Laccase-catalyzed carbon-carbon bond formation: oxidative dimerization of salicylic esters by air in aqueous solution

The laccase catalyzed oxidative dimerization of salicylic esters, a rare example of a laccase-catalyzed carbon-carbon bond formation, was studied. This reaction allows the use of air as stoichiometric oxidant and proceeds in aqueous solution. The preparative scope and the mechanism of the method, which provides a new and convenient access to functionalized biaryls under mild conditions, were investigated.     

Ion Chromatographic determination of trace anions and cations in high-purity water

An ion chromatographic measuring system for the off-line and on-line determination of some trace anions and cations in high-purity water is presented. The ng/L level of anions and cations in 20-130 mL high-purity water can be analyzed after preconcentration on ion exchange columns. The concentrated solutes are eluted by eluents from the trap column and separated using a Dionex analytical column. The quantification of each ion is achieved using the suppressor technique and conductivity detector. The influence of various parameters on the results is discussed. The detection limits of cations and anions are between 10 and 30 ng/L for chloride, bromide, nitrate, phosphate, sulphate, sodium, ammonium, potassium, magnesium and calcium ions.     

Probing macromolecular adsorbed layer structure and history dependence via the interfacial cavity function

Adsorbed layers of proteins and other macromolecules often relax structurally more slowly than they form, rendering layer growth an out-of-equilibrium process. We show here how the interfacial cavity function, Phi (the average Boltzmann factor for a single probe molecule), may be determined, using kinetic data available from optical waveguide lightmode spectroscopy, and used as a continuous, in situ measure of history dependent adsorbed layer structure. The increase of Phi observed with residence time for fibronectin and lysozyme layers suggests post-adsorption clustering on a time scale longer than that predicted by a surface diffusion model.     

Diffuse-reflectance laser flash photolysis of 16-(1-pyrene)-hexadecanoic acid adsorbed on silica

The absorption and fluorescence of 16-(1-pyrene)-hexadecanoicacid adsorbed on silica have been investigated. Time-resolved transient diffuse reflectance spectra were recorded following pulsed nanosecond laser excitation at 355 nm of pyrene, 1-methylpyrene and 16-(1-pyrene)-hexadecanoicacid adsorbed on silica. In addition to a rapidly decaying transient, absorbing at 420 nm assigned as the triplet state, and of the radical cation, absorbing at 460 nm, another long living transient species absorbing at 420 nm was observed for 16-(1-pyrene)-hexadecanoic acid. The decay is reversible but complete recovery takes several hours. Although no definitive assignment could be made for this transient several possibilities are discussed. The radical cations of the investigated molecules are formed by a biphotonic process. The non-exponential decay of the radical cations could be analyzed in the framework of a Gaussian distribution of free energy barriers.     

Topological study of intramolecular hydrogen bonding in beta-hydroxyethylperoxy radical and beta-hydroxyethoxy radical along its dissociation pathway

The presence of intramolecular hydrogen bond (IHB) in beta-hydroxyethylperoxy and beta-hydroxyethoxy radicals was investigated using the QTAIM topological study on UB3LYP/6-311++G(2d,2p) charge densities. Only one of the two conformers of beta-hydroxyethylperoxy radical which were previously considered to present IHB displays a bond critical point (BCP) associated to an IHB. Furthermore, the atomic energies and electron populations indicate no evidence of IHB in the second conformer. Nevertheless, very small differences in molecular energies were obtained using several one-step and multi-step methods (G3, G3B3) between both conformers. No BCP is found between the hydroxyl hydrogen and the oxygen in the most stable conformer or in the transition state for the dissociation path of beta-hydroxyethoxy radical. However, a BCP is formed in the last steps of this path, thereby yielding H-bonded products.     

Electron microscopic study of dehydration transformations. I. Twin formation and mosaic structure in hematite derived from goethite

A study by means of electron microscopy at high resolution has enabled the analysis of the microstructure of hematite as produced by dehydration from goethite. The apparently “polycrystalline” structure is found to consist of aggregates of well-oriented twin-related hematite crystals, separated by regularly spaced H(001) walls of voids resulting from the loss of water. The material is thus broken up into blocks with sizes smaller than 50 Å. The crystallography of the twinned structure can be analyzed by electron diffraction data and dark-field imaging. These aggregates of hematite crystals provide an ideal mosaic structure. In particular the effect of this mosaic structure on the individual reflections of the diffraction patterns has been analyzed and discussed.     

Mechanism of inactivation of inducible nitric oxide synthase by amidines. Irreversible enzyme inactivation without inactivator modification

Nitric oxide synthases (NOS) are hemoproteins that catalyze the reaction of L-arginine to L-citrulline and nitric oxide. N-(3-(Aminomethyl)benzyl)acetamidine (1400W) was reported to be a slow, tight-binding, and highly selective inhibitor of iNOS in vitro and in vivo. Previous mechanistic studies reported that 1400W was recovered quantitatively after iNOS fully lost its activity and modification to iNOS was not detected. Here, it is shown that 1400W is a time-, concentration-, and NADPH-dependent irreversible inactivator of iNOS. HPLC-electrospray mass spectrometric analysis of the incubation mixture of iNOS with 1400W shows both loss of heme cofactor and formation of biliverdin, as was previously observed for iNOS inactivation by another amidine-containing compound, N5-(1-iminoethyl)-L-ornithine (L-NIO). The amount of biliverdin produced corresponds to the amount of heme lost by 1400W inactivation of iNOS. A convenient MS/MS-HPLC methodology was developed to identify the trace amount of biliverdin produced by inactivation of iNOS with either 1400W or L-NIO to be biliverdin IXalpha out of the four possible regioisomers. Two mechanisms were previously proposed for iNOS inactivation by L-NIO: (1) uncoupling of the heme peroxide intermediate, leading to destruction of the heme to biliverdin; (2) abstraction of a hydrogen atom from the amidine methyl group followed by attachment to the heme cofactor, which causes the enzyme to catalyze the heme oxygenase reaction. The second mechanistic proposal was ruled out by inactivation of iNOS with d3-1400W, which produced no d2-1400W. Detection of carbon monoxide as one of the heme-degradation products further excludes the covalent heme adduct mechanism. On the basis of these results, a third mechanism is proposed in which the amidine inactivators of iNOS bind as does substrate L-arginine, but because of the amidine methyl group, the heme peroxy intermediate cannot be protonated, thereby preventing its conversion to the heme oxo intermediate. This leads to a change in the enzyme mechanism to one that resembles that of heme oxygenase, an enzyme known to convert heme to biliverdin IXalpha. This appears to be the first example of a compound that causes irreversible inactivation of an enzyme without itself becoming modified in any way.     

Liquid-liquid phase equilibria for some polystyrene-methylcyclohexane mixtures

Liquid-liquid cloud point diagrams of solutions of nearly monodisperse samples of polystyrene (PS), and binary mixtures of nearly monodisperse PS’s, both in methylcyclohexane (MCH), were determined for several polymer molecular weights (M_w) at 0.1 MPa. The bimodal mixtures (PS[M_w(1),ρ(1)] + PS[M_w(2),ρ(2)], M_w(1)=90×10³ g/mol, M_w(2)=13×10³ g/mol, 5.78 × 10³ g/mol, and 2.2 × 10³ g/mol, ρ=1.06) were prepared constraining 〈M_w〉=38.6×10³ g/mol, ρ=M_w/M_n is the polydispersity index. In each case the cloud point curves (CPC’s) for the bimodal mixtures are strongly skewed, lying well above CPC for 〈M_w〉 when φ<φ_CRITICAL, and below CPC for 〈M_w〉 when φ>φ_CRITICAL; φ is volume fraction polymer in the polymer/solvent mixture. The experimental results are discussed in the context of empirical and mean-field representations.     

Separation of actinides from Low Level Liquid Wastes (LLLW) by extraction chromatography using novel DMDOHEMA and TODGA impregnated resins

The uptake of several actinides [U(VI), Th(IV), Am(III), Cm(III)] and fission products was investigated from nitric acid solutions by two novel extraction chromatographic sorbents containing 2-(2-hexyloxy-ethyl)-N,N'-dimethyl-N,N'-dioctyl-malonamide (DMDOHEMA) and N,N,N',N'-tetraoctyl-3-oxapentane-1,5-diamide (TODGA), respectively. The kinetics of the uptake of actinides was studied. The sorption of metal ions fromz simulated Low Level Liquid Waste (LLLW) solutions was evaluated. The results of these experiments revealed that the actinides and lanthanides could be separated from the bulk of other fission products in simulated LLLW solutions on both sorbents.This revised version was published online in November 2005 with corrections to the Cover Date.     

Thermal conductivity, thermal effusivity, and specific heat capacity near the lower critical point of the binary liquid mixture n-butoxyethanol-water

Experimental investigations on binary liquid mixtures near the critical mixing point are presently leading to a controversy about the anomaly in the thermal conductivity. A photopyroelectric technique is used to determine the thermal conductivity and the effusivity of the binary liquid mixture n-butoxyethanol-water at its critical concentration near the critical mixing point. It is proven that, contrary to previous reports, there is no critical enhancement in the thermal conductivity. The specific heat capacity is calculated from these results and compared with the results from measurements performed by adiabatic scanning calorimetry.