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Far-Red Spectrum of Second Emerson Effect: A Study Using Dual-Wavelength Pulse Amplitude Modulation Fluorometry
摘要： Non-additive enhancement of the photosynthesis excited by simultaneous illumination with far-red light and light of shorter wavelengths is called as “second Emerson effect”. Its action spectra are well-known as a photosynthetic yield’s dependence on light wavelength in red (630-690 nm) spectral region at a constant-wavelength far-red illumination near 700-715 nm. However, the opposite dependence of the photosynthetic yield’s of shorter constant-wavelength light (red or blue) on light wavelength in far-red (690-760 nm) spectral region was never studied. In this study the action spectrum of second Emerson effect was studied using a fast-Fourier dual-wavelength Pulse Amplitude Modulation (PAM) fluorometry. Chlorophyll fluorescence in ailanthus (Ailanthus altissima Mill.) leaves was excited with blue modulated light. Far-red induced decrease of fluorescence (fluorescence shift-FRIFS) was studied in response to illumination of leaves with a background light from 690 to 760 nm (10 nm step), calculating FRIFS = (F0-Fs)/F0, where F0-fluorescence measured without and Fs-with far-red light. Maximum FRIFS was observed at 720 nm (11.8%), but it still remained considerable at 740, 750 nm and a low FRIFS values were revealed at 690 and even at 760 nm. Measurements carried out with blue saturating flashes during and after far-red illumination showed the increase of quantum yield of Photosystem II (PSII), calculated as Fv/Fm at 720 nm background light. FRIFS had lower values under excitation with red modulating light. It is concluded that FRIFS is a result of a photochemical quenching caused by an additional selective far-red excitation of PSI in conditions when PSII is preferably excited by blue light thus leading the PSI to limit non-cyclic electron flow. The contradiction between the known absorption spectra of PSI-light harvesting complex I and the observed action spectrum of second Emerson effect (FRIFS spectrum) is discussed.
关键词： Photosystem II,Ailanthus Altissima,Photosystem I,Second Emerson Effect,Fast-Fourier PAM-Fluorometry,Far-Red Light,Thylakoid Electron Transport
更新于2025-11-14 15:30:11
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New insights on ChlD1 function in Photosystem II from site-directed mutants of D1/T179 in Thermosynechococcus elongatus
摘要： The monomeric chlorophyll, ChlD1, which is located between the PD1PD2 chlorophyll pair and the pheophytin, PheoD1, is the longest wavelength chlorophyll in the heart of Photosystem II and is thought to be the primary electron donor. Its central Mg2+ is liganded to a water molecule that is H-bonded to D1/T179. Here, two site-directed mutants, D1/T179H and D1/T179V, were made in the thermophilic cyanobacterium, Thermosynechococcus elongatus, and characterized by a range of biophysical techniques. The Mn4CaO5 cluster in the water-splitting site is fully active in both mutants. Changes in thermoluminescence indicate that i) radiative recombination occurs via the repopulation of *ChlD1 itself; ii) non-radiative charge recombination reactions appeared to be faster in the T179H-PSII; and iii) the properties of PD1PD2 were unaffected by this mutation, and consequently iv) the immediate precursor state of the radiative excited state is the ChlD1+PheoD1? radical pair. Chlorophyll bleaching due to high intensity illumination correlated with the amount of 1O2 generated. Comparison of the bleaching spectra with the electrochromic shifts attributed to ChlD1 upon QA? formation, indicates that in the T179H-PSII and in the WT*3-PSII, the ChlD1 itself is the chlorophyll that is first damaged by 1O2, whereas in the T179V-PSII a more red chlorophyll is damaged, the identity of which is discussed. Thus, ChlD1 appears to be one of the primary damage site in recombination-mediated photoinhibition. Finally, changes in the absorption of ChlD1 very likely contribute to the well-known electrochromic shifts observed at ~430 nm during the S-state cycle.
关键词： ChlD1,Electrochromic shifts,P680,Photosystem II,Reaction center
更新于2025-11-14 15:14:40
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Electronic Structure of Chlorophyll a Solution Investigated by Photoelectron Yield Spectroscopy
摘要： Various bio-related processes are driven by electron transfer reactions. Therefore the electronic structures of bio-molecules in their living environment are keys of their functionalities. One significant example photosynthesis which has attracted much attention due to urgent necessity of clean energy source. In this study, we carried out photoelectron yield spectroscopy (PYS) measurements to demonstrate the electronic structures of oligomerizedChl-a molecules, which is known as an essential reaction center of the photosystem in general green plants, under the atmospheric environment. The ionization energies of the Chl-a aggregates are successfully derived.
关键词： Photoelectron Yield Spectroscopy,Polarization energy,Light harvesting antenna,Photosynthesis,Photosystem,Electronic structure,Ionization energy,Reaction center,Chlorophyll a
更新于2025-09-23 15:23:52
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Structure/Function/Dynamics of Photosystem II Plastoquinone Binding Sites
摘要： Photosystem II (PSII) continuously attracts the attention of researchers aiming to unravel the riddle of its functioning and efficiency fundamental for all life on Earth. Besides, an increasing number of biotechnological applications have been envisaged exploiting and mimicking the unique properties of this macromolecular pigment-protein complex. The PSII organization and working principles have inspired the design of electrochemical water splitting schemes and charge separating triads in energy storage systems as well as biochips and sensors for environmental, agricultural and industrial screening of toxic compounds. An intriguing opportunity is the development of sensor devices, exploiting native or manipulated PSII complexes or ad hoc synthesized polypeptides mimicking the PSII reaction centre proteins as biosensing elements. This review offers a concise overview of the recent improvements in the understanding of structure and function of PSII donor side, with focus on the interactions of the plastoquinone cofactors with the surrounding environment and operational features. Furthermore, studies focused on photosynthetic proteins structure/function/dynamics and computational analyses aimed at rational design of high-quality bio-recognition elements in biosensor devices are discussed.
关键词： plastoquinone binding site,molecular dynamics simulations,plastoquinone,Molecular docking,protein dynamics,Photosystem II
更新于2025-09-23 15:22:29
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The Catalytic Cycle of Water Oxidation in Crystallized Photosystem II Complexes: Performance and Requirements for Formation of Intermediates
摘要： Crystals of Photosystem II (PSII) contain the most homogeneous copies of the water-oxidizing reaction center where O2 is evolved (WOC). However, few functional studies of PSII operation in crystals have been carried out, despite their widespread use in structural studies. Here we apply oximetric methods to determine the quantum efficiency and lifetimes of intermediates of the WOC cycle as a function of added electron acceptors (quinones and ferricyanide), both aerobically and anaerobically. PSII crystals exhibit the highest quantum yield of O2 production yet observed of any native or isolated PSII (61.6%, theoretically 59,000 μmol O2/mg Chl/h). WOC cycling can be sustained for thousands of turnovers using an irreversible electron acceptor (ferricyanide). Simulations of the catalytic cycle identify four distinct photochemical inefficiencies in both PSII crystals and dissolved PSII cores that are nearly the same. The exogenous acceptors equilibrate with the native plastoquinone acceptor at the QB (or QC) site(s), for which two distinct redox couples are observable that regulate flux through PSII. Flux through the catalytic cycle of water oxidation is shown to be kinetically restricted by the QAQB two-electron gate. The lifetimes of the S2 and S3 states are greatly extended (especially S2) by electron acceptors and depend on their redox reversibility. PSII performance can be pushed in vitro far beyond what it is capable of in vivo. With careful use of precautions and monitoring of populations, PSII microcrystals enable the exploration of WOC intermediates and the mechanism of catalysis.
关键词： oxygen-evolving complex,electron acceptors,(micro)crystals,S states,quantum yield,Photosystem II
更新于2025-09-23 15:22:29
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Theoretical Model of Exciton States and Ultra-fast Energy Transfer in Heliobacterial Type-I Homodimeric Reaction Center
摘要： A simple theoretical model of exciton dynamics was proposed to interpret the fast excitation energy transfer process in the Type-I homodimeric reaction center of Heliobacterium modesticaldum (hRC); this structure was recently identified and shown to resemble that of the plant/cyanobacterial photosystem I (PSI) reaction center. The exciton state model, which mainly relies on the geometries of 54 bacteriochlorophyll (BChl) g, 4 BChl-g′ and 2 chlorophyll (Chl) a on hRC and assumes constant site energy values for the pigments, reproduced the absorption spectrum of hRC rather well. The model also enabled numerical analysis of the exciton dynamics on hRC, which can be compared with the decay-associated spectra obtained by the laser spectroscopy experiments. The model indicates that the stronger transition-dipole moment on BChl-g contributes to the faster energy transfer due to the higher coherency of the delocalized exciton states on hRC compared to that on PSI that arranges Chl-a at almost homologous locations.
关键词： chlorophyll,photosystem I,bacteriochlorophyll,exciton dynamics,energy transfer,reaction center
更新于2025-09-23 15:21:21
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Oxidation of P700 Ensures Robust Photosynthesis
摘要： In the light, photosynthetic cells can potentially suffer from oxidative damage derived from reactive oxygen species. Nevertheless, a variety of oxygenic photoautotrophs, including cyanobacteria, algae, and plants, manage their photosynthetic systems successfully. In the present article, we review previous research on how these photoautotrophs safely utilize light energy for photosynthesis without photo-oxidative damage to photosystem I (PSI). The reaction center chlorophyll of PSI, P700, is kept in an oxidized state in response to excess light, under high light and low CO2 conditions, to tune the light utilization and dissipate the excess photo-excitation energy in PSI. Oxidation of P700 is co-operatively regulated by a number of molecular mechanisms on both the electron donor and acceptor sides of PSI. The strategies to keep P700 oxidized are diverse among a variety of photoautotrophs, which are evolutionarily optimized for their ecological niche.
关键词： photosynthesis,reactive oxygen species,photoinhibition,P700 oxidation,photosystem I
更新于2025-09-23 15:21:21
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Self-Adaptable Quinone-Quinol Exchange Mechanism of Photosystem II
摘要： The step of plastoquinone (PQ) reduction to plastoquinol (PQH2) can regulate the photo-reaction rate of photosystem II (PSII). To experimentally unravel the PQ-PQH2 exchange mechanism of PSII, we investigate the reaction kinetics of plant PSII membranes and the subunits-trimmed PSII core complexes with various PQ analogues, and directly probe the reductions of PQ and other quinones by 257-nm resonance Raman scattering. Two phases of quinone concentration effect on the reaction rate originate from the quinone-quinol exchange mechanism. The results indicate that high concentrations of quinone, more than one movable quinone molecule per PSII reaction center, could trigger quinone-quinol exchange adapting to the unidirectional route: quinones enter through channel I and/or III, and quinols leave through channel II. A weak quinone binding site near QB probably plays a crucial role in pushing quinone-quinol exchange forward in the unidirectional route. Our work provides experimental proofs demonstrating a self-adaptable quinone-quinol exchange mechanism of PSII.
关键词： reaction kinetics,Photosystem II,plastoquinone,resonance Raman scattering,quinone-quinol exchange
更新于2025-09-23 15:21:01
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Biotechnology for Biofuel Production and Optimization || Photobiohybrid Solar Conversion with Metalloenzymes and Photosynthetic Reaction Centers
摘要： Sunlight provides an abundant and sustainable supply of energy to the Earth's surface, at levels that far exceed the yearly human global energy demand. However, the intermittency and geographic variability of solar irradiation, combined with the need for storage, limits the ability to provide practical alternatives to use of conventional fossil fuels. To better utilize the available solar energy, current photovoltaic technologies must be integrated with conversion technologies that produce storable chemical energy (fuels) that are easily distributed to meet regional demand as needed. In biological photosynthesis, conversion of solar energy into chemical energy is accomplished by the water-splitting and CO2 fixation reactions. The molecular machinery of the natural system provides ideal models for the design and development of artificial solar-to-fuel systems. The theoretical limit of biological photosynthesis is ~12%, and under optimal conditions, efficiencies of 7% have been achieved; however, 1% is a more typical benchmark. Photosynthetic reactions rely on four key components which are integrated to act in concert as a highly functional energy transduction network: (i) the antenna, where photons are absorbed; (ii) the charge separation site, where high-energy excitons (electron-hole pairs) are separated into positive and negative charge carriers; (iii) the reduction catalyst, where electrons are utilized in a fuel-forming reaction (e.g., NAD+ → NADH formation used for CO2 fixation in photosynthesis); and (iv) the oxidation catalyst, where holes are utilized to drive an oxidation reaction (e.g., water oxidation by photosystem II during photosynthesis). Efforts are underway to translate photobiological design principles to develop artificial systems for solar fuel generation that circumvent or eliminate unwanted side reactions and attain higher efficiencies. These efforts include the development of photochemical devices, inorganic biomimetic and bioinspired catalysts and light-harvesting complexes, and organic hybrid materials for photo-driven fuel production. Here, we discuss research focused on the development of hybrid materials that incorporate artificial and natural molecular components into unified functional systems for light-harvesting and conversion into reduced chemical fuels.
关键词： solar conversion,photobiohybrid,metalloenzymes,CO2 reduction,hydrogen production,photosystem I,photosynthetic reaction centers
更新于2025-09-23 15:21:01
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A Biomimetic-Computational Approach to Optimizing the Quantum Efficiency of Photovoltaics
摘要： The most advanced low-cost organic photovoltaic cells have a quantum efficiency of ~10%. This is in stark contrast to plant/bacterial light-harvesting systems which offer quantum efficiencies close to unity. Of particular interest is the highly effective quantum coherence-enabled energy transfer. Noting that quantum coherence is promoted by charged residues and local dielectrics, classical atomistic simulations and Time-Dependent Density Functional Theory (TD-DFT) can be used to identify charge/dielectric patterns and electronic coupling at exactly defined energy transfer interfaces incorporating structural information obtained on photosynthetic protein-pigment complexes. To this end, the project focuses on the first protein-pigment-redox carrier complex of the linear electron transport phosphorylation chain termed photosystem II [PSII]. PSII contains more than 10 major polypeptides in addition to hundreds of pigment molecules amounting to a molecular mass in excess of 1 Mio Dalton. Owing to the complexity and fragility of PSII, this project bases the overall architecture of PSII on in situ EM data providing structural clues about the entire, unperturbed PSII complex. Albeit not to high resolution when compared to X-ray crystallography and NMR spectroscopy, the EM tomographic results and projection maps provide an accurate delineation of the native complex suitable for fitting high-resolution X-ray data of PSII subcomplexes towards an atomistic model of the entire PSII complex. This must also include the light-harvesting antennae, i.e. the light-harvesting chlorophyll (Chl) a/b protein complex [LHCII]. With respect to LHCII one should take into account positioning LHCII next to PSII as well as in a separate, complementary membrane thus permitting to test for both, horizontal (intramembrane) and vertical (intermembrane) energy transfer, respectively. The presence of LHCII in a membrane different from PSII is supported by strong biochemical evidence and tomographic data, and it has also been noted that the organization of LHCII may change in response to environmental conditions.
关键词： photosystem II,biomimetic,computational approach,quantum efficiency,light-harvesting chlorophyll,photovoltaics
更新于2025-09-23 15:19:57