Positive Redox Research Articles

Structural, Electrochemical, and Spectroscopic Properties of a Class of Dodecasubstituted Iron Porphyrins Bearing Four Positive Charges Close to the Metal

AbstractThe first iron complexes of the tetracationic 2,3,7,8,12,13,17,18‐octaethyl‐5,10,15,20‐tetra‐N‐pyridiniumylporphyrin, (OEPy4P)4+, in which four pyridine molecules are attached to the meso‐carbon atoms through their nitrogen atoms, were synthesized in three steps starting from Zn(OEP) (Zn‐β‐octaethylporphyrin) with an overall yield of ca. 30 %. The X‐ray structure of [FeII(OEPy4P)(pyridine)2](CF3SO3)3(Br) established that the molecule adopts a severely distorted nonplanar saddle conformation. 1H NMR spectroscopy showed that the [FeII(OEPy4P)(B)2]4+ ligated complexes, B = pyridine (Py) or imidazole (Im), are low‐spin, FeII, S = 0. These complexes exhibit redox potentials for the FeIII/FeII couple that range between 0.62 and 0.38 V (vs. Ag/AgCl) and are shifted by as much as +0.7 V relative to those of the corresponding planar iron complexes of OEP and of the nonplanar complexes of OETPP. Such highly positive redox potentials explain the stability of these iron(II) porphyrins in air. As previously found for dodecasubstituted iron porphyrins bearing electron‐withdrawing groups, the UV/Vis spectra of the[FeII(OEPy4P)(B)2]4+ complexes exhibit Soret and visible bands which are redshifted by ca. 40 nm relative to those of Fe(OEP)(Py)2. These metallo‐OEPy4P4+complexes offer promising synthetic avenues to novel classes of highly charged porphyrins and multiporphyrin arrays, which are soluble in aqueous and polar organic solvents.(© Wiley‐VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2007)

Sensitively probing the cofactor redox species and photo-induced electron transfer of wild-type and pheophytin-replaced photosynthetic proteins reconstituted in self-assembled monolayers

An ultrathin, ordered, and packed protein film, consisting of the 2-mercaptoacetic acid (MAA), polydimethyldiallylammonium chloride (PDDA), and wild-type (WT) photosynthetic reaction center (RC; termed as WT-RC) or its pheophytin (Phe)-replaced counterpart (termed as Phe-RC), was fabricated by self-assembling technique onto gold electrode for facilitating the electron transfer (ET) between RC and the electrode surface. Near-infrared (NIR)-visible (Vis) absorption and fluorescence (FL) emission spectra revealed the influence of pigment substitution on the cofactors arrangement and excitation relaxation of the proteins, respectively. Square wave voltammetry (SWV) and photoelectric tests were employed to systematically address the differences between the WT-RC films and mutant ones on the direct and photo-induced ET. The electrochemical results demonstrated that ET initiated by the oxidation of the primary donor (P) was obviously slowed down, and the formed P+ had more population as well as more positive redox potential in the Phe-RC films compared with those in the WT ones. The photoelectrochemical results displayed the dramatically enhanced photoelectric performances of the mutant ones, further suggesting the slow-down formation of final charge-separated state in Phe-RC. The functionalized protein films introduced in this paper provided an efficient approach to sensitively probe the redox cofactors and ET differences resulting from only minor changes in pigment arrangement in the pigment–protein complex. The favored ET process observed for the membrane proteins RC was potentially valuable for a deep understanding of the multi-step biological ET process and development of versatile bioelectronic devices.

Charge Resonance Excitations in 1,3-Bis[di(4-methoxyphenyl)amino]azulene Radical Cations

The 1,3-bis[di(4-methoxyphenyl)amino]azulene 1 and 1,3-bis[di(4-methoxyphenyl)amino]azulene-2-carboxylic acid methyl ester 2 were synthesized and investigated by spectroscopic and electrochemical methods in order to estimate the electronic coupling between the attached amine redox centers. We found that the electronic coupling V which is mediated by a nonalternant 1,3-azulene bridge in the radical cation 2(+) (V = 3900 cm(-1)) is almost the same as in the isomeric alternant 1,4-naphthalene system 3(+) (V = 4000 cm(-1)). The electrochemical stability of azulene derivatives is drastically increased by substituents at the 2-position: whereas the oxidation of compound 1 is irreversible, the methylearboxylate derivative 2 undergoes four reversible oxidation processes in CH2Cl2 under semi-infinite conditions. The different redox states of the corresponding radical cations are well separated. However, under finite diffusion conditions only the first three oxidation processes are reversible. The absorption spectra of the radical cations of 2(+), 2(2+), and 2(3+) show intense absorption bands in the NIR region. The analysis of the optical spectra as well as DFT calculations indicate that in 2(+) the charge is symmetrically distributed similar to the naphthalene isomer 3(+). Thus, the 1,3-azulene unit and its derivatives are useful bridging units due to their ability to mediate a strong electronic coupling similar to naphthalene but with a less positive redox potential.

Utilization of AC Impedance Measurements for Electrochemical Glucose Sensing Using Glucose Oxidase to Improve Detection Selectivity

Abstract The electrochemical detection of glucose using glucose oxidase (GOx) has been conducted by means of AC impedance measurements. In the absence of glucose, the Nyquist plots were straight line due to a reversible redox reaction involving the electron mediator. On the other hand, the Nyquist plots became a circular arc in the presence of glucose, and its radius decreased with an increase in the glucose concentration. The shape of the Nyquist plots were explained with assistance of numerical calculations. Since the reaction impedance due to direct oxidation of interfering species was significantly higher than that of glucose oxidation catalyzed by glucose oxidase, unfavorable influence from interfering species could be eliminated without needing a complicated sensor device. When ferrocenecarboxylic acid was used as an electron mediator, the calibration curve obtained from the Nyquist plots was not affected by the presence of ascorbic acid. Based on those findings, a sensing system, which has no interference from both ascorbic acid and uric acid, was fabricated by using [Os(bpy)3]Cl2, which has a highly positive redox potential (0.65 V), as an electron mediator. A method to prepare calibration curves from AC impedance measurements with a fixed frequency was established so that this technique can be put to practical use.

Acellular Invertebrate Hemoglobins as Model Therapeutic Oxygen Carriers: Unique Redox Potentials

Natural acellular polymeric hemoglobins (Hb) provide oxygen transport and delivery within many terrestrial and marine invertebrate organisms. It has been our premise that these natural acellular Hbs may serve as models of therapeutic hemoglobin-based oxygen carriers (HBOC). Our attention has focused on the acellular Hb from the terrestrial invertebrate, Lumbricus terrestris (Lt), which possesses a unique hierarchical structure and a unique ability to function extracellularly without oxidative damage. Lumbricus Hb and Arenicola Hb are resistant to autoxidation, chemical oxidation by potassium ferricyanide, and have little or no capacity to transfer electrons to Fe(+3)-complexes at 37 degrees C. An understanding of how these invertebrate acellular oxygen carriers maintain their structural integrity and redox stability in vivo is vital for the design of a safe and effective red cell substitute. We report here a positive redox potential for these giant hemoglobins that may lie at the basis for its resistance to oxidation.

Tuning the redox properties of metalloporphyrin- and metallophthalocyanine-based molecular electrodes for the highest electrocatalytic activity in the oxidation of thiols

In this work we discuss different approaches for achieving electrodes modified with N(4) macrocyclic complexes for the catalysis of the electrochemical oxidation of thiols. These approaches involve adsorption, electropolymerization and molecular anchoring using self assembled monolayers. We also discuss the parameters that determine the reactivity of these complexes. Catalytic activity is associated with the nature of the central metal, redox potentials and Hammett parameters of substituents on the ligand. Correlations between catalytic activity (log i at constant E) and the redox potential of catalysts for complexes of Cr, Mn, Fe, Co, Ni and Cu are linear with an increase of activity for more positive redox potentials. For a great variety complexes bearing the same metal center (Co) correlations between log i and E(o') of the Co(II)/Co(I) couple have the shape of an unsymmetric volcano. This indicates that the potential of the Co(II)/Co(I) couple can be tuned using the appropriate ligand to achieve maximum catalytic activity. Maximum activity probably corresponds to a DeltaG of adsorption of the thiol on the Co center equal to zero, and to a coverage of active sites by the thiol equal to 0.5.

Photocatalytic reductive destruction of azo dyes by polyoxometallates: Naphthol blue black

Various polyoxometallates (POM) PW 12O 40 3−, SiW 12O 40 4−, P 2W 18O 62 6− and P 2Mo 18O 62 6− have been used as photocatalysts for the reductive destruction of the azo dye naphthol blue black (NB). In the process POM absorb light and mediate the electron transfer from a sacrificial donor, propan-2-ol, to the azo dye. NB is rapidly destructed in the presence of POM, while toxic intermediate products, such as aromatic amine derivatives (aniline, p-nitro-aniline and p-phenylene-diamine) are observed in a multi-electron process. Increase of catalyst and/or propan-2-ol concentration accelerates the photodegradation of dye till a saturation value, while an optimum concentration of the added dye is required. The nature of the POM catalyst dictates the efficiency of the reductive decoloration of NB, following the order PW 12O 40 3− > SiW 12O 40 4− > P 2W 18O 62 6− according to their photooxidizing ability. No reaction is noticed with the 2-e-reduced molybdate, P 2Mo 18O 62 8−, which exhibits a more positive redox potential.

The Redox-Bohr Group Associated with Iron-Sulfur Cluster N2 of Complex I

Proton pumping respiratory complex I (NADH:ubiquinone oxidoreductase) is a major component of the oxidative phosphorylation system in mitochondria and many bacteria. In mammalian cells it provides 40% of the proton motive force needed to make ATP. Defects in this giant and most complicated membrane-bound enzyme cause numerous human disorders. Yet the mechanism of complex I is still elusive. A group exhibiting redox-linked protonation that is associated with iron-sulfur cluster N2 of complex I has been proposed to act as a central component of the proton pumping machinery. Here we show that a histidine in the 49-kDa subunit that resides near iron-sulfur cluster N2 confers this redox-Bohr effect. Mutating this residue to methionine in complex I from Yarrowia lipolytica resulted in a marked shift of the redox midpoint potential of iron-sulfur cluster N2 to the negative and abolished the redox-Bohr effect. However, the mutation did not significantly affect the catalytic activity of complex I and protons were pumped with an unchanged stoichiometry of 4 H(+)/2e(-). This finding has significant implications on the discussion about possible proton pumping mechanism for complex I.

Aerobic degradation of acid orange 7 in a vertical-flow constructed wetland

Biological, aerobic degradation of an azo dye and of the resultant, recalcitrant, aromatic amines in a constructed wetland (CW) was demonstrated for the first time. A vertical-flow CW, planted with Phragmites sp. was fed with 127 mg l −1 of acid orange 7 (AO7) at hydraulic loads of 28, 40, 53 and 108 l m −2day −1. Color removal efficiencies of up to 99% clearly demonstrate cleavage of the azo bond, also confirmed by the similar AO7 removal and SO 4 2− release rates revealing that adsorption onto the matrix was constant. The positive redox potential at the outlet demonstrates that aerobic conditions were present. Chemical oxygen demand and total organic carbon removal efficiencies of up to 93% were also indicative of AO7 mineralization. The degradation of sulfanilic acid was confirmed by the presence of NO 3 −, SO 4 2− and secondary metabolites, which suggest at least two degradation pathways leading to a common compound, 3-oxoadipate.

Synthesis, spectroscopic and electrochemical studies of a series of transition metal complexes with amino- or bis(bromomethyl)-substituted dppz-ligands: Building blocks for fullerene-based donor–bridge–acceptor dyads

Transition metal complexes with ligands based on dipyrido[3,2- a:2′,3′- c]phenazine (dppz) have been synthesized. As metal fragments the [Ru(bpy) 2] +, Re(CO) 3Cl and the [Cu(PPh 3) 2] + moieties have been used. The complexes containing amino- or bis(bromomethyl) substituted dppz ligands can be used for fullerene-based donor–bridge–acceptor dyads. The electronic absorption spectra of these complexes and of the dppz ligands were investigated. The dppz ligands show strong absorptions in the 300 and 390 nm region. An additional absorption band in the visible region (∼440 nm) is observed for the amino-substituted dppz-ligands. Ruthenium complexes exhibited broad absorption bands at 350–500 nm arising from intraligand-based transitions and the MLCT transition. MLCT transitions of the Re(I) and Cu(I) complexes are observed as shoulders of the stronger ligand-based absorption band tailing out to 400–500 nm. The electrochemically active complexes and ligands were studied by cyclic voltammetry and square-wave voltammetry. All ligands show one first reversible one-electron reduction located at the phenazine portion. These reductions are shifted to more positive redox potentials upon complexation. Oxidation potentials for reversible processes could be determined for the Ru 2+/Ru 3+ couple. For rhenium(I) and copper(I) complexes one irreversible oxidation process is observed.

Di-imine copper(II) complexes as redox mediator and modulator in 2-deoxy-D-ribose oxidative damage

Redox properties of copper complexes are important for their catalytic functions in vitro and in biological systems, and can contribute to their reactivity toward selected targets. In order to evaluate the influence of different ligands on the reactivity of copper ions, comparative studies were carried out with some copper(II) complexes containing a tridentate imine, or a tetradentate di-Schiff base ligand with a mixed pyridine, pyrazine, or imidazole donor set, acting as catalysts in the oxidation of 2-deoxy-D-ribose. Addition of the reducing agent glutathione (gamma-glutamylcysteinylglycine; GSH), which can also act as a good ligand for copper(I), mediated the oxidation of the substrate. For some of these compounds, a reductive activation followed by competition for the metal ion was verified, with formation of copper(I)-glutathione complex monitored by fluorescence measurements. For others, however, the reduction of the metal by the glutathione seems to not occur. In the presence of hydrogen peroxide, the oxidative damage is significantly enhanced for all the complexes tested. Redox potential measurements by cyclic voltammetry corroborated partially these results, indicating that the most reactive complexes are those with more positive redox potential. Evidence for site-specific attack to 2-deoxy-D-ribose was also observed, consistent with the intermediary formation of a copper-hydroxyl species, [LCu(II)(*OH)], rather than 'free' hydroxyl radical.

Parametric study of the factors influencing simultaneous denitrification and enhanced biological phosphorus removal

AbstractIn this study, the effect of various factors such as C:N ratio, carbon source, percentage P content in the sludge influencing the simultaneous denitrification and enhanced biological phosphorus removal was investigated in batch tests on bean and tomato waste sludge from an upflow anaerobic sludge blanket reactor–anoxic/aerobic system and municipal sludge from a circulating fluidized bed bioreactor. A correlation between the change in redox potential and rate of P release was developed. Interestingly, maximum P release was observed at positive redox potential in some of the batch tests. Simultaneous denitrification and P release under anoxic conditions was observed during all the batch tests. Sludge acclimatization improved the efficiency of the sludge and proved independency of maximum specific denitrification rate and P content of sludges. The contribution of denitrifying PAOs to anoxic P uptake was determined through the denitrification control test at an initial level of PO4‐P of 100–120 mg dm−3. Copyright © 2006 Society of Chemical Industry

New PEG-ylated Mn(iii) porphyrins approaching catalytic activity of SOD enzyme

Two new tri(ethyleneglycol)-derivatized Mn(III) porphyrins were synthesized with the aim of increasing their bioavailability, and blood-circulating half-life. These are Mn(III) tetrakis(N-(1-(2-(2-(2-methoxyethoxy)ethoxy)ethyl)pyridinium-2-yl)porphyrin, MnTTEG-2-PyP5+ and Mn(III) tetrakis(N,N'-di(1-(2-(2-(2-methoxyethoxy)ethoxy)ethyl)imidazolium-2-yl)porphyrin, MnTDTEG-2-ImP5+. Both porphyrins have ortho pyridyl or di-ortho imidazolyl electron-withdrawing substituents at meso positions of the porphyrin ring that assure highly positive metal centered redox potentials, E1/2 = +250 mV vs. NHE for MnTTEG-2-PyP5+ and E1/2 = + 412 mV vs. NHE for MnTDTEG-2-ImP5+. As expected, from established E1/2 vs. log kcat(O2 *-) structure-activity relationships for metalloporphyrins (Batinic-Haberle et al., Inorg. Chem., 1999, 38, 4011), both compounds exhibit higher SOD-like activity than any meso-substituted Mn(III) porphyrins-based SOD mimic thus far, log kcat = 8.11 (MnTTEG-2-PyP5+) and log kcat = 8.55 (MnTDTEG-2-ImP5+), the former being only a few-fold less potent in disproportionating O2*- than the SOD enzyme itself. The new porphyrins are stable to both acid and EDTA, and non toxic to E. coli. Despite elongated substituents, which could potentially lower their ability to cross the cell wall, MnTTEG-2-PyP5+ and MnTDTEG-2-ImP5+ exhibit similar protection of SOD-deficient E. coli as their much smaller ethyl analogues MnTE-2-PyP5+ and MnTDE-2-ImP5+, respectively. Consequently, with anticipated increased blood-circulating half-life, these new Mn(III) porphyrins may be more effective in ameliorating oxidative stress injuries than ethyl analogues that have been already successfully explored in vivo.

Axial versus equatorial coordination of thioether sulfur: Mixed ligand copper(II) complexes of 2-pyridyl- N-(2′-methylthiophenyl)-methyleneimine with bidentate diimine ligands

The synthesis, structure and spectral and redox properties of the copper(II) complexes [Cu(pmtpm)Cl 2] ( 1) and [Cu(pmtpm) 2](ClO 4) 2 ( 6), where pmtpm is the linear tridentate ligand 2-pyridyl- N-(2′-methylthiophenyl)methyleneimine containing a thioether and two pyridine donors, are described. Also, the mixed ligand complexes [Cu(pmtpm)(diimine)](ClO 4) 2 ( 2– 5), where the diimine is 2,2′-bipyridine ( bpy) ( 2), 1,10-phenanthroline ( phen) ( 3), 2,9-dimethyl-1,10-phenanthroline ( 2,9-dmp) ( 4) or dipyrido-[3,2-d:2′,3′- f]-quinoxaline ( dpq) ( 5), have been isolated and studied. The X-ray crystal structures of the complexes 1, [Cu(pmtpm)(2,9-dmp)](ClO 4) 2 4 and 6 have been successfully determined. The complex 1 possesses a trigonal bipyramidal distorted square based pyramidal (TBDSBP) coordination geometry in which three corners of the square plane are occupied by two nitrogens and thioether s of pmtpm ligand and the remaining equatorial and the axial positions by two chloride ions. The complex 4 contains a CuN 4S chromophore also with a TBDSBP coordination geometry in which two nitrogens and the thioether sulfur of pmtpm ligand occupy three corners of the square plane. One of the two nitrogens of 2,9-dmp ligand is equatorially coordinated and the other axially to copper. On the other hand, the complex 6 is found to possess a square based pyramidal distorted trigonal bipyramidal (SPDTBP) coordination geometry. The CuN 2S trigonal plane in it is comprised of the pyridine and imine nitrogens and the thioether sulfur of the pmtpm ligand. The pyridine nitrogens of the ligand occupy the axial positions and the second thioether sulfur remains uncoordinated. On long standing in acetonitrile solution the mixed ligand complexes 2 and 3 undergo ligand disproportionation to provide the corresponding bis-complexes of bpy and phen, respectively, and 6. The electronic and EPR spectral parameters and the positive redox potential of complex 4 are consistent with the equatorial location of the thioether sulfur in the square-based coordination geometry around copper(II). On the other hand, the higher g ∥ and lower A ∥ values and lower E 1/2 values for the complexes 2, 3 and 5 are consistent with the axial coordination of the thioether sulfur. Also, the Cu(II)/Cu(I) redox potentials increase with increase in number of aromatic rings of the diimine ligand. The steric and electronic effects of the bidentate diimine ligands in orienting the thioether coordination to axial or equatorial position are discussed.

Regulation of nitrate reductase activity in Mycobacterium tuberculosis by oxygen and nitric oxide

Nitrate reduction by Mycobacterium tuberculosis is regulated by control of the transport of nitrate into the cell by NarK2. When oxygen was introduced into hypoxic cultures, nitrite production was quickly inhibited. The nitrate-reducing enzyme itself is relatively insensitive to oxygen, suggesting that the inhibition of nitrite production by oxygen was a result of interference with nitrate transport. This was not due to degradation of NarK2, as the inhibition was reversed by the removal of oxygen although chloramphenicol prevented new synthesis of NarK2. The oxidant potassium ferricyanide was added to anaerobic cultures to produce a positive redox potential in the absence of oxygen. Nitrite production decreased, signifying that oxidizing conditions, rather than oxygen itself, were responsible for the inhibition of nitrate transport. Nitric oxide added to cultures allowed NarK2 to be active even in the presence of oxygen. A similar result was obtained with hydroxylamine and ethanol, both of which interfere with oxygen utilization and the electron transport chain. It is proposed that NarK2 senses the redox state of the cell, possibly by monitoring the flow of electrons to cytochrome oxidase, and adjusts its activity so that nitrate is transported under reducing, but not under oxidizing, conditions.

Copper(II) complexes of sterically hindered Schiff base ligands: Synthesis, structure, spectra and electrochemistry

The synthesis, structure and spectral and redox behaviour of copper(II) complexes of sterically constrained N 3, N 3S 2 and N 2S 2 ligands are described. The ligands 2,6-bis(2,4,6-trimethylphenyliminomethyl)pyridine ( L1), 2,6-bis(2,6-diisopropylphenyliminomethyl)pyridine ( L2), 2,6-bis(2-methylthiophenyliminomethyl)pyridine ( L3), 1,3-bis(2-methylthiophenyliminomethyl)benzene ( L4), 1,3-bis(2,4,6-trimethylphenyliminomethyl)benzene ( L5) and 1,3-bis(2,6-diisopropyl-phenyliminomethyl)benzene ( L6) have been synthesized. The 1:1 copper(II) complexes of L1– L5 have been isolated and characterized by spectral and electrochemical techniques. The X-ray crystal structure of the complex [Cu(L2)Cl 2] ( 2) has been successfully determined and is found to possess a distorted square pyramidal coordination geometry. The replacement of isopropyl groups in this complex by the less sterically demanding methyl groups as in [Cu(L1)Cl 2] leads to a decreased geometric distortion. The complexes show relatively high positive redox potentials with a low reorganizational energy barrier for electron transfer involving the Cu(II)/Cu(I) couple. The redox cycle of the complexes 1 and 2 has been explained by assuming a square scheme. The spectral and electrochemical properties of CuN 3S 2 and CuN 2S 2 complexes generated by the incorporation of thioether sulfurs into L1/ L2 and L4/ L5, respectively, are also discussed. Both the complexes show positive redox potentials suggesting the coordination of at least one thioether sulfur to copper(II). The incorporation of a pyridine nitrogen donor into the CuN 2S 2 complexes leads to higher LF field strengths but with lower absorptivities. The coordination of Cl − ions to the CuN 3S 2 and CuN 2S 2 complexes significantly alter the spectral and redox properties of the complexes.

Nickel foam and carbon felt applications for sodium polysulfide/bromine redox flow battery electrodes

The first use of nickel foam (NF) as electrocatalytic negative electrode in a polysulfide/bromine battery (PSB) is described. The performance of a PSB employing NF and polyacrylonitrile (PAN)-based carbon felt (CF) as negative and positive electrode materials, respectively, was evaluated by constant current charge-discharge tests in a single cell. Charge/discharge curves of the cell, positive and negative electrodes show that the rapid fall in cell voltage is due to the drop of positive potential caused by depletion of Br2 dissolved in the catholyte at the end of discharge. Cell voltage efficiency was limited by the relatively high internal ohmic resistance drop (iR drop). Polarization curves indicated that both NF and CF have excellent catalytic activity for the positive and negative redox reactions of PSB. The average energy efficiency of the single cell designed in this work could be as high as 77.2% at 40mAcm−2 during 48 charge-discharge cycles.

Hierarchical Control of Anaerobic Gene Expression in Escherichia coli K-12: the Nitrate-Responsive NarX-NarL Regulatory System Represses Synthesis of the Fumarate-Responsive DcuS-DcuR Regulatory System

Hierarchical control ensures that facultative bacteria preferentially use the available respiratory electron acceptor with the most positive standard redox potential. Thus, nitrate is used before other electron acceptors such as fumarate for anaerobic respiration. Nitrate regulation is mediated by the NarX-NarL two-component system, which activates the transcription of operons encoding nitrate respiration enzymes and represses the transcription of operons for other anaerobic respiratory enzymes, including enzymes involved in fumarate respiration. These are fumarate reductase (encoded by the frdABCD operon), fumarase B, which generates fumarate from malate, and the DcuB permease for fumarate, malate, and aspartate. The transcription of the corresponding structural genes is activated by the DcuS-DcuR two-component system in response to fumarate or its dicarboxylate precursors. We report results from preliminary transcription microarray experiments that revealed two previously unknown members of the NarL regulon: the aspA gene encoding aspartate-ammonia lyase, which generates fumarate; and the dcuSR operon encoding the dicarboxylate-responsive regulatory system. We measured beta-galactosidase expression from monocopy aspA-lacZ, frdA-lacZ, and dcuS-lacZ operon fusions in response to added nitrate and fumarate and with respect to the dcuR and narL genotypes. Nitrate, acting through the NarX-NarL regulatory system, repressed the transcription of all three operons. Only frdA-lacZ expression, however, was responsive to added fumarate or a dcuR(+) genotype. Phospho-NarL protein protected operator sites in the aspA and dcuS promoter regions from DNase I cleavage in vitro. The overall results are consistent with the hypothesis that nitrate represses frdA operon transcription not only directly, by repressing frdA promoter activity, but also indirectly, by repressing dcuS promoter activity.

Diatom plastids possess a phosphoribulokinase with an altered regulation and no oxidative pentose phosphate pathway.

The chloroplast enzyme phosphoribulokinase (PRK; EC 2.7.1.19) is part of the Calvin cycle (reductive pentose phosphate pathway) responsible for CO(2) fixation in photosynthetic organisms. In green algae and vascular plants, this enzyme is light regulated via reversible reduction by reduced thioredoxin. We have sequenced and characterized the gene of the PRK from the marine diatom Odontella sinensis and found that the enzyme has the conserved cysteine residues necessary for thioredoxin-dependent regulation. Analysis of enzymatic activity of partially purified diatom enzyme and of purified protein obtained by native overexpression in Escherichia coli, however, revealed that under natural redox conditions the diatom enzyme is generally active. Treatment of the enzyme with strong oxidants results in inhibition of the enzyme, which is reversible by subsequent incubation with reducing agents. We determined the redox midpoint potentials of the regulatory cysteine in the PRK from O. sinensis in comparison to the respective spinach (Spinacia oleracea) enzyme and found a more positive redox potential for the diatom PRK, indicating that in vivo this enzyme might not be regulated by thioredoxin. We also demonstrate that in protease-treated diatom plastids, activities of enzymes of the oxidative pentose phosphate pathway are not detectable, thus reducing the need for a tight regulation of the Calvin cycle in diatoms. We discuss our results in the context of rearrangements of the subcellular compartmentation of metabolic pathways due to the peculiar evolution of diatoms by secondary endocytobiosis.

The effect of the redox-potential on the retention of phosphorus in a small constructed wetland

Building wetlands in small arable streams is a popular supplement to best management practice on arable fields. Particle bound phosphorus settles in the small constructed wetlands (CWs), receiving agricultural diffuse pollution. The sorption behavior of phosphorus is, however, redox-sensitive, and bound phosphorus may be remobilized in periods with low redox potential. This paper investigates changes in the redox potential in the free water of wetland Berg (Norway) during a three-year period, and how these redox changes affect the total phosphorus (TP) and total reactive phosphorus (TRP) retention. Despite eutrophic conditions in the wetland, the redox potential was never negative, and usually higher than 400 mV, indicating aerobic conditions. The relative retention was 440% and 43% for TP and TRP, respectively. The specific retention was 100 g TP and 43 g TRPm(-2) yr(-1). Loss of phosphorus was only observed during less than 19% of the total period of time. The net loss was less than 5% of the specific retention. The high positive redox potential probably conserves the redox-sensitive phosphorus in the wetland sediment as long as water fows through the CW.