Additional Electron Acceptor Research Articles

Complexation and chemical transformations in the ternary system silver-carbon tetrachloride-mesogenic cyanobiphenyl at low temperatures

The possibility of cryoformation and chemical transformations of new metallomesogenic complexes, exemplified by co-condensates of silver with carbon tetrachloride in the matrix of mesogenic 4-n-pentyl-4′-cyanobiphenyl, has been studied by EPR. Four types of paramagnetic products were detected in this system at low temperatures: biligand π-complexes of silver atoms with cyanobiphenyl molecules; σ-complexes of silver atoms with the cyanobiphenyl molecule stabilized by an additional electron acceptor ligand CCl4; trichloromethyl radical adducts of silver chloride — AgCl·CCl 3 • and silver nanoclusters formed as a result of aggregation of silver atoms. Thermal stability of these particles was examined and a possible mechanism of their chemical transformations was proposed.

Vertical and temporal variability of redox zonation in the water column of the Cariaco Basin: implications for organic carbon oxidation pathways

Temporal and vertical variability in the concentrations of major electron acceptors and donors (O 2, NO 3 −, particulate Mn and Fe, dissolved Mn 2+ and Fe 2+, and HS −) were investigated in the water column of the Cariaco Basin between 1995 and 1999. Vertical profiles of the redox species during upwelling and non-upwelling periods reflected the zonation of aerobic respiration, denitrification, metal reduction, and sulfate reduction in the water column and varied widely over time scales of months to years. Comparisons of these profiles with distributions of organic carbon production and acetate uptake suggest that the rate of organic carbon supply was the major control on the variability. Inhibition and enrichment experiments suggested that sulfate reduction was not a dominant pathway for organic carbon oxidation near the oxic/anoxic interface. Instead, the reduction of metal oxides, elemental sulfur, and/or thiosulfate is likely to be more important. Estimates of vertical fluxes showed that vertical diffusive transport of the major electron acceptors (oxygen, nitrate, and metal oxides) only accounted for less than 10% of the oxidation of organic carbon produced near the suboxic/anoxic transition zone. Thus, horizontal oxic mid-water intrusions coupled with strong internal redox recycling of the major redox species occur to supply additional electron acceptors for organic carbon oxidation near the redox transition zone.

Measurement and interpretation of molecular-level forces of interaction between the siderophore azotobactin and mineral surfaces

The forces of interaction were measured between the siderophore azotobactin and the minerals goethite (α-FeOOH) and diaspore (α-AlOOH) in aqueous solution using force microscopy. Azotobactin, a pyoverdin-type siderophore, was covalently linked to a hydrazide terminated atomic force microscope tip using a standard active ester protein coupling technique. Upon contact with each mineral surface, the adhesion force between azotobactin and goethite was two to three times the value observed for the isostructural Al-equivalent diaspore. The affinity for the solid iron oxide surface reflected in the force measurements correlates with the specificity of azotobactin for aqueous ferric iron. Further, the adhesion force between azotobactin and goethite significantly decreases (4 nN to 2 nN) when small amounts of soluble iron (0.1 μM FeCl 3 · 6H 2O) are added to the system at pH 3.5 suggesting a significant specific interaction between the chelating reactive center of azotobactin and the mineral surface. Changes in the force signature with pH and ionic strength were fairly predictable when considering mineral solubility, the charge character of the mineral surfaces, the molecular structure of azotobactin, and the intervening solution. For example, azotobactin–goethite adhesion values were consistently smaller at pH 3.5 relative to the forces at pH 7. At the lower pH, the large number of protons and the increase in the mineral solubility provides additional electron acceptors (e.g., H + and Fe 3+(aq)) that are free to compete for the basic oxygen chelating sites in the azotobactin structure. It is believed that this competition disrupts siderophore affinity for the surface resulting in decreased adhesion values.

Cometabolism of Cr(VI) by Shewanella oneidensis MR-1 produces cell-associated reduced chromium and inhibits growth.

Microbial reduction is a promising strategy for chromium remediation, but the effects of competing electron acceptors are still poorly understood. We investigated chromate (Cr(VI)) reduction in batch cultures of Shewanella oneidensis MR-1 under aerobic and denitrifying conditions and in the absence of an additional electron acceptor. Growth and Cr(VI) removal patterns suggested a cometabolic reduction; in the absence of nitrate or oxygen, MR-1 reduced Cr(VI), but without any increase in viable cell counts and rates gradually decreased when cells were respiked. Only a small fraction (1.6%) of the electrons from lactate were transferred to Cr(VI). The 48-h transformation capacity (Tc) was 0.78 mg (15 micromoles) Cr(VI) reduced. [mg protein](-1) for high levels of Cr(VI) added as a single spike. For low levels of Cr(VI) added sequentially, Tc increased to 3.33 mg (64 micromoles) Cr(VI) reduced. [mg protein](-1), indicating that it is limited by toxicity at higher concentrations. During denitrification and aerobic growth, MR-1 reduced Cr(VI), with much faster rates under denitrifying conditions. Cr(VI) had no effect on nitrate reduction at 6 microM, was strongly inhibitory at 45 microM, and stopped nitrate reduction above 200 microM. Cr(VI) had no effect on aerobic growth at 60 microM, but severely inhibited growth above 150 microM. A factor that likely plays a role in Cr(VI) toxicity is intracellular reduced chromium. Transmission electron microscopy (TEM) and electron energy loss spectroscopy (EELS) of denitrifying cells exposed to Cr(VI) showed reduced chromium precipitates both extracellularly on the cell surface and, for the first time, as electron-dense round globules inside cells.

Genes Encoding A-Type Flavoproteins Are Essential for Photoreduction of O2 in Cyanobacteria

O2 photoreduction by photosynthetic electron transfer, the Mehler reaction [1], was observed in all groups of oxygenic photosynthetic organisms [2–4], but the electron transport chain mediating this reaction remains unidentified. We provide the first evidence for the involvement of A-type flavoproteins that reduce O2 directly to water in vitro. Synechocystis sp. strain PCC 6803 mutants defective in flv1 and flv3, encoding A-type flavoproteins, failed to exhibit O2 photoreduction but performed normal photosynthesis and respiration. We show that the light-enhanced O2 uptake was not due to respiration or photorespiration. After dark acclimation, photooxidation of P700 was severely depressed in mutants Δflv1 and Δflv3 but recovered after light activation of CO2 fixation, which gives P700 an additional electron acceptor. Inhibition of CO2 fixation prevented recovery but scarcely affected P700 oxidation in the wild-type, where the Mehler reaction provides an alternative route for electrons. We conclude that the source of electrons for O2 photoreduction is PSI and that the highly conserved A-type flavoproteins Flv1 and Flv3 are essential for this process in vivo. We propose that in cyanobacteria, contrary to eukaryotes, the Mehler reaction produces no reactive oxygen species and may be evolutionarily related to the response of anaerobic bacteria to O2.

Reaction of 2,4-Difunctional Esters of 5-tert-Butylfuran-3-carboxylic Acid with Nucleophilic Reagents

Ethyl 5-tert-butyl-4-(chloromethyl)-2-methylfuran-3-carboxylate was brominated with N-bromo-succinimide to obtain the corresponding 2-bromomethyl derivative. The latter is selectively phosphorylatedwith trimethyl, triethyl phosphites by the bromomethyl group. The resulting [4-(chloromethyl)furyl]methyl-phosphonates in the presence of secondary amines and sodium butanethiolate behave as alkylating agents,while sodium phenolate causes their decomposition. 4-Acetoxymethyl- and 4-phenoxymethyl derivatives of the starting product are also selectively brominated with N-bromosuccinimide by the 2-methyl group. The first of the 2-(bromomethyl)furans formed is smoothly phosphorylated with trimethyl phosphite, while the second one under the action of triethyl phosphite gives a mixture of phosphorylation and debromination products. In all the cases, an additional electron-acceptor group in position 4 of alkyl 2-(bromomethyl)-5-tert-butylfuran-3-carboxylate considerably accelerates the Arbuzov reaction.

The biodiversity of lactic acid bacteria in Greek traditional wheat sourdoughs is reflected in both composition and metabolite formation.

Lactic acid bacteria (LAB) were isolated from Greek traditional wheat sourdoughs manufactured without the addition of baker's yeast. Application of sodium dodecyl sulfate-polyacrylamide gel electrophoresis of total cell protein, randomly amplified polymorphic DNA-PCR, DNA-DNA hybridization, and 16S ribosomal DNA sequence analysis, in combination with physiological traits such as fructose fermentation and mannitol production, allowed us to classify the isolated bacteria into the species Lactobacillus sanfranciscensis, Lactobacillus brevis, Lactobacillus paralimentarius, and Weissella cibaria. This consortium seems to be unique for the Greek traditional wheat sourdoughs studied. Strains of the species W. cibaria have not been isolated from sourdoughs previously. No Lactobacillus pontis or Lactobacillus panis strains were found. An L. brevis-like isolate (ACA-DC 3411 t1) could not be identified properly and might be a new sourdough LAB species. In addition, fermentation capabilities associated with the LAB detected have been studied. During laboratory fermentations, all heterofermentative sourdough LAB strains produced lactic acid, acetic acid, and ethanol. Mannitol was produced from fructose that served as an additional electron acceptor. In addition to glucose, almost all of the LAB isolates fermented maltose, while fructose as the sole carbohydrate source was fermented by all sourdough LAB tested except L. sanfranciscensis. Two of the L. paralimentarius isolates tested did not ferment maltose; all strains were homofermentative. In the presence of both maltose and fructose in the medium, induction of hexokinase activity occurred in all sourdough LAB species mentioned above, explaining why no glucose accumulation was found extracellularly. No maltose phosphorylase activity was found either. These data produced a variable fermentation coefficient and a unique sourdough metabolite composition.

Nonlinear optics. A semiempirical study of organic chromophores

Many donor–acceptor substituted organic compounds show internal charge separation upon excitation. Within this class, the following compounds are studied as suitable candidates for molecules with high second-order nonlinear optical (NLO) response: 4- N, N-dimethylamino-benzonitrile (DMABN), 6-propanoyl-2-(dimethylamino)-naphthalene (PRODAN), and various 4-(4- N, N-dimethylamino-phenyl)-1,3-diphenyl-pyrazolo[3,4-b]quinolines (DMA-DPPQ). The configuration interaction method with single and double excitations (CISD) is used in combination with the sum over states (SOS) approach within the semiempirical AM1 parametrization to calculate the vector component of the hyperpolarizability along the dipole axis β μ . The influence of changing molecular geometry on the second-order nonlinear properties is calculated for various intramolecular relaxation coordinates. Solvent effects are explicitly studied for DMABN by a self-consistent reaction field (SCRF) model revealing an increasing NLO-effect with increasing solvent polarity. The hyperpolarizability is discussed in relationship to the charge transfer properties of the excited state. The calculated values are in good agreement with experimental data for both DMABN and PRODAN. DMA-DPPQ is found to be a good NLO chromophore only when increasing either the donor or acceptor strength. Its nonlinear response can be drastically increased by an additional strong electron acceptor group, e.g. a NO 2-group, at the primary acceptor moiety.

Treatment of rinsing water from electroless nickel plating with a biologically active moving-bed sand filter

The MERESAFIN (MEtal REmoval by SAnd Filter INoculation) process presented here was designed to combine the optimum conditions for more than one of the well-known processes of biological metal immobilisation like biosorption and bioprecipitation in a treatment system for industrial waste water. The approach makes use of a continuously operated moving-bed Astrasand® filter which has been inoculated with a mixed population of selected metal biosorbing, bioprecipitating and biodegrading bacteria. One of four pilot plants has been erected at a metal plating company in Vienna (A) to treat waste water from an electroless nickel plating line. In addition to several mg/L of nickel the rinsing water also contains organic acids and inorganic phosphates, which make conventional treatment difficult. Metal laden biomass is continuously removed from the sand grains in the filter and settled in a lamella separator. The thickened biosludge contained 2% of Ni (at only 2–5 mg/L in the feed water), which could be recycled in a shaft furnace. Regeneration of biofilms on the sand is achieved by dosing a cheap carbon source; all other nutrients are available from that specific waste water. For the removal of 0.8 mg/L of nickel the biofilms consumed 8 mg carbon/L and, in addition to 8 mg/L of dissolved oxygen, 3.4 mg NO 3–N/L as additional electron acceptor. The process was shown to be economically favourable over comparable conventional techniques of metal removal. A further advantage of the biological system is its ability to cleave organo-metal complexes (e.g. nickel lactate in the presented case), to degrade organic molecules like organic acids, surface active substances, etc., often present in industrial waste waters, or to reduce ammonium, nitrite and nitrate. Proposed areas of application comprise the final polishing of industrial and mining water, but also the full treatment of contaminated ground water or drainage water.

Energetics of syntrophic propionate oxidation in defined batch and chemostat cocultures.

Propionate consumption was studied in syntrophic batch and chemostat cocultures of Syntrophobacter fumaroxidans and Methanospirillum hungatei. The Gibbs free energy available for the H(2)-consuming methanogens was <-20 kJ mol of CH(4)(-1) and thus allowed the synthesis of 1/3 mol of ATP per reaction. The Gibbs free energy available for the propionate oxidizer, on the other hand, was usually >-10 kJ mol of propionate(-1). Nevertheless, the syntrophic coculture grew in the chemostat at steady-state rates of 0.04 to 0. 07 day(-1) and produced maximum biomass yields of 2.6 g mol of propionate(-1) and 7.6 g mol of CH(4)(-1) for S. fumaroxidans and M. hungatei, respectively. The energy efficiency for syntrophic growth of S. fumaroxidans, i.e., the biomass produced per unit of available Gibbs free energy was comparable to a theoretical growth yield of 5 to 12 g mol of ATP(-1). However, a lower growth efficiency was observed when sulfate served as an additional electron acceptor, suggesting inefficient energy conservation in the presence of sulfate. The maintenance Gibbs free energy determined from the maintenance coefficient of syntrophically grown S. fumaroxidans was surprisingly low (0.14 kJ h(-1) mol of biomass C(-1)) compared to the theoretical value. On the other hand, the Gibbs free-energy dissipation per mole of biomass C produced was much higher than expected. We conclude that the small Gibbs free energy available in many methanogenic environments is sufficient for syntrophic propionate oxidizers to survive on a Gibbs free energy that is much lower than that theoretically predicted.

Random transposition by Tn916 in Desulfitobacterium dehalogenans allows for isolation and characterization of halorespiration-deficient mutants.

To allow for the molecular analysis of halorespiration by the strictly anaerobic gram-positive bacterium Desulfitobacterium dehalogenans, halorespiration-deficient mutants were selected and characterized following insertional mutagenesis by the conjugative transposon Tn916. To facilitate rapid screening of transconjugants, a highly efficient method for the growth of single colonies on solidified medium has been developed. A streptomycin-resistant mutant of D. dehalogenans was isolated and mated with Enterococcus faecalis JH2-2 carrying Tn916. Insertion of one or two copies of Tn916 into the chromosome of D. dehalogenans was observed. From a total of 2,500 transconjugants, 24 halorespiration-deficient mutants were selected based upon their inability to use 3-chloro-4-hydroxyphenylacetic acid as an electron acceptor. Physiological characterization led to the definition of three phenotypic classes of mutants that differed in their ability to use the additional terminal electron acceptors nitrate and fumarate. The activities of hydrogenase and formate dehydrogenase were determined, and the transposon insertion sites in selected mutants representing the different classes were analyzed on the sequence level following amplification by inverse PCR. The results of the molecular characterization as well as the pleiotropic phenotypes of most mutants indicate that genes coding for common elements shared by the different respiratory chains present in the versatile D. dehalogenans have been disrupted.

Zinc-tetrabenzoporphyrine-doped poly(methyl methacrylate):?a new photochromic recording medium

A photochromic reaction in Zn-tetrabenzoporphyrine-doped poly(methyl methacrylate) was investigated at room temperature. A one-quantum thermoreversible photoreaction could be initiated in the sample with 633-nm He-Ne laser irradiation. The photoreaction quantum yield depended strongly on the concentration of an additional electron acceptor. The changes of the refractive index were also measured.

Comparison of Energy and Growth Yields for Desulfitobacterium dehalogenans during Utilization of Chlorophenol and Various Traditional Electron Acceptors

Desulfitobacterium dehalogenans grew with formate as the electron donor and 3-chloro-4-hydroxyphenylacetate (3-Cl-4-OHPA) as the electron acceptor, yielding Y(X/formate), Y(X/2e), and Y(X/ATP) ranging from 3.2 to 11.3 g of biomass (dry weight)/mol, thus indicating that energy was conserved through reductive dechlorination. Pyruvate was utilized as the electron donor and acceptor, yielding stoichiometric amounts of acetate and lactate, respectively, and a Y(X/reduced acceptor) of 13.0 g of biomass (dry weight)/mol. The supplementation of pyruvate-containing medium with additional electron acceptors, such as 3-Cl-4-OHPA, nitrate, fumarate, or sulfite, caused pyruvate to be replaced as the electron acceptor and nearly doubled the Y(X/ATP) (Y(X/acetate formed)). A comparison of the yields for 3-Cl-4-OHPA with those for other traditional electron acceptors indicates that the dehalogenation reaction led to the formation of similar amounts of energy equivalents. The various electron acceptors were used concomitantly with 3-Cl-4-OHPA in nonacclimated cultures, but the utilization rates and amounts utilized differed.

Kinetics of pyrite formation by the H 2S oxidation of iron (II) monosulfide in aqueous solutions between 25 and 125°C: The mechanism

H 2S acts as an oxidizing agent in natural systems and compares with molecular oxygen as an electron acceptor. The experimentally determined lowest unoccupied molecular orbital (LUMO) for H 2S is −1.1 eV, which means that H 2S can be an excellent electron acceptor. In the oxidation of Fe(II) monosulfide by H 2S in aqueous solutions between 25 and 125°C, FeS + H 2 S ( aq ) = FeS 2 + H 2 ( g ) (where FeS is any Fe(II) monosulfide, H 2S (aq) is aqueous H 2S, FeS 2 is pyrite and H 2(g) is hydrogen gas), FeS is the electron donor (reductant) and aqueous H 2S is the electron acceptor (oxidant) and the product of the oxidation is H 2 gas. Because of the relative destabilization of H 2S caused by the presence of an antibonding LUMO orbital in a significantly bent molecule, electrons added to this LUMO orbital cause a weakening of both S H bonds as an S S bond forms. This allows the hydrogen atoms to combine to form H 2 because of their proximity and favorable interaction based on the original LUMO of H 2S. The reaction is transport-controlled. The mean Arrhenius energy for the reaction is 33.7 kJ mol −1. The Arrhenius energy is temperature dependent, which is consistent with electroactive, colloidal FeS being the FeS reactant. MO calculations suggest that the reaction proceeds through a FeS → SH 2 intermediate. The intermediate allows for the formation of an S S bond, the breaking of H S bonds with the formation of H 2 and the conversion of Fe(II) from high to low spin. The H 2 and FeS 2 formed interact with adsorption of H 2 onto the FeS 2 surface. The reaction mechanism can be summarised 1. FeS ( s ) → FeS ( aq ) ( fast < ca. 100 ° C > slow ) 2. FeS ( aq ) + H 2 S ( aq ) → { FeS → SH 2 } ( fast ) 3. { FeS → SH 2 } → [ FeS 2 ⋅ H 2 ( occluded ) ] ( fast ) 4. [ FeS 2 ⋅ H 2 ( occluded ) ] → FeS 2 ( s ) + H 2 ( g ) ( slow < ca. 100 ° C > fast ) The product pyrite forms as mixtures of individual cubes (up to 800 nm in size) and sub-spherical aggregates up to 1400 nm in diameter (“protoframboids”) on the surface of aggregates of particulate FeS. The lack of crystal growth observed in the pyrite products through 20 days of reaction at 125°C suggests that growth is nutrient limited. Observations show that initially, pyrite nucleates on the FeS but subsequently nucleation occurs on pre-formed pyrite crystals. Nucleation is rapid and kinetically favored over crystal growth leading to no significant increase in crystal size as the reaction progresses. There is some evidence that the crystal growth mechanism is through screw dislocation growth. HS − cannot be an electron acceptor because of the high positive calculated energy for the LUMO orbital (+8.015 eV). It is not possible to write an electronically balanced redox reaction involving HS − with FeS without the presence of an additional electron acceptor. In contrast, it is possible at high pH values that the complex FeSH + could react with HS − to form pyrite: however, the reaction is very slow and has not been observed experimentally. The net effect of these observations in natural systems is that reduced systems may change from oxidized to reduced merely by changing pH, from H 2S-dominant (oxidizing) at pH < 7 to HS − dominant (reducing) at pH > 7.

A model for the effects of primary substrates on the kinetics of reductive dehalogenation

A kinetic model that describes substrate interactions during reductive dehalogenation reactions is developed. This model describes how the concentrations of primary electron-donor and -acceptor substrates affect the rates of reductive dehalogenation reactions. A basic model, which considers only exogenous electron-donor and -acceptor substrates, illustrates the fundamental interactions that affect reductive dehalogenation reaction kinetics. Because this basic model cannot accurately describe important phenomena, such as reductive dehalogenation that occurs in the absence of exogenous electron donors, it is expanded to include an endogenous electron donor and additional electron acceptor reactions. This general model more accurately reflects the behavior that has been observed for reductive dehalogenation reactions. Under most conditions, primary electron-donor substrates stimulate the reductive dehalogenation rate, while primary electron acceptors reduce the reaction rate. The effects of primary substrates are incorporated into the kinetic parameters for a Monod-like rate expression. The apparent maximum rate of reductive dehalogenation (qm, ap) and the apparent half-saturation concentration (Kap) increase as the electron donor concentration increases. The electron-acceptor concentration does not affect qm, ap, but Kap is directly proportional to its concentration.

Complete Oxidation of Propionate, Valerate, Succinate, and Other Organic Compounds by Newly Isolated Types of Marine, Anaerobic, Mesophilic, Gram-Negative, Sulfur-Reducing Eubacteria

Anaerobic enrichment cultures with either propionate, succinate, lactate, or valerate and elemental sulfur and inocula from shallow marine or deep-sea sediments were dominated by rod-shaped motile bacteria after three transfers. By application of deep-agar dilutions, five eubacterial strains were obtained in pure culture and designated Kyprop, Gyprop, Kysw2, Gylac, and Kyval. All strains were gram negative and grew by complete oxidation of the electron donors and concomitant stoichiometric reduction of elemental sulfur to hydrogen sulfide. The isolates used acetate, propionate, succinate, lactate, pyruvate, oxaloacetate, maleate, glutamate, alanine, aspartate, and yeast extract. All isolates, except strain Gylac, used citrate as an electron donor but valerate was oxidized only by strain Kyval. Fumarate and malate were degraded by all strains without an additional electron donor or acceptor. Kyprop, Gyprop, and Gylac utilized elemental sulfur as the sole inorganic electron acceptor, while Kysw2 and Kyval also utilized nitrate, dimethyl sulfoxide, or Fe(III)-citrate as an electron acceptor.

Thylakoid photosystem II activity supported by the non-quinone acceptor Q 400 and an ancillary quinone acceptor Aq

The reaction center of Photosystem II is photoactive when its primary electron acceptor, Q a, or one or more of its ancillary electron acceptors are oxidized. In this work two ancillary electron acceptors are distinguished. One of these ancillary acceptors is Q 400. It was found to have no photoinduced absorption changes in the ultraviolet and so is not a quinone. An additional ancillary electron acceptor, Aq, was identified. Aq has the following characteristics: it is a quinone based on photoinduced absorption change in the ultraviolet, it has a midpoint potential of 318 mV that is independent of pH in the 6.5–8.0 range, the extent of its oxidation depends on pH and the concentration of ferricyanide, it has no C550 bandshift, and the stoichiometry of Aq:Q 400:Q a is 0.38:0.54:1.00 based on the area above the chlorophyll a fluorescence rise curve. When Aq is oxidized it participates in double advancement in transitions of oxygen-evolution S-states. A contribution to double advancement solely by Q 400 could not be found.

Numbers and functions of plastoquinone molecules associated with photosystem II preparations from Synechococcus sp.

The contents of plastoquinone and PS II electron carriers (Q B and Z) of four PS II preparations from the thermophilic cyanobacterium Synechococcus sp. were determined. (1) The oxygen-evolving preparations and the PS II reaction center complexes contained about three and two plastoquinone molecules per PS II, respectively. CP2-b, which represents the functional core of the reaction center complex (Yamagishi, A. and Katoh, S. (1985) Biochim. Biophys. Acta 807, 74–80), had a reduced amount of plastoquinone which is just comparable with the Q A content, whereas no plastoquinone was detected in CP2-c, the antenna chlorophyll-protein containing only the 40 kDa subunit. (2) The four preparations were essentially free from vitamin K 1. (3) The occurrence of Q B and Z in the PS II preparations were determined by measuring oxidation kinetics of Q − A, as well as effects of 3-(3,4-dichlorophenyl)-1,1-dimethylurea, ferricyanide and benzidine on the Q − A oxidation with repetitive flash technique. EPR measurements were also carried out to determine Signal II s and II f. Q B and Z were present in the oxygen-evolving preparations, while the PS II reaction center complexes had Z, but no or only a residual amount of Q B. CP2-b lacked both Q B and Z. (4) Q − A was oxidized partly by a back electron transport to Z + and partly by ferricyanide added in the reaction center complexes. No back reaction between Q − A and P-680 + was detected in CP2-b, which instead showed absorption changes indicative of the triplet formation. It is concluded that three plastoquinone molecules (but not vitamin K 1) function as Q A, Q B and Z and, besides them, there is no extra bound plastoquinone molecule which serves as an additional electron donor or acceptor in the vicinity of the PS II reaction center. The results also suggest that Q A, pheophytin and P-680 are located in the interior of the reaction center complexes, but are exposed on removal of the 40 kDa subunit.

On the function of the fluorescence quenchers in chloroplasts and their relation to the primary electron acceptor of photosystem II

Redox titrations of the fluorescence quenching components in chloroplasts indicate the presence of two components, one with E m 7.6 = + 25 m V and the second with E m 7.6 = -270 m V. These midpoint potentials are almost the same as those of two Photosystem II components previously shown to contribute to the chloroplast electrogenic reaction measured at 518 nm ( R. Malkin, 1978, FEBS Lett. 87, 329–333). Comparison of light-induced fluorescence yield changes with those obtained by redox titration suggests that both fluorescence quenchers are photoreduced. A direct demonstration of the photoreduction of the low-potential fluorescence quencher was observed in experiments at defined redox potentials. Fluorescence induction curves measured at low light intensity in the presence of 3-(3,4-dichlorophenyl)-1,1-dimethylurea (DCMU) also showed a contribution from both fluorescence quenchers. An additional electron acceptor, other than the two fluorescence quenchers, was also identified in the acceptor complex. These results are discussed in terms of several electron acceptors functioning in the Photosystem II reaction center complex, and the possible function of these acceptors is considered.

The anaerobic interaction of ferrocytochrome c with the “ferric” and “oxygenated” forms of purified cytochrome c oxidase. XVI. Cytochrome oxidase and its derivatives

1. The anaerobic interaction of ferrocytochrome c with “ferric” cytochrome c oxidase (EC 1.9.3.1) in the absence of ascorbate or ferro/ferricyanide cannot be interpreted as the reaction of a uniform ferric oxidase. The increase of the α-band at 605 mμ precedes that of the γ-band at 444 mμ and the γ/α ratio increases over a period of several minutes. 2. The biphasic form of the reaction is far less noticeable in the reaction of “oxygenated” oxidase with ferrocytochrome c. A similar difference has been previously found in the reduction by dithionite. 3. A spectrophotometric method is developed which allows calculations of the concentrations of ferrous and ferric cytochromes a, a 3 and c at various times after mixing oxidase and ferrocytochrome c. This is based on the extinction coefficients of cytochrome c and those of ferro- and ferricytochromes a and a 3 (refs. 1 and 2) at 605, 550 and 444 mμ. 4. With ferric oxidase, rapid reduction of ferricytochrome a is followed by a far slower reduction of ferricytochrome a 3 by ferrocytochromes c and a. No rapid equilibrium is established. Two equivalents of ferrocytochrome c are required for the reduction of one ferricytochrome a and two for that of ferricytochrome a 3 (4 per 2 moles haem a). The additional electron acceptor is probably the intrinsic copper of the oxidase. 5. With oxygenated oxidase, initial rapid conversion of the ferryl form of cytochrome a 3 is suggested by the fact that initially more than two equivalents of ferrocytochrome c are oxidized per mole of haem a. The ferricytochrome thus formed ( a 3 x 3+, ref. 2) is spectroscopically similar to or identical with ferricytochrome a 3 3+, but differs from it by its far more rapid reduction by ferrocytochromes c and a. 6. Ferricytochrome a 3 x 3+ probably plays a larger role in the enzymic reaction of the oxidase than does the ferricytochrome known as a 3 3+ present in the ferric oxidase preparation.