Aggregation-Driven Photoinduced α-C(sp3)-H Bond Hydroxylation/C(sp3)-C(sp3) Coupling of Boron Dipyrromethene Dye in Water Reported by Near-Infrared Emission.

Molecular aggregation is a powerful tool for tuning advanced materials' photophysical and electronic properties. Here we present a novel potential for the aqueous-solvated aggregated state of boron dipyrromethene (BODIPY) to facilitate phototransformations otherwise achievable only under harsh chemical conditions. We show that the photoinduced symmetry-breaking charge separation state can itself initiate catalyst-free redox chemistry, leading to selective α-C(sp3)-H bond activation/Csp3-Csp3 coupling on the BODIPY backbone. The photoproduction progress was tracked by monitoring the evolution of the strong Stokes-shifted near-infrared emission, resulting from selective self-assembly of the terminal heterodimeric photoproduct into well-ordered J-aggregates, as revealed by X-ray structural analysis. These findings provide a facile and green route to further explore the promising frontier of packing-triggered selective photoconversions via supramolecular engineering.

Copper Coordination and the Induced Morphological Changes in Covalent Organic Frameworks.

In this work, we reveal the coordination of copper ions absorbed by a series of covalent organic frameworks. The frameworks were synthesized through the nucleophilic substitution of either cyanuric chloride or phosphonitrilic chloride trimer by 4,4'-bipyridine, and they were utilized as absorbers for the removal of copper ions from aqueous solutions. The exfoliated counterpart of the layered network was compared to the bulk materials in terms of the copper retention capacity and efficiency. The ion absorption capacity of copper ranged from 100 to 290 mg/g depending on the morphology and chemical structure of the framework. As evidenced by the SEM and XRD analysis, the copper absorption induced certain morphological changes in the networks. EPR spectroscopy revealed the key finding of this study: the trigonal bipyramidal configuration of the copper ions in their divalent state, coordinated with the nitrogen of the core units, 4,4'-bipyridine, and chlorine ions. The analysis of the thoroughgoing experiments bridges the gap between coordination molecular chemistry and the field of covalent organic frameworks. EPR explores how the unique trigonal bipyramidal coordination could be suppressed in the end by the environment and, more specifically, by the addition of glycerol to the aqueous dispersions of the covalent organic frameworks.

Arrested Substrate Binding Resolves Catalytic Intermediates in Higher-Plant Water Oxidation.

Among the intermediate catalytic steps of the water-oxidizing Mn4 CaO5 cluster of photosystem II (PSII), the final metastable S3 state is critically important because it binds one substrate and precedes O2 evolution. Herein, we combine X- and Q-band EPR experiments on native and methanol-treated PSII of Spinacia oleracea and show that methanol-treated PSII preparations of the S3 state correspond to a previously uncharacterized high-spin (S=6) species. This is confirmed as a major component also in intact photosynthetic membranes, coexisting with the previously known intermediate-spin conformation (S=3). The high-spin intermediate is assigned to a water-unbound form, with a MnIV3 subunit interacting ferromagnetically via anisotropic exchange with a coordinatively unsaturated MnIV ion. These results resolve and define the structural heterogeneity of the S3 state, providing constraints on the S3 to S4 transition, on substrate identity and delivery pathways, and on the mechanism of O-O bond formation.

Energizing the light harvesting antenna: Insight from CP29.

How do plants cope with excess light energy? Crop health and stress tolerance are governed by molecular photoprotective mechanisms. Protective exciton quenching in plants is activated by membrane energization, via unclear conformational changes in proteins called antennas. Here we show that pH and salt gradients stimulate the response of such an antenna under low and high energization by all-atom Molecular Dynamics Simulations. Novel insight establishes that helix-5 (H5) conformation in CP29 from spinach is regulated by chemiosmotic factors. This is selectively correlated with the chl-614 macrocycle deformation and interactions with nearby pigments, that could suggest a role in plant photoprotection. Adding to the significance of our findings, H5 domain is conserved among five antennas (LHCB1-5). These results suggest that light harvesting complexes of Photosystem II, one of the most abundant proteins on earth, can sense chemiosmotic gradients via their H5 domains in an upgraded role from a solar detector to also a chemiosmotic sensor.

Theoretical study of the EPR spectrum of the S3TyrZ• metalloradical intermediate state of the O2-evolving complex of photosystem II.

The intermediates trapped during the transitions between the consecutive S-states of the oxygen-evolving complex (OEC) of photosystem II (PSII) contain the free radical TyrZ• interacting magnetically with the Mn-cluster (Mn4Ca). In this paper, we present a theoretical study of the EPR spectrum of the S3TyrZ• metalloradical intermediate state, which has been recently detected in MeOH-containing PSII preparations. For this analysis, we use two different approximations: the first, simpler one, is the point-dipole approach, where the two interacting spins are the S = 1/2 of TyrZ• and the ground spin state of S = 3 of the OEC being in the S3 state. The second approximation is based on previous proposals indicating that the ground spin state (S G = 3) of the S3 state arises from an antiferromagnetic exchange coupling between the S = 9/2 of the Mn(IV)3CaO4 and the S = 3/2 of the external Mn(IV) of the OEC. Under the above assumption, the second approximation involves three interacting spins, denoted S A(Mn(IV)3Ca) = 9/2, S B(Mn(IV)) = 3/2 and S C(TyrZ•) = 1/2. Accordingly, the tyrosine radical is exposed to dipolar interactions with both fragments of the OEC, while an antiferromagnetic exchange coupling within the "3 + 1" structural motif of the OEC is also considered. By application of the first-point-dipole approach, the inter-spin distance that simulates the experimental spectrum is not consistent with the theoretical models that were recently reported for the OEC in the S3 state. Instead, the recent models are consistent with the results of the analysis that is performed by using the second, more detailed, approach.

Could structural similarity of specific domains between animal globins and plant antenna proteins provide hints important for the photoprotection mechanism?

Non photochemical quenching is a fundamental mechanism in photosynthesis, which protects plants against excess excitation energy and is of crucial importance for their survival and fitness. In the last decades hundreds of papers have appeared that describe the role of antenna regulation in protection or the so called qE response. However, the exact quenching site is still obscure. Previously overlooked features of the antenna may provide hints towards the elucidation of its functionality and of the quenching mechanism. Recently it was demonstrated that the catalytic domain of human myoglobin that binds the pigment (i.e. heme) is similar in structure to the domain of the light harvesting complex II of pea that binds Chl a 614 (former known as b3). In addition, it is well accepted that conformational changes of the chlorophyll macrocycle result in reversible changes of fluorescence (the lowest fluorescence corresponds to non planar macrocycle). Here we put forward a hypothesis regarding the molecular mechanism that leads to the formation of a quenching center inside the antenna proteins. Our main suggestion is that a conformational change of helix H5 (known also as helix D) forces conformational changes in the macrocycle of Chl a 614 is implicated in the ΔA535 absorbance change and quenching during photoprotective qE. The specific features (some of them similar to those of heme domain of globins) of the b3 domain account for these traits. The model predicts that antenna proteins having b3 pigments (i.e. LHCII, CP29, CP26) can act as potential quenchers.

Polyamines in cell walls of chlorococcalean microalgae.

Biotechnology of microalgae represents a very attractive alternative as a source of energy and substances of high value when compared with plant cultivation. Cell walls of green microalgae have an extraordinary chemical and mechanical resistance and may impede some steps in the biotechnological/industrial exploitation of algae. The aim of the present contribution was to check the presence of polyamines in the cell walls of chlorococcalean green microalgae. Polyamines are nitrogenous compounds synthesized normally in cells and may affect the properties of the cell wall. Our work included strains either forming or not forming the polymer algaenan, allowing us to conclude that algaenan is not a prerequisite for the presence of polyamines in the cell walls. Polyamines were detected in isolated cell walls of Scenedesmus obliquus, Chlorella fusca, Chlorella saccharophila, and Chlorella vulgaris. Their concentration in isolated cell walls ranged between 0.4 and 8.4 nmol/mg dry weight.

Stress-related transcriptomic changes associated with GFP transgene expression and active transgene silencing in plants.

Plants respond to biotic and abiotic stress by activating and interacting with multiple defense pathways, allowing for an efficient global defense response. RNA silencing is a conserved mechanism of regulation of gene expression directed by small RNAs important in acquired plant immunity and especially virus and transgene repression. Several RNA silencing pathways in plants are crucial to control developmental processes and provide protection against abiotic and biotic stresses as well as invasive nucleic acids such as viruses and transposable elements. Various notable studies have shed light on the genes, small RNAs, and mechanisms involved in plant RNA silencing. However, published research on the potential interactions between RNA silencing and other plant stress responses is limited. In the present study, we tested the hypothesis that spreading and maintenance of systemic post-transcriptional gene silencing (PTGS) of a GFP transgene are associated with transcriptional changes that pertain to non-RNA silencing-based stress responses. To this end, we analyzed the structure and function of the photosynthetic apparatus and conducted whole transcriptome analysis in a transgenic line of Nicotiana benthamiana that spontaneously initiates transgene silencing, at different stages of systemic GFP-PTGS. In vivo analysis of chlorophyll a fluorescence yield and expression levels of key photosynthetic genes indicates that photosynthetic activity remains unaffected by systemic GFP-PTGS. However, transcriptomic analysis reveals that spreading and maintenance of GFP-PTGS are associated with transcriptional reprogramming of genes that are involved in abiotic stress responses and pattern- or effector-triggered immunity-based stress responses. These findings suggest that systemic PTGS may affect non-RNA-silencing-based defense pathways in N. benthamiana, providing new insights into the complex interplay between different plant stress responses.

Polyamines induce aggregation of LHC II and quenching of fluorescence in vitro.

Dissipation of excess excitation energy within the light-harvesting complex of Photosystem II (LHC II) is a main process in plants, which is measured as the non-photochemical quenching of chlorophyll fluorescence or qE. We showed in previous works that polyamines stimulate qE in higher plants in vivo and in eukaryotic algae in vitro. In the present contribution we have tested whether polyamines can stimulate quenching in trimeric LHC II and monomeric light-harvesting complex b proteins from higher plants. The tetramine spermine was the most potent quencher and induced aggregation of LHC II trimers, due to its highly cationic character. Two transients are evident at 100 µM and 350 µM for the fluorescence and absorbance signals of LHC II respectively. On the basis of observations within this work, some links between polyamines and the activation of qE in vivo is discussed.

Photocatalytic degradation of organic micropollutants under UV-A and visible light irradiation by exfoliated g-C3N4 catalysts.

In the present study, the photocatalytic performance of exfoliated graphitic carbon nitride (g-C3N4) catalysts, with enhanced properties and response in UV and visible light irradiation, was evaluated for the removal of selected contaminants i.e., diuron, bisphenol A and ethyl paraben. Commercial TiO2 Degussa P25 was also used as a reference photocatalyst. The g-C3N4 catalysts demonstrated good photocatalytic activity which in some cases is comparable to TiO2 Degussa P25 leading to high removal percentages of the studied micropollutants under UV-A light irradiation. In contrast to TiO2 Degussa P25, g-C3N4 catalysts were also able to degrade the studied micropollutants under visible light irradiation. For all the studied g-C3N4 catalysts under both UV-A and visible light irradiation, the overall degradation rate decreases in the order of bisphenol A > diuron > ethyl paraben. Among the studied g-C3N4, the chemically exfoliated catalyst (g-C3N4-CHEM) showed superior photocatalytic activity under UV-A light irradiation due to its enhanced characteristics, such as pore volume and specific surface area and ~ 82.0 % in 6 min, ~75.7 % in 15 min and ~ 96.3 % in 40 min removals were achieved for BPA, DIU and EP, respectively. Under visible light irradiation, the thermally exfoliated catalyst (g-C3N4-THERM) demonstrated the best photocatalytic performance and the degradation ranged from ~29.5 to 59.4 % after 120 min. EPR data revealed that the three g-C3N4 semiconductors generate mainly O2•-, whereas TiO2 Degussa P25 generates both HO• and O2•-, the latter only under UV-A light irradiation. Nevertheless, the indirect formation of HO• in the case of g-C3N4 should also be considered. Hydroxylation, oxidation, dealkylation, dechlorination and ring opening were the main degradation pathways. The process proceeded without significant alterations in toxicity levels. Based on the results, heterogeneous photocatalysis using g-C3N4 catalysts is a promising method for the removal of organic micropollutants without the formation of harmful transformation products.

Synthesis, characterization and photocatalytic evaluation of visible light activated C-doped TiO2 nanoparticles.

We have demonstrated heterogeneous photocatalytic degradation of microcystin-LR (MC-LR) by visible light activated carbon doped TiO(2) (C-TiO(2)) nanoparticles, synthesized by a modified sol-gel route based on the self-assembly technique exploiting oleic acid as a pore directing agent and carbon source. The C-TiO(2) nanoparticles crystallize in anatase phase despite the low calcination temperature of 350 °C and exhibit a highly porous structure that can be optimized by tuning the concentration of the oleic acid surfactant. The carbon modified nanomaterials exhibited enhanced absorption in the broad visible light region together with an apparent red shift in the optical absorption edge by 0.5 eV (2.69 eV), compared to the 3.18 eV of reference anatase TiO(2). Carbon species were identified by x-ray photoelectron spectroscopy analysis through the formation of both Ti-C and C-O bonds, indicative of substitution of carbon for oxygen atoms and the formation of carbonates, respectively. Electron paramagnetic resonance spectroscopy revealed the formation of two carbon related paramagnetic centers in C-TiO(2), whose intensity was markedly enhanced under visible light illumination, pointing to the formation of localized states within the anatase band gap, following carbon doping. The photocatalytic activity of C-TiO(2) nanomaterials was evaluated for the degradation of MC-LR at pH 3.0 under visible light (λ > 420 nm) irradiation. The doped materials showed a higher MC-LR degradation rate than reference TiO(2), behavior that is attributed to the incorporation of carbon into the titania lattice.

A nonsymmetric Dy2 single-molecule magnet with two relaxation processes triggered by an external magnetic field: a theoretical and integrated EPR study of the role of magnetic-site dilution.

The 1 : 2 reaction between Dy(O2CMe)3·4H2O and 1-acetyl-2-napthol (LH) in MeOH has provided access to the complex [Dy2L6(MeOH)]·MeOH (1·MeOH) in a good yield. The structures of the isomorphous complex 1·MeOH and its doped diamagnetic yttrium analogue [Dy0.14Y1.86L6(MeOH)]·MeOH (Dy@Y2) have been determined by single-crystal X-ray crystallography and characterized based on elemental analyses, IR spectra, and powder X-ray patterns. Combined dc and ac magnetic susceptibility and the magnetization data for 1 suggest that this complex shows slow magnetic relaxation. Under a 0 Oe dc field, a single relaxation mechanism is seen while two magnetization relaxation processes are evident under a 1500 G external magnetic field. The fit to the Arrhenius law has been performed using the 1.8-10 K ac data, affording an effective barrier for the magnetization reversal of 13 K and 7 K under the external dc field. Theoretical studies have been performed using ab initio and density functional methodologies to understand the electronic structure and the magnetic relaxation dynamics resulting from the single DyIII ion as well as from the dinuclear exchange-coupled states. Rich powder EPR spectra at the X-band and Q-band were obtained from Dy@Y2, as well as from the 1·MeOH dimer, while simulation studies revealed the ferromagnetic nature of the interaction between the DyIII ions in accordance with theoretical studies.

Conversion of the g=4.1 EPR signal to the multiline conformation during the S(2) to S(3) transition of the oxygen evolving complex of Photosystem II.

The oxygen evolving complex of Photosystem II undergoes four light-induced oxidation transitions, S(0)-S(1),...,S(3)-(S(4))S(0) during its catalytic cycle. The oxidizing equivalents are stored at a (Mn)(4)Ca cluster, the site of water oxidation. EPR spectroscopy has yielded valuable information on the S states. S(2) shows a notable heterogeneity with two spectral forms; a g=2 (S=1/2) multiline, and a g=4.1 (S=5/2) signal. These oscillate in parallel during the period-four cycle. Cyanobacteria show only the multiline signal, but upon advancement to S(3) they exhibit the same characteristic g=10 (S=3) absorption with plant preparations, implying that this latter signal results from the multiline configuration. The fate of the g=4.1 conformation during advancement to S(3) is accordingly unknown. We searched for light-induced transient changes in the EPR spectra at temperatures below and above the half-inhibition temperature for the S(2) to S(3) transition (ca 230K). We observed that, above about 220K the g=4.1 signal converts to a multiline form prior to advancement to S(3). We cannot exclude that the conversion results from visible-light excitation of the Mn cluster itself. The fact however, that the conversion coincides with the onset of the S(2) to S(3) transition, suggests that it is triggered by the charge-separation process, possibly the oxidation of tyr Z and the accompanying proton relocations. It therefore appears that a configuration of (Mn)(4)Ca with a low-spin ground state advances to S(3).

Polyamines stimulate non-photochemical quenching of chlorophyll a fluorescence in Scenedesmus obliquus.

Polyamines (PAs) are small metabolites that are produced and oxidized in chloroplasts with an obscure mode of action. Recently, we showed that qE is stimulated by PAs in higher plants (Nicotiana tabacum) and in genetically modified plants with elevated thylakoid-associated PAs (Ioannidis and Kotzabasis Biochim Biophys Acta 1767:1371-1382, 2007; Ioannidis et al. Biochim Biophys Acta 1787:1215-1222, 2009). Here, we investigated further their quenching properties both in vivo in green algae and in vitro is isolated LHCII. In vivo spermine up-regulates NPQ in Scenedesums obliquus about 30%. In vitro putrescine--the obligatory metabolic precursor of PAs--has a marginal quenching effect, while spermidine and spermine exhibit strong quenching abilities in isolated LHCII up to 40%. Based on available 3D models of LHCII we report a special cavity of about 600 Å(3) and a near-by larger pocket in the trimeric LHCII that could be of importance for the stimulation of qE by amines.

Remodeling of tobacco thylakoids by over-expression of maize plastidial transglutaminase.

Transglutaminases (TGases, EC 2.3.2.13) are intra- and extra-cellular enzymes that catalyze post-translational modification of proteins by establishing epsilon-(gamma-glutamyl) links and covalent conjugation of polyamines. In chloroplast it is well established that TGases specifically polyaminylate the light-harvesting antenna of Photosystem (PS) II (LHCII, CP29, CP26, CP24) and therefore a role in photosynthesis has been hypothesised (Della Mea et al. [23] and refs therein). However, the role of TGases in chloroplast is not yet fully understood. Here we report the effect of the over-expression of maize (Zea mays) chloroplast TGase in tobacco (Nicotiana tabacum var. Petit Havana) chloroplasts. The transglutaminase activity in over-expressers was increased 4 times in comparison to the wild-type tobacco plants, which in turn increased the thylakoid associated polyamines about 90%. Functional comparison between Wt tobacco and tgz over-expressers is shown in terms of fast fluorescence induction kinetics, non-photochemical quenching of the singlet excited state of chlorophyll a and antenna heterogeneity of PSII. Both in vivo probing and electron microscopy studies verified thylakoid remodeling. PSII antenna heterogeneity in vivo changes in the over-expressers to a great extent, with an increase of the centers located in grana-appressed regions (PSIIalpha) at the expense of centers located mainly in stroma thylakoids (PSIIbeta). A major increase in the granum size (i.e. increase of the number of stacked layers) with a concomitant decrease of stroma thylakoids is reported for the TGase over-expressers.

Kinetic and mechanistic investigation of water taste and odor compound 2-isopropyl-3-methoxy pyrazine degradation using UV-A/Chlorine process.

The present study was launched as a continuation of global efforts to tackle problems associated with two important aesthetic characteristics, taste and odor (T&O), of drinking water. The UV-A/Chlorine process, a promising advanced oxidation process (AOP), was evaluated for the first time for the removal of 2-isopropyl-3-methoxy pyrazine (IPMP), a widely reported compound in the literature that causes unpleasant taste and odor when present in water at or below the ng L-1 level. It was found that the studied process was efficient for the removal of IPMP in both ultrapure and drinking water. The initial chlorine dosage influenced significantly the degradation efficiency under initial neutral pH values. Degradation efficiency of IPMP was slightly inhibited by using drinking water as matrix. Scavenging experiments highlighted the significant role of various reactive species (e.g. HO, ClO, Cl, Cl2-) generated during the process that have not been studied comprehensively until now. In addition, the significant role of HO was further verified by Electron paramagnetic resonance spectroscopy (EPR) experiments. Overall, the formation of diverse radicals during the UV-A/Chlorine treatment enhanced the degradation of IPMP, promoting mainly the formation of hydroxy, hydroperoxy and dealkylated derivatives. In contrast, chlorinated by-products were only identified in traces.

Proton Translocation via Tautomerization of Asn298 During the S2-S3 State Transition in the Oxygen-Evolving Complex of Photosystem II.

In biological water oxidation, a redox-active tyrosine residue (D1-Tyr161 or YZ) mediates electron transfer between the Mn4CaO5 cluster of the oxygen-evolving complex and the charge-separation site of photosystem II (PSII), driving the cluster through progressively higher oxidation states S i ( i = 0-4). In contrast to lower S-states (S0, S1), in higher S-states (S2, S3) of the Mn4CaO5 cluster, YZ cannot be oxidized at cryogenic temperatures due to the accumulation of positive charge in the S1 → S2 transition. However, oxidation of YZ by illumination of S2 at 77-190 K followed by rapid freezing and charge recombination between YZ• and the plastoquinone radical QA•- allows trapping of an S2 variant, the so-called S2trapped state (S2t), that is capable of forming YZ• at cryogenic temperature. To identify the differences between the S2 and S2t states, we used the S2tYZ• intermediate as a probe for the S2t state and followed the S2tYZ•/QA•- recombination kinetics at 10 K using time-resolved electron paramagnetic resonance spectroscopy in H2O and D2O. The results show that while S2tYZ•/QA•- recombination can be described as pure electron transfer occurring in the Marcus inverted region, the S2t → S2 reversion depends on proton rearrangement and exhibits a strong kinetic isotope effect. This suggests that YZ oxidation in the S2t state is facilitated by favorable proton redistribution in the vicinity of YZ, most likely within the hydrogen-bonded YZ-His190-Asn298 triad. Computational models show that tautomerization of Asn298 to its imidic acid form enables proton translocation to an adjacent asparagine-rich cavity of water molecules that functions as a proton reservoir and can further participate in proton egress to the lumen.

Synthesis, characterization and antimicrobial activity of N-acetyl-3-acetyl-5-benzylidene tetramic acid-metal complexes. X-ray analysis and identification of the Cd(II) complex as a potent antifungal agent.

This study aims at the further expansion of knowledge on the antimicrobial activities of the tetramic acid moiety and the effect of metal complexation. Complexes of the N-acetyl-3-acetyl-5-benzylidenetetramic acid with Mn, Co, Ni, Cu, Zn and Cd were synthesized and screened against 5 key ESKAPE pathogens (Escherichia coli, methicillin-resistant Staphylococcus aureus (MRSA), Klebsiella pneumoniae, Acinetobacter baumannii and Pseudomonas aeruginosa) and 2 fungi (Cryptococcus neoformans and Candida albicans). The cadmium complex was found to effectively inhibit the fungus Cryptococcus neoformans with minimum inhibitory concentration (MIC) of 8 µg/mL, with no human cell toxicity and hemolytic activity within the tested concentration range. The biologically active tetramic acid­cadmium complex was structurally characterized by single-crystal X-ray analysis. Furthermore, the thermal stability of the ligand and the complexes was investigated along with NMR and EPR studies of the Cd(II) and Co(II) complexes respectively.

The EPR spectrum of tyrosine Z* and its decay kinetics in O2-evolving photosystem II preparations.

The O2-evolving complex of photosystem II, Mn 4Ca, cycles through five oxidation states, S0,..., S4, during its catalytic function, which involves the gradual abstraction of four electrons and four protons from two bound water molecules. The direct oxidant of the complex is the tyrosine neutral radical, YZ(*), which is transiently produced by the highly oxidizing power of the photoexcited chlorophyll species P680. EPR characterization of YZ(*) has been limited, until recently, to inhibited (non-oxygen-evolving) preparations. A number of relatively recent papers have demonstrated the trapping of YZ(*) in O2-evolving preparations at liquid helium temperatures as an intermediate of the S0 to S1, S1 to S2, and S2 to S3 transitions. The respective EPR spectra are broadened and split at g approximately 2 by the magnetic interaction with the Mn cluster, but this interaction collapses at temperatures higher than about 100K [Zahariou et al. (2007) Biochemistry 46, 14335 -14341]. We have conducted a study of the Tyr Z(*) transient in the temperature range 77-240 K by employing rapid or slow EPR scans. The results reveal for the first time high-resolution X-band spectra of Tyr Z(*) in the functional system and at temperatures close to the onset of the S-state transitions. We have simulated the S 2Y Z(*) spectrum using the simulation algorithm of Svistunenko and Cooper [(2004) Biophys. J. 87, 582 -595]. The small g(x) = 2.00689 value inferred from the analysis suggests either a H-bonding of Tyr Z (*) (presumably with His190) that is stronger than what has been assumed from studies of Tyr D(*) or Tyr Z(*) in Mn-depleted preparations or a more electropositive environment around Tyr Z(*). The study has also yielded for the first time direct information on the temperature variation of the YZ(*)/QA(-) recombination reaction in the various S states. The reaction follows biphasic kinetics with the slow phase dominating at low temperatures and the fast phase dominating at high temperatures. It is tentatively proposed that the slow phase represents the action of the YZ(*)/YZ(-) redox couple while the fast phase represents that of the YZ(*)/YZH couple; it is inferred that Tyr Z at elevated temperatures is protonated at rest. It is also proposed that YZ(*)/YZH is the couple that oxidizes the Mn cluster during the S1-S2 and S2-S3 transitions. A simple mechanism ensuring a rapid (concerted) protonation of Tyr Z upon oxidation of the Mn cluster is discussed, and also, a structure-based molecular model suggesting the participation of His190 into two hydrogen bonds is proposed.

Effects of polyamines on the functionality of photosynthetic membrane in vivo and in vitro.

The three major polyamines are normally found in chloroplasts of higher plants and are implicated in plant growth and stress response. We have recently shown that putrescine can increase light energy utilization through stimulation of photophosphorylation [Ioannidis et al., (2006) BBA-Bioenergetics, 1757, 821-828]. We are now to compare the role of the three major polyamines in terms of chloroplast bioenergetics. There is a different mode of action between the diamine putrescine and the higher polyamines (spermidine and spermine). Putrescine is an efficient stimulator of ATP synthesis, better than spermidine and spermine in terms of maximal % stimulation. On the other hand, spermidine and spermine are efficient stimulators of non-photochemical quenching. Spermidine and spermine at high concentrations are efficient uncouplers of photophosphorylation. In addition, the higher the polycationic character of the amine being used, the higher was the effectiveness in PSII efficiency restoration, as well as stacking of low salt thylakoids. Spermine with 50 microM increase F(V) as efficiently as 100 microM of spermidine or 1000 microM of putrescine or 1000 microM of Mg(2+). It is also demonstrated that the increase in F(V) derives mainly from the contribution of PSIIalpha centers. These results underline the importance of chloroplastic polyamines in the functionality of the photosynthetic membrane.