Local mean field models of uniform to nonuniform density fluid-crystal transitions

We investigate the existence of nontranslation invariant (periodic) density profiles, for systems interacting via translation invariant long-range potentials, as minimizers of local mean field free energy functionals. The existence of a second-order transition from a uniform to a nonuniform density at a specified temperature is proven for a class of model systems.     

Diode or tunnel-diode characteristics? Resolving the catalytic consequences of proton coupled electron transfer in a multi-centered oxidoreductase

Protein film voltammetry has been employed to define multiple catalytic consequences of proton coupled electron transfer (PCET) in a cytochrome c nitrite reductase. Current-potential profiles reflecting the steady-state rate of nitrite-limited reduction have been defined from pH 4 to 8. Lowering the electrode potential at pH 8 causes the catalytic current to increase and then decrease before it takes a value independent of any further lowering of electrode potential. By comparison, at pH 4, catalysis is initiated at more positive electrode potentials in an approximately sigmoidal fashion with no attenuation of the catalytic rate evident at more negative electrode potentials. The results show that activity is turned on by the coupled transfer of two electrons and one proton to the enzyme. The decreased rate of catalysis at lower electrode potentials under more alkaline conditions shows that this rate attenuation occurs only when reduction is not coupled to compensating protonation(s) of the enzyme. Sites within the enzyme whose reduction and/or protonation may contribute to the definition of these activities are discussed.     

Preparation,and infrared and thermal properties of six adducts of tetrakis-(8-hydroxyquinolinato)thorium(IV)

Six adducts of the tctrakis(8-hydroxyquinolinato) complex of thorium(IV) with 8-hydroxyquinoline, pyridine, urea, dimethylsulphoxide, dimethylformamide and 1,10-phenanthroline were prepared and characterized, the infrared bands for the room-temperature forms are given and their particular thermal properties are reported. The thermal analyses were performed mostly in the presence of air, but in some cases an argon atmosphere was used for comparison. The adducts were lost from the principal complex at temperatures varying between 110 and 290°. Intermediate oxygenated complexes were formed in each case at about 400°. The thermal data suggest the standardization of a procedure for the formation of Th(C₉H₆NO)₄.

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Zusammenfassung Sechs Addukte des Tetrakis(8-hydroxyquinolinato)-Komplexes von Thorium(IV) mit 8-Hydroxyquinolin, Pyridin, Harnstoff, Dimethylsulphoxid, Dimethylformamid und 1,10-Phenanthrolin wurden dargestellt und charakterisiert. Infrarotbanden und spezielle thermische Eigenschaften der Raumtemperatur-Formen sind angegeben. Die thermischen Analysen wurden meist in Luft ausgeführt, in einigen Fällen zu Vergleichszwecken aber auch in Argonatmosphäre. Die Addukte wurden vom Komplex bei Temperaturen zwischen 110 und 290 °C abgegeben. Intermediäre oxydierte Komplexe wurden in jedem Fall bei etwa 400 °C gebildet. Die thermischen Daten ermöglichen die Standardisierung eines Verfahrens zur Darstellung von Th(C₉H₆NO)₄.

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-(8- ) (IV) 8- , , , - , 1,10-. - , . , , . 110–290°. 400° . Th(C₉H₆NO)₄.

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We are grateful to the Bangladesh University Grants Commission for a Fellowship Grant to AR.     

Photoactive metallocyclodextrins: sophisticated supramolecular arrays for the construction of light activated miniature devices

The introduction of photoactive metal centres onto cyclodextrin receptors opens up new possibilities for the design of sensors, wires and energy conversion systems. This tutorial review focuses on strategies involving such metallocyclodextrins for the construction of supramolecular arrays with light-activated functions. The assembly procedures for building such arrays are presented, together with the features required for their functions both as sensors for ion or small molecule detection and as wires for photoinduced long-range energy or electron transport. Systems for metal ion sensing are described where the cyclodextrin plays a mediating role in influencing the luminescence properties of an organic probe, responsive to metal binding. Small molecule sensing by the cyclodextrin cavity is realised using luminescent lanthanide or transition metal functionalised cyclodextrins. The light signal of the photoactive metal is switched on or off upon binding an analyte in the cyclodextrin cavity. The metallocyclodextrin systems that function as wires are distinguished by the controlled assembly of transition metal polypyridine and metalloporphyrin units. These units have inherent photoactivity that defines the vectorial direction of energy or electron transfer processes through the wire.     

Equilibrium interaction of solid surfaces across a polymer melt

Forces across polymer melts are poorly understood despite their importance for adhesion and fabricating composite materials. Using an atomic force microscope (AFM), this interaction was measured for poly(dimethyl siloxane) (PDMS). The structure of the polymer at the surface changed during the first approximately 10 h. Afterward, short-range attractive forces were observed with short-chain PDMS (M(w) = 4200 g/mol). Using PDMS with a molecular weight (M(w) = 18 000 g/mol) above the entanglement limit, we measured a monotonically decaying repulsive force, which indicates that a quasi-immobilized layer had formed at the solid surface. Due to the small radius of curvature of the tip, forces could be measured in equilibrium.     

Reductive activation of nitrate reductases

Protein film voltammetry of Paracoccus pantotrophus respiratory nitrate reductase (NarGH) and Synechococcus elongatus assimilatory nitrate reductase (NarB) shows that reductive activation of these enzymes may be required before steady state catalysis is observed. For NarGH complementary spectroscopic studies suggest a structural context for the activation. Catalytic protein film voltammetry at a range of temperatures has allowed quantitation of the activation energies for nitrate reduction. For NarGH with an operating potential of ca. 0.05 V the activation energy of ca. 35 kJ mol-1 is over twice that measured for NarB whose operating potential is ca. -0.35 V.     

Novel products from attempted michael addition to 1-methyl-1,2,5,6-tetrahydropyridine-4-carbonitrile

The reaction of some phenols with the title compound, in the presence of sodium, gives 2-(2-hydroxyaryl_piperidine derivatives. Geometrical isomers have been separated, which differ in having an equatorial (A) or axial (B) cyano group on the piperidine chair (the methyl and aryl groups are equatorial in both forms). The x-ray crystallographic structures of an example of each of A and B are reported and the proton NMR spectra are assigned.     

Photosensitized Oxygenation of Abieta-7,9(11)-dien-13β-ol

Dehydration of abiet-8-ene-7β, 13β-diol (ibozol, 1 ) leads to abieta-7,9(11)-dien-13β-ol ( 2 ) which aromatizes slowly to the known abieta-8,11,13-triene ( 3 ). Photosensitized oxygenation of the heteroannular diene 2 yields a mixture from which three compounds were identified; abiet-7-ene-9α, 11α, 13β-triol ( 4 ), abieta-8,11,13-trien-7-one ( 5 ), and abieta-8,11,13-trien-7α-ol ( 6 ).     

Microdrops on atomic force microscope cantilevers: evaporation of water and spring constant calibration

The evaporation of water drops with radii approximately 20 microm was investigated experimentally by depositing them onto atomic force microscope (AFM) cantilevers and measuring the deflection versus time. Because of the surface tension of the liquid, the Laplace pressure inside the drop, and the change of interfacial stress at the solid-liquid interface, the cantilever is deflected by typically a few hundred nanometers. The experimental results are in accordance with an analytic theory developed. The evaporation process could be monitored with high accuracy even at the last stage of evaporation because (1) cantilever deflections can be measured with nanometer resolution and (2) the time resolution, given by the inverse of the resonance frequency of the cantilever of approximately 0.3 ms, is much faster than the typical evaporation time of 1 s. Experimental results indicate that evaporation of the last thin layer of water is significantly slower than the rest of the drop, which can be due to surface forces. This drop-on-cantilever system can also be used to analyze the drop impact dynamics on a surface and to determine the spring constant of cantilevers.