Enzyme electrokinetics: electrochemical studies of the anaerobic interconversions between active and inactive states of Allochromatium vinosum [NiFe]-hydrogenase

The cycling between active and inactive states of the catalytic center of [NiFe]-hydrogenase from Allochromatium vinosum has been investigated by dynamic electrochemical techniques. Adsorbed on a rotating disk pyrolytic graphite "edge" electrode, the enzyme is highly electroactive: this allows precise manipulations of the complex redox chemistry and facilitates quantitative measurements of the interconversions between active catalytic states and the inactive oxidized form Ni(r) (also called Ni-B or "ready") as functions of pH, H(2) partial pressure, temperature, and electrode potential. Cyclic voltammograms for catalytic H(2) oxidation (current is directly related to turnover rate) are highly asymmetric (except at pH > 8 and high temperature) due to inactivation being much slower than activation. Controlled potential-step experiments show that the rate of oxidative inactivation increases at high pH but is independent of potential, whereas the rate of reductive activation increases as the potential becomes more negative. Indeed, at 45 degrees C, activation takes just a few seconds at -288 mV. The cyclic asymmetry arises because interconversion is a two-stage reaction, as expected if the reduced inactive Ni(r)-S state is an intermediate. The rate of inactivation depends on a chemical process (rearrangement and uptake of a ligand) that is independent of potential, but sensitive to pH, while activation is driven by an electron-transfer process, Ni(III) to Ni(II), that responds directly to the driving force. The potentials at which fast activation occurs under different conditions have been analyzed to yield the potential-pH dependence and the corresponding entropies and enthalpies. The reduced (active) enzyme shows a pK of 7.6; thus, when a one-electron process is assumed, reductive activation at pH < 7 involves a net uptake of one proton (or release of one hydroxide), whereas, at pH > 8, there is no net exchange of protons with solvent. Activation is favored by a large positive entropy, consistent with the release of a ligand and/or relaxation of the structure around the active site.     

Hexakis(isopropylthio)-1,5-hexadien-3-yne and its fluorescent Ag(I) coordination polymers: assembly of helicates with thioether sites

A new thioether-rich ligand with a conjugated dienyne backbone and its fluorescent Ag(I) coordination networks have been synthesized and characterized by single crystal X-ray diffraction studies, which reveal that the supramolecular architectures of the networks contain assembled helicates with thioether sites.     

The thermodynamic properties of acetals+heptane mixtures at 298.15 K

Excess enthalpies and excess volumes were determined at 298.15 K for: dimethoxymethane+heptane, diethoxymethane+heptane, 1,1-dimethoxyethane+heptane, 1,1-diethoxyethane+heptane, 2,2-dimethoxypropane+heptane and 1,1-diethoxypropane+heptane.     

Oil core-polymer shell microcapsules prepared by internal phase separation from emulsion droplets. I. Characterization and release rates for microcapsules with polystyrene shells

Microcapsules with an oil core surrounded by a polymeric shell have been prepared by the controlled phase separation of polymer dissolved within the oil droplets of an oil-in-water emulsion. The dispersed oil phase consists of the shell polymer (polystyrene), a good solvent for the polymer (dichloromethane), and a poor solvent for the polymer (typically hexadecane). Removal of the good solvent results in phase separation of the polymer within the oil droplets. If the three interfacial tensions between the core oil, the shell-forming polymer, and the continuous phase are of the required relative magnitudes, a polymer shell forms surrounding the poor solvent. A UV-responsive organic molecule was added to the oil phase, prior to emulsification, to investigate the release of a model active ingredient from the microcapsules. This molecule should be soluble in the organic core but also have some water solubility to provide a driving force for release into the continuous aqueous phase. As the release rate of the active ingredient is a function of the thickness of the polymeric shell, for controlled release applications, it is necessary to control this parameter. For the preparative method described here, the thickness of the shell formed is directly related to the mass of polymer dissolved in the oil phase. The rate of volatile solvent removal influences the porosity of the polymer shell. Rapid evaporation leads to cracks in the shell and a relatively fast release rate of the active ingredient. If a more gentle evaporation method is employed, the porosity of the polymer shell is decreased, resulting in a reduction in release rate. Cross-linking the polymer shell after capsule formation was also found to decrease both the release rate and the yield of the active ingredient. The nature of the oil core also affected the release yield.     

Direct evaluation of multicomponent phase equilibria using flat-histogram methods

We present a method for directly locating density-driven phase transitions in multicomponent systems. Phase coexistence conditions are determined through manipulation of a total density probability distribution evaluated over a density range that includes both coexisting phases. Saturation quantities are determined through appropriate averaging of density-dependent mean values of a given property of interest. We discuss how to implement the method in both the grand-canonical and isothermal-isobaric semigrand ensembles. Calculations can be conducted using any of the recently introduced flat-histogram techniques. Here, we combine the general algorithm with a transition-matrix approach to produce an efficient self-adaptive technique for determining multicomponent phase equilibrium properties. To assess the performance of the new method, we generate phase diagrams for a number of binary and ternary Lennard-Jones mixtures.     

A short access to the macrocyclic core of cycloviracin and glucolipsin

The macrocyclic core of cycloviracin and glucolipsin has been synthesised in ten steps from levoglucosan and (S)-(−)-dimethyl malate. The limited number of steps to obtain this macrolide makes it a valuable procedure for the synthesis of analogues of cycloviracin and glucolipsin.     

Experimental and theoretical studies of the quenching of Li(3p,4p) by N2

Quenching mechanisms of the Li3p and Li4p states in collision with the nitrogen molecule are studied by laser-induced fluorescence spectroscopy and by a quantum chemical calculation. The Li3p state is observed to be efficiently quenched to the Li3s state detected as intense 3s-->2p emission. The Li4p state is efficiently quenched to the Li4s and Li3d states detected as 4s-2p and 3d-2p emissions, respectively. The potential-energy surfaces for the Li(2s-4p)N2 states show a large number of conical intersections and avoided crossings resulting from the couplings between the ionic [Li+(N2)-] and covalent configurations. There are a large number of stable excited states, and we give here the spectroscopic constants for the lowest two stable isomers correlating to Li2p+N2.     

Conformational analysis of a stereochemically complete set of cis-enediol peptide analogues

A conformational analysis of a stereochemically complete set of peptide analogues based on a cis-enediol unit is presented. The cis-enediol unit, which can replace a two or a three amino acid segment of a peptide, contains two "side chains", four asymmetrical carbon atoms, and six free dihedral angles. To determine the accessible conformational space, the molecules are divided into three fragments, each containing two free dihedral angles. The energy surfaces are computed for all dihedral angle values, and the possible conformations of the cis-enediol unit analogues are built using all combinations of the surface minima. Such a "build-up" procedure, which is very fast, is able to reproduce 75% of the minima obtained from a full dihedral angle exploration of the conformational space. The cis-enediol unit minima are compared with the corresponding di- and tripeptide minima; all peptide minima can be closely matched by a cis-enediol unit minimum of low energy (less than 2.2 kcal/mol above the lowest energy conformer). However, there are low energy minima of the cis-enediol unit that have no corresponding minima in peptides. The results are shown to depend strongly on the chirality of the analogues. The ability of each of the stereoisomers to mimic natural peptides, evaluated by the present approach, is correlated with its experimental activity in a renin inhibition assay.