Optimized spin crossings and transition states for short-range electron transfer in transition metal dimers

Electron-transfer reactions in eight mixed-valence manganese dimers are studied using B3LYP. One of the dimers is a model of the active site of manganese catalase, while another represents a basic building block of the oxygen-evolving complex in photosystem II. The adiabatic reactions are characterized by fully optimized transition states where the single imaginary frequency represents the electron-transfer coordinate. When there is antiferromagnetic coupling between different high-spin centers, electron transfer must be accompanied by a spin transition. Spin transitions are characterized by minimum-energy crossing points between spin surfaces. Three reaction mechanisms have been investigated. First, a single-step reaction where spin flip is concerted with electron transfer. Second, an initial transition to a center with intermediate spin that can be followed by electron transfer. Third, an initial transition to a ferromagnetic state from which the electron can be transferred adiabatically. The complexes prefer the third route with rate-determining barriers ranging from 5.7 kcal/mol to 17.2 kcal/mol for different complexes. The origins of these differences are discussed in terms of oxidation states and ligand environments. Many DFT functionals overestimate charge-transfer interactions, but for the present complexes, the error should be limited because of short Mn-Mn distances.     

Comparative study of the photochemistry of chloroplast membranes and photosystem II particles

A comparative study of the photoreducing potentials of spinach thylakoid membranes and spinach photosystem II particles has been made. Hexachloroplatinate ions have been used as electron acceptors in a Hill-like assay for oxygen evolution measurements with both thylakoid membranes and photosystem II particles. However, unlike other Hill acceptors, such as ferricyanide, hexachloroplatinate can be fully reduced to metallic platinum that is catalytically active for hydrogen evolution. This is experimentally confirmed in the ability of chloroplast membranes to photoprecipitate platinum and photoproduce molecular hydrogen. Although similar experiments with photosystem II particles resulted in hexachloroplatinate-supported oxygen evolution, hydrogen evolution was not observed. Moreover, photosystem II particles coupled to ferredoxin and hydrogenase resulted in neither hydrogen nor oxygen evolution—a distinct contrast to the results obtained with chloroplast membranes.

     

PHOTOSYSTEM II HETEROGENEITY IN THE MARINE DIATOM Phaeodactylum tricornutum

Abstract— The kinetics of photosystem II photochemistry are analyzed in the marine diatom Phaeodacfylum tricornutum by measurement of fluorescence induction in cell suspensions treated with 3–(3,4-dichlorophenyl)-1,1-dimethylurea. Photosystem II kinetics are found to be biphasic, the sum of two exponential components, suggesting that biphasic energy conversion in photosystem II may be a general consequence of thylakoid membrane appression. The emission wavelength-dependence of fluorescence induction suggests that the two photosystem II components have different variable fluorescence emission spectra. The slower component exhibits characteristic emission of the diatom light-harvesting complexes while emission from the faster component resembles that of the photosystem II reaction center. Variable fluorescence emission (293 K) at wavelengths > 700 nm is assigned to photosystem II. Application of model equations indicates that the two photosystem II unit types differ primarily in antenna size. A new analytical procedure is presented which eliminates ambiguities in the kinetic analysis associated with the incorrect assignment of the maximal fluorescence yield.     

Effects of light on the photosynthetic apparatus and a novel type of degradation of the photosystem I peripheral antenna complexes under darkness

A novel type of degradation of photosystem I peripheral antenna complexes has been observed in rice leaves under darkness in the present study. Photosynthesis, chlorophyll content, the chlorophyll a/b ratio, and relative amounts of ribulose-1,5-bisphosphate carboxylase/oxygenase decrease during dark treatment. The levels of photosystem II reaction-center complex and cytochrome f on the basis of units of chlorophyll also decline rapidly under darkness. In contrast, the levels of photosystem I reaction-center complex remain stable under darkness for six days. Low-temperature fluorescence emission spectra ascribed to photosystem I antennae clearly show a blue shift. A similar shift is also observed in the photosystem I complexes resolved with dodecyl maltoside-polyacrylamide gel electrophoresis. Moreover, polypeptide analysis of the thylakoids and photosystem I complexes isolated from the green gels shows that some polypeptides originating from photosystem I peripheral antenna complexes disappear during the dark treatment. A curve-fitting method also displays remarkable changes in the chlorophyll components between the light and dark treatments. It is likely that these results indicate the disconnection/disassembly of the photosystem I antenna as well as the photosystem II complexes induced by dark treatment. Moreover, these findings also imply the existence of different degradation mechanisms for the photosystem I and II complexes.     

A biomimetic approach to artificial photosynthesis: Ru(II)-polypyridine photo-sensitisers linked to tyrosine and manganese electron donors

The paper describes recent advances towards the construction of functional mimics of the oxygen evolving complex in photosystem II (PSII) that are coupled to photoinduced charge separation. Some key principles of PSII and artificial systems for light-induced charge accumulation are discussed. Systems are described where biomimetic electron donors--manganese complexes and tyrosine--have been linked to a Ru(II)-polypyridine photosensitiser. Oxidation of the donors by intramolecular electron transfer from the photo-oxidised Ru(III) complex has been studied using optical flash photolysis and EPR experiments. A step-wise electron transfer Mn(III,III)-->tyrosine Ru(III) has been demonstrated, in analogy to the reaction on the donor side of PSII. Electron transfer from the tyrosine to Ru(III) was coupled to tyrosine deprotonation. This resulted in a large reorganisation energy and thus a slow reaction rate, unless the tyrosine was hydrogen bonded or already deprotonated. A comparison with analogous reactions in PSII is made. Finally, light-induced oxidation of a manganese dimer linked to a Ru(II)-photosensitiser has been observed. Preliminary results suggest the possibility of photo-oxidising manganese dimers in several steps, which is an important advancement towards water oxidation.     

Changes in the energy distribution in mutant thylakoid membranes of pea with modified pigment content. II. Changes due to magnesium ions concentration

Low-temperature (77K) steady-state chlorophyll fluorescence emission spectra, room temperature fluorescence and light scattering of thylakoid membranes isolated from pea mutants were studied as a function of Mg2+ concentration. The mutants have modified pigment content and altered structural organization of the pigment-protein complexes, distinct surface electric properties and functions. The analysis of the 77K emission spectra revealed that Mg2+-depletion of the medium caused not only an increased energy flow toward photosystem I in all investigated membranes but also changes in the quenching of the fluorescence, most probably by internal conversion. The results indicated that the macroorganization of the photosynthetic apparatus of mutants at supramolecular level (distribution and segregation of two photosystems in thylakoid membranes) and at supermolecular level (stacking of photosystem II supercomplexes) required different Mg ion concentrations. The data confirmed that the segregation of photosystems and the stacking of thylakoid membranes are two distinct phenomena and elucidated some features of their mechanisms. The segregation is initiated by changes in the lateral microorganization of light harvesting complexes II, their migration (repulsion from photosystem I) and subsequent separation of the two photosystems. Most likely 3D aggregation and formation of macrodomains, containing only photosystem II antenna complexes, play a certain precursory role for the increasing degree of the membrane stacking and the energy coupling between the light harvesting complexes II and the core complexes of photosystem II in the frame of photosystem II supercomplexes.     

Synchronous cultures of the green alga Scenedesmus obliquus: Comparison of picosecond decay associated emission spectra and fluorescence induction kinetics

Decay-associated emission spectra of synchronized cultures of Scenedesmusobliquus have been studied at two stages of their life cycle corresponding to the maximum and minimum of photosynthetic capacity. These decay-associated spectra comprise three kinetic components. The two components which are assigned to photosystem II show variations in their relative amplitudes depending on the life cycle of the cells. From the correlations observed in the decay-associated fluorescence spectra on the one hand and the fluorescence induction parameters on the other hand we obtained further evidence that the two photosystem II fluorescence components are directly related to the two fluorescence induction phases. This correlation supports our previous assignment of the two photosystem II fluorescence decay components of about 0.3 ns and about 0.6 ns lifetimes at the F₀ level (open photosystem II reaction centres to photosystem II α units and photosystem II β units respectively. The most pronounced difference between cells at the 8th hour of the life cycle and those at the 16th hour consists in the size of the photosystem II β units which are about 30% larger for the latter. In agreement with previous studies it was found that at these two stages the photosystem I units do not differ in size.     

General synthesis of di-mu-oxo dimanganese complexes as functional models for the oxygen evolving complex of photosystem II

A series of complexes with the formula [Mn(III/IV)2(mu-O)2(L)2(X)2]3+ have been prepared in situ from Mn(II)LCl2 precursors by a general preparative method (L = terpy, Cl-terpy, Br-terpy, Ph-terpy, tolyl-terpy, mesityl-terpy, t Bu3-terpy, EtO-terpy, py-phen, dpya, Me2N-terpy, or HO-terpy, and X = a labile ligand such as water, chloride, or sulfate). The parent complex, where L = terpy and X = water, is a functional model for the oxygen-evolving complex of photosystem II (Limburg, et al. J. Am. Chem. Soc. 2001, 123, 423-430). Crystals of Mn(II)(dpya)Cl2, Mn(II)(Ph-terpy)Cl2, Mn(II)(mesityl-terpy)Cl2, and an organic-soluble di-mu-oxo di-aqua dimanganese complex, [Mn(III/)(IV)2(mu-O)2(mesityl-terpy)2(OH2)2](NO3)3, were obtained and characterized by X-ray crystallography. Solutions of the in situ-formed di-mu-oxo dimanganese complexes were characterized by electrospray mass spectrometry, EPR spectroscopy, and UV-visible spectroscopy, and the rates of catalytic oxygen-evolving activity were assayed. The use of Mn(II)LCl2 precursors leads to higher product purity of the Mn dimers while achieving the 1:1 ligand to Mn stoichiometry appropriate for catalytic activity assay. These methods can be used to screen the catalytic activity of other di-mu-oxo dimanganese complexes generated by using a ligand library.     

The structure of dimers and the nature of azulene chromaticity

The structure of supramolecular azulene dimers responsible for the blue coloration of its crystals and solutions has been discussed on the basis of result of optical spectroscopy and literature data. It is established that two types of these dimers (I and II) absorb the light in the red region of the visible (VIS) spectrum and differ by the mutual orientation of the molecules. Dimers I have a VIS band with a vibronic structure; the azulene molecules in dimers I match their own seven-membered rings in type (stacking structure), and five-membered rings of the molecules are separated so that the molecular C _2v axes form an obtuse angle. The spectra of dimers II do not have a vibronic structure in the VIS band. The dipole moments of the molecules in these dimers are oriented antiparallel (five-membered rings are located over (or under) seven-membered rings, forming a structure with a center of inversion). It is concluded that due to their structure, dimers I should have a certain dipole moment, while dimers II have no dipole moment.     

UVB effects on the photosystem II-D1 protein of phytoplankton and natural phytoplankton communities

The reaction center of photosystem II is susceptible to photodamage. In particular the D1 protein located in the photosystem II core has a rapid, light-dependent turnover termed the photosystem II repair cycle that, under illumination, degrades and resynthesizes D1 protein to limit accumulation of photodamaged photosystem II. Most studies concerning the effects of UVB (280-320 nm) on this cycle have been on cyanobacteria or specific phytoplankton species rather than on natural communities of phytoplankton. During a 5-year multidisciplinary project on the effects of UV radiation (200-400 nm) on natural systems, the effects of UVB on the D1 protein of natural phytoplankton communities were assessed. This review provides an overview of photoinhibitory effects of light on cultured and natural phytoplankton, with an emphasis on the interrelation of UVB exposure, D1 protein degradation and the repair of photosystem II through D1 resynthesis. Although the UVB component of the solar spectrum contributes to the primary photoinactivation of photosystem II, we conclude that, in natural communities, inhibition of the rate of the photosystem II repair cycle is a more important influence of UVB on primary productivity. Indeed, exposing tropical and temperate phytoplankton communities to supplemented UVB had more inhibitory effect on D1 synthesis than on the D1 degradation process itself. However, the rate of net D1 damage was faster for the tropical communities, likely because of the effects of high ambient light and water temperature on mechanisms of protein degradation and synthesis.     

Polymer-Modified Single-Walled Carbon Nanotubes Affect Photosystem II Photochemistry,Intersystem Electron Transport Carriers and Photosystem I End Acceptors in Pea Plants

Single-walled carbon nanotubes (SWCNT) have recently been attracting the attention of plant biologists as a prospective tool for modulation of photosynthesis in higher plants. However, the exact mode of action of SWCNT on the photosynthetic electron transport chain remains unknown. In this work, we examined the effect of foliar application of polymer-grafted SWCNT on the donor side of photosystem II, the intersystem electron transfer chain and the acceptor side of photosystem I. Analysis of the induction curves of chlorophyll fluorescence via JIP test and construction of differential curves revealed that SWCNT concentrations up to 100 mg/L did not affect the photosynthetic electron transport chain. SWCNT concentration of 300 mg/L had no effect on the photosystem II donor side but provoked inactivation of photosystem II reaction centres and slowed down the reduction of the plastoquinone pool and the photosystem I end acceptors. Changes in the modulated reflection at 820 nm, too, indicated slower re-reduction of photosystem I reaction centres in SWCNT-treated leaves. We conclude that SWCNT are likely to be able to divert electrons from the photosynthetic electron transport chain at the level of photosystem I end acceptors and plastoquinone pool in vivo. Further research is needed to unequivocally prove if the observed effects are due to specific interaction between SWCNT and the photosynthetic apparatus.     

Characterization of the Photosynthetic Activity of Platinized Chloroplasts: A Study Using Fluorescence and Photoacoustic Techniques

In this report, the effect of platinization on the photosynthetic activity of the chloroplast membranes is studied. Oxygen evolution, fluorescence emission and thermal de-activation processes are modified after platinization. It is shown that photosystem II activity is affected by the hydrogen purging involved in the platinization procedure as seen by the reduced rates of oxygen evolution and a decrease in variable fluorescence. Depletion of bicarbonate from photosystem II during purging is suggested to be responsible partly for the decreased electron transfer rates and for a lower half-saturation light intensity required for energy storage as measured by photoacoustic spectroscopy. On the other hand, the electron sink created by the reduction of hydrogen at the acceptor side of photosystem I is shown to reoxidize efficiently the plas-toquinone pool of photosystem II.     

Nitrogen‐Bridged Metallodiazaporphyrin Dimers: Synergistic Effects of Nitrogen Bridges and meso‐Nitrogen Atoms on Structure and Properties

NH‐bridged and pyrazine‐fused metallodiazaporphyrin dimers have been prepared from nickel(II) and copper(II) complexes of 3‐amino‐5,15‐diazaporphyrin by Pd‐catalyzed C−N cross‐coupling and oxidative dimerization reactions, respectively. The synergistic effects of the nitrogen bridges and meso ‐nitrogen atoms play major roles in enhancing the light‐harvesting properties and delocalization of an electron spin over the entire π‐skeletons of the metallodiazaporphyrin dimers.     

Functional Models for the Oxygen-Evolving Complex of Photosystem II

In the last ten years, a number of advances have been made in the study of the oxygen-evolving complex (OEC) of photosystem II (PSII). Along with this new understanding of the natural system has come rapid advance in chemical models of this system. The advance of PSII model chemistry is seen most strikingly in the area of functional models where the few known systems available when this topic was last reviewed has grown into two families of model systems. In concert with this work, numerous mechanistic proposals for photosynthetic water oxidation have been proposed. Here, we review the recent efforts in functional model chemistry of the oxygen-evolving complex of photosystem II.     

PROTEIN COMPOSITION OF SPINACH CHLOROPLASTS AND THEIR PHOTOSYSTEM I AND PHOTOSYSTEM II SUBFRAGMENTS*

Abstract— The proteins of spinach chloroplasts and their subfragments containing photosystem I and photosystem II, obtained by Triton X-100 treatment or French-pressure rupture, were separated by sodium dodecyl sulfate (SDS)-acrylamide electrophoresis at pH 7·0 in phosphate buffer. The individual protein bands were identified where possible by comparing them with known, isolated and characterized proteins from chloroplasts, and their molecular weights were determined. The protein composition of the chloroplast fragments were correlated to the functional properties of these fragments. Distinct patterns were obtained for photosystem I and photosystem II particles. The major protein of photosystem II is expressed in the 23 kilodalton range and photosystem I proteins seem to be clustered mainly in the 50–70 kilodalton range.     

Comparison of the action spectra and quantum requirements for photosynthesis and photo-reduction of Scenedesmus

Abstract -The action spectrum and the quantum requirement of photoreduction (P.R) by hydrogen-adapted Scenedesmus have been determined under conditions where photosystem II is inoperative. The action spectrum of P.R., when compared to that of photosynthesis, shows approximately a 20 nm shift in the long wavelength maxima and an apparent decreased utilization of light in the wavelength region 600–650 nm. The quantum requirement for P.R. is high at wavelengths below 650 nm and decreases to a minimum between 705–720 nm; the best value obtained was 10. These observations indicate that this process is exclusively a photosystem I reaction and furthermore offer additional evidence for a lack of appreciable transfer of excitation energy between the pigments of photosystems I and II. The relatively high requirement photophosphorylation or to the inadequacy of H₂-hydrogenase system as the electron donor mechanism for photosystem I.     

Assembling phthalocyanine dimers through a platinum(II) acetylide linker

Phthalocyanine (Pc) dimers connected through trans-platinum(II) diacetylide linkers have been prepared by reaction of the corresponding ethynylphthalocyanines with trans-bis(triethylphosphine)platinum(II) chloride. Special emphasis was placed on the analysis of the ground- and excited-state features of these compounds in relation to butadiyne-bridged Pc dimers and the corresponding monomers. Both Zn(II)-containing Pc dimers exhibit long-lived triplet excited states. The insertion of σ-bonded trans-platinum(II) diacetylide spacers decoupled the two Pc groups and led to an appreciable acceleration (by a factor of up to 10) of the radiative and nonradiative decay rate of the singlet and triplet excited states.     

ENERGY DISSIPATION AND PHOTOPROTECTION MECHANISMS DURING CHLOROPHYLL PHOTOBLEACHING IN THYLAKOID MEMBRANES

The kinetics of chlorophyll photobleaching were followed in whole thylakoid membranes as well as in photosystem I and photosystem II submembrane fractions. The onset of photobleaching was characterized by a slow rate which indicated the presence of energy traps implicated in the photoprotection of the bulk pigments. The pigments in photosystem I submembrane fractions bleached at a faster rate than those in photosystem II counterparts, the latter being more sensitive towards photoinhibition. An analysis of the pigment-protein complexes isolated from whole thylakoid membranes during the course of a photobleaching experiment has shown that the core-antenna complexes, including CP29, are more sensitive to illumination than the peripheral complexes. The absorption spectra of the CPI and CP29 complexes presented a blue shift of the red absorption maximum after partial photobleaching, indicative of a non-homogeneous bleaching of the holochromes in these complexes. An analysis of these data points towards the involvement of CP29 in a photoprotection mechanism at the level of photosystem II. The weaker resistance of photosystem I to photobleaching relative to photosystem II and its stronger resistance to photoinhibition is discussed in terms of an energy dissipation pathway in thylakoid membranes.     

Light emission originating from photosystem II radical pair recombination is sensitive to zeaxanthin related non-photochemical quenching (NPQ)

We have used chlorophyll fluorescence, delayed luminescence and thermoluminescence measurements to study the influence of an artificial DeltapH in the presence or absence of zeaxanthin on photosystem II reactions. Energization of the pea thylakoid membranes induced non-photochemical fluorescence quenching and an increase in the overall luminescence emission of PSII during delayed luminescence and thermoluminescence measurements. This DeltapH-induced overall luminescence increase was caused by a strongly enhanced delayed luminescence in the seconds range before sample heating. In the subsequent thermoluminescence measurements the intensity of the B-band decreased after one and increased after two or more single turnover flashes. We propose that strong membrane energization shifted the redox potential of photosystem II radical pairs to more negative values causing the high delayed luminescence. The zeaxanthin-dependent non-photochemical fluorescence quenching component, however, did not alter thermoluminescence B-bands but decreased the delayed luminescence intensity by 30%. To our knowledge this is the first report that the radiative radical pair recombination, exhibited as delayed luminescence but not thermoluminescence emission, is sensitive to the antenna located zeaxanthin related non-photochemical fluorescence quenching. Our data can be interpreted within the frame of the exciton/radical pair equilibrium model that describes photosystem II as a shallow trap and incorporates the transfer of energy from the re-excitated reaction centre to the antenna of photosystem II.     

Useful parameter describing magnetic interactions in extended bis‐bidentate bridged dimers

A useful parameter describing magnetic coupling interactions has been inspected in transition metal dimers, in which two transition metal ions are bridged by extended bis‐bidentate ligands. This parameter is the square of overlap integral between the spatial parts of magnetic orbitals in the broken symmetry state. The oxalato‐bridged Cu (II) dimers, in which the Cu (II) atom is either in tetrahedral or square pyramid coordination environment, have been calculated with the density functional theory coupling the broken symmetry approach. © 2003 Wiley Periodicals, Inc. Int J Quantum Chem, 2003