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.     

Analysis of oligomeric peroxides in synthetic triacetone triperoxide samples by tandem mass spectrometry

Oligomeric peroxides formed in the synthesis of triacetone triperoxide (TATP) have been analyzed by mass spectrometry utilizing both electrospray ionization (ESI) and chemical ionization (CI) to form sodiated adducts (by ESI) and ammonium adducts (by CI and ESI). Tandem mass spectrometry and deuterium isotopic labeling experiments have been used to elucidate the collision‐induced dissociation (CID) mechanisms for the adducts. The CID mechanisms differ for the sodium and ammonium adducts and vary with the size of the oligoperoxide. The sodium adducts of the oligoperoxides, H[OOC(CH₃)₂]_nOOH, do not cyclize under CID, whereas the ammonium adducts of the smaller oligoperoides (n < 6) do form the cyclic peroxides under CID. Larger oligoperoxide adducts with both sodium and ammonium undergo dissociation through cleavage of the backbone under CID to form acyl‐ and hydroperoxy‐terminated oligomers of the general form CH₃C(O)[OOC(CH₃)₂]_xOOH, where x is an integer less than the original oligoperoxide degree of oligomerization. The oligoperoxide distribution is shown to vary batch‐to‐batch in the synthesis of TATP and the post‐blast distribution differs slightly from the distribution in the uninitiated material. The oligoperoxides are shown to be decomposed under gentle heating. Copyright © 2009 John Wiley & Sons, Ltd.     

Electrode materials and reaction mechanisms in solid oxide fuel cells: a brief review

Following previous surveys of the solid electrolyte ceramics and electrode reaction mechanisms in solid oxide fuel cells, this review is focused on the comparative analysis of electrochemical performance, thermal expansion, oxygen ionic and electronic transport, and durability-determining factors in the major groups of electrode materials. The properties of mixed-conducting oxide phases with perovskite-related and fluorite structures, ceramic–metal and oxide composites, and catalytically active additives are briefly discussed, with emphasis on the approaches and findings reported during the last 10–15 years. The performance of conventional and alternative electrode materials in the cells with ZrO₂-, CeO₂-, LaGaO₃-, and La₁₀Si₆O₂₇-based electrolytes is appraised in the context of potential optimization strategies. Particular attention is centered on the cathode and anode compositions providing maximum electrochemical activity and stability and on the critical aspects relevant for electrode microstructure engineering.     

A mathematical model of the photodynamic fullerene-oxygen action on biological tissues

A kinetic model of the photodynamic fullerene-oxygen action on biological tissues is developed. The efficiency of generation of singlet oxygen is studied in relation to the intensity of exciting radiation and the concentrations of fullerene and oxygen. The spectral efficiencies of singlet oxygen generation upon irradiation of a biological tissue by a lamp and different lasers are studied and compared with each other. The power of luminescence of singlet oxygen is calculated. The spatial distributions of singlet oxygen in allantoic fluid and in murine sarcoma are studied. The oxidation of lipids of external membranes is assumed to be the basic mechanism of cell damage. The characteristic time of this oxidation is calculated. The dose curve of cell survival is estimated. The results of modeling are compared with experimental data in the literature.     

Functionally Rigid and Degenerate Molecular Shuttles

The preparation and dynamic behavior of two functionally rigid and degenerate [2]rotaxanes ( 1⋅ 4 PF₆ and 2⋅ 4 PF₆) in which a π-electron deficient tetracationic cyclophane, cyclobis(paraquat-p-phenylene) (CBPQT⁴⁺) ring, shuttles back and forth between two π-electron-rich naphthalene (NP) stations by making the passage along an ethynyl-phenylene-(PH)-ethynyl or butadiyne rod, are described. The [2]rotaxanes were synthesized by using the clipping approach to template-directed synthesis, and were characterized by NMR spectroscopic and mass spectrometric analyses. ¹H NMR spectra of both [2]rotaxanes show evidence for the formation of mechanically interlocked structures, resulting in the upfield shifts of the resonances for key protons on the dumbbell-shaped components. In particular, the signals for the peri protons on the NP units in the dumbbell-shaped components experienced significant upfield shifts at low temperatures, just as has been observed in the flexible [2]rotaxanes. Interestingly, the resonances for the same protons did not exhibit a significant upfield shift at 298 K, but rather only a modest shift. This phenomenon arises from the much reduced binding of the ethynyl-NP unit compared to the 1,5-dioxy-NP unit. This effect, in turn, increases the shuttling rate when compared to the 1,5-dioxy-NP-based rotaxane systems investigated previously. The kinetic and thermodynamic data of the shuttling behavior of the CBPQT⁴⁺ ring between the NP units were obtained by variable-temperature NMR spectroscopy and using the coalescence method to calculate the free energies of activation (ΔG_c^≠) of 9.6 and 10.3 kcal mol⁻¹ for 1⋅ 4 PF₆ and 2⋅ 4 PF₆, respectively, probed by using the rotaxane's α-bipyridinium protons. The solid-state structure of the free dumbbell-shaped compound ( 3 ) shows the fully rigid ethynyl-PH-ethynyl linker with a length (8.1 Å) twice as long as that (3.8 Å) of the butadiyne linker. Full-atomistic simulations were carried out with the DREIDING force field (FF) to probe the degenerate molecular shuttling processes, and afforded shuttling energy barriers (ΔG^≠=10.4 kcal mol⁻¹ for 1⋅ 4 PF₆ and 2⋅ 4 PF₆) that are in good agreement with the experimental values (ΔG_c^≠=9.6 and 10.3 kcal mol⁻¹ for 1⋅ 4 PF₆ and 2⋅ 4 PF₆, respectively, probed by using their α-bipyridinium protons).     

The Uncommon Active Site of D-Amino Acid Transaminase from Haliscomenobacter hydrossis: Biochemical and Structural Insights into the New Enzyme

Among industrially important pyridoxal-5’-phosphate (PLP)-dependent transaminases of fold type IV D-amino acid transaminases are the least studied. However, the development of cascade enzymatic processes, including the synthesis of D-amino acids, renewed interest in their study. Here, we describe the identification, biochemical and structural characterization of a new D-amino acid transaminase from Haliscomenobacter hydrossis (Halhy). The new enzyme is strictly specific towards D-amino acids and their keto analogs; it demonstrates one of the highest rates of transamination between D-glutamate and pyruvate. We obtained the crystal structure of the Halhy in the holo form with the protonated Schiff base formed by the K143 and the PLP. Structural analysis revealed a novel set of the active site residues that differ from the key residues forming the active sites of the previously studied D-amino acids transaminases. The active site of Halhy includes three arginine residues, one of which is unique among studied transaminases. We identified critical residues for the Halhy catalytic activity and suggested functions of the arginine residues based on the comparative structural analysis, mutagenesis, and molecular modeling simulations. We suggested a strong positive charge in the O-pocket and the unshaped P-pocket as a structural code for the D-amino acid specificity among transaminases of PLP fold type IV. Characteristics of Halhy complement our knowledge of the structural basis of substrate specificity of D-amino acid transaminases and the sequence-structure-function relationships in these enzymes.     

Comparison of photonic nanojets key parameters produced by nonspherical microparticles

Photonic nanojet (PNJ) phenomenon arising near transparent dielectric microparticles subject to plane wave illumination in the visible is considered. The near-field light scattering patterns produced by shaped wavelength-sized particles (hexahedron, cuboid, sphere, hemisphere, axicon, assembled particles) are numerically simulated and key PNJ parameters are analyzed. Particle shape influence on the peak intensity and spatial resolution of produced PNJ is investigated. We demonstrate that due to the reciprocal action of spherical-type and conical-type focusing of the special type of composite particles constituted of a hemisphere and an axicon can produce highly localized PNJ with peak intensity considerable higher than that for isolated regular particle (sphere, microaxicon, hemisphere).     

Nitrogen-rich La-Si-Al-O-N oxynitride glass structures probed by solid state NMR

We investigate the local structures of oxynitride La-Si-(Al)-O-N glasses by ²⁹Si and ²⁷Al magic-angle spinning (MAS) solid state Nuclear Magnetic Resonance (NMR). The glasses studied span an unprecedented range of compositions, up to >50 at.% lanthanum and nitrogen out of the cations and anions, respectively, and achievable through a recently introduced glass preparation route. Transmission as well as scanning electron microscopy verified homogeneous samples over length-scales down to 20 nm. As the nitrogen content of the glasses increased, ²⁹Si NMR evidenced a progressive formation of Si-N bonds, with SiO₂N₂ tetrahedra dominating in the nitrogen-rich glass networks. In the oxygen-rich end of the series, aluminum is predominantly present in tetrahedral coordination as AlO₄, whereas the glasses with highest nitrogen contents have a major fraction of AlO₃N structural units. Trends in isotropic ²⁹Si and ²⁷Al chemical shifts and ²⁷Al quadrupolar couplings are compared with results of La-Si-Al-O glasses and are discussed in relation to the glass compositions and their proposed structures.     

Reduction of phosphine oxides to phosphines

The digest is devoted to the most widespread reducing agents which are used for the reduction of tertiary and secondary phosphine oxides, phospholene oxides, phospholane oxides, phosphonates, phosphinates, α- and β-hydroxy and thiomethyl phosphine oxides, α-aminophosphine oxides. Stereoselectivity of reactions is described. Methods of stabilization of phosphines which are prone to re-oxidation by the formation of borane-adduct or metal complexes are considered.