N2 fixation by purified components of the N2-fixing system of Clostridium pasteurianum

1. N₂ fixation, reductant-supported ATP utilization and ATP-dependent H₂ evolution are catalyzed by combined molybdoferredoxin and azoferredoxin, the two purified components of the N₂-fixing system of Clostridium pasteurianum. No additional protein components are needed. At this stage of purity, neither of the two components of the N₂-fixing system catalyzed any of these reactions without the other.

2. The utilization of ATP by the N₂-fixing system involves a single P_i elimination; the rate of P_i release from ATP is exactly matched by ADP formation. The product ADP inhibits each of the reactions catalyzed by the enzyme components.

3. Aided by a more sensitive colorimetric analysis for NH₃ with ninhydrin, N₂ fixation is routinely obtained with ATP as the sole source of energy. No ATP-generating system is needed.

4. With the purified components, the stoichiometry of P_i released per electron pair for H₂ evolution and N₂ fixation is 4.     

A dicopper complex with distant metal centers. Structure, magnetic properties, electrochemistry and catecholase activity

The crystal structure and magnetic properties of a dinuclear copper(II) complex of the ligand [2,8-dimethyl-5,11-di-(dimethylethyleneamine) 1,4,5,6,7,10,11,12-octahydroimidazo [4,5-h] imidazo [4,5-c] [1,6]diazecine] dimeim have been investigated. Also, its catecholase activity has been explored in different solvent mixtures: MeCN/H₂O and OH/H₂O, each at several pH values. In CH₃OH/H₂O, where the activity was superior, the optimal pH value for the catalytic activity was found to be lower than in CH₃CN/H₂O. The study of the complex’s electrochemical behavior (cyclic voltammetry) which was also investigated in these various media, revealed that although an increase in pH in both solvent mixtures results in an increase both in Me oxidizing power (E_1/2) and reversibility (i_pa/i_pc) the change of solvent system seems to be a more influencing factor. The superior catalytic activity found in MeOH/H₂O_pH=8.0, is associated with a significantly more reversible behavior displayed in this medium. Potentiometric determination of the overall formation constant and three successive pK_as for the complex, suggest the formation of stable hydroxo complexes which could be the catalytically active species.     

Turning point in apoptosis/necrosis induced by hydrogen peroxide

The turning point between apoptosis and necrosis induced by hydrogen peroxide (H₂O₂) have been investigated using human T-lymphoma Jurkat cells. Cells treated with 50 μM H₂O₂ exhibited caspase-9 and caspase-3 activation, finally leading to apoptotic cell death. Treatment with 500 μM H₂O₂ did not exhibit caspase activation and changed the mode of death to necrosis. On the other hand, the release of cytochrome c from the mitochondria was observed under both conditions. Treatment with 500 μM H₂O₂, but not with 50 μM H₂O₂, caused a marked decrease in the intracellular ATP level; this is essential for apoptosome formation. H₂O₂-reducing enzymes such as cellular glutathione peroxidase (cGPx) and catalase, which are important for the activation of caspases, were active under the 500 μM H₂O₂ condition. Prevention of intracellular ATP loss, which did not influence cytochrome c release, significantly activated caspases, changing the mode of cell death from necrosis to apoptosis. These results suggest that ATP-dependent apoptosome formation determines whether H₂O₂-induced cell death is due to apoptosis or necrosis.     

Activation of CO2 fixation in isolated spinach chloroplasts

1. 1. The effect of the Mg²⁺ concentration on the CO₂ fixation activity in situ in isolated and intact spinach chloroplasts upon suspension in hypotonic medium was examined. CO₂ fixation in the dark was activated 25–100 fold by 20 mM Mg²⁺ in the presence of added ATP plus either ribulose 5-phosphate or ribose 5-phosphate. 20 mM Mg²⁺-stimulated fixation only 2–3 fold in the presence of the substrate of fixation, ribulose 1,5-diphosphate. The highest Mg²⁺-stimulated rate of fixation in the dark observed with chloroplasts was 480 μmoles CO₂ fixed per mg chlorophyll per h.

2. 2. The concentration of bicarbonate at half of the maximal velocity (apparent K_m) during the Mg²⁺-stimulated fixation of CO₂ was 0.4 mM in the presence of ATP plus ribose 5-phosphate and 0.6 mM with ribulose 1,5-diphosphate.

3. 3. Dithioerythritol or light enhanced Mg²⁺-stimulated CO₂ fixation 1–3 fold in the presence of ATP plus ribose 5-phosphate but not ribulose 1,5-diphosphate.

4. 4. These results indicate that Mg²⁺ fluxes in the stroma of the chloroplast could control the activity of the phosphoribulokinase with a lesser effect on the ribulosediphosphate carboxylase. An increase in Mg²⁺ of 6–10 mM in the stroma region of the chloroplast would be enough to activate CO₂ fixation during photosynthesis.

Oxygen inhibition of nitrogen fixation in cell-free preparations of blue-green algae

1. Factors inhibiting N₂ fixation in cell-free preparations of two blue-green algae, Anabaena cylindrica and Chlorogloea fritschii were investigated.

2. ¹⁵N uptake by particulate cell-free fractions of A. cylindrica was less in the light than in the dark, indicating some photo-inhibition of N₂ fixation.

3. Anaerobic conditions during preparation and incubation of particulate cell-free fractions of C. fritschii greatly increased their N₂-fixing ability.

4. Soluble cell-free material was found to have an inhibitory effect on N₂ fixation by particulate cell-free preparations obtained from A. cylindrica. This inhibition was approximately proportional to the amount of soluble fraction supplied to the particulate material. Dialysis of the soluble cell-free fraction did not remove the inhibition which has therefore been attributed to the presence of a soluble enzyme, possibly an oxidase.

5. Dialysis of the soluble cell-free fraction or of particulate material suspended with soluble cell-free fraction resulted in a slight decrease of ¹⁵N uptake, indicating the removal by dialysis of small-molecule factors required for N₂ fixation.

6. The presence of sulphydryl agents increased the rate of N₂ fixation in cell-free preparations of A. cylindrica.     

A high-activity ATP translocator in mesophyll chloroplasts of Digitaria sanguinalis, a plant having the C-4 dicarboxylic acid pathway of photosynthesis

The effect of exogenous adenine nucleotides on CO₂ fixation and oxygen evolution was studied with mesophyll protoplast extracts of the C₄ plant Digitaria sanguinalis. Exogenous ATP was found to stimulate the rate of pyruvate and pyruvate + oxalacetate induced CO₂ fixation, as well as reverse the inhibition of CO₂ fixation by carbonyl cyanide m-chlorophenyl hydrazone and several electron transport inhibitors. The ATP-dependent stimulation of CO₂ fixation varied from 40 to 70 μmol CO₂ fixed/mg chlorophyll per h, suggesting that ATP was crossing the chloroplast membranes at rates of 80–140 μmol/mg chlorophyll per h, since 2 ATP are required for each CO₂ fixed. Fixation of CO₂ could also be induced in the dark by exogenous ATP, in which case ADP accumulated outside the chloroplasts. This suggests that external ATP is exchanging for internal ADP. In contrast, ADP and AMP were found not to traverse chloroplast membranes, on the basis that neither nucleotide inhibited CO₂ fixation or stimulated oxygen evolution that was limited by available ADP for phosphorylation. Further evidence that ATP can enter the chloroplasts was obtained by direct measurements of the increase in ATP in the chloroplasts due to addition of exogenous ATP in the dark. These studies yielded minimal rates of ATP uptake on the order of 30–40 μmol/mg chlorophyll per h. It is suggested that a membrane translocator exists that specifically transports ATP into the chloroplasts in exchange for ADP. The significance of these findings are considered with respect to the C₄ pathway of photosynthesis.     

Acetylene reduction with physiological electron donors by extracts and particulate fractions from nitrogen-fixing Azotobacter chroococcum

1. 1. Preparations were obtained from Azotobacter chroococcum which reduced acetylene to ethylene using physiological electron donors instead of sodium dithionite. These preparations fell into two categories: those which required catalytic amounts of benzyl viologen for acetylene reduction and those that did not.

2. 2. Acetylene reduction without benzyl viologen or sodium dithionite was observed only with particles that sedimented at 40 000 × g after disrupting bacteria in the French press or with preparations obtained by disrupting bacteria protected by a mixture of defatted bovine serum albumin-Ficoll-MgCl₂ with liquid N₂; supernatant fractions required benzyl viologen for acetylene reduction.

3. 3. Added ATP inhibited acetylene reduction by large particles; ATP and MgCl₂ were necessary for maximum acetylene reduction with bovine serum albumin-protected preparations.

4. 4. NADH and carbon substrates acted as electron donors but H₂ did not; NAD⁺ was necessary for maximum acetylene reduction with carbon substrates.

5. 5. Anaerobic conditions were necessary for maximum acetylene reduction in all cases.

The acceptor specificity of flavins and flavoproteins. II. Free flavins

1. For comparison with flavoprotein oxidases, a study has been made of free flavins in the reduced form with respect to the specificity and stoichiometry of their oxidation by a series of acceptors.

2. Reduced flavins uncombined with proteins show very little acceptor specificity and react very rapidly with nearly all the commonly used acceptors. Their behaviour resembles that of dithionite very closely indeed, and it differs considerably from that of flavoproteins. Like dithionite, free reduced flavins reduce O₂ quantitatively to H₂O₂; this oxidizes a further molecule of flavin.

3. H₂O₂ and cytochrome c react more slowly than most acceptors with reduced flavins. Nitrate and NDA⁺ do not act at all and require special activation.

4. Catalase can act as a catalyst for the aerobic oxidation of flavins by converting slowly-reacting H₂O₂ into rapidly-reacting O₂.

5. In the absence of catalytic metals ascorbate reacts with acceptors much more slowly than reduced flavins do.     

Thermal and photochemical substitution reactions of CpRe(PPh3)2H2 and CpRe(PPh3)H4. Catalytic insertion of ethylene into the C-H bond of benzene

The thermal and photochemical reactions of CpRe(PPh₃)₂H₄ and CpRe(PPh₃)H₄ (Cp = η⁵-C₅H₅) with PMe₃, P(p-tolyl)₃, PMe₂Ph, DMPE, DPPE, DPPM, CO, 2,6-xylylisocyanide and ethylene have been examined. While CpRe(PPh₃)₂H₂ is thermally inert, it will undergo photochemical substitution of one or two PPh₃ ligands. With ethylene, substitution is followed by insertion of the olefin into the C-H bond of benzene, giving ethylbenzene. CpRe(PPh₃)H₄ undergoes thermal loss of PPh₃, which leads to substituted products of the type CpRe(L) H₄. Photochemically, reductive elimination of dihydrogen occurs preferentially. The complex trans-CpRe(DMPE)H₂ was structurally characterized, crystallizing in the monoclinic space group P2₁/n (No. 14) with a = 6.249(6), b = 16.671(8), c = 13.867(7) Å, β = 92.11(6)°, V = 1443.7(2.9) Å and Z = 4. The complex trans-CpRe(PMe₂Ph)₂H₂ was structurally characterized, crystallizing in the monoclinic space group P2₁/n (No. 14) with a = 7.467(3), b = 23.874(14), c = 11.798(6) Å, β = 100.16(4)°, V = 2070.2(3.4) ų and Z = 4.     

One-dimensional polymeric chlorocadmate(II) systems of N-methylpiperazinium and N,N′-dimethylpiperazinium compounds: synthesis, structural and thermal properties

The chlorocadmate(II) systems of (H₂me₂pipz)[Cd₂Cl₆(H₂O)₂] (1) and (H₂mepipz)₂[Cd₃Cl₁₀(H₂O)] (2) (L = me₂pipz = N,N′-dimethylpiperazine; L′ = mepipz = N-methylpiperazine) were prepared and their structural and thermal properties investigated. Compound 1 is monoclinic, space group P2₁/c, A = 7.664(1), B = 7.472(4), C = 15.347(1) Å, β = 99.468(7)°, Z = 2, R = 0.024. The crystal structure consists of organic cations and infinite one-dimensional chains of [CdCl₃(H₂O)]_n³⁻ anions. Each Cd atom is octahedrally surrounded by bridged and terminal chlorine atoms and by a water molecule, which is in trans position with respect to the terminal chlorine atom. Inter- and intrachain hydrogen bond interactions between the terminal chlorine atoms and the water molecules contribute to the crystal packing. Compound 2 is orthorhombic, space group Cmc2₁, A = 15.286(3), B = 13.354(3), C = 13.154(3) Å, R = 0.023. The crystal structure consists of organic dications and infinite chains of [Cd₂Cl₆(CdCl₄H₂O]_n⁴⁻ units running along the [001] axis. Each unit is formed of regularly alternate six-coordinated Cd atoms, one of them linking one pentacoordinated Cd atom which completes its coordination througha water molecule. A strong hydrogen bond interaction involving the organic dication and the inorganic chain contributes to the crystal packing. Differential hydrogen bond interaction involving the organic dication and the inorganic chain contributes to the crystal packing. Differential scanning calorimetry measurements did not show the presence of any structural phase transitions. The structures are compared with those of (H₂pipz)[Cd₂Cl₆(H₂O)₂] (3), (H₂mepipz)[Cd₂Cl₆(H₂O)₂]·H₂O (4) and (H₂mepipz)[Cd₂Cl₆] (5) (L = pipz = piperazine, L′ = mepipz = N-ethylpiperazine).     

Aging, cytochrome oxidase activity, and hydrogen peroxide release by mitochondria

The objective of this study was to explore the possible cause(s) underlying the previously observed, age-related increase in the rate of mitochondrial H₂O₂ release in the housefly. The hypothesis that an imbalance between different respiratory complexes may be a causal factor was tested. Cytochrome c oxidase activity was found to sharply decline in the latter part of the life span of the flies. Effects of different substrates and respiratory inhibitors were determined in order to ascertain if a decrease in cytochrome c oxidase activity could be responsible for the increased H₂O₂ release. H₂O₂ was measured spectrofluorometrically using horseradish peroxidase and p-hydrophenylacetate as an indicator. Neither NADH-linked substrates nor succinate caused a stimulation of H₂O₂ production. H₂O₂ release by mitochondria, inhibited with rotenone and antimycin A, was greatly increased upon supplementation with -glycerophosphate; however, the further addition of KCN or myxothiazol, to such preparations, caused a depression of H₂O₂ generation. In contrast, relatively low concentrations of KCN or myxothiazol were found to stimulate H₂O₂ release in insect mitochondria supplemented with -glycerophosphate and exposed to rotenone, but not antimycin A. Results are interpreted to suggest that partial inhibition of cytochrome c oxidase activity can lead to the stimulation of mictochondrial H₂O₂ production in the housefly at site(s) other than NADH dehydrogenase and ubisemiquinone/ cytochrome b region; a possible source may be glycerophosphate dehydrogenase.     

低温对离体大鼠心室肌复极时程的影响及其机制

目的: 观察低温对离体大鼠心室肌复极时程及Kir2.1蛋白表达的影响,探讨Kir2.1蛋白在其中的作用。 方法:18只健康雄性SD大鼠, 随机分为3组(n=6),即正常对照组(C组)、35℃低温组(H₁组)和32℃低温组(H₂组)。制备Langendorff离体心脏灌注模型,37℃ K-H液平衡灌注15 min后,C组继续灌注37℃ K-H液30 min;H₁组继续灌注35℃的K-H液30 min,H₂组继续灌注32℃的K-H液30 min。记录各组平衡灌注15 min(T₁)和继续灌注30 min(T₂)时HR、左心室前壁三层心肌单相动作电位,计算单相动作电位复极50％、90％的时程(MAPD₅₀、MAPD₉₀)和跨室壁复极离散度(TDR),同时记录心律失常发生情况。取测量电生理的心室肌部位组织以Western blot测定Kir2. 1蛋白表达,以免疫组织化学法测量Kir2. 1蛋白的平均光密度值(AOD)及分布情况。结果: 与T₀比较,T₁时H₁组和H₂组HR显著减慢(P＜0.05), MAPD₅₀、MAPD₉₀显著延长(P＜0.05),TDR显著增大(P＜ 0.05);与C组比较,T₁时H₁组和H₂组HR显著减慢(P＜0.05),MAPD₉₀显著延长(P＜0.05),TDR显著增大(P＜ 0.05),Kir2.1蛋白表达显著减少(P＜0.05), AOD值显著减少(P＜0.05)。与H₁组比较,T₁时H₂组心率显著减慢(P＜0.05),MAPD₅₀、MAPD₉₀显著延长(P＜0.05),TDR显著增大(P＜0.05)。C组Kir2.1蛋白分布正常,H₁、H₂组蛋白分布紊乱。结论: 低温会延长心室复极时程,增加复极离散度,其机制与低温下调Kir2.1蛋白表达、改变Kir2.1蛋白分布有关。     

Lethality of hydrogen peroxide in wild type and superoxide dismutase mutants of Escherichia coli. (A hypothesis on the mechanism of H2O2-induced inactivation of Escherichia coli)

The toxicity of H₂O₂ in Escherichia coli wild type and superoxide dismutase mutants was investigated under different experimental conditions. Cells were either grown aerobically, and then treated in M9 salts or K medium, or grown anoxically, and then treated in K medium. Results have demonstrated that the wild type and superoxide dismutase mutants display a markedly different sensitivity to both modes of lethality produced by H₂O₂ (i.e. mode one killing, which is produced by concentrations of H₂O₂ lower than 5 mM, and mode two killing which results from the insult generated by concentrations of H₂O₂ higher than 10 mM). Although the data obtained do not clarify the molecular basis of H₂O₂ toxicity and/or do not explain the specific function of superoxide ions in H₂O₂-induced bacterial inactivation, they certainly demonstrate that the latter species plays a key role in both modes of H₂O₂ lethality. A mechanism of H₂O₂ toxicity in E. coli is proposed, involving the action of a hypothetical enzyme which should work as an O₂^-• generating system. This enzyme should be active at low concentrations of H₂O₂ (<5 mM) and high concentrations of the oxidant (>5 mM) should inactivate the same enzyme. Superoxide ions would then be produced and result in mode one lethality. The resistance at intermediate H₂O₂ concentrations may be dependent on the inactivation of such enzyme with no superoxide ions being produced at levels of H₂O₂ in the range 5–10 mM. Mode two killing could be produced by the hydroxyl radical in concert with superoxide ions, chemically produced via the reaction of high concentrations of H₂O₂ (>10 mM) with hydroxyl radicals. The rate of hydroxyl radical production may be increased by the higher availability of Fe²⁺ since superoxide ions may also reduce trivalent iron to the divalent form.     

Purification of cytochrome c peroxidase for monitoring H2O2 production.

One of the most precise methods of determining hydrogen peroxide (H₂O₂) formation by biological systems is based on measuring the rate of enzyme-substrate complex formation between H₂O₂ and cytochrome c peroxidase (CCP). The main problem with this method is that CCP is not commercially available and has to be prepared in the laboratory. We have modified some currently available methods for purifying a highly active preparation of CCP in about 4 d. It includes a batch extraction of protein using DEAE-sepharose followed by concentration either by lyophilization or by passing the extract through a small DEAE-sepharose column instead of by ultrafiltration. The concentrated preparation is passed through a Sephadex G-75 column and the final CCP crystallized against water. The final preparations had a purity index (PI, ratio of absorbance at 408 nm/280 nm, equivalent to heme/protein ratio) above 1.2. These changes make the overall procedure very simple, preserving enzyme activity and spectral properties. In addition, we point out that special care has to be taken to eliminate cytochrome c from crude CCP extracts. Cytochrome c not only introduces an artifact when determining PI, but is also may act as a hydrogen donor for CCP when monitoring H₂O₂ formation, thus decreasing the sensitivity of this method.     

Phenotypic analysis of the ccp1Delta and ccp1Delta-ccp1W191F mutant strains of Saccharomyces cerevisiae indicates that cytochrome c peroxidase functions in oxidative-stress signaling

Yeast cytochrome c peroxidase (CCP) efficiently catalyzes the reduction of H₂O₂ to H₂O by ferrocytochrome c in vitro. The physiological function of CCP, a heme peroxidase that is targeted to the mitochondrial intermembrane space of Saccharomyces cerevisiae, is not known. CCP1-null-mutant cells in the W303-1B genetic background (ccp1Δ) grew as well as wild-type cells with glucose, ethanol, glycerol or lactate as carbon sources but with a shorter initial doubling time. Monitoring growth over 10 days demonstrated that CCP1 does not enhance mitochondrial function in unstressed cells. No role for CCP1 was apparent in cells exposed to heat stress under aerobic or anaerobic conditions. However, the detoxification function of CCP protected respiring mitochondria when cells were challenged with H₂O₂. Transformation of ccp1Δ with ccp1^W191F, which encodes the CCP^W191F mutant enzyme lacking CCP activity, significantly increased the sensitivity to H₂O₂ of exponential-phase fermenting cells. In contrast, stationary-phase (7-day) ccp1Δ-ccp1^W191F exhibited wild-type tolerance to H₂O₂, which exceeded that of ccp1Δ. Challenge with H₂O₂ caused increased CCP, superoxide dismutase and catalase antioxidant enzyme activities (but not glutathione reductase activity) in exponentially growing cells and decreased antioxidant activities in stationary-phase cells. Although unstressed stationary-phase ccp1Δ exhibited the highest catalase and glutathione reductase activities, a greater loss of these antioxidant activities was observed on H₂O₂ exposure in ccp1Δ than in ccp1Δ-ccp1^W191F and wild-type cells. The phenotypic differences reported here between the ccp1Δ and ccp1Δ-ccp1^W191F strains lacking CCP activity provide strong evidence that CCP has separate antioxidant and signaling functions in yeast.     

Bactericidal Activity of Catecholamine Copper Complexes

Washed or growing E. coli cells are killed by epinephrine, norepinephrine or dopamine in the presence of non lethal concentrations of Cu(II). Killing is enhanced by anoxia and by sublethal Concentrations of H₂O₁. The rate of killing is proportional to the rate of catecholamine oxidation. The copper epinephrine complex binds to E. coli cells, induces membrane damage and depletion of the cellular ATP pool. The cells may be partially protected by SOD or catalase but not by OH radical scavengers. Addition of H²O₂ to cells which were sensitized by preincubation with the epinephrine-copper complex, causes rapid killing and DNA degradation. Sensitized cells are not protected by BSA.     

Development and characterization in vitro of a catalase-based biosensor for hydrogen peroxide monitoring

There is increasing evidence that hydrogen peroxide (H₂O₂) may act as a neuromodulator in the brain, as well as contributing to neurodegeneration in diseased states, such as Parkinson's disease. The ability to monitor changes in endogenous H₂O₂ in vivo with high temporal resolution is essential in order to further elucidate the roles of H₂O₂ in the central nervous system. Here, we describe the in vitro characterization of an implantable catalase-based H₂O₂ biosensor. The biosensor comprises two amperometric electrodes, one with catalase immobilized on the surface and one without enzyme (blank). The analytical signal is then the difference between the two electrodes. The H₂O₂ sensitivity of various designs was compared, and ranged from 0 to 56 ± 4 mA cm⁻² M⁻¹. The most successful design incorporated a Nafion^® layer followed by a poly-o-phenylenediamine (PPD) polymer layer. Catalase was adsorbed onto the PPD layer and then cross-linked with glutaraldehyde. The ability of the biosensors to exclude interference from ascorbic acid, and other interference species found in vivo, was also tested. A variety of the catalase-based biosensor designs described here show promise for in vivo monitoring of endogenous H₂O₂ in the brain.     

Structures and magnetism of rubidium and cesium salts of dimeric oxovanadium(IV) tartrate(4−) complexes

Complexes of type A₄[VO(tart)]₂·nH₂O, where A = Rb or Cs and tart =d,l-tartrate(4−) (n = 2) or d,d-tartrate(4−) (n = 2 for Rb and n = 3 for Cs), were prepared from an aqueous mixture of V₂O₅, AOH and H₄tart. These complexes were studied by single-crystal X-ray diffraction methods: Rb₄[VO(d,l-tart)]₂·2H₂O, space group P1 with a = 8.156(1),b = 8.246(1),c = 8.719(1)Å, = 66.09(1)°, β = 65.07(1)°, γ = 82.40(1)°,Z = 2, 1917 observed reflections, and final R_w = 0.035; Cs₄[VO(d,l-tart)]₂·2H₂O, space group P₂₁/c with a = 9.350(1),b = 13.728(2),c = 8.479(1)Å, β = 106.77(1)°,Z = 4, 2235 observed reflections, and final R_w = 0.054; Rb₄[VO(d,d-tart)]₂·2H₂O, space group P₄₁₂₂ with a = 8.072(1),c = 32.006(3)Å,Z = 8, 1014 observed reflections and final R_w = 0.038; Cs₄[VO(d,d-tart)]₂·3H₂O, space group P₁₂₂ with a = 8.184(1),c = 33.680(5)Å,Z = 8, 1310 observed reflections, and final R_w = 0.063. Bulk magnetic susceptibility data (1.5–300 K) for these compounds and A₄[VOl,l-tart)]₂·nH₂O (A = Rb, Cs) were obtained on polycrystalline samples. These data were analyzed in terms of a Van Vleck exchange coupled S = 1/2 model which was modified to include an interdimer exchange parameters Θ. Analysis of the low-temperature (1.5–20 K) susceptibility data gave 2J = +1.30 cm⁻¹ and Θ = −1.86 K for Rb₄[VO(d,l-tart)]₂·2H₂O, 2J = +1.16 cm⁻¹ and Θ = −1.69 K for Cs₄[VO(d,l-tart)]₂·2H₂O, 2J = +1.90 cm⁻¹ and Θ = −0.82 K for Rb₄[VO(d,d-tart)]₂·2H₂O, 2J = +2.04 cm⁻¹ and Θ = −0.80 K for Rb₄[VO(l,l-tart)]₂·2H₂O, 2J = +1.52 cm⁻¹ and Θ = −0.25 K for Cs₄[VO(d,d-tart)]₂·3H₂O, and 2J = +1.64 cm⁻¹ and Θ = −0.31 K for Cs₄[VO(l,l-tart)]₂·3H₂O. These results suggest the magnitudes of intradimer (ferromagnetic and interdimer (antiferromagnetic) exchange interactions are similar in these complexes, as observed for the analogous Na salts.     

Evidence for a chemiosmotic mechanism of ATP synthesis in methanogenic bacteria

Recent studies on methanogenesis from methanol and H₂ in Methanosarcina barkeri have provided the first proof of a chemiosmotic mechanism of ATP synthesis in methanogenic bacteria.     

The nitrogen fixation system of photosynthetic bacteria II. Chromatium nitrogenase activity linked to photochemically generated assimilatory power

The nitrogenase activity (measured by N₂ or acetylene reduction) of cell-free extracts of the photosynthetic bacterium Chromatium was coupled to photochemically generated ATP and reductant. The ATP was formed through cyclic photophosphorylation by bacterial chromatophores. The reductant (reduced ferredoxin) was generated by a heated preparation (incapable of O₂ and ATP production) of spinach chloroplasts. The nitrogenase activity of Chromatium extracts was supported by reduced Chromatium or Clostridium pasteurianum ferredoxin but not by that of spinach chloroplasts.