C2H2 Reduction Research Articles

Ultrathin Metal-Organic Framework Nanosheets for Selective Photocatalytic C2 H2 Semihydrogenation in Aqueous Solution.

The selective semihydrogenation of C2 H2 to C2 H4 in crude C2 H4 (with ~1 vol % C2 H2 contamination) is a crucial process in the manufacture of polyethylene. Comparing to conventional thermalcatalytic route with Pd as catalyst under high temperature with H2 as hydrogen source, photocatalytic C2 H2 reduction reaction with H2 O as hydrogen source can achieve high selectivity under milder conditions, but has rarely been reported. Here, we present a kind of ultrathin metal-organic framework nanosheets (Cu-Co-MNSs) that demonstrate excellent catalytic activities in the semihydrogenation of C2 H2 . Employing Ru(bpy)3 2+ as the photosensitizer, this catalyst attains a noteworthy turnover number (TON) of 2124 for C2 H4 , coupled with an impressive selectivity of 99.5 % after 12 h visible light irradiation. This performance is comparable to molecular catalysts and notably surpasses the efficiency of bulk metal-organic framework materials. Furthermore, Cu-Co-MNSs achieve a 99.95 % conversion of C2 H2 under industrial relevant conditions (1.10 % C2 H2 in C2 H4 ) with 90.3 % selectivity for C2 H4 over C2 H6 , demonstrating a great potential for polymer-grade C2 H4 production.

In-situ laser diagnostic and numerical investigations of soot formation characteristics in ethylene and acetylene counterflow diffusion flames blended with dimethyl carbonate and methyl formate

Oxygenated fuels have emerged as promising energy carriers for storing renewable energy given their compatibility with current energy systems and their potential of reducing pollutant emissions when burning in blends with traditional fossil fuels. In particular, dimethyl carbonate (DMC) and methyl formate (MeFo) have raised increasing interest for their use as gasoline substitutes in recent years. However, the effect of fuel blending on soot formation remains unclear, which prevents the utilization of the full potential of these fuels. In this work, soot formation of ethylene-based and acetylene-based fuel blended with DMC and MeFo in laminar counterflow diffusion flames was investigated by laser-induced incandescence (LII) in combination with laser-induced breakdown spectroscopy (LIBS) and spontaneous Raman scattering (RS). When substituting the base fuels with DMC or MeFo, the flame temperature profiles remain unchanged, allowing to exclusively focus on the chemical kinetic effect of fuel blending on soot formation. The soot formation in the ethylene-based flames can be enhanced by blending a small amount of DMC, while the soot volume fraction begins to decline at high substitution rates of DMC. Blending DMC with acetylene shows no soot enhancement effect. In comparison, the blending of ethylene with MeFo leads to a direct reduction of soot production. This soot formation behavior was further examined using a reaction pathway analysis based on chemical kinetic modeling. The propargyl recombination pathway plays a more important role than the HACA pathway in the formation of benzene and naphthalene, especially for ethylene-based flames. Different from MeFo, DMC can decompose to generate CH3 radicals and subsequently enhance propargyl formation. Therefore, blending DMC means a reduction of C2H2 and an increase of CH3, which results in the soot formation characteristics of DMC blends. Our kinetic analysis also implies the need for improving the decomposition kinetics of C2H4 to C2H2.

Experimental and numerical study of soot volume fraction and number density in laminar co-flow diffusion flames of n-decane/n-butanol blends

Normal decane was selected as a surrogate fuel for RP-3 aviation fuel, and soot formation characteristics and effects of n-decane (D) /n-butanol (B) cofiring were investigated experimentally and numerically. Combustion experiments of six n-decane/n-butanol blends (i.e., D100, D80, D60, D40, D20, D00, indicating the volumetric fraction of n-butanol in blends) were performed on co-flow diffusion flames. The volume fraction and morphological characteristics of soot at different heights of the flame axis were obtained using thermophoretic sampling and transmission electron microscopy analysis. A newly developed n-decane/n-butanol/PAH kinetic model consisted of 73 species and 275 reactions were further applied to model the soot formation process and ascertain the effects of n-butanol blending. Experimental and simulation results showed that soot loading (soot volume fraction and number density) decreased with increasing n-butanol blending ratios. Chemical kinetic analysis suggested that n-butanol addition decreased the rate of the HACA reaction as a result of a reduction of C2H2 and C2H3 (+M) ⇄ C2H2 + H (+M), ultimately leading to a reduction of soot loading.

Functionalization of TiO2 inverse opal structure with atomic layer deposition grown Cu for photocatalytic and antibacterial applications

TiO2 inverse opal (IO) structure surfaces were functionalized with a sub-monolayer amount of Cu by atomic layer deposition (ALD) and tested for photocatalytic and antimicrobial applications. Decomposition of acetylene (C2H2) into CO2 and reduction of CO2 into CH4 were tested in the gas phase and photodegradation of methylene blue (MB) was tested in the liquid phase. Antimicrobial activity was tested against Gram-positive Staphylococcus aureus (S. aureus) bacteria. ALD Cu without any post-deposition heat treatment (HT) decreased the photo degradation rate of both C2H2 and MB but improved the activity towards CO2 reduction. ALD Cu increased MB photodegradation rate and antimicrobial activity only after HT at 550 °C, which was linked to the improved chemical stability Cu after the HT. The same HT decreased the activity towards CO2 reduction and decomposition of C2H2. The HT induced desorption of loosely bound ALD Cu+/2+ from the TiO2 IO surface and the remaining Cu+/2+ was reduced to Cu+. The photocatalytic and antimicrobial activity of TiO2 IO can be tailored by the addition of a sub-monolayer amounts of Cu with performance depending on the targeted reaction.

Investigating the performance and mechanism of nitrogen gas fixation and conversion to ammonia based on biocathode bioelectrochemistry system

AbstractBackgroundNowadays, microbial electrochemistry is widely used to produce valuable products (CH4, CH3CH2OH, CH3COOH, etc.) from carbon dioxide. Meanwhile, given that low‐value nitrogen (N2) is abundantly available as a feedstock for chemicals, thus ammonia (NH3) synthesis using bioelectrochemistry is still a promising technology. However, considering N2 stability and ammonium (NH4+) adsorption with a proton exchange membrane, developing suitable microbial systems for NH3 synthesis still faces challenges. Based on this, we developed a single‐chamber biocathode bioelectrochemical system for efficient NH3 production.ResultsFor the reaction system that we constructed, NH4+ production reached 6.31 mg L−1 within 10 days, and NH4+ production was positively correlated with current density. A C2H2 reduction assay further verified the activity of nitrogenase enzyme. Microbial community analysis revealed that Clostridia plays a crucial role in the N2 fixation process, and the synergistic interaction between different microorganisms favors electron transfer and an increase in NH3 production.ConclusionsThe results of a series of experiments have proved that this green and economical method can be used to efficiently generate NH4+. This reaction system not only utilizes the advantages of obtaining electrons for reduction reaction, but also improves electron transfer and avoids the shortcomings of NH4+ adsorption. Additionally, this in‐depth study of N2 fixation mechanism provides a theoretical reference for other applications of bioelectrochemical systems. © 2022 Society of Chemical Industry (SCI).

Method for High Frequency Underway N2 Fixation Measurements: Flow-Through Incubation Acetylene Reduction Assays by Cavity Ring Down Laser Absorption Spectroscopy (FARACAS).

Because of the difficulty in resolving the large variability of N2 fixation with current methods which rely on discrete sampling, the development of new methods for high-resolution measurements is highly desirable. We present a new method for high-frequency measurements of aquatic N2 fixation by continuous flow-through incubations and spectral monitoring of the acetylene (C2H2, a substrate analog for N2) reduction to ethylene (C2H4). In this method, named Flow-through Incubation Acetylene Reduction Assays by Cavity Ring-Down Laser Absorption Spectroscopy (FARACAS), dissolved C2H2 is continuously admixed with seawater upstream of a continuous-flow stirred-tank reactor (CFSR) in which C2H2 reduction takes place. Downstream of the flow-through incubator, the C2H4 gas is stripped using a bubble column contactor and circulated with a diaphragm pump into a wavelength-scanned cavity ring down laser absorption spectrometer (CRDS). Our method provides high-resolution and precise mapping of aquatic N2 fixation, its diel cycle, and its response to environmental gradients, and can be adapted to measure other biological processes. The short-duration of the flow-through incubations without preconcentration of cells minimizes potential artifacts such as bottle containment effects while providing near real-time estimates for adaptive sampling. We expect that our new method will improve the characterization of the biogeography and kinetics of aquatic N2 fixation rates.

Combining a Nitrogenase Scaffold and a Synthetic Compound into an Artificial Enzyme

AbstractNitrogenase catalyzes substrate reduction at its cofactor center ([(Cit)MoFe7S9C]n−; designated M‐cluster). Here, we report the formation of an artificial, nitrogenase‐mimicking enzyme upon insertion of a synthetic model complex ([Fe6S9(SEt)2]4−; designated Fe6RHH) into the catalytic component of nitrogenase (designated NifDKapo). Two Fe6RHH clusters were inserted into NifDKapo, rendering the conformation of the resultant protein (designated NifDKFe) similar to the one upon insertion of native M‐clusters. NifDKFe can work together with the reductase component of nitrogenase to reduce C2H2 in an ATP‐dependent reaction. It can also act as an enzyme on its own in the presence of EuIIDTPA, displaying a strong activity in C2H2 reduction while demonstrating an ability to reduce CN− to C1–C3 hydrocarbons in an ATP‐independent manner. The successful outcome of this work provides the proof of concept and underlying principles for continued search of novel enzymatic activities based on this approach.

Combining a Nitrogenase Scaffold and a Synthetic Compound into an Artificial Enzyme.

Nitrogenase catalyzes substrate reduction at its cofactor center ([(Cit)MoFe7S9C](n-); designated M-cluster). Here, we report the formation of an artificial, nitrogenase-mimicking enzyme upon insertion of a synthetic model complex ([Fe6S9(SEt)2](4-); designated Fe6(RHH)) into the catalytic component of nitrogenase (designated NifDK(apo)). Two Fe6(RHH) clusters were inserted into NifDK(apo), rendering the conformation of the resultant protein (designated NifDK(Fe)) similar to the one upon insertion of native M-clusters. NifDK(Fe) can work together with the reductase component of nitrogenase to reduce C2H2 in an ATP-dependent reaction. It can also act as an enzyme on its own in the presence of Eu(II)DTPA, displaying a strong activity in C2H2 reduction while demonstrating an ability to reduce CN(-) to C1-C3 hydrocarbons in an ATP-independent manner. The successful outcome of this work provides the proof of concept and underlying principles for continued search of novel enzymatic activities based on this approach.

Mutagenesis at α-423Ile of MoFe protein reduces the catalytic activity of nitrogenase in Klebsiella oxytoca

Nitrogenase catalyzes the ATP-dependent reduction of dinitrogen to ammonia, the central process of biological nitrogen fixation. The mechanism of electron transfer from the P-cluster to the iron-molybdenum cofactor (FeMo-co) during the process of substrate reduction remains unclear. Based on our previous hypothesis, the dual pathway model, three mutants of Klebsiella oxytoca strain M5al (termed Nβ102A, Kα431H and Iα423P) were constructed to map the H+/e− transfer pathway, in which β-102Asn, α-431Lys and α-423Ile of the molybdenum-iron (MoFe) protein of nitrogenases were replaced by alanine, histidine and proline, respectively. All three substitutions affected the diazotrophic growth, especially of the Iα423P strain with very low capability of diazotrophic growth and C2H2 reduction activity. In a 42-L fermenter, the Iα423P strain was induced to the maximal C2H2 reduction activity, which was only ~17 % of that of M5al. However, the transcription level of the nifD gene was approximately four-fold higher than that of the M5al strain, indicating that the low C2H2 reduction activity of Iα423P was not because of downregulation of nifD expression. The maximal C2H2 and H+ reduction activities of the Iα423P MoFe protein were 13 % and 21 % of those of the wild-type MoFe protein, respectively. In conclusion, we propose that the changed properties of the Iα423P MoFe protein are related to the disruption of hydrogen bond formation between α-423Ile and homocitrate, and that α-423Ile serves as a gate to the Mo site in FeMo-co during the H+/e− transfer in nitrogenase.

REGULATION OF NITROGEN FIXATION BY THE METABOLISM OF CARBON COMPOUNDS

ABSTRACT Nitrogen fixation in Rhodopseudomonas capsulata was subject to regulation by carbon metabolism. Citric acid cycle intermediates participated in this regulation, either directly or through accumulation of unknown derivatives of their metabolism. The carboxylic acids appeared to inhibit the A (or active) form of nitrogenase that predominates in nitrogen-starved cells. In exponentially growing cells in lactate-glutamate medium, where the R (or regulated) form of nitrogenase predominates, the citric acid cycle intermediates inhibited C2H2 reduction activity by only 10–35%. Citrate and isocitrate were the most potent inhibitors, though full inhibition was not achieved before periods of one to several hours after their addition. The inhibitory effect of the 6-carbon carboxylic acids was not due to repression of enzyme synthesis, nor was it accompanied by changes in the adenylylation state of glutamine synthetase. Oxaloacetate, the immediate precursor of citrate, also caused inhibition of C2H2 reduction...

Effects of Elevated CO2 and N Addition on Growth and N2 Fixation of a Legume Subshrub (Caragana microphylla Lam.) in Temperate Grassland in China

It is well demonstrated that the responses of plants to elevated atmospheric CO2 concentration are species-specific and dependent on environmental conditions. We investigated the responses of a subshrub legume species, Caragana microphylla Lam., to elevated CO2 and nitrogen (N) addition using open-top chambers in a semiarid temperate grassland in northern China for three years. Measured variables include leaf photosynthetic rate, shoot biomass, root biomass, symbiotic nitrogenase activity, and leaf N content. Symbiotic nitrogenase activity was determined by the C2H2 reduction method. Elevated CO2 enhanced photosynthesis and shoot biomass by 83% and 25%, respectively, and the enhancement of shoot biomass was significant only at a high N concentration. In addition, the photosynthetic capacity of C. microphylla did not show down-regulation under elevated CO2. Elevated CO2 had no significant effect on root biomass, symbiotic nitrogenase activity and leaf N content. Under elevated CO2, N addition stimulated photosynthesis and shoot biomass. By contrast, N addition strongly inhibited symbiotic nitrogenase activity and slightly increased leaf N content of C. microphylla under both CO2 levels, and had no significant effect on root biomass. The effect of elevated CO2 and N addition on C. microphylla did not show interannual variation, except for the effect of N addition on leaf N content. These results indicate that shoot growth of C. microphylla is more sensitive to elevated CO2 than is root growth. The stimulation of shoot growth of C. microphylla under elevated CO2 or N addition is not associated with changes in N2-fixation. Additionally, elevated CO2 and N addition interacted to affect shoot growth of C. microphylla with a stimulatory effect occurring only under combination of these two factors.

Hydrogen cycling by the unicellular marine diazotroph Crocosphaera watsonii strain WH8501.

The hydrogen (H₂) cycle associated with the dinitrogen (N₂) fixation process was studied in laboratory cultures of the marine cyanobacterium Crocosphaera watsonii. The rates of H₂ production and acetylene (C₂H₂) reduction were continuously measured over the diel cycle with simultaneous measurements of fast repetition rate fluorometry and dissolved oxygen. The maximum rate of H₂ production was coincident with the maximum rates of C₂H₂ reduction. Theoretical stoichiometry for N₂ fixation predicts an equimolar ratio of H₂ produced to N₂ fixed. However, the maximum rate of net H₂ production observed was 0.09 nmol H₂ μg chlorophyll a (chl a)⁻¹ h⁻¹) compared to the N₂ fixation rate of 5.5 nmol N₂ μg chl a⁻¹ h⁻¹, with an H₂ production/N₂ fixation ratio of 0.02. The 50-fold discrepancy between expected and observed rates of H₂ production was hypothesized to be a result of H₂ reassimilation by uptake hydrogenase. This was confirmed by the addition of carbon monoxide (CO), a potent inhibitor of hydrogenase, which increased net H₂ production rates ∼40-fold to a maximum rate of 3.5 nmol H₂ μg chl a⁻¹ h⁻¹. We conclude that the reassimilation of H₂ by C. watsonii is highly efficient (> 98%) and hypothesize that the tight coupling between H₂ production and consumption is a consequence of fixing N₂ at nighttime using a finite pool of respiratory carbon and electrons acquired from daytime solar energy capture. The H₂ cycle provides unique insight into N₂ fixation and associated metabolic processes in C. watsonii.

Catalytic activities of NifEN: Implications for nitrogenase evolution and mechanism

NifEN is a key player in the biosynthesis of nitrogenase MoFe protein. It not only shares a considerable degree of sequence homology with the MoFe protein, but also contains clusters that are homologous to those found in the MoFe protein. Here we present an investigation of the catalytic activities of NifEN. Our data show that NifEN is catalytically competent in acetylene (C(2)H(2)) and azide (N(3)(-)) reduction, yet unable to reduce dinitrogen (N(2)) or evolve hydrogen (H(2)). Upon turnover, C(2)H(2) gives rise to an additional S = 1/2 signal, whereas N(3)(-) perturbs the signal originating from the NifEN-associated FeMoco homolog. Combined biochemical and spectroscopic studies reveal that N(3)(-) can act as either an inhibitor or an activator for the binding and/or reduction of C(2)H(2), while carbon monoxide (CO) is a potent inhibitor for the binding and/or reduction of both N(3)(-) and C(2)H(2). Taken together, our results suggest that NifEN is a catalytic homolog of MoFe protein; however, it is only a "skeleton" version of the MoFe protein, as its associated clusters are simpler in structure and less versatile in function, which, in turn, may account for its narrower range of substrates and lower activities of substrate reduction. The resemblance of NifEN to MoFe protein in catalysis points to a plausible, sequential appearance of the two proteins in nitrogenase evolution. More importantly, the discrepancy between the two systems may provide useful insights into nitrogenase mechanism and allow reconstruction of a fully functional nitrogenase from the "skeleton" enzyme, NifEN.

Effect of acetylene and ethylene gases released from encapsulated calcium carbide on growth and yield of wheat and cotton

Calcium carbide (CaC2) is a rich source of the nitrification inhibitor acetylene (C2H2) and plant hormone ethylene (C2H4). C2H4 formed from biotic reduction of C2H2 released from CaC2 may accumulate in soil at physiologically active concentrations. Laboratory studies were conducted to evaluate the potential of encapsulated CaC2 for gradually releasing C2H2 and its product C2H4 in soil. The GC-FID analysis revealed that encapsulated CaC2 released a copious amount of C2H2 (up to 23700 nmol kg−1 soil), which was gradually reduced to C2H4 over a period of time via a strictly biotic reaction as no C2H4 was detected in CaC2-amended sterilized soil. Ammonium oxidation was suppressed by the encapsulated CaC2 indicating that C2H2 acted as a nitrification inhibitor. Results of pot trials conducted in the net house indicated that encapsulated CaC2 applied at 30 mg kg−1 soil significantly increased the number of tillers (up to 45.5%), root weight (up to 14.9%), straw (up to 32.8%) and grain yield (up to 37.3%) of wheat over the fertilizer application alone. In the case of cotton, the number of bolls, root, shoot and seed weight were also significantly increased in response to the application of encapsulated CaC2. Moreover, application of encapsulated CaC2 resulted in greater N-use efficiency (NUE) (up to 61.1%) by both wheat and cotton crops than that observed at the same rates of N fertilizer alone. These findings imply that CaC2 affects plant growth through hormonal action of C2H4 as well as improved NUE; however, the latter factor might be a relatively more contributing. It is desirable that CaC2 is formulated for gradually slow release of C2H2 and C2H4 in soil air.

Catalytic behavior of a polynuclear Mg-Mo complex and nitrogenase active site (FeMoco) isolated from the enzyme in reactions with C2H2, N2, and CO: A comparative study

In order to identify common and distinctive features in the catalytic behavior of natural and artificial nitrogen-fixation clusters, the kinetics of the catalytic reduction of C2H2 in the presence of Mg-Mo-cluster (1) was investigated and compared with the kinetics of acetylene reduction catalyzed by the cluster FeMoco (2) isolated from the enzyme nitrogenase we studied previously. The reactions were conducted in the presence of Zn/Hg and Eu/Hg as reducing agents and PhSH and C6F5SH as proton donors, i.e., under the same conditions as had been used in the case of 2. Both polynuclear Mg-Mo-complex and the europium amalgam-reduced FeMoco have multiple interdependent binding sites for substrates and/or inhibitors. Carbon monoxide inhibits the acetylene reduction much less efficiently in systems with cluster 1 than in systems with cluster 2, although the type of inhibition is mixed in both systems: CO binds to multiple sites of the cluster and affects both C2H2 complexation to the reduced cluster and decomposition of the catalyst-substrate complex to give the products. Unlike isolated FeMoco, the Mg-Mo-cluster efficiently catalyzes the reduction of molecular nitrogen. The reaction is greatly inhibited by acetylene, while no inhibiting effect of N2 is observed in acetylene reduction, as was found earlier for a system with the natural cluster as the catalyst.

Model for Acetylene Reduction by Nitrogenase Derived from Density Functional Theory

The catalytic cycle of acetylene reduction at the FeMo cofactor of nitrogenase has been investigated on the basis of density functional theory. C2H2 binds to the same site as N2, but it binds to a less reduced state of the cofactor. In a manner similar to that of N2 binding, one of the sulfur bridges opens during acetylene binding. The model explains the strong noncompetitive inhibition of N2 reduction by C2H2 and the weak competitive inhibition of C2H2 reduction by N2. Our proposed mechanism is consistent with experimentally observed stereoselectivity and the ability of C2H2 to suppress H2 production by nitrogenase.

Effect of the potential of an external electron donor on C2H2 reduction catalyzed by the nitrogenase active center (FeMoco) isolated from the enzyme

The effect of potential value and chemical properties of an external electron donor on C2H2 reduction catalyzed by nitrogenase active center (cluster [(μ6-N)Fe7MoS9·homocitrate] FeMoco isolated from the enzyme) has been investigated in the presence of proton donors of different acidity. The temperature—reaction rate dependences of these reactions have been studied. It has been shown that the rate-limiting steps of the reactions differ depending on the proton donor used. When thiophenol or water are used as proton donors, and electrochemical step — the electron transfer from cathode to adsorbed catalytic cluster — has been found to be a rate-limiting one. The effective activation energy of ethane formation as a product of four-electron C2H2 reduction is found to be 1.5 times lower than that of ethylene, namely, ∼13 kcal mol−1. When stronger acid, pentafluorothiophenol, is used as a proton donor, the chemical step of intramolecular rearrangement of the catalyst—substrate complex taking place in solution becomes a rate-limiting one. The effective activation energies of both ethylene and ethane become equal to 32 kcal mol−1.

Construction and characterization of double mutants in nitrogenase ofKlebsiella pneumoniae

Two mutants in nitrogenase ofKlebsiella pneumoniae are constructed by site-directed mutagenesis and gene replacement procedure, which express the nitrogenases with Lysine and Glutamine substituting for α-Glutamine 190 and α-Histidine 194 respectively (Kp-Q α190 K and Kp-H α194 Q). The above two substitutions are respectively introduced into anifV mutant (expressing a citrate-containing nitrogenase) and sequentially two double mutants are obtained (Kp-Q α190 K-nifV − and Kp-H α194 Q-nifV −). All four mutants exhibit strict Nif-phenotype under the N2-fixation condition and fail to grow diazotrophically. Altered nitrogneases are effectively depressed and the C2H2 reduction analysis shows that the double substitutions in Kp-Q α190 K-nifV abolish cell C2H2 reduction activity, but Kp-H α194 Q-nifV − cells maintain a C2H2 reduction activity at 10% of that of wild type. Whole cell C2D2 reduction by all four mutants in comparison to the wild type andnifV mutant is also detected. The results show that only single α-Gln194 substitution does not perturb the stereospecificity of protonation of C2D2. These results indicate that the α-Glutamine 190 and its combination with homocitrate are essential to the catalytic activity of nitrogenase and it is proposed that α-Glutamine 190 and its combination with homocitrate are involved in the proton and/or electron transfer to FeMoco. The nitrogenases from these double mutants will be useful in further analysis of the entry of the proton and/or electron to FeMoco and the substrate binding sites.

Influence of diazotrophic inoculations on nitrogen nutrition of rice

An experiment was conducted in microplots (7 by 7m) to investigate the effect of 2 non-symbiotic N2-fixing bacteria [Azotobacter (strain CS1) and Azospirillum (strain CM4)] in the presence of 50 kg N/ha on the performances of the diazotrophs with respect to nitrogen accretion and its transformation in the rhizosphere soils of rice (Oryza sativa L. cv. IR-36). In most cases, a successful inoculation of the diazotrophs was recorded, with the proliferation of Azotobacter and Azospirillum, either alone or in combination, in the rhizosphere soils, and nitrogenase activity (C2H2 reduction) of the microbes was present in rice roots. The uninoculated soil receiving 100 kg N/ha recorded the highest amount of total nitrogen, non-hydrolysable organic nitrogen, available nitrogen, and hydrolysable organic nitrogen content in the rhizosphere soils, resulting in greater yield of the crop. Inoculation of the diazotrophs substantially increased different fractions of nitrogen content in the rhizosphere soils, and the increase in total nitrogen, non-hydrolysable organic nitrogen, and hydrolysable organic nitrogen was greater due to Azotobacter than either Azospirillum or a combination of Azotobacter and Azospirillum. Total and mineral nitrogen content increased at maximum tillering to flowering stages of the crop, followed by a decline at maturity, whereas, hydrolysable organic nitrogen decreased with a concomitant increase in non-hydrolysable fraction with the age of the crop.

Kinetics and Mechanism of Acetylene Reduction with Europium Amalgam Catalyzed by Isolated Active Center of Nitrogenase

The reaction kinetics of C2H2 reduction with europium amalgam (Eu/Hg) catalyzed by nitrogenase active center separated from the enzyme, the molybdenum–iron–sulfur cluster [MoFe7S9 · homocitrate] (FeMoco), was studied. The dependence of the rates of ethylene and ethane formation on the concentrations of catalyst, substrate, protonating agent, and amalgam was studied. The stereospecificity of the reaction was studied by Fourier transform IR spectroscopy. It was found that the reaction occurred at the amalgam surface via the adsorption of the compound [FeMoco · PhSH]. Upon reduction, this compound can simultaneously coordinate several substrate molecules to activate them for the subsequent reactions. A study of the IR spectra of the gas phase of the reaction demonstrated that cis-1,2-dideuterioethylene is the main product of C2D2 reduction. Taking into account this fact and the dependence of the reaction rate on the concentration of a protonating agent, we concluded that substrate molecules bound to the cofactor underwent protonation by intramolecular hydrogen transfer from the iron or sulfur atoms of FeMoco to coordinated C2H2.