Identification of iron(III) peroxo species in the active site of the superoxide reductase SOR from Desulfoarculus baarsii

The active site of superoxide reductase SOR consists of an Fe2+ center in an unusual [His4 Cys1] square-pyramidal geometry. It specifically reduces superoxide to produce H2O2. Here, we have reacted the SOR from Desulfoarculus baarsii directly with H2O2. We have found that its active site can transiently stabilize an Fe3+-peroxo species that we have spectroscopically characterized by resonance Raman. The mutation of the strictly conserved Glu47 into alanine results in a stabilization of this Fe3+-peroxo species, when compared to the wild-type form. These data support the hypothesis that the reaction of SOR proceeds through such a Fe3+-peroxo intermediate. This also suggests that Glu47 might serve to help H2O2 release during the reaction with superoxide.     

Simple enzyme-amplified amperometric detection of a 38-base oligonucleotide at 20 pmol L(-1) concentration in a 30- microL droplet

A 38-base DNA sequence has been detected at 20 pmol L(-1) concentration in 15-35- microL droplets by means of an electrochemical enzyme-amplified sandwich-type assay on a mass-manufacturable screen-printed carbon electrode. Formation of the sandwich brought the horseradish peroxidase-label of the detection sequence into electrical contact with a pre-electrodeposited redox polymer, making the sandwich an electrocatalyst for the reduction of hydrogen peroxide to water at +0.2 V (Ag/AgCl). Sensitivity twenty times better than that of a related system resulted from: 1. fivefold reduction of the noise by substituting the formerly used poly( N-vinyl imidazole)-co-acrylamide comprising redox co-polymer with poly(4-vinyl pyridine)-co-acrylamide comprising redox polymer, enabling use of the electrodes at a more oxidizing potential at which noise (the rate of non-enzyme catalyzed electroreduction currents of dissolved oxygen and hydrogen peroxide) was lower; 2. doubling of the catalytic electroreduction current upon electrodeposition of a second layer of the redox polymer on the capture sequence-containing film; and 3. doubling of the current by increasing the coverage by the capture sequence.     

Sulfur K-edge XANES study of dihydrolipoic acid capped gold nanoparticles: dihydrolipoic acid is bound by both sulfur ends

Dihydrolipoic acid (DHLA) capped gold nanoparticles (Au@DHLA) are characterized in solid and liquid states by sulfur K-edge XANES spectroscopy; it clearly shows that DHLA is anchored to gold thanks to both sulfur ends.     

Transition-metal nanocluster size vs formation time and the catalytically effective nucleus number: a mechanism-based treatment

A mechanism-based equation for the size of a forming transition-metal nanocluster vs time has been derived based on the Finke-Watzky two-step mechanism for transition-metal nanocluster nucleation (A --> B, rate constant k1) and autocatalytic growth (A + B --> 2B, rate constant k2), where A is the nanocluster precursor and B is the growing nanocluster. The resultant equation expresses nanocluster diameter as a function of time, D(t), in terms of k1, k2, the initial concentration of the nanocluster precursor complex, [A]0, and the number of catalytically effective nuclei derived from either (i) the final nanocluster size, D(f), or (ii) the number of atoms in the average catalytically effective nucleus, N*, and the induction period time, t ind (N* being by definition the number of atoms present in the average size nucleus at the end of the induction period and when observable catalysis begins). By fitting experimentally determined nanocluster size vs time data using this equation, evidence for the validity of the equation is obtained for Ir(0) nanoclusters formed from the well-studied system of H2 reduction of the precursor [(1,5-COD)Ir x P2W15Nb3O62](8-). The D(t) equation is then used to determine N* for nine prior Ir(0) nanocluster preparations from five different [(1,5-COD)Ir(+)]n [anion(n-)] precursors. Also given is a relationship allowing one to interconvert between nanocluster size data and nanocluster precursor concentration data, again when the two-step nucleation and growth mechanism has been shown to apply. Some of the key experimental factors that are known to affect the kinetics of nanocluster formation, and therefore nanocluster size, are also summarized. A look ahead to needed future work is also provided.     

Structure and composition of the nacre–prisms transition in the shell of <Emphasis Type="Italic">Pinctada margaritifera</Emphasis> (Mollusca,Bivalvia)

A microstructural, mineralogical, and chemical study of the nacre–prisms boundary in the shells of Pinctada margaritifera shows that this boundary is not an abrupt transition, but that there exists a distinct fibrous layer with clear topographic structures and evidence of growth lines. A three-step biomineralization process is proposed that involves changes in the chemical and biochemical composition of the last growth increments of the calcite prisms, formation of the fibrous layer, and development of regular tablets in the nacreous layer.     

One-dimensional inorganic arrangement in the bismuth oxalate hydroxide Bi(C2O4)OH

Single crystals of Bi(C₂O₄)OH were obtained by the slow diffusion of Bi³⁺ cations through silica gel impregnated with oxalic acid. The structure was solved in the Pnma space group with a=6.0853(2) Å, b=11.4479(3) Å, c=5.9722(2) Å, leading to R=0.0188 and wR=0.0190 from 513 unique reflections. The bismuth coordination polyhedron is a BiO₆E pentagonal bipyramid with the lone pair E sitting at an axial vertex. The opposite axial vertex is occupied by a hydroxyl oxygen atom, which is also an equatorial corner of a neighboring BiO₆E bipyramid. The sharing of the hydroxyl oxygen atoms build zig-zag chains running down the [100] direction. These chains are aligned in a sheet parallel to the (010) plane and are further connected through oxalate ions to form a three-dimensional arrangement. On heating, Bi(C₂O₄)OH decomposes to the meta-stable quadratic β-Bi₂O₃ phase.     

Synthesis of New Organophosphorus Derivatives of Acylureas and Acylsemicarbazides

A convenient route to phosphorus acylureas and acylsemicarbazides is reported. The syntheses of the new compounds involve the reaction between a chloroacetyl compound and various phosphorus starting materials (dithiophosphate, phosphine, phosphite and phosphinite).     

Contribution toward comprehension of contact angle values on single polydimethylsiloxane and poly(ethylene oxide) polymer networks

The large application ranges of polydimethylsiloxane (PDMS) and poly(ethylene oxide) (PEO) based materials justify the importance of controlling polymer surface properties including morphology and wettability behavior. However, it appears that the reported contact angle values of PDMS surfaces show significant scattering which cannot always be interpreted in terms of sole chemical data. In addition, few values are reported concerning pure PEO surfaces, since the polymer generally swells in the presence of water. Thus, in order to correlate surface properties with sample preparation, several single PDMS and PEO polymer networks were synthesized with varying cross-linkers and different cross-linking densities. First, the sample surface topography was systematically analyzed by atomic force microscopy (AFM). It was proven that the removal process of the polymer film from the mold plays a significant role in surface topography according to the vitreous or rubbery state of the given polymer network at room temperature irrespective of mold surface treatment. AFM-scale smooth surfaces can be obtained for all the samples by removing them systematically from the mold at a temperature below the α-relaxation temperature. Dynamic water contact angles were then measured and the values analyzed as a function of cross-linker nature and cross-linking density.     

Theoretical Modeling of Low‐Energy Electronic Absorption Bands in Reduced Cobaloximes

The reduced Co^I states of cobaloximes are powerful nucleophiles that play an important role in the hydrogen‐evolving catalytic activity of these species. In this work we analyze the low‐energy electronic absorption bands of two cobaloxime systems experimentally and use a variety of density functional theory and molecular orbital ab initio quantum chemical approaches. Overall we find a reasonable qualitative understanding of the electronic excitation spectra of these compounds but show that obtaining quantitative results remains a challenging task.