Quenching of chlorophyll fluorescence by substituted anthraquinones

Investigation of quinone reduction by microalgae using fluorescence - do "lake" and "puddle" mechanisms matter?

Photosynthesis is a fundamental process used by Nature to convert solar energy into chemical energy. For the last twenty years, many solutions have been explored to provide electrical power from the photosynthetic chain. In this context, the coupling between microalgae and exogenous quinones is an encouraging strategy because of the capability of quinones to be reduced by the photosynthetic chain. The ability of a quinone to be a good or bad electron acceptor can be evaluated by fluorescence measurements. Fluorescence analyses are thus a convenient tool helping to define a diverting parameter for some quinones. However, this parameter is implicitly designed on the basis of a particular light capture mechanism by algae. In this paper, we propose to revisit previous fluorescence experimental data by considering the two possible mechanisms (lake vs. puddle) and discussing their implication on the conclusions of the analysis. In particular, we show that the maximum extraction efficiency depends on the mechanism (in the case of 2,6-dichlorobenzoquinone - 2,6-DCBQ, (0.45 ± 0.02) vs (0.61 ± 0.03) for lake and puddle mechanisms respectively) but that the trends for different quinones remain correlated to the redox potentials independently of the mechanism.

Phenazines as model low-midpoint potential electron shuttles for photosynthetic bioelectrochemical systems

Bioelectrochemical approaches for energy conversion rely on efficient wiring of natural electron transport chains to electrodes. However, state-of-the-art exogenous electron mediators give rise to significant energy losses and, in the case of living systems, long-term cytotoxicity. Here, we explored new selection criteria for exogenous electron mediation by examining phenazines as novel low-midpoint potential molecules for wiring the photosynthetic electron transport chain of the cyanobacterium Synechocystis sp. PCC 6803 to electrodes. We identified pyocyanin (PYO) as an effective cell-permeable phenazine that can harvest electrons from highly reducing points of photosynthesis. PYO-mediated photocurrents were observed to be 4-fold higher than mediator-free systems with an energetic gain of 200 mV compared to the common high-midpoint potential mediator 2,6-dichloro-1,4-benzoquinone (DCBQ). The low-midpoint potential of PYO led to O2 reduction side-reactions, which competed significantly against photocurrent generation; the tuning of mediator concentration was important for outcompeting the side-reactions whilst avoiding acute cytotoxicity. DCBQ-mediated photocurrents were generally much higher but also decayed rapidly and were non-recoverable with fresh mediator addition. This suggests that the cells can acquire DCBQ-resistance over time. In contrast, PYO gave rise to steadier current enhancement despite the co-generation of undesirable reactive oxygen species, and PYO-exposed cells did not develop acquired resistance. Moreover, we demonstrated that the cyanobacteria can be genetically engineered to produce PYO endogenously to improve long-term prospects. Overall, this study established that energetic gains can be achieved via the use of low-potential phenazines in photosynthetic bioelectrochemical systems, and quantifies the factors and trade-offs that determine efficacious mediation in living bioelectrochemical systems.

Mediator-Microorganism Interaction in Microbial Solar Cell: a Fluo-Electrochemical Insight

Microbial solar cells that mainly rely on the use of photosynthesic organisms are a promising alternative to photovoltaics for solar electricity production. In that way, we propose a new approach involving electrochemistry and fluorescence techniques. The coupled setup Electro-Pulse-Amplitude-Modulation ("e-PAM") enables the simultaneous recording of the produced photocurrent and fluorescence signals from the photosynthetic chain. This methodology was validated with a suspension of green alga Chlamydomonas reinhardtii in interaction with an exogenous redox mediator (2,6-dichlorobenzoquinone; DCBQ). The balance between photosynthetic chain events (PSII photochemical yield, quenching) and the extracted electricity can be monitored overtime. More particularly, the nonphotochemical quenching induced by DCBQ mirrors the photocurrent. This setup thus helps to distinguish the electron harvesting from some side effects due to quinones in real time. It therefore paves the way for future analyses devoted to the choice of the experimental conditions (redox mediator, photosynthetic organisms, and so on) to find the best electron extraction.

Investigation of photocurrents resulting from a living unicellular algae suspension with quinones over time

Plants, algae, and some bacteria convert solar energy into chemical energy by using photosynthesis. In light of the current energy environment, many research strategies try to benefit from photosynthesis in order to generate usable photobioelectricity. Among all the strategies developed for transferring electrons from the photosynthetic chain to an outer collecting electrode, we recently implemented a method on a preparative scale (high surface electrode) based on a Chlamydomonas reinhardtii green algae suspension in the presence of exogenous quinones as redox mediators. While giving rise to an interesting performance (10-60 μA cm-2) in the course of one hour, this device appears to cause a slow decrease of the recorded photocurrent. In this paper, we wish to analyze and understand this gradual fall in performance in order to limit this issue in future applications. We thus first show that this kind of degradation could be related to over-irradiation conditions or side-effects of quinones depending on experimental conditions. We therefore built an empirical model involving a kinetic quenching induced by incubation with quinones, which is globally consistent with the experimental data provided by fluorescence measurements achieved after dark incubation of algae in the presence of quinones.

The artificial humic substance HS1500 does not inhibit photosynthesis of the green alga Desmodesmus armatus in vivo but interacts with the photosynthetic apparatus of isolated spinach thylakoids in vitro

Humic substances (HSs) can influence the growth and composition of freshwater phytoplankton assemblage. Since HSs contain many phenolic and quinonic moieties and cause growth reductions in eco-physiological field experiments, HSs are considered photosystem II herbicides. To test this specific mode of action in vivo and in vitro, respectively, we used intact cells of the green alga Desmodesmus armatus, as well as thylakoids isolated from spinach (Spinacia oleracea) as a model system for the green algal chloroplast. Photosynthetic electron transport was measured as oxygen evolution and variable chlorophyll fluorescence. The in vivo effect of the artificial humic substance HS1500 on algae consisted of no impact on photosynthesis-irradiance curves of intact green algae compared to untreated controls. In contrast, addition of HS1500 to isolated thylakoids resulted in light-induced oxygen consumption (Mehler reaction) as an in vitro effect. Fluorescence induction kinetics of HS-treated thylakoids revealed a large static quenching effect of HS1500, but no inhibitory effect on electron transport. For the case of intact algal cells, we conclude that the highly hydrophilic and rather large molecules of HS1500 are not taken up in effective quantities and, therefore, cannot interfere with photosynthesis. The in vitro tests show that HS1500 has no inhibitory effect on photosystem II but operates as a weak, oxygen-consuming Hill acceptor at photosystem I. Hence, the results indicate that eco-physiological field experiments should focus more strongly on effects of HSs on extracellular features, such as reducing and red-shifting the underwater light field or influencing nutrient availability by cation exchange within the plankton network.

Spectral dependence of irreversible light-induced fluorescence quenching: Chlorophyll forms with maximal emission at 700-702 and 705-710nm as spectroscopic markers of conformational changes in the core complex

The spectral dependence of the irreversible non-photochemical fluorescence quenching associated with photoinhibition in vitro has been comparatively investigated in thylakoid membranes, PSII enriched particles and PSII core complexes isolated from spinach. The analysis of the fluorescence emission spectra of dark-adapted and quenched samples as a function of the detection temperature in the 280-80K interval, indicates that Chlorophyll spectral forms having maximal emission in the 700-702nm and 705-710nm ranges gain relative intensity in concomitance with the establishment of irreversible light-induced quenching, acting thereby as spectroscopic markers. The relative enhancement of the 700-702nm and 705-710nm forms emission could be due either to an increase of their stoichiometric abundance or to their intrinsically low fluorescence quantum yields. These two factors, that can also coexist, need to be promoted by light-induced alterations in chromophore-protein as well as chromophore-chromophore interactions. The bands centred at about 701 and 706nm are also observed in the PSII core complex, suggesting their, at least partial, localisation in proximity to the reaction centre, and the occurrence of light-induced conformational changes in the core subunits.

Evaluation of photosynthetic electrons derivation by exogenous redox mediators

Oxygenic photosynthesis is the complex process that occurs in plants or algae by which the energy from the sun is converted into an electrochemical potential that drives the assimilation of carbon dioxide and the synthesis of carbohydrates. Quinones belong to a family of species commonly found in key processes of the Living, like photosynthesis or respiration, in which they act as electron transporters. This makes this class of molecules a popular candidate for biofuel cell and bioenergy applications insofar as they can be used as cargo to ship electrons to an electrode immersed in the cellular suspension. Nevertheless, such electron carriers are mostly selected empirically. This is why we report on a method involving fluorescence measurements to estimate the ability of seven different quinones to accept photosynthetic electrons downstream of photosystem II, the first protein complex in the light-dependent reactions of oxygenic photosynthesis. To this aim we use a mutant of Chlamydomonas reinhardtii, a unicellular green alga, impaired in electron downstream of photosystem II and assess the ability of quinones to restore electron flow by fluorescence. In this work, we defined and extracted a "derivation parameter" D that indicates the derivation efficiency of the exogenous quinones investigated. D then allows electing 2,6-dichlorobenzoquinone, 2,5-dichlorobenzoquinone and p-phenylbenzoquinone as good candidates. More particularly, our investigations suggested that other key parameters like the partition of quinones between different cellular compartments and their propensity to saturate these various compartments should also be taken into account in the process of selecting exogenous quinones for the purpose of deriving photoelectrons from intact algae.

Reconstituted CP29: multicomponent fluorescence decay from an optically homogeneous sample

The multiexponential fluorescence decay of the CP29 complex in which the apoprotein and pigments were reconstituted in vitro was examined. Of the three decay components observed only the two dominant ones, with about 3 and 5 ns lifetimes, were studied. The main question addressed was whether the multicomponent decay was associated with sample optical heterogeneity. To this end, we examined the optical absorption and fluorescence of the CP29 sample by means of two different and independent experimental strategies. This approach was used as the wavelength positions of the absorption/fluorescence spectral forms has recently been shown to be a sensitive indicator of the binding site-induced porphyrin ring deformation (Zucchelli et al. Biophys J 93:2240-2254, 2007) and hence of apoprotein conformational changes. The data indicate that this CP29 sample is optically homogeneous. It is hypothesised that the different lifetimes are explained in terms of multiple detergent/CP29 interactions leading to different quenching states, a suggestion that allows for optical homogeneity.

Limited sensitivity of pigment photo-oxidation in isolated thylakoids to singlet excited state quenching in photosystem II antenna

Light-induced pigment oxidation and its relation to excited state quenching in photosystems antennae have been investigated in isolated thylakoids. The results indicate that (i) chlorophyll oxidation takes place in two sequential steps. A slow initial phase is followed by a steep increase in the bleaching rate when more than one quarter of the chromophores are oxidised. (ii) During the initial slow phase, the carotenoid pool is bleached with an apparent rate which is about three times faster than that found for chlorophyll a and more than six times faster than that of chlorophyll b. (iii) Pigment bleaching has been observed both in photosystem I and photosystem II, and it has been possible to estimate a similar carotenoid bleaching rate in the two photosystems. (iv) The protection conferred by singlet state quenchers in the initial slow phase of pigment oxidation is modest. Taking into consideration that both the photosystems are subjected to the oxidative treatment, a somewhat larger protective effect than those estimated for photo-inhibition in thylakoids [S. Santabarbara, F.M. Garlaschi, G. Zucchelli, R.C. Jennings, Biochim. Biophys. Acta 1409 (1999) 165-170] can be computed, although it is less than 50% of the expected level on the basis of the observed reciprocity to the number of incident photons. (v) Pigment oxidation is associated with the loss of membrane ultra-structure, which is interpreted as originating from a decrease in grana stacking. The dynamics of loss of membrane ultra-structure parallel the phases observed for chlorophyll photo-bleaching.

The size of the population of weakly coupled chlorophyll pigments involved in thylakoid photoinhibition determined by steady-state fluorescence spectroscopy

On the basis of experiments with singlet quenchers and in agreement with previous data, it is suggested that a population of energetically weakly coupled chlorophylls may play a central role in photoinhibition in vivo and in vitro. In the present study, we have used steady state fluorescence techniques to gain direct evidence for these uncoupled chlorophylls. Due to the presence of their emission maxima, near 650 nm and more prominently in the 670--675 nm interval both chlorophylls b and a seem to be involved. A straightforward mathematical model is developed to describe the data which allows us to conclude that the uncoupled/weakly coupled population size is in the range of 1--3 molecules per photosystem.

Role of LHCII organization in the interaction of substituted 1,4-anthraquinones with thylakoid membranes

The chlorophyll fluorescence, photochemical activity and surface electric properties of thylakoid membranes with different stoichiometry of pigment-protein complexes and organization of the light-harvesting chlorophyll a/b protein complex of photosystem II (LHCII) were studied in the presence of substituted 1,4-anthraquinones. Data show strong dependence of the quenching of the chlorophyll fluorescence on the structural organization of LHCII. The increase of the LHCII oligomerization, which is associated with significant reduction of the transmembrane electric charge asymmetry and electric polarizability of the membrane, correlates with enhanced quenching effect of substituted 1,4-athraquinones. Crucial for the large quinone-induced changes in the membrane electric dipole moments is the structure of the quinone molecule. The strongest reduction in the values of the dipole moments is observed after interaction of thylakoids with 3-chloro-9-hydroxy-1,4-anthraquinone (TF33) which has the highest quenching efficiency. The quinone induced changes in the photochemical activity of photosystem II (PSII) correlate with the total amount of the supramolecular LHCII-PSII complex and depend on the number of substituents in the 1,4-anthraquinone molecule.

Quenching of excited states of chlorophyll molecules in submembrane fractions of Photosystem I by exogenous quinones

The ability of three substituted quinones, 2,5-dibromo-3-methyl-6-isopropyl-p-benzoquinone (DBMIB), 2,6-dichloro-p-benzoquinone (DCBQ), and tetramethyl-p-benzoquinone (duriquinone) to quench the excited states of chlorophyll (Chl) molecules in Photosystem I (PSI) was studied. Chl fluorescence emission measured with isolated PSI submembrane fractions was reduced following the addition of exogenous quinones. This quenching progressively increased with rising concentrations of the exogenous quinones according to the Stern-Volmer law. The values of Stern-Volmer quenching coefficients were found to be 3.28 x 10(5) M(-1) (DBMIB), 1.31 x 10(4) M(-1) (DCBQ), and 3.7 x 10(3) M(-1) (duroquinone). The relative quenching capacities of the various exogenous quinones in PSI thus strictly coincided to those found for the quenching of Fo level of Chl fluorescence in isolated thylakoids, which is emitted largely by Photosystem II (PSII) [Biochim. Biophys. Acta (2003) 1604, 115-123]. Quenching of Chl excited states in PSI submembrane fractions by exogenous quinones slowed down the rate of P700, primary electron donor of PSI, photooxidation measured at limiting actinic light irradiances thus revealing a reduced photochemical capacity of absorbed quanta. The possible involvement of non-photochemical quenching of excited Chl states by oxidized phylloquinones, electron acceptors of PSI, and oxidized plastoquinones, mobile electron carriers between PSII and the cytochrome b(6)/f complex, into the control of photochemical activity of PSI is discussed.

Interaction of exogenous quinones with membranes of higher plant chloroplasts: modulation of quinone capacities as photochemical and non-photochemical quenchers of energy in Photosystem II during light-dark transitions

Light modulation of the ability of three artificial quinones, 2,5-dibromo-3-methyl-6-isopropyl-p-benzoquinone (DBMIB), 2,6-dichloro-p-benzoquinone (DCBQ), and tetramethyl-p-benzoquinone (duroquinone), to quench chlorophyll (Chl) fluorescence photochemically or non-photochemically was studied to simulate the functions of endogenous plastoquinones during the thermal phase of fast Chl fluorescence induction kinetics. DBMIB was found to suppress by severalfold the basal level of Chl fluorescence (F(o)) and to markedly retard the light-induced rise of variable fluorescence (F(v)). After irradiation with actinic light, Chl fluorescence rapidly dropped down to the level corresponding to F(o) level in untreated thylakoids and then slowly declined to the initial level. DBMIB was found to be an efficient photochemical quencher of energy in Photosystem II (PSII) in the dark, but not after prolonged irradiation. Those events were owing to DBMIB reduction under light and its oxidation in the dark. At high concentrations, DCBQ exhibited quenching behaviours similar to those of DBMIB. In contrast, duroquinone demonstrated the ability to quench F(v) at low concentration, while F(o) was declined only at high concentrations of this artificial quinone. Unlike for DBMIB and DCBQ, quenched F(o) level was attained rapidly after actinic light had been turned off in the presence of high duroquinone concentrations. That finding evidenced that the capacity of duroquinone to non-photochemically quench excitation energy in PSII was maintained during irradiation, which is likely owing to the rapid electron transfer from duroquinol to Photosystem I (PSI). It was suggested that DBMIB and DCBQ at high concentration, on the one hand, and duroquinone, on the other hand, mimic the properties of plastoquinones as photochemical and non-photochemical quenchers of energy in PSII under different conditions. The first model corresponds to the conditions under which the plastoquinone pool can be largely reduced (weak electron release from PSII to PSI compared to PSII-driven electron flow from water under strong light and weak PSI photochemical capacity because of inactive electron transport on its reducing side), while the second one mimics the behaviour of the plastoquinone pool when it cannot be filled up with electrons (weak or moderate light and high photochemical competence of PSI).

Influence of substituted 1,4-anthraquinones on the chlorophyll fluorescence and photochemical activity of pea thylakoid membranes

The effect of substituted 1,4-anthraquinones on the photochemical activity and chlorophyll fluorescence of thylakoid membranes was examined. Both the fluorescence and the photochemical activity depend on the 1,4-anthraquinone substituent. Stronger quinone-induced quenching of the chlorophyll fluorescence than quinone-induced changes in the activity of photosystem II is observed. The type (Cl or Br) and the position (Cl) of the chalogen atom strongly influence the degree of inhibition of PSII electron transport and the quenching of chlorophyll fluorescence. The data suggest that the quenching of chlorophyll fluorescence is due rather to the interaction of the 1,4-anthraquinones and chlorophyll molecules than to an indirect effect caused by stimulation of the photochemistry.

Synthetic 1,4-anthracenediones, which block nucleoside transport and induce DNA fragmentation, retain their cytotoxic efficacy in daunorubicin-resistant HL-60 cell lines

Anthracene-1,4-dione and 6,7-dichloro-1,4-anthracenedione (code names AQ1 and AQ4, respectively) are cytostatic (IC50: 53 and 110 nM, respectively) and cytotoxic (IC50: 100 and 175 nM, respectively) in wild-type drug-sensitive HL-60-S tumor cells at day 4 in vitro. Therefore, the antitumor effects of these drugs were assessed and compared to those of daunorubicin (DAU) in HL-60-RV and HL-60-R8 tumor cells, which are, respectively, P-glycoprotein-positive and -negative multidrug-resistant (MDR) sublines. In contrast to DAU, which loses its cytostatic [resistance factors (RFs): 30.3-31.8] and cytotoxic (RFs: 48.8-58.1) activities in MDR sublines, AQ1 inhibits cell proliferation (RFs: 0.9-1.3) and cell viability (RFs: 1.4-1.6) as effectively in HL-60-RV and HL-60-R8 as in HL-60-S cells. Similarly, DAU decreases the rate of DNA synthesis less effectively in MDR sublines (RFs: 8.0-13.3) but AQ1 inhibits the incorporation of [3H]thymidine into DNA to the same degree in HL-60-S as in HL-60-RV and HL-60-R8 cells (RFs: 0.9-1.1). In contrast to DAU, which is ineffective, the advantage of AQ1 is its ability to block the cellular transport of purine and pyrimidine nucleosides in HL-60-S cells, an effect which persists in the MDR sublines (RFs: 1.1). AQ4, which mimics to a lesser degree all the antitumor effects of AQ1, except the inhibition of adenosine transport, also retains its effectiveness in MDR sublines (RFs: 1.1-3.1). The peaks of DNA cleavage caused by DAU and AQ1 in HL-60-S cells shift to lower concentrations with increasing times of drug exposure but DAU loses most of its ability to induce DNA fragmentation in MDR sublines, whereas the levels of AQ1-induced DNA cleavage at 16 and 24 h are nearly equivalent in HL-60-S, HL-60-RV and HL-60-R8 cells. Because they not only mimic the antitumor effects of DAU in the nM range but also block nucleoside transport and remain effective in tumor cells that have developed different mechanisms of MDR, AQ1 and AQ4 analogs might be valuable to develop new means of polychemotherapy.

Quinone isomers of the WS-5995 antibiotics: synthetic antitumor agents that inhibit macromolecule synthesis, block nucleoside transport, induce DNA fragmentation, and decrease the growth and viability of L1210 leukemic cells more effectively than ellagic acid and genistein in vitro

Antibiotic WS-5995A (code name J4) and two of its synthetic analogs, o-quinone J1 and model p-quinone J7, which show some structural similarity with both ellagic acid (EA) and genistein (GEN), were compared for their antileukemic activity in L1210 cells in vitro. Overall, J4 is more cytostatic and cytotoxic than J1 and J7, suggesting that methyl and methoxy substitutions, a p-quinone moiety, and a hydrogen bonding phenolic group may enhance the antitumor potential of these naphthoquinone lactones, which are all more potent than EA and GEN. For instance, the lead compound J4 inhibits tumor cell proliferation and viability at day 4 (IC(50): 0.24--0.65 microM) more effectively than EA (IC(50): 5--6 microM) and GEN (IC(50): 7 microM). Since J4 does not increase but rather decreases the mitotic index of L1210 cells at 24 h, it is not an antitubulin drug but might arrest early stages of cell cycle progression like EA and GEN. A 1.5- to 3-h pretreatment with J4 is sufficient to inhibit the rates of DNA, RNA and protein syntheses (IC(50): 2.0--2.5 microM) determined over 30- to 60-min periods of pulse-labeling in L1210 cells in vitro, whereas EA (IC(50): 20-130 microM) and GEN (IC(50): 40--115 microM) are less effective against macromolecule synthesis. In contrast to 156 microM EA, which is inactive, a 15-min pretreatment with 10--25 microM J4 has the advantage of also inhibiting the cellular transport of both purine and pyrimidine nucleosides over a 30 s period in vitro, an effect which can be mimicked by 156 microM GEN. Hence, the WS-5995 analogs and GEN may prevent the incorporation of [(3)H]adenosine and [(3)H]thymidine into DNA because they rapidly block the uptake of these nucleosides by the tumor cells. After 24 h, the concentration-dependent induction of DNA cleavage by J4 peaks at 10 microM and declines at 25 microM, whereas EA and GEN are ineffective at 10 microM but maximally stimulate DNA cleavage at 62.5 microM. Like EA and GEN, the mechanism by which J4 induces DNA fragmentation is inhibited by actinomycin D, cycloheximide, benzyloxycarbonyl-Val-Ala-Asp-fluoromethyl ketone, N-tosyl-L-phenylalanine chloromethyl ketone and ZnSO(4), suggesting that J4 triggers apoptosis by caspase and endonuclease activation. Because they are more potent than EA and GEN, and affect both nucleoside transport and DNA cleavage, the WS-5995 antitumor antibiotics might be valuable in polychemotherapy to potentiate the action of antimetabolites and sensitize multidrug-resistant tumor cells.

1,4-Anthraquinone: an anticancer drug that blocks nucleoside transport, inhibits macromolecule synthesis, induces DNA fragmentation, and decreases the growth and viability of L1210 leukemic cells in the same nanomolar range as daunorubicin in vitro

1,4-Anthraquinone (AQ) was synthesized and shown to prevent L1210 leukemic cells from synthesizing macromolecules and growing in vitro. In contrast, its dihydroxy-9,10anthraquinone precursor, quinizarin, was inactive. The antitumor activity of AQ was compared to that of daunorubicin (DAU), which is structurally different from AQ but also contains a quinone moiety. AQ is equipotent to DAU against L1210 tumor cell proliferation (IC50: 25 nM at day 2 and 9 nM at day 4) and viability (IC50: 100 nM at day 2 and 25 nM at day 4), suggesting that its cytostatic and cytotoxic activities are a combination of drug concentration and duration of drug exposure. Since AQ does not increase but rather decreases the mitotic index of L1210 cells at 24 h, it is not an antitubulin drug but might arrest early stages of cell cycle progression. Like DAU, a 1.5-3 h pretreatment with AQ is sufficient to inhibit the rates of DNA, RNA and protein syntheses (IC50: 2 microM) determined over 30-60 min periods of pulse-labeling in L1210 cells in vitro. In contrast to DAU, which is inactive, a 15 min pretreatment with AQ has the advantage of also inhibiting the cellular transport of both purine and pyrimidine nucleosides (IC50: 2.5 microM) over a 30 s period in vitro. Hence, AQ may prevent the incorporation [3H]thymidine into DNA because it rapidly blocks the uptake of these nucleosides by the tumor cells. After 24 h, AQ induces as much DNA cleavage as camptothecin and DAU, two anticancer drugs producing DNA strand breaks and known to, respectively, inhibit topoisomerase I and II activities. However, the concentration-dependent induction of DNA cleavage by AQ, which peaks at 1.6-4 microM and disappears at 10-25 microM, resembles that of DAU. The mechanism by which AQ induces DNA cleavage is inhibited by actinomycin D, cycloheximide and aurintricarboxylic acid, suggesting that AQ activates endonucleases and triggers apoptosis. The abilities of AQ to block nucleoside transport, inhibit DNA synthesis and induce DNA fragmentation are irreversible upon drug removal, suggesting that this compound may rapidly interact with various molecular targets in cell membranes and nuclei to disrupt the functions of nucleoside transporters and nucleic acids, and trigger long-lasting antitumor effects which persist after cessation of drug treatment. Because of its potency and dual effects on nucleoside transport and DNA cleavage, the use of bifunctional AQ with antileukemic activity in the nM range in vitro might provide a considerable advantage in polychemotherapy to potentiate the action of antimetabolites and sensitize multidrug-resistant tumor cells.

Exogenous quinones inhibit photosynthetic electron transfer in Chloroflexus aurantiacus by specific quenching of the excited bacteriochlorophyll c antenna

In the photosynthetic green filamentous bacterium Chloroflexus aurantiacus, excitation energy is transferred from a large bacteriochlorophyll (BChl) c antenna via smaller BChl a antennas to the reaction center. The effects of substituted 1,4-naphthoquinones on BChl c and BChl a fluorescence and on flash-induced cytochrome c oxidation were studied in whole cells under aerobic conditions. BChl c fluorescence in a cell suspension with 5.4 microM BChl c was quenched to 50% by addition of 0.6 microM shikonin ((R)-2-(1-hydroxy-4-methyl-3-pentenyl)-5,8-dihydroxy-1, 4-naphthoquinone), 0.9 microM 5-hydroxy-1,4-naphthoquinone, or 4 microM 2-acetyl-3-methyl-1,4-naphthoquinone. Between 25 and 100 times higher quinone concentrations were needed to quench BChl a fluorescence to a similar extent. These quinones also efficiently inhibited flash-induced cytochrome c oxidation when BChl c was excited, but not when BChl a was excited. The quenching of BChl c fluorescence induced by these quinones correlated with the inhibition of flash-induced cytochrome c oxidation. We concluded that the quinones inhibited electron transfer in the reaction center by specifically quenching the excitation energy in the BChl c antenna. Our results provide a model system for studying the redox-dependent antenna quenching in green sulfur bacteria because the antennas in these bacteria inherently exhibit a sensitivity to O(2) similar to the quinone-supplemented cells of Cfx. aurantiacus.

The effect of excited state population in photosystem II on the photoinhibition-induced changes in chlorophyll fluorescence parameters

The photoinhibition-induced changes in Photosystem II fluorescence parameters of spinach thylakoids were only slightly sensitive to the excited state population in Photosystem II antenna, as modulated by either quinone quenching or energy spillover. The possibility that this may be due to a small fraction of chlorophyll molecules which are poorly coupled to the antenna is discussed.

Non-photochemical quenching of chlorophyll fluorescence in photosynthesis. 5-hydroxy-1,4-naphthoquinone in spinach thylakoids as a model for antenna based quenching mechanisms

In vivo mechanisms of non-photochemical quenching that contribute to energy dissipation in higher plants are still a source of some controversy. In the present study we used an exogenous oxidized quinone, 5-hydroxy-1,4-naphthoquinone to induce quenching of chlorophyll excited states in photosynthetic light-harvesting antenna and to elucidate the mechanism of non-photochemical quenching of chlorophyll fluorescence by this quinone. Excitation dynamics in isolated spinach thylakoids in the presence of an exogenous fluorescence quencher was studied by a combined analysis of data gathered from independent techniques (fluorescence yields, effective absorption cross-sections and picosecond kinetics). The application of a kinetic model for photosystem II to a combined data set of fluorescence decay kinetics and absorbance cross-section measurements was used to quantify antenna quenching by a model antenna quencher, 5-hydroxy-1,4-naphthoquinone. We observed depressions in F0 and photosystem II absorption cross-sections, paralleled with an increase of the rate constant for excitation decay in antenna. This approach is a first step towards quantifying the amount of antenna quenching contributing to non-photochemical quenching in vivo, evaluation of the contributions of antenna and reaction centre mechanisms to it and localization of the sites of non-photochemical energy dissipation in intact plant systems. Copyright 1998 Elsevier Science B.V.

Alternative measures of photosystem II electron transfer inhibition in anthraquinone-treated chloroplasts

We have previously used chlorophyll fluorescence measurements at Fmax conditions (i.e. with Photosystem II electron acceptor QA reduced) to monitor the action of 9,10-anthraquinones on photosynthetic electron transport in plant chloroplasts. The present investigation employs two additional techniques to characterize the extent of electron transport inhibition induced by the addition of substituted anthraquinones to the suspending medium of spinach chloroplasts. Results are presented for spectrophotometric assays of the rate of electron transfer to an exogenous electron acceptor, 2,6-dichloroindophenol (DCIP) and for electrochemical determinations of the rate of oxygen evolution in anthraquinone-treated chloroplasts. In general, amino-substituted anthraquinones are ineffective inhibitors, maintaining electron transfer rates to DCIP at levels ranging from 50 to 90% of normal rates and yielding rates of O2 evolution averaging at 70% of the rate in untreated chloroplasts. In contrast, hydroxy-substituted anthraquinones efficiently block Photosystem II electron transport, resulting in low rates of DCIP photoreduction ranging from 0 to 20% of normal values and reducing O2 evolution rates to an average of 30% of the rate observed for untreated chloroplasts. Relative rates of DCIP photoreduction for anthraquinone-treated chloroplasts show a strong linear correlation with the reported relative Fmax chlorophyll fluorescence intensities. Relative O2 evolution rates are observed to correlate with the Stern-Volmer fluorescence quenching parameter Ksv. We suggest that slight differences in the extent of inhibitory activity of an anthraquinone as measured by the three techniques are consistent with certain known Photosystem II heterogeneities. The similarities in relative rankings of inhibitory effects for the 9, 10-anthraquinones, however, demonstrate that the three techniques employed (measurements of Fmax chlorophyll fluorescence, DCIP photoreduction rates, and O2 evolution rates) are alternative assays of anthraquinone-induced Photosystem II electron transport inhibition.

Chlorophyll fluorescence measurements to assess the competition of substituted anthraquinones for the QB binding site

As analogs of the Photosystem II plastoquinone electron acceptor, QB, substituted quinones compete with QB for a common binding domain and thereby inhibit QB function. Substituted quinones interact with the QB binding niche via hydrogen bonds, and the extent of hydrogen bond formation is determined by quinone structure. We have previously shown that the quinone inhibitory activity can be quantitated using measurements of chlorophyll fluorescence quenching. To assess competition for the QB binding site, we report here measurements of the action of various pairs of substituted anthraquinones on the chlorophyll fluorescence emission of barley chloroplasts. The degree of competition between quinones for the QB binding site is classified as competition, partial competition, or no competition. Two quinones were classified as undergoing competition, i.e., interacting for the same or overlapping sites, if the chlorophyll fluorescence level in the presence of the two quinones was not as low as that achieved in the presence of either one of the quinones individually. Non-competitive quinones with different binding sites quenched chlorophyll fluorescence to the level expected if the quenching effects of the individual quinones were additive. Partial competition, or some interaction for the same or overlapping sites, was characterized by an extent of fluorescence quenching in the presence of two quinones that was more effective than either quinone alone but not as sizable as that expected when the two quinones act independently. These results reflect an interesting situation whereby substitution patterns can alter the binding characteristics within a single class of inhibitors. In an accompanying manuscript we report the results of CNDO molecular orbital calculations to demonstrate that the pi charge distribution in substituted quinones governs their binding properties.

Analysis of pi charge distribution in substituted anthraquinones to assess affinity for the QB binding site

In the accompanying paper (Biochim. Biophys. Acta (1990) 1020, 163-168), we have determined the degree of competition between substituted 9,10-anthraquinones for the QB binding niche through measurements of the additivity of quinone-quenching effects on chlorophyll fluorescence. Quinones inhibit QB function by competitively displacing QB through hydrogen-bond formation with the QB binding protein. The sign of the net pi-charge density on atoms adjacent to the carbonyl moieties is believed to determine the particular hydrogen-bond(s) that result(s). In this study we report CNDO molecular orbital calculations of pi electronic charge distribution in substituted 9,10-anthraquinones to explore the relationship of inhibitor activity and competition to sign of net pi-charge density. We find that the substitution patterns of 9,10-anthraquinones alter the signs of the net pi-charge densities on the carbon atoms adjacent to the carbonyl moieties and thus determine the binding properties of the anthraquinones in the QB niche. While most experimentally studied 9,10-anthraquinones use both carbonyl oxygens to hydrogen bond to the histidine-215 and serine-264 regions of the D-1 QB binding protein, some quinones appear to hydrogen-bond to only one site. Thus, 9,10-anthraquinones constitute a class of QB inhibitors that function as either members of the histidine or serine family of QB inhibitors or as simultaneous representatives of both inhibitor groups.

Determination of four apparent mercury interaction sites in photosystem II by using a new modification of the Stern-Volmer analysis

We used the Stern-Volmer method to analyze the mercury fluorescence quenching effect in the green alga Dunaliella tertiolecta. To this end, we introduced a new modification of the Stern-Volmer equation on the basis of the Lineweaver-Burk analysis used to characterize allosteric enzyme activity. This modification was useful to determine the Stern-Volmer constant, the parameter indicating the fraction of PSII fluorescence susceptible to the mercury quenching effect (Fs), and to estimate the apparent number of mercury binding sites (Napp = 3.72) on PSII which affect the variable fluorescence. This value of Napp indicates the possibility of four mercury binding sites in the PSII complex. We suggested that this may be related to the mercury inhibition of the oxygen-evolving complex containing four Mn active sites.

Chlorophyll a fluorescence measurements of isolated spinach thylakoids obtained by using single-laser-based flow cytometry

Flow cytometry data of spinach thylakoid membrane preparations indicate the presence of a homogeneous thylakoid population. Fluorescence data from a flow cytometer and comparison with data from two other fluorometers show that chlorophyll a fluorescence detected with a flow cytometer has the character of maximum fluorescence (Fmax), not of the constant component (Fo). This conclusion is important since Fo measures fluorescence that is affected mostly by changes in excitation energy transfer and Fmax-Fo (the variable fluorescence) by changes in photochemistry. This was demonstrated by: 1) The light intensity as well as diffusion rate dependence of the quenching effect of various quinones (p-benzoquinone, phenyl-benzoquinone, and 2,5-dibromo-3-methyl-6-isopropyl-p-benzoquinone, DBMIB) on fluorescence yield; quenching for the same concentration of these quinones was lower at the higher than at the lower light intensities. 2) Temperature dependence of the fluorescence yield; increasing the temperature from 20 to 70 degrees C did not show an increase in fluorescence yield using a flow cytometer in contrast to measurements with weak excitation light, but similar to those obtained for Fmax. 3) Addition of an inhibitor diuron up to 100 microM did not change the fluorescence intensity. A comparison of quenching of fluorescence by various quinones obtained by flow cytometry with those by other fluorometers suggests that the high intensity used in the cytometry produces unique results: the rate of reduction of quinones in much larger than the rate of equilibration with the bulk quinones.

Reversal of quinone-induced chlorophyll fluorescence quenching

We have used two methods to investigate the reversibility of the interaction of substituted quinones with the thylakoid membrane of plant chloroplasts. Treatment of chloroplasts with added quinones lowers the room-temperature Photosystem II chlorophyll fluorescence intensity by variable amounts depending on the identity and concentration of the quinone. The extent of restoration of the chlorophyll fluorescence level is used as a measure of the effectiveness of the reversal technique. One reversal method involves the addition of thiols to quinone-treated chloroplasts to alter the quinone in a chemical way via a nucleophilic 1,4-Michael addition. In general, the modified quinones exhibit a lower affinity for the thylakoid membrane, as evidenced by an accompanying increase in chlorophyll fluorescence. The thiol concentrations necessary for quenching reversal are found to be in the order [dithiothreitol] less than [2-mercaptoethanol] less than [glutathione]. The second reversal method examines the extent to which added quinones can be removed from thylakoid membranes using a concentration gradient established by resuspension of quinone-treated chloroplasts in quinone-free media. The results further support the reversible nature of the quinone inhibition and indicate that the extent of recovery is dependent upon the degree of fluorescence inhibition originally induced by the added quinone.