IsiA18 supercomplexes allowed us to make inferences about EET from the IsiA18 ring to the PSI3 core and demonstrate that the number of entry points varies between sample preparations studied by different groups. In our most recent samples, there most likely are three entry points for EET from the IsiA18 ring per the PSI core monomer, with two of these entry points likely being located next to each other. Therefore, there are nine entry points from the IsiA18 ring to the PSI3 trimer. We anticipate that the data discussed below will stimulate further research in this area, providing even more insight into the structure-based models of these important cyanobacterial photosystems.

Site-Directed Mutagenesis of the Chlorophyll-Binding Sites Modulates Excited-State Lifetime and Chlorophyll-Xanthophyll Energy Transfer in the Monomeric Light-Harvesting Complex CP29.

We combine site-directed mutagenesis with picosecond time-resolved fluorescence and femtosecond transient absorption (TA) spectroscopies to identify excitation energy transfer (EET) processes between chlorophylls (Chls) and xanthophylls (Xant) in the minor antenna complex CP29 assembled inside nanodiscs, which result in quenching. When compared to WT CP29, a longer lifetime was observed in the A2 mutant, missing Chl a612, which closely interacts with Xant Lutein in site L1. Conversely, a shorter lifetime was obtained in the A5 mutant, in which the interaction between Chl a603 and Chl a609 is strengthened, shifting absorption to lower energy and enhancing Chl-Xant EET. Global analysis of TA data indicated that EET from Chl a Qy to a Car dark state S* is active in both the A2 and A5 mutants and that their rate constants are modulated by mutations. Our study provides experimental evidence that multiple Chl-Xant interactions are involved in the quenching activity of CP29.

Excitation energy transfer and vibronic relaxation through light-harvesting dendrimer building blocks: A nonadiabatic perspective.

The light-harvesting excitonic properties of poly(phenylene ethynylene) (PPE) extended dendrimers (tree-like π-conjugated macromolecules) involve a directional cascade of local excitation energy transfer (EET) processes occurring from the "leaves" (shortest branches) to the "trunk" (longest branch), which can be viewed from a vibronic perspective as a sequence of internal conversions occurring among a connected graph of nonadiabatically coupled locally excited electronic states via conical intersections. The smallest PPE building block that is able to exhibit EET, the asymmetrically meta-substituted PPE oligomer with one acetylenic bond on one side and two parallel ones on the other side (hence, 2-ring and 3-ring para-substituted pseudo-fragments), is a prototype and the focus of the present work. From linear-response time-dependent density functional theory electronic-structure calculations of the molecule as regards its first two nonadiabatically coupled, optically active, singlet excited states, we built a (1 + 2)-state-8-dimensional vibronic-coupling Hamiltonian model for running subsequent multiconfiguration time-dependent Hartree wavepacket relaxations and propagations, yielding both steady-state absorption and emission spectra as well as real-time dynamics. The EET process from the shortest branch to the longest one occurs quite efficiently (about 80% quantum yield) within the first 25fs after light excitation and is mediated vibrationally through acetylenic and quinoidal bond-stretching modes together with a particular role given to the central-ring anti-quinoidal rock-bending mode. Electronic and vibrational energy relaxations, together with redistributions of quantum populations and coherences, are interpreted herein through the lens of a nonadiabatic perspective, showing some interesting segregation among the foremost photoactive degrees of freedom as regards spectroscopy and reactivity.

Lipid-induced quenching of chlorophyll singlet excitation in lipid-nanodisc accommodated FCP complex from diatom Chaetoceros gracilis

Diatom is the most flourishing group of ocean photosynthetic organism, whose living success relies largely on the light-harvesting and photoprotection potency of its antenna complex, i.e. the fucoxanthin chlorophyll (Chl) a/c binding protein (FCP). With reference to the light-harvesting antennae of higher plants and green algae, FCP binds weakly coupled Chls, among which the intra-complex excitation energy transfer (EET) is more susceptive to the protein conformation and surrounding. Particularly, the fucoxanthin molecules bound in FCP are highly interactive with the thylakoid membrane, which may influence the intra-complex EET processes. To investigate the effect of lipid microenvironment on the conformation and the energetics of FCP, we reconstituted Chaetoceros gracilis FCP dimer into L-α-PC lipid nanodisc, and investigated the fluorescence dynamics among different pools of Chls using time-resolved fluorescence spectroscopy. The lipid-protein interactions lead to obvious conformational change of the blue-absorbing fucoxanthin molecule, and about 30 % decrease in Chl a-fluorescence quantum yield and 20 % shortening in fluorescence lifetime, which is ascribed to the quenching of major fluorescent component at 677 nm by non-radiative quencher.

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.

First-principles simulation of excitation energy transfer and transient absorption spectroscopy in the CP29 light-harvesting complex.

The dynamics of delocalized excitons in light-harvesting complexes (LHCs) can be investigated using different experimental techniques, and transient absorption (TA) spectroscopy is one of the most valuable methods for this purpose. A careful interpretation of TA spectra is essential for the clarification of excitation energy transfer (EET) processes occurring during light-harvesting. However, even in the simplest LHCs, a physical model is needed to interpret transient spectra as the number of EET processes occurring at the same time is very large to be disentangled from measurements alone. Physical EET models are commonly built by fittings of the microscopic exciton Hamiltonians and exciton-vibrational parameters, an approach that can lead to biases. Here, we present a first-principles strategy to simulate EET and transient absorption spectra in LHCs, combining molecular dynamics and accurate multiscale quantum chemical calculations to obtain an independent estimate of the excitonic structure of the complex. The microscopic parameters thus obtained are then used in EET simulations to obtain the population dynamics and the related spectroscopic signature. We apply this approach to the CP29 minor antenna complex of plants for which we follow the EET dynamics and transient spectra after excitation in the chlorophyll b region. Our calculations reproduce all the main features observed in the transient absorption spectra and provide independent insight on the excited-state dynamics of CP29. The approach presented here lays the groundwork for the accurate simulation of EET and unbiased interpretation of transient spectra in multichromophoric systems.

Interfacial self-assembly of Cd2+-tetraphenylethylene coordination polymers and their photophysical properties in Langmuir-Blodgett films

Molecular arrangement and orientation of tetraphenylethylene (TPE) unit in the highly ordered thin films and aggregates can effectively limit its conformation that results in strong effect on the excited energy transfer process and luminescent behaviors under irradiation. We report here an interfacial self-assembly of metal-TPE coordination polymer (CP) ultrathin films and their aggregate-induced emission (AIE) behaviors. After connected with pyridyl or benzimidazole substituents, the tetradentate TPEs as synthesized are used as the multidentate linkers to coordinate with the Cd2+ ions to produce aggregates in the solutions, and to form well-defined two- and three-dimensional organized Cd-TPE CP multilayers at the air-water interfaces by the Langmuir-Blodgett (LB) method. It is revealed that, in addition to the aggregation-induced emission, the following external factors have also large effects on the luminescence of TPEs, including the coordination bonding in the CPs, in plane and interlayer molecular interaction in the LB films; both of them can restrict the intramolecular motion or rotation of TPE unit. Because of these external effects, the emission lifetime of TPEs reaches to approximately 5–10 times longer in some Cd-TPE CP LB films than that in the solutions. Further, the luminescent emission wavelengths and colors of LB films can be tuned to some extent by the in-plane and interlayer molecular interaction. Finally, it is demonstrated that the luminescent emissions and colors of the Cd-TPPE CP LB films is selectively sensing to methanol and formic acid in the gas phase with good reversibility and reproductivity.

Long-term light adaptation of light-harvesting and energy-transfer processes in the glaucophyte Cyanophora paradoxa under different light conditions.

In response to fluctuation in light intensity and quality, oxygenic photosynthetic organisms modify their light-harvesting and excitation energy-transfer processes to maintain optimal photosynthetic activity. Glaucophytes, which are a group of primary symbiotic algae, possess light-harvesting antennas called phycobilisomes (PBSs) consistent with cyanobacteria and red algae. However, compared with cyanobacteria and red algae, glaucophytes are poorly studied and there are few reports on the regulation of photosynthesis in the group. In this study, we examined the long-term light adaptation of light-harvesting functions in a glaucophyte, Cyanophora paradoxa, grown under different light conditions. Compared with cells grown under white light, the relative number of PBSs to photosystems (PSs) increased in blue-light-grown cells and decreased in green-, yellow-, and red-light-grown cells. Moreover, the PBS number increased with increment in the monochromatic light intensity. More energy was transferred from PBSs to PSII than to PSI under blue light, whereas energy transfer from PBSs to PSII was reduced under green and yellow lights, and energy transfer from the PBSs to both PSs decreased under red light. Decoupling of PBSs was induced by intense green, yellow, and red lights. Energy transfer from PSII to PSI (spillover) was observed, but the contribution of the spillover did not distinctly change depending on the culture light intensity and quality. These results suggest that the glaucophyte C. paradoxa modifies the light-harvesting abilities of both PSs and excitation energy-transfer processes between the light-harvesting antennas and both PSs during long-term light adaption.

On the Nature of Stationary and Time-Resolved Fluorescence Spectroscopy of Collagen Powder from Bovine Achilles Tendon.

This paper presents a more systematic study of steady-state and time-resolved autofluorescence spectroscopy of collagen isolated from bovine Achilles tendon. In steady-state fluorescence measurements, the excitation and emission spectra of collagen powder, recorded at different fluorescence excitation and detection wavelengths, were compared with the fluorescence excitation and emission spectra of the amino acids phenylalanine, tyrosine, and tryptophan, as well as with similar spectra for 13 autofluorescent collagen cross-links, which have been identified and described in the literature so far. In time-resolved studies, fluorescence was excited by the pulsed light of different wavelengths, and for each excitation wavelength, fluorescence decay was recorded for several detection wavelengths. Data analysis allowed recovery of the fluorescence decay times for each experimental excitation detection event. The obtained information on the decay times of the measured fluorescent signals was discussed, taking into account the available literature data from similar studies of isolated collagen and collagen-rich tissues. Based on the obtained results, it was found that the shape and position of the measured fluorescence excitation and emission spectra of collagen strongly depend on the emission and excitation wavelengths selected in the measurements. From the recorded excitation and emission bands of collagen, it can be concluded with high probability that there are additional, so far unidentified, collagen cross-links, which can be excited at longer excitation wavelengths. In addition, the collagen excitation spectra were measured at longer emission wavelengths at which the collagen cross-links emit fluorescent light. In addition to the emission spectra obtained for excitation in the deep-UV region, the results of time-resolved fluorescence studies with excitation in the deep-UV region and detection at longer wavelengths suggest that fluorescence excitation energy transfer processes occur from the amino acids to the collagen cross-links, and also between the cross-links themselves.

An efficient time-domain implementation of the multichromophoric Förster resonant energy transfer method.

The excitation energy transfer (EET) process for photosynthetic antenna complexes consisting of subunits, each comprised of multiple chromophores, remains challenging to describe. The multichromophoric Förster resonance energy transfer theory is a popular method to describe the EET process in such systems. This paper presents a new time-domain method for calculating energy transfer based on the combination of multichromophoric Förster resonance energy transfer theory and the Numerical Integration of the Schrödinger Equation method. After validating the method on simple model systems, we apply it to the Light-Harvesting antenna 2 (LH2) complex, a light harvesting antenna found in purple bacteria. We use a simple model combining the overdamped Brownian oscillators to describe the dynamic disorder originating from the environmental fluctuations and the transition charge from the electrostatic potential coupling model to determine the interactions between chromophores. We demonstrate that with this model, both the calculated spectra and the EET rates between the two rings within the LH2 complex agree well with experimental results. We further find that the transfer between the strongly coupled rings of neighboring LH2 complexes can also be well described with our method. We conclude that our new method accurately describes the EET rate for biologically relevant multichromophoric systems, which are similar to the LH2 complex. Computationally, the new method is very tractable, especially for slow processes. We foresee that the method can be applied to efficiently calculate transfer in artificial systems as well and may pave the way for calculating multidimensional spectra of extensive multichromophoric systems in the future.