Rapid-Scan Fourier Transform Infrared Difference Spectroscopy with Two-Dimensional Correlation Analysis to Show the Build-Up of Light-Adapted States in Bacterial Photosynthetic Reaction Centers

Time-resolved, rapid-scan Fourier transform infrared (FT-IR) difference spectra have been recorded upon illumination on photosynthetic reaction centers (RCs) from Rhodobacter sphaeroides under fixed hydration conditions (relative humidity = 76%). Two different illumination schemes were adopted. Whereas the use of a laser flash (duration: 7 ns) made it possible to follow the kinetics of recombination of the light-induced state P+QA- to the neutral state PQA, the use of a 20.5 s continuous light from a lamp made it possible to follow both the build-up of a steady-state P+QA- population and its decay to PQA. Comparison between P+QA-/PQA FT-IR difference spectra obtained under (or 650 ms after) continuous illumination and obtained after one laser flash show small but meaningful differences, reflecting structural changes in the light-adapted state produced by the 20.5 s period of illumination. These differences are strikingly similar to those observed when comparing FT-IR difference spectra reflecting charge separation in photosystem II in light-adapted states and non-light-adapted states (c.f. Sipka et al., "Light-Adapted Charge-Separated State of Photosystem II: Structural and Functional Dynamics of the Closed Reaction Center". Plant Cell. 2021. 33(4): 1286-1302). Two-dimensional correlation spectroscopy analysis revealed that in all the observed series of time-resolved FT-IR difference spectra (under illumination, after illumination, and after a laser flash), marker bands at 1749, 1716, and 1668 cm-1 all evolve synchronously, demonstrating that electron transfer reactions and protein backbone response (at least the one reflected by the 1668 cm-1 band) are strongly correlated. Conversely, for spectra under and after continuous illumination, many asynchronicities are observed for (still unassigned) bands throughout the whole 1740-1200 cm-1 region, reflecting a more complicated molecular scenario in the RC upon build-up of the light-adapted state and during its relaxation to the resting neutral state.

Fifty Shades of Phenanthroline: Synthesis Strategies to Functionalize 1,10-Phenanthroline in All Positions

1,10-Phenanthroline (phen) is one of the most popular ligands ever used in coordination chemistry due to its strong affinity for a wide range of metals with various oxidation states. Its polyaromatic structure provides robustness and rigidity, leading to intriguing features in numerous fields (luminescent coordination scaffolds, catalysis, supramolecular chemistry, sensors, theranostics, etc.). Importantly, phen offers eight distinct positions for functional groups to be attached, showcasing remarkable versatility for such a simple ligand. As a result, phen has become a landmark molecule for coordination chemists, serving as a must-use ligand and a versatile platform for designing polyfunctional arrays. The extensive use of substituted phenanthroline ligands with different metal ions has resulted in a diverse array of complexes tailored for numerous applications. For instance, these complexes have been utilized as sensitizers in dye-sensitized solar cells, as luminescent probes modified with antibodies for biomaterials, and in the creation of elegant supramolecular architectures like rotaxanes and catenanes, exemplified by Sauvage's Nobel Prize-winning work in 2016. In summary, phen has found applications in almost every facet of chemistry. An intriguing aspect of phen is the specific reactivity of each pair of carbon atoms ([2,9], [3,8], [4,7], and [5,6]), enabling the functionalization of each pair with different groups and leading to polyfunctional arrays. Furthermore, it is possible to differentiate each position in these pairs, resulting in non-symmetrical systems with tremendous versatility. In this Review, the authors aim to compile and categorize existing synthetic strategies for the stepwise polyfunctionalization of phen in various positions. This comprehensive toolbox will aid coordination chemists in designing virtually any polyfunctional ligand. The survey will encompass seminal work from the 1950s to the present day. The scope of the Review will be limited to 1,10-phenanthroline, excluding ligands with more intracyclic heteroatoms or fused aromatic cycles. Overall, the primary goal of this Review is to highlight both old and recent synthetic strategies that find applicability in the mentioned applications. By doing so, the authors hope to establish a first reference for phenanthroline synthesis, covering all possible positions on the backbone, and hope to inspire all concerned chemists to devise new strategies that have not yet been explored.

Probing the Influence of Novel Organometallic Copper(II) Complexes on Spinach PSII Photochemistry Using OJIP Fluorescence Transient Measurements

Modern agricultural cultivation relies heavily on genetically modified plants that survive after exposure to herbicides that kill weeds. Despite this biotechnology, there is a growing need for new sustainable, environmentally friendly, and biodegradable herbicides. We developed a novel [CuL₂]Br₂ complex (L = bis{4H-1,3,5-triazino[2,1-b]benzothiazole-2-amine,4-(2-imidazole) that is active on PSII by inhibiting photosynthetic oxygen evolution on the micromolar level. [CuL₂]Br₂ reduces the F_V of PSII fluorescence. Artificial electron donors do not rescind the effect of [CuL₂]Br₂. The inhibitory mechanism of [CuL₂]Br₂ remains unclear. To explore this mechanism, we investigated the effect of [CuL₂]Br₂ in the presence/absence of the well-studied inhibitor DCMU on PSII-containing membranes by OJIP Chl fluorescence transient measurements. [CuL₂]Br₂ has two effects on Chl fluorescence transients: (1) a substantial decrease of the Chl fluorescence intensity throughout the entire kinetics, and (2) an auxiliary "diuron-like" effect. The initial decrease dominates and is observed both with and without DCMU. In contrast, the "diuron-like" effect is small and is observed only without DCMU. We propose that [CuL₂]Br₂ has two binding sites for PSII with different affinities. At the high-affinity site, [CuL₂]Br₂ produces effects similar to PSII reaction center inhibition, while at the low-affinity site, [CuL₂]Br₂ produces effects identical to those of DCMU. These results are compared with other PSII-specific classes of herbicides.

Anacardic Acid Derivatives Affect the in Vitro Reactions of Photosynthesis

The dichloromethane extract of the cashew nuts from Anacardium occidentale was fractionated by rotation locular countercurrent chromatography aimed at discovering metabolites that could be useful as new models for photosynthesis inhibitors. The chemical fractionation afforded a complex mixture of anacardic acids, which upon catalytic hydrogenation yielded anacardic acid (1). Methylation of 1 via reaction with diazomethane afforded an ester 2. Both compounds were evaluated using polarographic approaches and fluorescence studies of chlorophyll a (ChL a). The in vitro assays informed the decision for the classification of 1 and 2 as Hill reaction inhibitors. Besides that, 1 inhibited the donor side of the PSII, while 2 acted as an energy transfer inhibitor. Therefore, this study is important for the development of herbicides.

Photoferrotrophy and phototrophic extracellular electron uptake is common in the marine anoxygenic phototroph Rhodovulum sulfidophilum

Photoferrotrophy allows anoxygenic phototrophs to use reduced iron as an electron donor for primary productivity. Recent work shows that freshwater photoferrotrophs can use electrons from solid-phase conductive substances via phototrophic extracellular electron uptake (pEEU), and the two processes share the underlying electron uptake mechanism. However, the ability of marine phototrophs to perform photoferrotrophy and pEEU, and the contribution of these processes to primary productivity is largely unknown. To fill this knowledge gap, we isolated 15 new strains of the marine anoxygenic phototroph Rhodovulum sulfidophilum on electron donors such as acetate and thiosulfate. We observed that all of the R. sulfidophilum strains isolated can perform photoferrotrophy. We chose strain AB26 as a representative strain to study further, and find that it can also perform pEEU from poised electrodes. We show that during pEEU, AB26 transfers electrons to the photosynthetic electron transport chain. Furthermore, systems biology-guided mutant analysis shows that R. sulfidophilum AB26 uses a previously unknown diheme cytochrome c protein, which we call EeuP, for pEEU but not photoferrotrophy. Homologs of EeuP occur in a range of widely distributed marine microbes. Overall, these results suggest that photoferrotrophy and pEEU contribute to the biogeochemical cycling of iron and carbon in marine ecosystems.

The hydrophobicity of mutations targeting D1:Val219 modifies formate and diuron binding in the quinone-Fe-acceptor complex of Photosystem II

The D1:Val219 residue of Photosystem II in the cyanobacterium Synechocystis sp. PCC 6803 was mutated to alanine or isoleucine, creating the V219A and V219I mutants, respectively. Oxygen evolution was slowed in these mutants, while chlorophyll a fluorescence induction assays indicated slowed electron transfer. As previously observed [Erickson J.M., Rahire, M., Rochaix, J.-D. and Mets. L. (1985) Science, 228, 204-207], the V219I mutant was resistant to 3,4-dichloro-1,1-dimethyl urea (DCMU); however, the V219A strain displayed no DCMU resistance. Additionally, the V219A strain was less sensitive to the addition of formate than the control, while the V219I strain was more sensitive to formate. Both mutant strains were susceptible to photodamage and required protein synthesis for recovery. We hypothesize that the sensitivity to DCMU and the extent of bicarbonate-reversible formate-induced inhibition, as well as the capacity for recovery in cells following photodamage, are influenced by the hydrophobicity of the environment associated with the Val219 residue in D1.

Energetics of the exchangeable quinone, QB, in Photosystem II

Photosystem II (PSII), the light-driven water/plastoquinone photooxidoreductase, is of central importance in the planetary energy cycle. The product of the reaction, plastohydroquinone (PQH₂), is released into the membrane from the Q_B site, where it is formed. A plastoquinone (PQ) from the membrane pool then binds into the Q_B site. Despite their functional importance, the thermodynamic properties of the PQ in the Q_B site, Q_B, in its different redox forms have received relatively little attention. Here we report the midpoint potentials (E_m ) of Q_B in PSII from Thermosynechococcus elongatus using electron paramagnetic resonance (EPR) spectroscopy: E_m Q_B/Q_B ^•- ≈ 90 mV, and E_m Q_B ^•-/Q_BH₂ ≈ 40 mV. These data allow the following conclusions: 1) The semiquinone, Q_B ^•-, is stabilized thermodynamically; 2) the resulting E_m Q_B/Q_BH₂ (∼65 mV) is lower than the E_m PQ/PQH₂ (∼117 mV), and the difference (ΔE ≈ 50 meV) represents the driving force for Q_BH₂ release into the pool; 3) PQ is ∼50× more tightly bound than PQH₂; and 4) the difference between the E_m Q_B/Q_B ^•- measured here and the E_m Q_A/Q_A ^•- from the literature is ∼234 meV, in principle corresponding to the driving force for electron transfer from Q_A ^•- to Q_B The pH dependence of the thermoluminescence associated with Q_B ^•- provided a functional estimate for this energy gap and gave a similar value (≥180 meV). These estimates are larger than the generally accepted value (∼70 meV), and this is discussed. The energetics of Q_B in PSII are comparable to those in the homologous purple bacterial reaction center.

Thylakoid Containing Artificial Cells for the Inhibition Investigation of Light-Driven Electron Transfer during Photosynthesis

The fabrication of artificial cells containing nature components is challenging. Herein we construct a thylakoid containing artificial cell (TA-cell) by forming multicompartmental structure inside giant unilamellar vesicles (GUVs) using osmotic stress. The thylakoids are selectively loaded inside each compartment in GUVs to mimic "chloroplast". The TA-cells are able to carry out photosynthesis upon light on. The TA-cells keep their 50% functionality of electron transfer for 12 days, which is twice of those of free thylakoids. Using TA-cells the inhibition of 3-(3,4-dichlorophenyl)-1,1-dimethylurea (DCMU) and heavy metal ions (Hg²⁺, Cu²⁺, Cd²⁺, Pb²⁺ and Zn²⁺) on the electron transfer process in TA-cells is systematically investigated. Their half maximal inhibitory concentration (IC₅₀) values are 36.23 ± 1.87, 0.02 ± 0.01, 0.42 ± 0.08, 0.82 ± 0.12, 1.97 ± 0.21, and 4.08 ± 0.18 μM, respectively. Hg²⁺ is the most toxic ion for the photosynthesis process among these five heavy metal ions. This biomimetic system can be expanded to study other processes during the photosynthesis. The TA-cells pave a way to fabricate more complicated nature component containing artificial cells.

Water-splitting-based, sustainable and efficient H2 production in green algae as achieved by substrate limitation of the Calvin-Benson-Bassham cycle

BACKGROUND

Photobiological H₂ production has the potential of becoming a carbon-free renewable energy source, because upon the combustion of H₂, only water is produced. The [Fe-Fe]-type hydrogenases of green algae are highly active, although extremely O₂-sensitive. Sulphur deprivation is a common way to induce H₂ production, which, however, relies substantially on organic substrates and imposes a severe stress effect resulting in the degradation of the photosynthetic apparatus.

RESULTS

We report on the establishment of an alternative H₂ production method by green algae that is based on a short anaerobic induction, keeping the Calvin-Benson-Bassham cycle inactive by substrate limitation and preserving hydrogenase activity by applying a simple catalyst to remove the evolved O₂. Cultures remain photosynthetically active for several days, with the electrons feeding the hydrogenases mostly derived from water. The amount of H₂ produced is higher as compared to the sulphur-deprivation procedure and the process is photoautotrophic.

CONCLUSION

Our protocol demonstrates that it is possible to sustainably use algal cells as whole-cell catalysts for H₂ production, which enables industrial application of algal biohydrogen production.

Synthesis and evaluation of heterocyclic analogues of bromoxynil

One attractive strategy to discover more active and/or crop-selective herbicides is to make structural changes to currently registered compounds. This strategy is especially appealing for those compounds with limited herbicide resistance and whose chemistry is accompanied with transgenic tools to enable herbicide tolerance in crop plants. Bromoxynil is a photosystem II (PSII) inhibitor registered for control of broadleaf weeds in several agronomic and specialty crops. Recently at the University of Tennessee-Knoxville several analogues of bromoxynil were synthesized including a previously synthesized pyridine (2,6-dibromo-5-hydroxypyridine-2-carbonitrile sodium salt), a novel pyrimidine (4,6-dibromo-5-hydroxypyrimidine-2-carbonitrile sodium salt), and a novel pyridine N-oxide (2,6-dibromo-1-oxidopyridin-1-ium-4-carbonitrile). These new analogues of bromoxynil were also evaluated for their herbicidal activity on soybean (Glycine max), cotton (Gossypium hirsutum), redroot pigweed (Amaranthus retroflexus), velvetleaf (Abutilon theophrasti), large crabgrass (Digitaria sanguinalis), and pitted morningglory ( Ipomoea lacunose ) when applied at 0.28 kg ha(-1). A second study was conducted on a glyphosate-resistant weed (Amaranthus palmeri) with the compounds being applied at 0.56 kg ha(-1). Although all compounds were believed to inhibit PSII by binding in the quinone binding pocket of D1, the pyridine and pyridine-N-oxide analogues were clearly more potent than bromoxynil on Amaranthus retroflexus. However, application of the pyrimidine herbicide resulted in the least injury to all species tested. These variations in efficacy were investigated using molecular docking simulations, which indicate that the pyridine analogue may form a stronger hydrogen bond in the pocket of the D1 protein than the original bromoxynil. A pyridine analogue was able to control the glyphosate-resistant Amaranthus palmeri with >80% efficacy. The pyridine analogues of bromoxynil showed potential to have a different weed control spectrum compared to bromoxynil. A pyridine analogue of bromoxynil synthesized in this research controlled several weed species greater than bromoxynil itself, potentially due to enhanced binding within the PSII binding pocket. Future research should compare this analogue to bromoxynil using optimized formulations at higher application rates.

Effects of dehydration on light-induced conformational changes in bacterial photosynthetic reaction centers probed by optical and differential FTIR spectroscopy

Following light-induced electron transfer between the primary donor (P) and quinone acceptor (Q(A)) the bacterial photosynthetic reaction center (RC) undergoes conformational relaxations which stabilize the primary charge separated state P(+)Q(A)(-). Dehydration of RCs from Rhodobacter sphaeroides hinders these conformational dynamics, leading to acceleration of P(+)Q(A)(-) recombination kinetics [Malferrari et al., J. Phys. Chem. B 115 (2011) 14732-14750]. To clarify the structural basis of the conformational relaxations and the involvement of bound water molecules, we analyzed light-induced P(+)Q(A)(-)/PQ(A) difference FTIR spectra of RC films at two hydration levels (relative humidity r=76% and r=11%). Dehydration reduced the amplitude of bands in the 3700-3550cm(-1) region, attributed to water molecules hydrogen bonded to the RC, previously proposed to stabilize the charge separation by dielectric screening [Iwata et al., Biochemistry 48 (2009) 1220-1229]. Other features of the FTIR difference spectrum were affected by partial depletion of the hydration shell (r=11%), including contributions from modes of P (9-keto groups), and from NH or OH stretching modes of amino acidic residues, absorbing in the 3550-3150cm(-1) range, a region so far not examined in detail for bacterial RCs. To probe in parallel the effects of dehydration on the RC conformational relaxations, we analyzed by optical absorption spectroscopy the kinetics of P(+)Q(A)(-) recombination following the same photoexcitation used in FTIR measurements (20s continuous illumination). The results suggest a correlation between the observed FTIR spectral changes and the conformational rearrangements which, in the hydrated system, strongly stabilize the P(+)Q(A)(-) charge separated state over the second time scale.

Deactivation processes in PsbA1-Photosystem II and PsbA3-Photosystem II under photoinhibitory conditions in the cyanobacterium Thermosynechococcus elongatus

The sensitivity to high light conditions of Photosystem II with either PsbA1 (WT*1) or PsbA3 (WT*3) as the D1 protein was studied in whole cells of the thermophilic cyanobacterium Thermosynechococcus elongatus. When the cells are cultivated under high light conditions the following results were found: (i) The O(2) evolution activity decreases faster in WT*1 cells than in WT*3 cells both in the absence and in the presence of lincomycin, a protein synthesis inhibitor; (ii) In WT*1 cells, the rate constant for the decrease of the O(2) evolution activity is comparable in the presence and in the absence of lincomycin; (iii) The D1 content revealed by western blot analysis decays similarly in both WT*1 and WT*3 cells and much slowly than O(2) evolution; (iv) The faster decrease in O(2) evolution in WT*1 than in WT*3 cells correlates with a much faster inhibition of the S(2)-state formation; (v) The shape of the WT*1 cells is altered. All these results are in agreement with a photo-inhibition process resulting in the loss of the O(2) activity much faster than the D1 turnover in PsbA1-PSII and likely to a greater production of reactive oxygen species under high light conditions in WT*1 than in WT*3. This latter result is discussed in view of the known effects of the PsbA1 to PsbA3 substitution on the redox properties of the Photosystem II cofactors. The observation that under low light conditions WT*3 cells are able to express the psbA(3) gene, whereas under similar conditions wild type cells are expressing mainly the psbA(1) gene is also discussed. This article is part of a Special Issue entitled: Photosynthesis Research for Sustainability: from Natural to Artificial.

On the relation between the Kautsky effect (chlorophyll a fluorescence induction) and Photosystem II: basics and applications of the OJIP fluorescence transient

Chlorophyll a fluorescence is a highly sensitive, non-destructive, and reliable tool for measuring, rather quickly, photosynthetic efficiency, particularly of Photosystem II (PSII), the water-plastoquinone oxidoreductase. We briefly review here the connection between the fast (up to 2 s) chlorophyll fluorescence rise and PSII, as well as the empirical use of the fluorescence rise kinetics in understanding photosynthetic reactions, particularly of PSII. When dark-adapted photosynthetic samples are exposed to light, a fluorescence induction is observed, known as the Kautsky effect, after Hans Kautsky, the discoverer of the phenomenon showing the existence of variable fluorescence. The chlorophyll fluorescence intensity rises from a minimum level (the O level), in less than 1 s, to a maximum level (the P-level) via two intermediate steps labeled J and I. This is followed by a decline to a lower semi-steady state level, the S level, which is reached in about one minute. We provide here an educational review on how this phenomenon has been exploited through analysis of the fast OJIP fluorescence transient, by discussing basic assumptions, derivation of equations, as well as application to PSII-related questions.

Molecular Basis of Herbicide Resistance in Amaranthus hybridus

Resistance of different species of weeds to s-triazines, a commonly used class of herbicides, has been shown to involve a change in the binding affinity of the herbicide to a chloroplast polypeptide of 32,000 daltons. A single amino acid difference in this 32,000-dalton protein appears to be responsible for resistance to the herbicide in Amaranthus hybridus.

Charge separation, stabilization, and protein relaxation in photosystem II core particles with closed reaction center

The fluorescence kinetics of cyanobacterial photosystem II (PSII) core particles with closed reaction centers (RCs) were studied with picosecond resolution. The data are modeled in terms of electron transfer (ET) and associated protein conformational relaxation processes, resolving four different radical pair (RP) states. The target analyses reveal the importance of protein relaxation steps in the ET chain for the functioning of PSII. We also tested previously published data on cyanobacterial PSII with open RCs using models that involved protein relaxation steps as suggested by our data on closed RCs. The rationale for this reanalysis is that at least one short-lived component could not be described in the previous simpler models. This new analysis supports the involvement of a protein relaxation step for open RCs as well. In this model the rate of ET from reduced pheophytin to the primary quinone Q(A) is determined to be 4.1 ns(-1). The rate of initial charge separation is slowed down substantially from approximately 170 ns(-1) in PSII with open RCs to 56 ns(-1) upon reduction of Q(A). However, the free-energy drop of the first RP is not changed substantially between the two RC redox states. The currently assumed mechanistic model, assuming the same early RP intermediates in both states of RC, is inconsistent with the presented energetics of the RPs. Additionally, a comparison between PSII with closed RCs in isolated cores and in intact cells reveals slightly different relaxation kinetics, with a approximately 3.7 ns component present only in isolated cores.

D1-arginine257 mutants (R257E, K, and Q) of Chlamydomonas reinhardtii have a lowered QB redox potential: analysis of thermoluminescence and fluorescence measurements

Arginine257 (R257), in the de-helix that caps the Q(B) site of the D1 protein, has been shown by mutational studies to play a key role in the sensitivity of Photosystem II (PS II) to bicarbonate-reversible binding of the formate anion. In this article, the role of this residue has been further investigated through D1 mutations (R257E, R257Q, and R257K) in Chlamydomonas reinhardtii. We have investigated the activity of the Q(B) site by studying differences from wild type on the steady-state turnover of PS II, as assayed through chlorophyll (Chl) a fluorescence yield decay after flash excitation. The effects of p-benzoquinone (BQ, which oxidizes reduced Q(B), Q(B)(-) ) and 3-(3,4-dichlorophenyl)-1,1-dimethylurea (DCMU, which blocks electron flow from Q(A)(-) to Q(B)) were measured. The equilibrium constants of the two-electron gate were obtained through thermoluminescence measurements. The thermoluminescence properties were changed in the mutants, especially when observed after pretreatment with 100 microM BQ. A theoretical analysis of the thermoluminescence data, based mainly on the recombination pathways model of Rappaport et al. (2005), led to the conclusion that the free-energy difference for the recombination of Q(B)(-) with S(2) was reduced by 20-40 mV in the three mutants (D1-R257K, D1-R257Q, and D1-R257E); this was interpreted to be due to a lowering of the redox potential of Q(B)/Q(B)(-). Further, since the recombination of Q(A)(-) with S(2) was unaffected, we suggest that no significant change in redox potential of Q(A)/Q(A)(-) occurred in these three mutants. The maximum variable Chl a fluorescence yield is lowered in the mutants, in the order R257K > R257Q > R257E, compared to wild type. Our analysis of the binary oscillations in Chl a fluorescence following pretreatment of cells with BQ showed that turnover of the Q(B) site was relatively unaffected in the three mutants. The mutant D1-R257E had the lowest growth rate and steady-state activity and showed the weakest binary oscillations. We conclude that the size and the charge of the amino acid at the position D1-257 play a role in PS II function by modulating the effective redox potential of the Q(B)/Q(B)(-) pair. We discuss an indirect mechanism mediated through electrostatic and/or surface charge effects and the possibility of more pleiotropic effects arising from decreased stability of the D1/D2 and D1/CP47 interfaces.

Turnover of ubiquinone-0 at the acceptor side of photosynthetic reaction center

The steady-state operation of photosynthetic reaction center from Rhodobacter sphaeroides was investigated by measuring the rate of cytochrome photo-oxidation under intensive continuous illumination (808 nm, 5 W cm(-2)). The native quinone UQ10 in Q(B) binding site of the reaction center was substituted by tailless UQ0 and the binding parameters and the turnover rate of the UQ0 was studied to test the recently discovered light-intensity dependent acceptor side effect (Gerencsér and Maróti 2006). The binding parameters of UQ0 (k(on) = 2.1 x 10(5) M(-1) s(-1) and k(off) = 100 s(-1)) were characteristic to the RC exposed to high light-intensity. The dissociation constant (K (D) = 480 microM) determined under high light intensity is 2-3 times larger than that determined from flash-experiments. The light-intensity dependent acceleration of cytochrome turnover measured on reaction center of inhibited proton binding was independent of the type of the quinone and was sensitive only to the size ("pressure") of the quinone pool. The dissociation constants of different types of semiquinones show similarly high (several orders of magnitude) increase in the modified conformation of the Q(B) binding pocket due to high intensity of illumination. This result indicates the exclusive role of the quinone headgroup in the binding of semiquinone to different conformations of the protein.

Inhibition of photophosphorylation and electron transport chain in thylakoids by lasiodiplodin, a natural product from Botryosphaeria rhodina

Four natural products were isolated from the fungus Botryosphaeria rhodina, and their effects on photosynthesis were tested. Only lasiodiplodin (1) inhibited ATP synthesis and electron flow from water to methylviologen; therefore, it acts as a Hill reaction inhibitor in freshly lysed spinach thylakoids. Photosystem I and II and partial reactions as well as ATPase were measured in the presence of 1. Three new different sites of 1 interaction and inhibition were found: one at CF1, the second in the water-splitting enzyme, and the third at the electron-transfer path between P680 and QA; these targets are different from that of the synthetic herbicides present. Electron transport chain inhibition by 1 was corroborated by fluorescence induction kinetics studies.

Fluorescence lifetime imaging microscopy of Chlamydomonas reinhardtii: non-photochemical quenching mutants and the effect of photosynthetic inhibitors on the slow chlorophyll fluorescence transient

Fluorescence lifetime-resolved images of chlorophyll fluorescence were acquired at the maximum P-level and during the slower transient (up to 250 s, including P-S-M-T) in the green photosynthetic alga Chlamydomonas reinhardtii. At the P-level, wild type and the violaxanthin-accumulating mutant npq1 show similar fluorescence intensity and fluorescence lifetime-resolved images. The zeaxanthin-accumulating mutant npq2 displays reduced fluorescence intensity at the P-level (about 25-35% less) and corresponding lifetime-resolved frequency domain phase and modulation values compared to wild type/npq1. A two-component analysis of possible lifetime compositions shows that the reduction of the fluorescence intensity can be interpreted as an increase in the fraction of a short lifetime component. This supports the important photoprotection function of zeaxanthin in photosynthetic samples, and is consistent with the notion of a 'dimmer switch'. Similar, but quantitatively different, behaviour was observed in the intensity and fluorescence lifetime-resolved imaging measurements for cells that were treated with the electron transport inhibitor 3-(3,4-dichlorophenyl)-1,1-dimethyl urea, the efficient PSI electron acceptor methyl viologen and the protonophore nigericin and. Lower fluorescence intensities and lifetimes were observed for all npq2 mutant samples at the P-level and during the slow fluorescence transient, compared to wild type and the npq1 mutant. The fluorescence lifetime-resolved measurements during the slow fluorescence changes after the P level up to 250 s for the wild type and the two mutants, in the presence and absence of the above inhibitors, were analyzed with a graphical procedure (polar plots) to determine lifetime compositions. At higher illumination intensity, wild type and npq1 cells show a rise in fluorescence intensity and corresponding rise in the species concentration of the slow lifetime component after the initial decrease following the P level. This reversal is absent in the npq2 mutant, and for all samples in the presence of the inhibitors. Lifetime heterogeneities were observed in experiments averaged over multiple cells as well as within single cells, and these were followed over time. Cells in the resting state (induced by several hours of darkness), instead of the normal swimming state, show shortened lifetimes. The above results are discussed in terms of a superposition of effects on electron transfer and protonation rates, on the so-called 'State Transitions', and on non-photochemical quenching. Our data indicate two major populations of chlorophyll a molecules, defined by two 'lifetime pools' centred on slower and faster fluorescence lifetimes.

Influence of the Redox Potential of the Primary Quinone Electron Acceptor on Photoinhibition in Photosystem II

We report the characterization of the effects of the A249S mutation located within the binding pocket of the primary quinone electron acceptor, Q(A), in the D2 subunit of photosystem II in Thermosynechococcus elongatus. This mutation shifts the redox potential of Q(A) by approximately -60 mV. This mutant provides an opportunity to test the hypothesis, proposed earlier from herbicide-induced redox effects, that photoinhibition (light-induced damage of the photosynthetic apparatus) is modulated by the potential of Q(A). Thus the influence of the redox potential of Q(A) on photoinhibition was investigated in vivo and in vitro. Compared with the wild-type, the A249S mutant showed an accelerated photoinhibition and an increase in singlet oxygen production. Measurements of thermoluminescence and of the fluorescence yield decay kinetics indicated that the charge-separated state involving Q(A) was destabilized in the A249S mutant. These findings support the hypothesis that a decrease in the redox potential of Q(A) causes an increase in singlet oxygen-mediated photoinhibition by favoring the back-reaction route that involves formation of the reaction center chlorophyll triplet. The kinetics of charge recombination are interpreted in terms of a dynamic structural heterogeneity in photosystem II that results in high and low potential forms of Q(A). The effect of the A249S mutation seems to reflect a shift in the structural equilibrium favoring the low potential form.

Functionality of Photosynthetic Reaction Centers in Polyelectrolyte Multilayers: Toward an Herbicide Biosensor

The bacterial reaction center (RC), a membrane photosynthetic protein, has been adsorbed onto a glass surface by alternating deposition with the cationic polymer poly(dimethyldiallylammonium chloride) (PDDA) obtaining as an end result an ordinate polyelectrolyte multilayer (PEM) where the protein retains its integrity and photoactivity over a period of several months. Such a system has been characterized from the functional point of view by checking the protein photoactivity at different hydration conditions, from extensive drought to full hydration. The kinetic analysis of charge recombination indicates that incorporation of RCs into dehydrated PEM hinders the conformational dynamics gating QA- to QB electron-transfer leaving unchanged the protein relaxation that stabilizes the primary charge separated state P+QA-. The herbicide-induced inhibition of the QB activity was studied in some detail. By dipping the PEM in herbicide solutions for short times, kinetics of herbicide binding and release have been determined; binding isotherms have been studied using PEM immersed in herbicide solution. QB functionality of RC has been restored by rinsing the PEM with water, thus allowing the reuse of the same sample. This last point has been exploited to design a simple optical biosensor for herbicides. A suitable kinetic model has been proposed to describe the interplay between forward and back electron-transfer processes upon continuous illumination, and the use of the PDDA-RC multilayers in herbicide bioassays was successfully tested.

Quantitative analysis of the experimental O-J-I-P chlorophyll fluorescence induction kinetics. Apparent activation energy and origin of each kinetic step

Fluorescence induction has been studied for a long time, but there are still questions concerning what the O-J-I-P kinetic steps represent. Most studies agree that the O-J rise is related to photosystem II primary acceptor (Q(A)) reduction, but several contradictory theories exist for the J-I and I-P rises. One problem with fluorescence induction analysis is that most work done to date has used only qualitative or semiquantitative data analysis by visually comparing traces to observe the effects of different chemicals or treatments. Although this method is useful to observe major changes, a quantitative method must be used to detect more subtle, yet important, differences in the fluorescence induction trace. To achieve this, we used a relatively simple mathematical approach to extract the amplitudes and half-times of the three major fluorescence induction phases obtained from traces measured in thylakoid membranes kept at various temperatures. Apparent activation energies (E(A)) were also obtained for each kinetic step. Our results show that each phase has a different E(A), with E(A O-J) <E(A J-I) < E(A I-P), and thus a different origin. The effects of two well-known chemicals, 3-(3,4-dichlorophenyl)-1,1-dimethylurea, which blocks electron transfer to the photosystem II secondary electron acceptor (Q(B)), and decylplastoquinone, which acts similarly to endogenous reducible plastoquinones, on the quantitative parameters are discussed in terms of the origin of each kinetic phase.

Scanning Electrochemical Microscopy of the Photosynthetic Reaction Center of Rhodobacter sphaeroides in Different Environmental Systems

The present work uses a scanning electrochemical microscopy technique to study systems containing the membrane-bound reaction center protein (RC) from the purple photosynthetic bacteria Rhodobacter spheroides to chromatophores (spherical reorganization of cell membrane following its mechanical rupture) and liposomes (reconstituted membrane systems at lower degree of complexity). Scanning electrochemical microscopy is a useful tool to investigate redox processes involving a RC, because the effective heterogeneous rate constants for the redox reaction with different mediators can be measured. The technique is also able to provide information on the role of the outer cell membrane permeation on the kinetics of the electron-transfer processes and to obtain more insight into the nature of the species involved.

Uncoupling of electron and proton transfers in the photocycle of bacterial reaction centers under high light intensity

Photosynthetic reaction centers produce and export oxidizing and reducing equivalents in expense of absorbed light energy. The formation of fully reduced quinone (quinol) requires a strict (1:1) stoichiometric ratio between the electrons and H(+) ions entering the protein. The steady-state rates of both transports were measured separately under continuous illumination in the reaction center from the photosynthetic bacterium Rhodobacter sphaeroides. The uptake of the first proton was retarded by different methods and made the rate-limiting reaction in the photocycle. As expected, the rate constant of the observed proton binding remained constant (7 s(-)(1)), but that of the cytochrome photooxidation did show a remarkably large increase from 14 to 136 s(-)(1) upon increase of the exciting light intensity up to 5 W/cm(2) (808 nm) at pH 8.4 in the presence of NiCl(2). This corresponds to about 20:1 (e(-):H(+)) stoichiometric ratio. The observed enhancement is linearly proportional to the light intensity and the rate constant of the proton uptake by the acceptor complex and shows saturation character with quinone availability. For interpretation of the acceleration of cytochrome turnover, an extended model of the photocycle is proposed. A fraction of photochemically trapped RC can undergo fast (>10(3) s(-)(1)) conformational change where the semiquinone loses its high binding affinity (the dissociation constant increases by more than 5 orders of magnitude) and dissociates from the Q(B) binding site of the protein with a high rate of 4000 s(-)(1). Concomitantly, superoxide is being produced. No H(+) ion is taken up, and no quinol is created by the photocycle which is operating in about 25% of the reaction centers at the highest light intensity (5500 s(-)(1)) and slowest proton uptake (3.5 s(-)(1)) used in our experiments. The possible physical background of the light-induced conformational change and the relationship between the energies of dissociation and redox changes of the quinone in the Q(B) binding sites are discussed.

Secondary Quinone in Photosystem II of Thermosynechococcus elongatus: Semiquinone−Iron EPR Signals and Temperature Dependence of Electron Transfer

The secondary quinone acceptor, Q(B), has been studied in photosystem II (PSII) isolated from Thermosynechococcus (T.) elongatus. Thermoluminescence indicated that Q(B) was present in this preparation. An EPR signal observed at low temperature at g = 1.9 was attributed to Fe2+ Q(B)- on the basis of the characteristic period-of-two variations in its intensity depending on the number of laser flashes given at 20 degrees C. When samples showing the Fe2+ Q(B)- signal were illuminated at 77 K, an EPR signal at g = 1.66 appeared with an amplitude proportional to that of the Fe2+ Q(B)- signal. This signal is attributed to the Q(A)- Fe2+ Q(B)- state. While these attributions have been made previously in PSII from other origins, they have remained relatively tentative since the characteristic period-of-two oscillations of Q(B) had not previously been observed. The flash experiments indicated that more than one exchangeable plastoquinone is associated with the isolated PSII. The g = 1.66 signal from the Q(A)- Fe2+ Q(B)- state was used to study the temperature dependence of electron transfer between the two quinones. Electron transfer occurred in half of the centers (after 30 s incubation) at -28 degrees C for Q(A)- to Q(B) but at -58 degrees C for Q(A)- to Q(B)-. This marked difference for the two electron transfer reactions indicates different types of rate-limiting reactions. In the better studied but homologous system, the purple bacterial reaction center, the Q(A)- to Q(B) step is limited by a gating process, while the Q(A)- to Q(B)- step is limited by protonation events. Similar reactions in PSII could give rise to the observed temperature dependence.

Herbicide-induced changes in charge recombination and redox potential of Q(A) in the T4 mutant of Blastochloris viridis

To gain new insights into the function of photosystem II (PSII) herbicides DCMU (a urea herbicide) and bromoxynil (a phenolic herbicide), we have studied their effects in a better understood system, the bacterial photosynthetic reaction center of the terbutryn-resistant mutant T4 of Blastochloris (Bl.) viridis. This mutant is uniquely sensitive to these herbicides. We have used redox potentiometry and time-resolved absorption spectroscopy in the nanosecond and microsecond time scale. At room temperature the P(+)(*)Q(A)(-)(*) charge recombination in the presence of bromoxynil was faster than in the presence of DCMU. Two phases of P(+)(*)Q(A)(-)(*) recombination were observed. In accordance with the literature, the two phases were attributed to two different populations of reaction centers. Although the herbicides did induce small differences in the activation barriers of the charge recombination reactions, these did not explain the large herbicide-induced differences in the kinetics at ambient temperature. Instead, these were attributed to a change in the relative amplitude of the phases, with the fast:slow ratio being approximately 3:1 with bromoxynil and approximately 1:2 with DCMU at 300 K. Redox titrations of Q(A) were performed with and without herbicides at pH 6.5. The E(m) was shifted by approximately -75 mV by bromoxynil and by approximately +55 mV by DCMU. As the titrations were done over a time range that is assumed to be much longer than that for the transition between the two different populations, the potentials measured are considered to be a weighted average of two potentials for Q(A). The influence of the herbicides can thus be considered to be on the equilibrium of the two reaction center forms. This may also be the case in photosystem II.

Internal dynamics and protein–matrix coupling in trehalose-coated proteins

We review recent studies on the role played by non-liquid, water-containing matrices on the dynamics and structure of embedded proteins. Two proteins were studied, in water-trehalose matrices: a water-soluble protein (carboxy derivative of horse heart myoglobin) and a membrane protein (reaction centre from Rhodobacter sphaeroides). Several experimental techniques were used: Mossbauer spectroscopy, elastic neutron scattering, FTIR spectroscopy, CO recombination after flash photolysis in carboxy-myoglobin, kinetic optical absorption spectroscopy following pulsed and continuous photoexcitation in Q(B) containing or Q(B) deprived reaction centre from R. sphaeroides. Experimental results, together with the outcome of molecular dynamics simulations, concurred to give a picture of how water-containing matrices control the internal dynamics of the embedded proteins. This occurs, in particular, via the formation of hydrogen bond networks that anchor the protein surface to the surrounding matrix, whose stiffness increases by lowering the sample water content. In the conclusion section, we also briefly speculate on how the protein-matrix interactions observed in our samples may shed light on the protein-solvent coupling also in liquid aqueous solutions.

In intact leaves, the maximum fluorescence level (FM) is independent of the redox state of the plastoquinone pool: A DCMU-inhibition study

The effects of DCMU (3-(3',4'-dichlorophenyl)-1,1-dimethylurea) on the fluorescence induction transient (OJIP) in higher plants were re-investigated. We found that the initial (F(0)) and maximum (F(M)) fluorescence levels of DCMU-treated leaves do not change relative to controls when the treatment is done in complete darkness and DCMU is allowed to diffuse slowly into the leaves either by submersion or by application via the stem. Simultaneous 820 nm transmission measurements (a measure of electron flow through Photosystem I) showed that in the DCMU-treated samples, the plastoquinone pool remained oxidized during the light pulses whereas in uninhibited leaves, the F(M) level coincided with a fully reduced electron transport chain. The identical F(M) values with and without DCMU indicate that in intact leaves, the F(M) value is independent of the redox state of the plastoquinone pool. We also show that (i) the generally observed F(0) increase is probably due to the presence of (even very weak) light during the DCMU treatment, (ii) vacuum infiltration of leaf discs leads to a drastic decrease of the fluorescence yield, and in DCMU-treated samples, the F(M) decreases to the I-level of their control (leaves vacuum infiltrated with 1% ethanol), (iii) and in thylakoid membranes, the addition of DCMU lowers the F(M) relative to that of a control sample.

Protein/Lipid Interaction in the Bacterial Photosynthetic Reaction Center: Phosphatidylcholine and Phosphatidylglycerol Modify the Free Energy Levels of the Quinones

The role of characteristic phospholipids of native membranes, phosphatidylcholine (PC), phosphatidylglycerol (PG), and cardiolipin (CL), was studied in the energetics of the acceptor quinone side in photosynthetic reaction centers of Rhodobacter sphaeroides. The rates of the first, k(AB)(1), and the second, k(AB)(2), electron transfer and that of the charge recombination, k(BP), the free energy levels of Q(A)(-)Q(B) and Q(A)Q(B)(-) states, and the changes of charge compensating protein relaxation were determined in RCs incorporated into artificial lipid bilayer membranes. In RCs embedded in the PC vesicle, k(AB)(1) and k(AB)(2) increased (from 3100 to 4100 s(-1) and from 740 to 3300 s(-1), respectively) and k(BP) decreased (from 0.77 to 0.39 s(-1)) compared to those measured in detergent at pH 7. In PG, k(AB)(1) and k(BP) decreased (to values of 710 and 0.26 s(-1), respectively), while k(AB)(2) increased to 1506 s(-1) at pH 7. The free energy between the Q(A)(-)Q(B) and Q(A)Q(B)(-) states decreased in PC and PG (DeltaG degrees (Q)A-(Q)B(-->)(Q)A(Q)B- = -76.9 and -88.5 meV, respectively) compared to that measured in detergent (-61.8 meV). The changes of the Q(A)/Q(A)(-) redox potential measured by delayed luminescence showed (1) a differential effect of lipids whether RC incorporated in micelles or vesicles, (2) an altered binding interaction between anionic lipids and RC, (3) a direct influence of PC and PG on the free energy levels of the primary and secondary quinones probably through the intraprotein hydrogen-bonding network, and (4) a larger increase of the Q(A)/Q(A)(-) free energy in PG than in PC both in detergent micelles and in single-component vesicles. On the basis of recent structural data, implications of the binding properties of phospholipids to RC and possible interactions between lipids and electron transfer components will be discussed.

Modulation of the free energy of the primary quinone acceptor (QA) in reaction centers from Rhodobacter sphaeroides: contributions from the protein and protein-lipid(cardiolipin) interactions

The redox midpoint potential (E(m)) of Q(A), the primary quinone of bacterial reaction centers, is substantially modulated by the protein environment. Quite subtle mutations in the Q(A) binding site, e.g., at residues M218, M252 and M265, cause significant increases in the equilibrium constant for electron transfer to Q(B), which indicate relative lowering of the E(m) of Q(A). However, reports of functional linkage between the Q(A) and Q(B) sites make it difficult to partition such effects between Q(A) and Q(B) from purely relative changes. We report here measurements on the yield of delayed fluorescence emission from the primary donor (P) accompanying the thermally activated charge recombination of P(+)Q(A)(-) to form the excited singlet state of the primary donor, P*. The results show that for mutations of the Q(A) site residues, Met(M218) and Ile(M265), essentially all the substantial thermodynamic effect is localized at Q(A), with no evidence for a significant effect of these residues on the properties of Q(B) or the mutual influence (linkage) of Q(A) and Q(B). We also report a significant lowering of the E(m) of Q(A) by the native lipid, cardiolipin, which brings the E(m) in isolated reaction centers more in line with that seen in native membrane vesicles (chromatophores). Possible origins of this effect are discussed in the context of the Q(A) binding site structure.

Acceptor and donor-side interactions of phenolic inhibitors in Photosystem II

Certain phenolic compounds represent a distinct class of Photosystem (PS) II Q(B) site inhibitors. In this paper, we report a detailed study of the effects of 2,4,6-trinitrophenol (TNP) and other phenolic inhibitors, bromoxynil and dinoseb, on PS II energetics. In intact PS II, phenolic inhibitors bound to only 90-95% of Q(B) sites even at saturating concentrations. The remaining PS II reaction centers (5-10%) showed modified Q(A) to Q(B) electron transfer but were sensitive to urea/triazine inhibitors. The binding of phenolic inhibitors was 30- to 300-fold slower than the urea/triazine class of Q(B) site inhibitors, DCMU and atrazine. In the sensitive centers, the S(2)Q(A)(-) state was 10-fold less stable in the presence of phenolic inhibitors than the urea/triazine herbicides. In addition, the binding affinity of phenolic herbicides was decreased 10-fold in the S(2)Q(A)(-) state than the S(1)Q(A) state. However, removal of the oxygen-evolving complex (OEC) and associated extrinsic polypeptides by hydroxylamine (HA) washing abolished the slow binding kinetics as well as the destabilizing effects on the charge-separated state. The S(2)-multiline electron paramagnetic resonance (EPR) signal and the 'split' EPR signal, originating from the S(2)Y(Z) state showed no significant changes upon binding of phenolic inhibitors at the Q(B) site. We thus propose a working model where Q(A) redox potential is lowered by short-range conformational changes induced by phenolic inhibitor binding at the Q(B) niche. Long-range effects of HA-washing eliminate this interaction, possibly by allowing more flexibility in the Q(B) site.

Novel psbA1 gene from a naturally occurring atrazine-resistant cyanobacterial isolate

A naturally occurring atrazine-resistant cyanobacterial isolate, strain SG2, was isolated from an atrazine-containing wastewater treatment system at the Syngenta atrazine production facility in St. Gabriel, La. Strain SG2 was resistant to 1,000 microg of atrazine per ml but showed relatively low resistance to diuron [3-(3,4-dichlorophenyl)-1,1-dimethyl urea]. Analyses of 16S ribosomal DNA indicated that strain SG2 falls into the Synechocystis/Pleurocapsa/Microcystis group. Photosynthetically driven oxygen evolution in strain SG2 was only slightly inhibited (about 10%) by 2,000 microg of atrazine per ml, whereas in the control strain Synechocystis 6803, oxygen evolution was inhibited 90% by 1,000 microg of atrazine per ml. No atrazine accretion, mineralization, or metabolites were detected when strain SG2 was grown with [(14)C]atrazine. Strain SG2 contained three copies of the psbA gene, which encodes the D(1) protein of the photosystem II reaction center. Nucleotide sequence analyses indicated that the psbA2 and psbA3 genes encoded predicted proteins with the same amino acid sequence. However, the psbA1 gene product contained five extra amino acids, which were not found in PsbA proteins from five other cyanobacteria. Moreover, the PsbA1 protein from strain SG2 had an additional 13 amino acid changes compared to the PsbA2/PsbA3 proteins and contained 10 amino acid alterations compared to conserved residues found in other cyanobacteria. Reverse transcriptase PCR analysis indicated that the psbA1 gene and the psbA2/psbA3 gene(s) were expressed in photosynthetically grown cells in the presence of atrazine. These results suggest that strong selection pressure conferred by the continual input of atrazine has contributed to the evolution of a herbicide-resistant, yet photosynthetically efficient, psbA gene in a cyanobacterium.

Herbicide-induced oxidative stress in photosystem II

Some herbicides act by binding to the exchangeable quinone site in the photosystem II (PSII) reaction centre, thus blocking electron transfer. In this article, it is hypothesized that the plant is killed by light-induced oxidative stress initiated by damage caused by formation of singlet oxygen in the reaction centre itself. This occurs when light-induced charge pairs in herbicide-inhibited PSII decay by a charge recombination route involving the formation of a chlorophyll triplet state that is able to activate oxygen. The binding of phenolic herbicides favours this pathway, thus increasing the efficiency of photodamage in this class of herbicides.

Absorption changes induced by the binding of triazines to the QB pocket in reaction centers of Rhodobacter capsulatus

Inhibitors which block electron transfer from the primary (Q(A)) to the secondary (Q(B)) quinone of the bacterial reaction center are competing with the pool ubiquinones for binding at the Q(B) pocket. Due to the much greater stability of the semiquinone state Q(B)(-) compared with fully oxidized or reduced quinone, a displacement of the inhibitors takes place after one flash from state Q(A)(-)I to state Q(A)Q(B)(-). This process can be monitored from near-IR absorption changes which reflect local absorption shifts specific to Q(A)(-) and Q(B)(-). An anomalous behavior was observed when using triazines in chromatophores of R. capsulatus: the IR absorption change reflecting the formation of Q(B)(-) after one flash was absent. A normal transient decay of this signal was, however, triggered by a second flash, followed by a rapid return to the baseline. We show that this phenomenon is due to an absorption change induced by inhibitor binding (thus present in the dark baseline), with a spectrum close to that of Q(B)(-), so that the Q(B)(-) changes are canceled out during the inhibitor displacement process. On the second flash, one monitors the destruction of the semiquinone, leading transiently to the Q(A)Q(B) state, followed by inhibitor rebinding. This allows a direct measurement of the binding kinetics. This behavior was observed both in chromatophores and in isolated reaction centers from R. capsulatus, but not in R. sphaeroides.

Equilibrium and kinetic parameters for the binding of inhibitors to the QB pocket in bacterial chromatophores: dependence on the state of QA

The equilibrium and kinetic parameters for the binding of various inhibitors to the Q(B) pocket of the bacterial reaction center were investigated in chromatophores from Rhodobacter capsulatus and Rhodobacter sphaeroides. By monitoring the near-IR absorption changes specific to Q(A)(-) and Q(B)(-), we measured the fraction of inhibited centers in the dark and the kinetics and extent of inhibitor displacement after one flash due to the formation of the Q(A)Q(B)(-) state. The inhibitor release rate was much faster for triazines and o-phenanthroline (t(1/2) in the 50 ms to 1 s range) than for stigmatellin (t(1/2) approximately 20 s). For inhibitors with a rapid release rate, the fast phase of P(+) decay observed in the absence of secondary donor reflects the competition between P(+)Q(A)(-) recombination and inhibitor release: it is thus faster than the P(+)Q(A)(-) recombination, and its relative extent is smaller than the fraction of initially inhibited centers. At appropriate inhibitor concentrations, one can have almost total binding in the dark and almost total inhibitor displacement after one flash. Under such conditions, a pair of closely spaced flashes resets the two-electron gate in a single state (Q(A)Q(B)(-)), irrespective of the initial state. The apparent dissociation constant of terbutryn was significantly increased (by a factor of 4-7) in the presence of Q(A)(-), in agreement with the conclusion of Wraight and co-workers [Stein, R. R., et al. (1984) J. Cell. Biochem. 24, 243-259]. We suggest that this effect is essentially due to a tighter binding of ubiquinone in the Q(A)(-) state.