Catalytic behavior of the nitrogenase iron-molybdenum cofactor extracted from the enzyme in the reduction of C2H2 under nonenzymatic conditions

To compare the catalytic effect of the active center of nitrogenase (iron-molybdenum cofactor (FeMoco)) under nonenzymatic conditions with the behavior of FeMoco incorporated in a protein, the kinetics of C₂H₂ reduction with Zn and Eu amalgams was examined in the presence of the cofactor extracted from the MoFe protein of nitrogenase (the specific activity of the extracted FeMoco after its integration into the cofactordeficient MoFe protein ofKp 5058 was 200 ± 20 mol of C₂H₄ (mol of Mo)^-1 min^-1. It was found that under exposure to reducing agents of different strength—Zn amalgam (I) (−0.84 V with respect to a normal hydrogen electrode (NHE)) and Eu amalgam (II) (−1.4 V with respect to NHE)—different reduction states of FeMoco were produced. They differed in the number and properties of substrateand inhibitor-coordinating active sites. For I, the rate of ethylene formation was described by a hyperbolic function of substrate concentration (K _M = 0.045 atm). Carbon monoxide reversibly inhibited the reduction of acetylene(K _i - 0.05). For II, a sigmoid relationship between the rate of accumulation of C₂H₄ or C₂H₆ and substrate concentration was found. This relationship was explained by the occurrence of three interrelated sites of acetylene coordination and reduction with the apparent constantK _M = 0.08 atm in the FeMoco cluster reduced by europium amalgam. In this case, the specific activity was 40–60 mol of C₂H₄ (mol of Mo)⁻¹ min⁻¹. For the system with Eu (Hg), the CO inhibition constants were 0.004 and 0.009 atm for the formation of ethylene and ethane, respectively. The behavior of FeMoco as a catalyst for acetylene reduction and the inhibition of this reaction by carbon monoxide in various reducing protein and nonprotein media were compared. This comparison demonstrated that typical features of the catalytic behavior of FeMoco depend primarily on its composition and structure and only secondarily on the type of the reducing agent and on the reaction medium.     

Mutual Effects of Substrates and Inhibitors in Reactions Catalyzed by Isolated Iron–Molybdenum Cofactor of Nitrogenase

The inhibiting effects of CO and N₂ on the ability of the nitrogenase iron–molybdenum cofactor (FeMoco) to catalyze acetylene reduction outside the protein were studied to obtain data on the mechanism of substrate reduction at the active center of the enzyme nitrogenase. It was found that CO and N₂ reacted with FeMoco that was separated from the enzyme and reduced by zinc amalgam (E = –0.84 V relative to a normal hydrogen electrode (NHE)) (I) or europium amalgam (E = –1.4 V relative to NHE) (II). In system I, CO reversibly inhibited the reaction of acetylene reduction to ethylene with K _i = 0.05 atm CO. In system II, CO inhibited the formation of the two products of C₂H₂ reduction in different manners: the mixed-type or competitive inhibition was found for ethylene formation with K _i = 0.003 atm CO and the incomplete competitive inhibition was found for ethane formation with K _i = 0.006 atm CO. The fraction of C₂H₆ in the reaction products was greater than 50% at a CO pressure of 0.05 atm because of the stronger inhibiting effect of CO on the formation of C₂H₄. The change in the product specificity of acetylene-reduction centers under influence of CO was explained by some stabilization of the intermediate complex [FeMoco · C₂H₂] upon the simultaneous coordination of CO to the catalytic cluster. Because of this, the fraction value of ethane as a multielectron reduction product increased. The experimental results suggest that several active sites at the FeMoco cluster reduced outside the protein can be simultaneously occupied by substrates and (or) inhibitors. The inhibition of both ethane and ethylene formation by molecular nitrogen in system II is competitive with K _i = 0.5 atm N₂ for either product. That is, N₂ and C₂H₂ as ligands compete for the same coordination site at the reduced FeMoco cluster. The inhibiting effects of CO and N₂ on the catalytic behaviors of both isolated FeMoco and that in the enzyme were compared.     

Mutual Effects of Substrates and Inhibitors in Reactions Catalyzed by the Nitrogenase Iron–Molybdenum Cofactor outside the Enzyme

The inhibiting effects of CO and N₂ on the ability of the nitrogenase iron–molybdenum cofactor (FeMoco) to catalyze acetylene reduction outside the protein were studied to obtain data on the mechanism of substrate reduction at the active center of the enzyme nitrogenase. It was found that CO and N₂ reacted with FeMoco that was separated from the enzyme and reduced by zinc amalgam (E = –0.84 V with reference to a normal hydrogen electrode (NHE)) (I) or europium amalgam (E = –1.4 V with reference to NHE) (II). In system I, CO reversibly inhibited the reaction of acetylene reduction to ethylene with K _i = 0.05 atm CO. In system II, CO inhibited the formation of the two products of C₂H₂ reduction in different manners: the mixed-type or competitive inhibition of ethylene formation with K _i = 0.003 atm CO and the incomplete competitive inhibition of ethane formation with K _i = 0.006 atm CO. The fraction of C₂H₆ in the reaction products was higher than 50% at a CO pressure of 0.05 atm because of the stronger inhibiting effect of CO on the formation of C₂H₄. A change in the product specificity of acetylene-reduction centers under exposure to CO was explained by some stabilization of the intermediate complex [FeMoco · C₂H₂] upon the simultaneous coordination of CO to the catalytic cluster. Because of this, the fraction of the many-electron reduction product (ethane) increased. The experimental results suggest that several active sites in the FeMoco cluster reduced outside the protein can be simultaneously occupied by substrates and (or) inhibitors. The inhibition of both ethane and ethylene formation by molecular nitrogen in system II is competitive with K _i = 0.5 atm N₂ for either product. That is, N₂ and C₂H₂ as ligands compete for the same coordination site in the reduced FeMoco cluster. The inhibiting effects of CO and N₂ on the catalytic behaviors of FeMoco outside the protein and as an enzyme constituent were compared.     

Effect of the acidity and chemical nature of the protonating agent on the rate of acetylene reduction catalyzed by the nitrogenase active site isolated from the enzyme

The effect of the acidity (pK _a) of the source of protons on the rate and selectivity of acetylene reduction has been investigated in order to elucidate the mechanism of protonation of substrate molecules coordinated to the reduced FeMoco cluster. A number of compounds whose pK _a in DMF varies between 6 and 20 have been examined as protonating agents. The rate of the reaction is almost independent of the acidity of the proton donor in a wide pK _a range. This can be explained in terms of the specific features of substrate protonation catalyzed by iron-sulfur clusters. Active protonating agents in the system are those which react with the catalyst to form hydrogen-bonded association species or those which are ligands reversibly binding to the cluster and are capable of donating protons, likely with simultaneous electron transfer.     

Effect of the potential of an external electron donor on C<Subscript>2</Subscript>H<Subscript>2</Subscript> 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 [(₆-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.Published in Russian in Izvestiya Akademii Nauk. Seriya Khimicheskaya, No. 8, pp. 1583–1591, August, 2004.     

Reaction patterns of cyclopentadienylcobalt(I) carbonyl and acetylene derivatives

Studies have been made of photochemical and thermal reaction sequences through which bisubstituted acetylenes are transformed in (C₅H₅)Co-carbonyl reaction systems into cyclobutadiene and cyclopentadienone complexes and hexasubstituted benzenes. A primary intermediate observed by its IR spectrum in low-temperature photochemical reactions of (C₅H₅)Co(CO)₂ with diphenyl alkynes RCCR is the mixed mononuclear species (C₅H₅)Co(CO)(RCCR). At room temperature this species is converted by excess alkyne into the cyclopentadienone complex (C₅H₅)Co(R₄C₄CO). We have isolated from these reactions systems an important intermediate the mixed binuclear compound(C₅H₅)₂Co₂(μ-CO)(RCCR). In the presence of excess alkyne this compound is thermally converted either to the cyclobutadiene or to the cyclopentadienone complex of (C₅H₅)Co, depending on the partial pressure of CO in the reaction system. The mixed binuclear compound forms a catalyst for the cyclotrimerization of excess 2-butyne. The fluxional binuclear metallocycle (C₅H₅)₂Co₂[(CH₃)₄C₄], which is formed by sodium amalgam reduction of (C₅H₅)Co(CO)I₂ in the presence of 2-butyne, is a true catalyst for alkyne cyclotrimerization.     

Synthesis,Structure, and Physicochemical Properties of Lithium Uranovanadate

A new chemical compound, lithium uranovanadate, was synthesized and studied by X-ray diffraction, IR spectroscopy, and thermography. Standard enthalpies of formation of lithium uranovanadate and its dihydrate at 298.15 K were determined by means of reaction and adiabatic vacuum calorimetry. Temperature dependence of heat capacity in the range 80-300 K was obtained for crystalline LiVUO₆·2H₂O and thermodynamic characteristics of its synthesis were calculated.     

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. Published in Russian in Izvestiya Akademii Nauk. Seriya Khimicheskaya, No. 5, pp. 766–774, May, 2006.     

Ab initio quantum-chemical calculations of interactions of ions and hydrides of alkali metals with C3H6 and C4H8 molecules

Calculations of the C₃H₆ · LiH, C₄H₈ · M⁺, and C₄H₈ · MH systems and of C₂H₂ · MH complexes (M = Li or Na) were carried out by the unrestricted Hartree-Fock-Roothaan (UHF) method with partial optimization of the geometry using fixed geometric parameters of the C₃H₆ and C₄H₈ molecules. The standard 3-21G and 6-31G* basis sets were used. Unlike the C₃H₆ · LiH structure, the C₄H₈ · M⁺ and C₄H₈ · MH systems are typical complexes. It was found that the C₄H₈ · M⁺, C₄H₈ · MH, and C₂H₂ · MH complexes are similar in coordination of M⁺ ions and MH molecules by carbon atoms in spite of considerable differences in the interatomic distances (–1 A) between these atoms in the C₄H₈ and C₂H₂ molecules. The heats of formation (Q), which were calculated in the UHF/6-31G* approximation and using second- and fourth-order Möller-Plesset perturbation theory taking into account the electron correlation energy in the MP2/6-31G*. MP4(SDQ)/6-31G*, and MP4(SDTQ)/6-31G* approximations, satisfy the following relationships: Q(C₂H₃ · MH) < Q(C₄H₈ · MH) < Q(C₄H₈ · M⁺). It was observed that in going from Li to Na the corresponding values of Q tend to decrease.Translated fromIzvestiya Akademii Nauk. Seriya Khimicheskaya. No. 7, pp. 1636–1640, July, 1996.     

Crystal and Molecular Structure of <Emphasis Type="Italic">Bis</Emphasis>(2,3-pentamethylene-3,4-dihydro-4-oxoquinazolinium) Tetrachlorocuprate(II) Sesquihydrate

The structure of bis-(2,3-pentamethylene-3,4-dihydro-4-oxoquinazolinium) tetrachlorocuprate (II) sesquihydrate, (C₁₃H₁₅N₂O)₂[CuCl₄]·1.5H₂O, was determined by single crystal X-ray diffraction. In contrast to the previously studied analogs, the compound contains crystallization water molecules.Original Russian Text Copyright © 2004 by K. K. Turgunov, B. Tashkhodzhaev, L. V. Molchanov, and Kh. M. Shakhidoyatov__________Translated from Zhurnal Strukturnoi Khimii, Vol. 45, No. 5, pp. 955–959, September–October, 2004.     

Reactions of photogenerated fluorine atoms with molecules trapped in solid argon. 4. Spectroscopic characteristics of β-C2H2F· radicals generated in reactions of mobile F atoms with C2H2 molecules trapped in solid argon

Reactions of mobile fluorine atoms with C₂H₂, C₂D₂, and C₂HD molecules in solid argon were studied by ESR and IR spectroscopic techniques. Highly resolved ESR spectra of the stabilized radicals CHF=^·CH, CDF=^·CD, CHF=^·CD, and CDF=^·CH were obtained for the first time. Isotropic hyperfine constants on fluorine and proton nuclei were measured. It was found that the radicals formed in the reaction F + C₂H₂ correspond to the cis--C₂H₂F^· isomer. A comparison of the measured HFC constants with the values calculated by modern quantum-chemical methods allows the identification of the isomeric form of the radical, whereas vibrational analysis of the IR absorption spectra gives unreliable results. The calculation of the energy of the radical isomers predicts that cis--C₂H₂F^· is more stable than trans--C₂H₂F^· by 1.0 kJ mol^–1.     

Computer study of the absorption of ethane by aqueous ultradispersed medium: IR spectra

Absorption of ethane molecules by water clusters containing 10–20 molecules is studied by the molecular dynamics method. The (H₂O) _n (I), C₂H₆(H₂O) _n (II), and (C₂H₆)₂(H₂O) _n (III) cluster systems are composed on the basis of specific statistical weights. Spectral characteristics of system and single clusters are determined in the frequency range of 0 ≤ ω ≤ 1000 cm^?1. In this frequency range, both real and imaginary parts of dielectric permittivity decrease monotonically after the absorption of C₂H₆ molecules by an aqueous ultradispersed system. Integral coefficient of IR absorption increases, while average (over frequency) reflection coefficient decreases after the absorption of ethane molecules. The intensity of IR scattering by the systems of clusters containing C₂H₆ molecules lowers. Maximal values of radiation power for water clusters with various sizes are balanced with the capture of ethane molecules by the clusters, whereas oscillations in the size dependence of the density of electrons that are active with respect to IR radiation decrease.     

Benzyl cleavage in the oxidation of cycleanine by mercuric acetate

The dehydrogenation of cycleanine with mercuric acetate in 10% AcOH (at the boil) has been studied. It has been shown that it is accompanied by oxidative cleavage similar to benzyl cleavage under the action of electron impact. The main reaction product (without complexone) is the 8-(4-hydroxymethylphenoxy)-6,7-dimethoxy-2-methylisoquinolinium salt C₁₉H₂₀NO₄Cl·1.5H₂O, mp 196°C. The oxidation of cycleanine in the presence of ethylenediaminetetraacetic acid gave the mercuride C₁₉H₁₉NO₃I₂Hg·2H₂O, mp 240°C (water).All-Union Scientific-Research Institute of Medicinal Plants, Moscow. Translated from Khimiya Prirodnykh Soedinenii, No. 5, pp. 676–680, September–October, 1979.     

Spectrophotometric and x-ray crystallographic studies of 1-carbamoylbenz-[c,d]indol-2-ones

On the basis of a measurement of the frequencies and intensities of the stretching vibrations of the amino, amido, and ester groups in the IR spectra, it was established that products of the reaction of N-benzenesulfonylhydroxynaphthalimides with ammonia in alcohol solutipns have a structure of the type of 1-carbamoylbenz[c,d]-indol-2-ones and are not molecules with a seven-membered heterocycle, by analogy with 3-oxido-2-alkylnaphtho[1,8-d,e][1,3] oxoniazepin-1-ones. A determination of the molecular and crystal structure of one of these compounds (C₁₃H₁₀N₂O₂) confirmed this conclusion: The molecule is 1-(N-methylcarbamoyl)benz[c,d]indol-2-one. In the naphthalene ring of the molecule the bond angles at the central bond are deformed analogously to that found in acenaphthene. The intramolecular hydrogen bond O···HN closes a six-membered ring in the molecule. The crystal is constructed from dimers formed by intermolecular hydrogen bonds O···HN.Translated from Khimiya Geterotsiklicheskikh Soedinenii, No. 8, pp. 1110–1114, August, 1984.     

Solubility Polytherm of the Ternary System Hexamethylenetetramine-Ammonium Dihydrogen Phosphate-Water

The solubility polytherm of the ternary system hexamethylenetetramineammonium dihydrogen phosphate-water was studied in a wide temperature range. The concentration and temperature ranges of crystallization of the starting components and a new compound, NH₄H₂PO₄·2(CH₂)₆N₄·8H₂O, were determined. The properties and structure of the new compound were studied by X-ray phase, thermal, and IR spectroscopic analyses.     

Crystal and molecular structure of the solvate of 2,4,7-trinitro-9,9,10,10-tetrahydroxyphenanthrene with hexamethylphosphortriamide (I) and the solvate of 2,4,7-trinitro-10,10-dihydroxyphenanthren-9-one with dimethylformamide (II)

Two products of the addition reactions of H₂O at the carbonyl bonds of 2,4,7-trinitro-9,10-phenanthrenequinone, formed under mild conditions when it is dissolved in aqueous hexamethylphosphortriamide (HMPTA) and dimethylformamide (DMF), have been studied by x-ray diffraction. The crystalline solvates of 2,4,7-trinitro-9,9,10,10-tetrahydroxyphenanthrene (C₁₄H₉N₃O₁₀·3HMPTA) and 2,4,7-trinitro-10,10-dihydroxyphenanthren-9-one (C₁₄H₇N₃O₉·2DMF) are formed. In the first of these, all the molecules of the included solvent are disordered in complex fashion. Both crystal structures are stabilized by strong hydrogen bonds between the solvated and solvating molecules.Translated from Izvestiya Akademii Nauk SSSR, Seriya Khimicheskaya, No. 3, pp. 575–580, March, 1990.     

Sorption of amides by mixed niobyl-vanadyl phosphate

The layered mixed niobyl-vanadyl phosphate [(V_0.l4Nb_0.86)OPO₄·2.7 H₂O] can intercalate different amide molecules. In all cases, the amide I bands [v(CO)] of the original compound shift to lower frequencies, thereby indicating that the amide molecules interact with the layers probablyvia hydrogen bonds linking the coordinated water molecules to the metal atoms. To a lesser extent, primary amides show protonated molecules and tertiary amides show some molecules directly coordinated to the metals, as can be inferred from the bands appearing at 1732 and 1617 cm^–1 in the IR spectrum, respectively.     

Synthesis of the complex of chlorodiphenylborane with dimethylformamide

Summary A complex compound between chlorodiphenylborane and dimethylformamide was prepared by reaction in solvent, nonsolvent, and excess of dimethylformamide; it has the composition (C₆H₅)₂BC1 ·HCON(CH₃)₂. Some of its properties were examined.Deceased.Translated from Izvestiya Akademii Nauk SSSR, Seria Khimicheskaya, No. 10, pp. 1838–1843, October, 1964     

Reaction of Organyltrifluorosilanes with Dimethylsulfoxide and Domethylformamide and Its Spectroscopic Investigation

Reaction of organyltrifluorosilanes RSiF₃ (R = C₆H₅, 3-O₂NC₆H₄, and C₆H₅CH₂) with DMSO and DMF (B) results in formation of the complexes 2B·SiF₄ and R₂SiF₂. Besides, biphenyl, benzene, methyl(fluoromethyl)sulfoxide, and S,S'-dimethyldisulfide-S,S'-dioxide CH₃S(O)S(O)CH₃ were either isolated or identified by chromatomass-spectrometry. Speculative mechanism of the reaction proceeding is discussed. IR spectra of the reaction mixtures and those of 2B·SiF₄ adduct were studied in details; they indicate octahedron structure of the complex with cis arrangement of B ligands.