Photosynthetic Center Research Articles

Femtopicosecond relaxation of zinc porphyrinate trimer linked by the triazole bridge

The femtosecond dynamics of excitation relaxation of the zinc porphyrinate trimer was studied by the pumping—scanning method. The obtained results were examined using semi-empirical quantum chemical calculations. The dynamics of excitation relaxation shows that the application of similar systems as models for studying photosystems and development of artificial analogs of natural photosynthetic centers is promising. An explanation for the observed coherent dynamics of exciton bands was proposed.

Creating electrochemical gradients by light: from bio-inspired concepts to photoelectric conversion.

Light is harvested by natural photosynthetic systems to generate electrochemical gradients that power various reactions. Implementing nature's lessons in photosynthesis holds great promise for technological advances. With a focus on designs and concepts, recent progress in generating electrochemical gradients by light, mimicking the two general types of photosynthetic centers in nature that make use of either light-induced charge separation or photo-isomerization are summarized here. Light induced electrochemical gradients pave new ways for photoelectric conversion. While extensive research in this direction has focused on light-induced charge separation, recent work has shown that energy conversion based on photo-isomerization is very promising. Photoswitchable compounds have been found in nature, such as the retinal molecule in bacteriorhodopsin. These compounds may form an attractive molecular basis for future progress in this field.

Molecular model of LH1 light-harvesting complex of the photosynthetic center of Thermochromatium tepidum bacteria

A three-dimensional all-atom structure of the LH1 light-harvesting complex of the photosynthetic center of the Thermochromatium tepidum bacteria was constructed with molecular modeling techniques.

Phenomenon of quantum entanglement in a system composed of two minimal protocells.

The quantum mechanical self-assembly of two separate photoactive supramolecular systems with different photosynthetic centers was investigated by means of density functional theory methods. Quantum entangled energy transitions from one subsystem to the other and the assembly of logically controlled artificial minimal protocells were modeled. The systems studied were based on different photoactive sensitizer molecules covalently bonded to a non-canonical oxo-guanine::cytosine supramolecule with the precursor of a fatty acid (pFA) molecule attached via Van der Waals forces, all surrounded by water molecules. The electron correlation interactions responsible for the weak hydrogen and Van der Waals chemical bonds increased due to the addition of polar water solvent molecules. The distances between the separated sensitizer, nucleotide, pFA, and water molecules are comparable to Van der Waals and hydrogen bonding radii. As a result, the overall system becomes compressed, resulting in photo-excited electron tunneling from the sensitizer (bis(4-diphenylamine-2-phenyl)-squarine or 1,4-bis(N,N-dimethylamino)naphthalene) to the pFA molecules. Absorption spectra as well as electron transfer trajectories associated with the different excited states were calculated using time dependent density functional theory methods. The results allow separation of the quantum entangled photosynthetic transitions within the same minimal protocell and with the neighboring minimal protocell. The transferred electron is used to cleave a "waste" organic molecule resulting in the formation of the desired product. A two variable, quantum entangled AND logic gate was proposed, consisting of two input photoactive sensitizer molecules and one output (pFA molecule). It is proposed that a similar process might be applied for the destruction of tumor cancer cells or to yield building blocks in artificial cells.

Artificial Light‐Harvesting System Based on Multifunctional Surface‐Cross‐Linked Micelles

In the natural photosynthetic centers of bacteria and plants, antenna chromophores absorb solar light and transfer the excitation energy to the reaction center by highly efficient singlet–singlet energy transfer. Spatial organization of individual chromophores is key to such efficiency: chromophores need to be separated enough to minimize selfquenching without sacrificing the dipole–dipole couplingmediated energy transfer. In addition to its important role in photosynthesis, efficient transfer of energy from multiple chromophores to a single acceptor is of potential significance to solar cells, photocatalysts, optical sensors and light-emitting devices. For these reasons, there has been a great deal of interest in mimicking the natural light-harvesting process. A variety of scaffolds have been used including dendrimers, organogels, porphyrin arrays/assemblies, biopolymer assemblies, and organic–inorganic hybrid materials. Although impressive results have been obtained with the above scaffolds, the multistep synthesis of the complex architectures hampers their scale up and widespread application. Nature relies on a combination of covalent and noncovalent interactions to create the photosynthetic centers. Covalent structures possess excellent stability and noncovalent self-assembled constructs provide order and synthetic efficiency. Herein, we report a biomimetic approach to construct artificial light-harvesting systems. We combined two self-assembling strategies and covalent fixation to prepare a highly efficient antenna system from readily available building blocks. The entire synthesis was achieved by a one-pot reaction, and the product precipitated spontaneously out of the reaction mixture at the end of the reaction. The synthesis of the light-harvesting system is shown in Scheme 1, and is based on the recently reported method to cross-link surfactant micelles. Our model antenna chromophore is 9,10-bis(4-methylphenyl)anthracene (DPA), a compound with high fluorescence quantum yield (90%). Eosin Y disodium salt (EY) is the energy acceptor. Cationic surfactant, 4-(dodecyloxy)benzyltripropargylammonium bromide (1), forms micelles at concentrations of above 0.14 mm in water. Because the surface of the micelle is covered with a dense layer of alkynyl groups, 1,4-diazidobutane-2,3-diol (2) could easily capture the micelle by 1,3dipolar cycloaddition with a Cu catalyst. When 1 and 2 were used in a 1:1 ratio, the resulting surface-cross-linked micelles (SCMs) are water-soluble nanoparticles with numerous alkynes on the surface. Surface functionalization occurred readily upon addition of a THF solution of DPA–N3 (obtained from commercially available DPA by partial bromination and azidation, see the Supporting Information). After 18 hours at room temperature, the DPA-functionalized SCMs (DPA– SCMs) precipitated spontaneously from the 2:1 THF/water mixture, apparently as a result of the increased hydrophobicity of the product. The IR spectrum of the DPA–SCMs showed nearly complete disappearance of the alkyne peaks in the starting SCMs (Figure S1, in the Supporting Information). DLS (dynamic light scattering) indicated an increase in size for the SCMs upon DPA-functionalization (Figure S2, in the Supporting Information). The absorption band of the DPA–SCMs is at 330–420 nm in THF and the emission band at 390–520 nm. These spectra match almost exactly with those of the free, monomeric DPA in solution (Figure S3, in the Supporting Information). Therefore, the DPA concentration ([DPA]SCMs) in this system can be determined from the absorption spectrum and the molar coefficient extinction of DPA. A frequent issue in light-harvesting systems with multiple donors is the self-quenching and/or excimer formation caused by the proximity of the chromophores. These pathways interfere with the energy transfer and lower the overall efficiency, and often require elaborate strategies to overcome. Excitingly, the fluorescence quantum yield was 0.80 and 0.90 for the micelle-bound DPA and the free chromophore, respectively (see the Supporting Information). Clearly, neither self-quenching nor excimer formation was significant in the highly crowded system. We suspect there are [*] H.-Q. Peng, Dr. Y.-Z. Chen, Prof. Q.-Z. Yang, Prof. L.-Z. Wu, Prof. C.-H. Tung, Prof. L.-P. Zhang Key Laboratory of Photochemical Conversion and Optoelectronic Materials, Technical Institute of Physics and Chemistry Chinese Academy of Sciences 29 Zhongguancun East Road, Beijing 100190 (China) E-mail: qzyang@mail.ipc.ac.cn H.-Q. Peng, Prof. Q.-X. Tong Department of Chemistry, Shantou University Shantou, Guangdong 515063 (China) E-mail: qxtong@stu.edu.cn

Time-Resolved Investigation of Excitation Energy Transfer in Carbon Nanotube–Porphyrin Compounds

Single-Wall Carbon Nanotubes (SWNTs) non-covalently functionalized with organic molecules have proven to be a very promising light-harvesting system. The goal is to combine the unique transport and optical properties of nanotubes with the wide tunability of organic molecules properties. Among those molecules, porphyrins, a precursor of photosynthetic centers, show a strong absorption in the visible that is already used in dye sensitized hybrid solar cells [1]. Porphyrins link to SWNTs through π-stacking interaction which preserves most of the intrinsic properties of the SWNTs, in contrast with covalent functionalization. Moreover, the coupling is still strong enough to induce new functionalities. It was recently shown that efficient excitation energy transfer (EET) can occur in such system [2,3]. The very high quantum yield reported in this system suggests that the EET mechanism must occur on an ultrafast time-scale [4]. In order to characterize the nature of the EET mechanism, we investigate its dynamics at a subpicosecond time scale, by means of pump-probe spectroscopy. We show that the ground state recovery time of porphyrin in the compound is reduced by several orders of magnitude compared to the case or free porphyrin (Fig. 1). Concomitantly, a strong bleaching signal is observed on the optical resonances of the nanotubes showing an ultrafast population build-up upon excitation of the porphyrin (Fig. 2). We conclude that the energy transfer occurs on a time scale shorter than 100fs and that the surrounding of SWNTs by porphyrins does not affect the relaxation dynamics.

Diverse Arrangement of Photosynthetic Gene Clusters in Aerobic Anoxygenic Phototrophic Bacteria

BackgroundAerobic anoxygenic photototrophic (AAP) bacteria represent an important group of marine microorganisms inhabiting the euphotic zone of the ocean. They harvest light using bacteriochlorophyll (BChl) a and are thought to be important players in carbon cycling in the ocean.Methodology/Principal FindingsAerobic anoxygenic phototrophic (AAP) bacteria represent an important part of marine microbial communities. Their photosynthetic apparatus is encoded by a number of genes organized in a so-called photosynthetic gene cluster (PGC). In this study, the organization of PGCs was analyzed in ten AAP species belonging to the orders Rhodobacterales, Sphingomonadales and the NOR5/OM60 clade. Sphingomonadales contained comparatively smaller PGCs with an approximately size of 39 kb whereas the average size of PGCs in Rhodobacterales and NOR5/OM60 clade was about 45 kb. The distribution of four arrangements, based on the permutation and combination of the two conserved regions bchFNBHLM-LhaA-puhABC and crtF-bchCXYZ, does not correspond to the phylogenetic affiliation of individual AAP bacterial species. While PGCs of all analyzed species contained the same set of genes for bacteriochlorophyll synthesis and assembly of photosynthetic centers, they differed largely in the carotenoid biosynthetic genes. Spheroidenone, spirilloxanthin, and zeaxanthin biosynthetic pathways were found in each clade respectively. All of the carotenoid biosynthetic genes were found in the PGCs of Rhodobacterales, however Sphingomonadales and NOR5/OM60 strains contained some of the carotenoid biosynthetic pathway genes outside of the PGC.Conclusions/SignificanceOur investigations shed light on the evolution and functional implications in PGCs of marine aerobic anoxygenic phototrophs, and support the notion that AAP are a heterogenous physiological group phylogenetically scattered among Proteobacteria.

Quantum processes in 8-Oxo-Guanine-Ru(bipyridine)32+ photosynthetic systems of artificial minimal cells

Abstract Density functional theory methods were used to investigate various self-assembled photoactive bioorganic systems of interest for artificial minimal cells. The cell systems studied are based on nucleotides or their compounds and consisted of up to 123 atoms (not including the associated water or methanol solvent shells) and are up to 2.5 nm in diameter. The electron correlation interactions responsible for the weak hydrogen and Van derWaals chemical bonds increase due to the addition of a polar solvent (water or methanol). The precursor fatty acid molecules of the system also play a critical role in the quantum mechanical interaction based self-assembly of the photosynthetic center and the functioning of the photosynthetic processes of the artificial minimal cells. The distances between the separated sensitizer, fatty acid precursor, and methanol molecules are comparable to Van derWaals and hydrogen bonding radii. As a result the associated electron correlation interactions compress the overall system, resulting in an even smaller gap between the highest occupied molecular orbital (HOMO), and lowest unoccupied molecular orbital (LUMO) electron energy levels and photoexcited electron tunnelling occurs from the sensitizer (either Ru(bpy)32+ or [Ru(bpy)2(4-Bu-4’-Me-2,2’-bpy)]2++ derivatives) to the precursor fatty acid molecules (notation used: Me = methyl; Bu = butyl; bpy = bipyridine). The shift of the absorption spectrum to the red for the artificial protocell photosynthetic centers might be considered as the measure of the complexity of these systems.

Ultrafast excitation transfer and charge stabilization in a newly assembled photosynthetic antenna-reaction center mimic composed of boron dipyrrin, zinc porphyrin and fullerene

A self-assembled supramolecular triad as a model to mimic the light-induced events of the photosynthetic antenna-reaction center, that is, ultrafast excitation transfer followed by electron transfer ultimately generating a long-lived charge-separated state, has been accomplished. Boron dipyrrin (BDP), zinc porphyrin (ZnP) and fullerene (C60), respectively, constitute the energy donor, electron donor and electron acceptor segments of the antenna-reaction center imitation. Unlike in the previous models, the BDP entity was placed between the electron donor, ZnP and electron acceptor, C60 entities. For the construction, benzo-18-crown-6 functionalized BDP was synthesized and subsequently reacted with 3,4-dihydroxyphenyl functionalized ZnP through the central boron atom to form the crown-BDP-ZnP dyad. Next, an alkyl ammonium functionalized fullerene was used to self-assemble the crown ether entity of the dyad via ion–dipole interactions. The newly formed supramolecular triad was fully characterized by spectroscopic, computational and electrochemical methods. Steady-state fluorescence and excitation studies revealed the occurrence of energy transfer upon selective excitation of the BDP in the dyad. Further studies involving the pump–probe technique revealed excitation transfer from the 1BDP* to ZnP to occur in about 7 ps, much faster than that reported for other systems in this series of triads, as a consequence of shorter distance between the entities. Upon forming the supramolecular triad by self-assembling fullerene, the 1ZnP* produced by direct excitation or by energy transfer mechanism resulted in an initial electron transfer to the BDP entity. The charge recombination resulted in the population of the triplet excited state of C60, from where additional electron transfer occurred to produce C60•−:crown-BDP-ZnP•+ ion pair as the final charge-separated species. Nanosecond transient absorption studies revealed the lifetime of the charge-separated state to be ∼100 μs, the longest ever reported for this type of antenna-reaction center mimics, indicating better charge stabilization as a result of the different disposition of the entities of the supramolecular triad.

Electron Affinities and Electronic Structures of o-, m-, and p-Hydroxyphenoxyl Radicals: A Combined Low-Temperature Photoelectron Spectroscopic and Ab Initio Calculation Study

Hydroxyl substituted phenoxides, o-, m-, p-HO(C(6)H(4))O(-), and the corresponding neutral radicals are important species; in particular, the p-isomer pair, i.e., p-HO(C(6)H(4))O(-) and p-HO(C(6)H(4))O*, is directly involved in the proton-coupled electron transfer in biological photosynthetic centers. Here we report the first spectroscopic study of these species in the gas phase by means of low-temperature photoelectron spectroscopy (PES) and ab initio calculations. Vibrationally resolved PES spectra were obtained at 70 K and at several photon energies for each anion, directly yielding electron affinity (EA) and electronic structure information for the corresponding hydroxyphenoxyl radical. The EAs are found to vary with OH positions, from 1.990 +/- 0.010 (p) to 2.315 +/- 0.010 (o) and 2.330 +/- 0.010 (m) eV. Theoretical calculations were carried out to identify the optimized molecular structures for both anions and neutral radicals. The electron binding energies and excited state energies were also calculated to compare with experimental data. Excellent agreement is found between calculations and experiments. Molecular orbital analyses indicate a strong OH antibonding interaction with the phenoxide moiety for the o- as well as the p-isomer, whereas such an interaction is largely missing for the m-anion. The variance of EAs among three isomers is interpreted primarily due to the interplay between two competing factors: the OH antibonding interaction and the H-bonding stabilization (existed only in the o-anion).

Rationally tuning the reduction potential of a single cupredoxin beyond the natural range

Redox processes are at the heart of numerous functions in chemistry and biology, from long-range electron transfer in photosynthesis and respiration to catalysis in industrial and fuel cell research. These functions are accomplished in nature by only a limited number of redox-active agents. A long-standing issue in these fields is how redox potentials are fine-tuned over a broad range with little change to the redox-active site or electron-transfer properties. Resolving this issue will not only advance our fundamental understanding of the roles of long-range, non-covalent interactions in redox processes, but also allow for design of redox-active proteins having tailor-made redox potentials for applications such as artificial photosynthetic centres or fuel cell catalysts for energy conversion. Here we show that two important secondary coordination sphere interactions, hydrophobicity and hydrogen-bonding, are capable of tuning the reduction potential of the cupredoxin azurin over a 700 mV range, surpassing the highest and lowest reduction potentials reported for any mononuclear cupredoxin, without perturbing the metal binding site beyond what is typical for the cupredoxin family of proteins. We also demonstrate that the effects of individual structural features are additive and that redox potential tuning of azurin is now predictable across the full range of cupredoxin potentials.

Synthesis at the interface of chemistry and biology.

As the focus of synthesis increasingly shifts from its historical emphasis on molecular structure to function, improved strategies are clearly required for the generation of molecules with defined physical, chemical, and biological properties. In contrast, living organisms are remarkably adept at producing molecules and molecular assemblies with an impressive array of functions - from enzymes and antibodies to the photosynthetic center. Thus, the marriage of Nature's synthetic strategies, molecules, and biosynthetic machinery with more traditional synthetic approaches might enable the generation of molecules with properties difficult to achieve by chemical strategies alone. Here we illustrate the potential of this approach and overview some opportunities and challenges in the coming years.

Chloroplast parameters differ in wild type and transgenic poplars overexpressing gsh1 in the cytosol

Poplar mutants overexpressing the bacterial genes gsh1 or gsh2 encoding the enzymes of glutathione biosynthesis are among the best-characterised transgenic plants. However, this characterisation originates exclusively from laboratory studies, and the performance of these mutants under field conditions is largely unknown. Here, we report a field experiment in which the wild-type poplar hybrid Populus tremula x P. alba and a transgenic line overexpressing the bacterial gene gsh1 encoding gamma-glutamylcysteine synthetase in the cytosol were grown for 3 years at a relatively clean (control) field site and a field site contaminated with heavy metals. Aboveground biomass accumulation was slightly smaller in transgenic compared to wild-type plants; soil contamination significantly decreased biomass accumulation in both wild-type and transgenic plants by more than 40%. Chloroplasts parameters, i.e., maximal diameter, projection area and perimeter, surface area and volume, surface/volume ratio and a two-dimensional form coefficient, were found to depend on plant type, leaf tissue and soil contamination. The greatest differences between wild and transgenic poplars were observed at the control site. Under these conditions, chloroplast sizes in palisade tissue of transgenic poplar significantly exceeded those of the wild type. In contrast to the wild type, palisade chloroplast volume exceeded that of spongy chloroplasts in transgenic poplars at both field sites. Chlorophyll content per chloroplast was the same in wild and transgenic poplars. Apparently, the increase in chloroplast volume was not connected to changes in the photosynthetic centres. Chloroplasts of transgenic poplar at the control site were more elongated in palisade cells and close to spherical in spongy mesophyll chloroplasts. At the contaminated site, palisade and spongy cell chloroplasts of leaves from transgenic trees and the wild type were the same shape. Transgenic poplars also had a smaller chloroplast surface/volume ratio, both at the control and the contaminated site. Chloroplast number per cell did not differ between wild and transgenic poplars at the control site. Soil contamination led to suppression of chloroplast replication in wild-type plants. From these results, we assume that overexpressing the bacterial gsh1 gene in the cytosol interacts with processes in the chloroplast and that sequestration of heavy metal phytochelatin complexes into the vacuole may partially counteract this interaction in plants grown at heavy metal-contaminated field sites. Further experiments are required to test these assumptions.

Synthesis of an amide-linked chlorin-anthraquinone dyad and its structural and spectroscopic properties

An enantiomerically pure chlorin-anthraquinone dyad 6 was synthesized in respect to a model compound for light-induced electron transfer in the photosynthetic center. Therefore, the enantiomerically pure chlorin 4 was linked with the anthraquinone amine 5 to form a peptide-like dyad. The conformation of the dyad follows NMR investigations and a PM3 calculation. Observed quenching of the fluorescence indicates an intramolecular energy and/or electron transfer from the chlorin donor moiety to the anthraquinone subunit of the dyad.

1P-230 光合成酸素発生反応におけるCP43-354位グルタミン酸のFTIRによる構造解析(光生物-光合成,第47回日本生物物理学会年会)

1P-230 光合成酸素発生反応におけるCP43-354位グルタミン酸のFTIRによる構造解析(光生物-光合成,第47回日本生物物理学会年会)

Quantum Mechanical Control of Artificial Minimal Living Cells

Artificial minimal living cells and their substructures are quantum mechanically self-assembling due to competition of weak electrostatic forces and weak attraction Van der Waals dispersion forces, and hydrogen bonds originated due to electron correlation interactions among biological and water molecules. The best available method to simulate in minimal cells these weak electrostatic and Van der Waals dispersion forces, and hydrogen bonds is to perform quantum mechanical non-local density functional potential calculations of artificial minimal living cells consisting of around 400 atoms. The cell systems studied are based on peptide nucleic acid and are 3.0 – 4.5 nm in diameter. The electron tunneling and associated light absorption of most intense transitions as calculated by the time dependent density functional theory method (TD DFT) differs from spectroscopic experiments by only 0.3 nm, which are within the value of experiment errors. This agreement implies that the quantum mechanically self-assembled structure of artificial minimal living cells very closely approximate the realistic ones. Analysis of TD DFT method calculated absorption spectrum and images of electron transfer trajectories in the different excited states allow to separate two different logically controlled functions of molecular device consisting of guaninecytosine-PNA-1,4-dihydroquinoxaline-1,4-bis(N,N-dimethylamino)naphthalene supermolecule and Van der Waals bonded precursor of fatty acid molecule. These two different logically controlled functions of artificial minimal living cells are: 1) initiation of metabolic fatty acid production in five excited states or 2) initiation of gene dehybridization in one excited state. This designed supermolecule works in the artificial minimal cell as molecular electronics classic OR logic function (Boolean logics OR gate). TD DFT were used to investigate various photoactive systems of artificial minimal cells based on Ru(bipyridine)32+ sensitizer. The electron correlation interactions associated with the weak Van der Waals and hydrogen bonds increase due to the addition of a polar solvent (water or methanol) shells. The addition of precursor fatty acid or cytosine molecules plays a role in the quantum mechanical interaction based self-assembly of the photosynthetic center and the functioning of the photosynthetic processes of the artificial minimal cells. As a result the associated nonlinear quantum interactions compress the overall system resulting in an even smaller gap between the HOMO and LUMO electron energy levels and photoexcited electron tunneling occurs from the sensitizer (covalently bonded 8-oxo-guanine molecule to the [Ru(bpy)2(4-Bu-4'-Me-2,2'-bpy)]2+) to pFA molecule (notation used: Me = methyl; Bu = butyl; bpy = bipyridine). The electron tunneling and associated light absorption for the most intense transitions as calculated by the TD DFT method are within the values of experiment errors.

Effect of different Fe(III) compounds on photosynthetic electron transport in spinach chloroplasts and on iron accumulation in maize plants

AbstractSynthesis and spectral characteristics of [Fe(nia)3Cl3] and [Fe(nia)3(H2O)2](ClO4)3 are described. The effect of these compounds as well as of FeCl3·6H2O on photosynthetic electron transport in spinach chloroplasts was investigated using EPR spectroscopy. It was found that due to the interaction of these compounds with tyrosine radicals situated at the 161st position in D1 (TyrZ) and D2 (TyrD) proteins located at the donor side of photosystem (PS) II, electron transport between the photosynthetic centres PS II and PS I was interrupted. In addition, the treatment with [Fe(nia)3(H2O)2](ClO4)3 resulted in a release of Mn(II) from the oxygen evolving complex situated on the donor side of PS II. Moreover, the effect of the Fe(III) compounds studied on some production characteristics of hydroponically cultivated maize plants and on Fe accumulation in plant organs was investigated. In general, the production characteristic most inhibited by the presence of Fe(III) compounds was the leaf dry mass and [Fe(nia)3(H2O)2](ClO4)3 was found to be the most effective compound. The highest Fe amount was accumulated in the roots, and the leaves treated with Fe(III) compounds contained more Fe than the stems. The treatment with FeCl3·6H2O caused the most effective translocation of Fe into the shoots. Comparing the effect of nicotinamide complexes, [Fe(nia)3(H2O)2](ClO4)3 was found to facilitate the translocation of Fe into the shoots more effectively than [Fe(nia)3Cl3]. This could be connected with the different structure of these complexes. [Fe(nia)3(H2O)2](ClO4)3 has ionic structure and, in addition, coordinated H2O molecules can be easily substituted by other ligands.

Singlet Oxygen Quenching by Phenylamides and their Parent Compounds

This paper demonstrates for the first time that plant metabolites of the phenylamide type, conjugates of putrescine with hydroxycinnamic acids (p-coumaric, caffeic and ferulic), possess 1O2 quenching properties. Data were obtained confirming that their acidic parent compounds were also able to quench 1O2, as did polyamines (putrescine, spermidine and spermine), and that this ability depends on the number of amino groups. Potentiation of the 1O2 quenching ability of the conjugates relative to both parent components was established. The importance of polyamines and phenylamides in the plant non-enzymatic antioxidant defence at sites of intensive 1O2 generation, such as the photosynthetic centers, was suggested.

Differential changes in photosynthetic capacity, 77 K chlorophyll fluorescence and chloroplast ultrastructure between Zn‐efficient and Zn‐inefficient rice genotypes (Oryza sativa) under low zinc stress

The relationship of zinc (Zn) efficiency in rice to differential tolerance of photosynthetic capacity and chloroplast function to low Zn stress was studied using Zn-efficient (IR8192) and Zn-inefficient (Erjiufeng) rice genotypes (Oryza sativa L.). Zinc deficiency caused extensive declines in leaf chlorophyll (Chl) content, ratios of chl a:b, Pn, Fv/Fm and Fv/Fo, indicating that the intrinsic quantum efficiency of the photosystem II (PSII) units was damaged. A greater decline was observed in the inefficient genotype (Erjiufeng) than the efficient genotype (IR8192). The 77 K chl fluorescence emission spectrum revealed that Zn deficiency blocked energy spillover from PSII to PSI and more excitation energy was distributed to PSII in IR8192 than Erjiufeng. The spectrum of Zn-deficient Erjiufeng was completely disordered, implying that the photosynthetic centers were seriously damaged. Electron microscopy showed that Zn deficiency caused a severe damage to the fine structure of chloroplasts, but IR8192 had a better preserved chloroplast ultrastructure as compared with Erjiufeng. These differences may result from the higher levels of the antioxidant enzyme activities and lower oxidant stress level in IR8192. These results indicate that Zn deficiency decreases leaf photosynthetic capacity primarily by reducing the number of PSII units per unit leaf area, and also reducing the photochemical capacity of the remaining PSII units. Therefore, the maintenance of more efficient photochemical capacity under low Zn stress is a key factor for the high Zn efficiency in rice, which may result from less antioxidant damage caused by low Zn to the chloroplast ultrastructure.

Intramolecular photoinduced processes of newly synthesized dual zinc porphyrin-fullerene triad with flexible linkers

Zinc porphyrin-fullerene-zinc porphyrin triad, in which two zinc porphyrin ( ZnP ) moieties and a fullerene ( C 60) moiety are linked by flexible bonds and which is intended to be a working model of the photosynthetic antenna-reaction centre, has been newly synthesized. Its photophysical properties have been investigated by both time-resolved emission and transient absorption techniques. Excitation of the zinc porphyrin moiety of the triad induced charge separation, generating the radical ion pair, in which the electron localizes on the C 60 moiety and the hole localizes on the zinc porphyrin moiety. In polar solvents, the charge-separated states decayed with lifetimes of 300-600 ns returning to the ground state. Compared with ZnP - C 60 dyad, ZnP - C 60- ZnP triad showed longer lifetimes for the radical ion pair due to the conformation of the two ZnP moieties. The effects of the coordinating reagents on the zinc atom have been studied, with the expectation of conformational change of the two ZnP moieties with respect to C 60.