The outstanding solvatochromic properties of the complex Ru(bpy)(CN)(4)(2-), deriving from the ability of the nitrile goup to participate in H-bond interactions, were explored to develop a sensor for low concentrations of water on common organic aprotic solvents. In dry solvents, the maximum wavenumber of the low energy transition of Ru(bpy)(CN)(4)(2-) presents linear correlations with the Kamlet-Taft solvent acidity parameter (slope = 0.59 +/- 0.02, r > 0.99) and Gutman acceptor number (slope = (1.7 +/- 0.1) x 10(-2), r > 0.99). The slopes and regression coefficients of the correlations measure the sensitivity of the compound to H-bond donation by the solvent and confirm its high specificity towards the sensing of H-bond donating solvents, such as water. In acetonitrile, tetrahydrofuran, N,N-dimethylformamide, dichloromethane and toluene, the sensor presents spectral changes that respond linearly to water concentrations in the range 5-70 ppm and compete favorably with the widely spread Karl-Fisher method with a detection limit of similar to 50 ppm. The high sensitivity and the much simpler spectrophotometric titration, portends the use of this complex as an optical water sensor for determination of low water contents in organic solvents. (c) 2005 Elsevier B.V. All rights reserved.

The objective of this work was to prepare polysaccharide-based gels exhibiting liquid crystalline properties. Such systems may be used in some optical or in biomedical applications, where biodegradability is required. Chitosan is a derivative of chitin, widely used in a series of medical applications. Due to its rigid structure, chitosan or its derivatives may show lyotropic mesophases in certain conditions. In this work, chitosan solutions were prepared by mixing completely the polysaccharide with different concentration of formic, acetic and monochloroacetic acids at room temperature. X-ray diffraction patterns of the gels did not show the existence of a crystalline structure. Finger-prints texture observed by polarised optical microscopy was attributed to a cholesteric liquid crystalline phase that usually develops in concentrated solutions. Values of the nematic chiral pitch (P) were determined in function of acid solution concentration. The critical concentrations (C*) to form a lyotropic liquid crystalline phase in formic, acetic and monochloroacetic acids were determined, and the obtained values were confronted with the expected critical concentration based on the Flory formalism. The critical concentration values were found to be dependent upon the acid used.

The facultative aerobic bacterium Geobacter sulfurreducens produces a small periplasmic c-type triheme cytochrome with 71 residues (PpcA) under anaerobic growth conditions, which is involved in the iron respiration. The thermodynamic properties of the PpcA redox centers and of a protonatable center were determined using NMR and visible spectroscopy techniques. The redox centers have negative and different reduction potentials (−162, −143, and −133 mV for heme I, III, and IV, respectively, for the fully reduced and protonated protein), which are modulated by redox interactions among the hemes (covering a range from 10 to 36 mV) and by redox−Bohr interactions (up to −62 mV) between the hemes and a protonatable center located in the proximity of heme IV. All the interactions between the four centers are dominated by electrostatic effects. The microscopic reduction potential of heme III is the one most affected by the oxidation of the other hemes, whereas heme IV is the most affected by the protonation state of the molecule. The thermodynamic properties of PpcA showed that pH strongly modulates the redox behavior of the individual heme groups. A preferred electron transfer pathway at physiologic pH is defined, showing that PpcA has the necessary thermodynamic properties to perform e-/H+ energy transduction, contributing to a H+ electrochemical potential gradient across the periplasmic membrane that drives ATP synthesis. PpcA is 46% identical in sequence to and shares a high degree of structural similarity with a periplasmic triheme cytochrome c7 isolated from Desulfuromonas acetoxidans, a bacterium closely related to the Geobacteracea family. However, the results obtained for PpcA are quite different from those published for D. acetoxidans c7, and the physiological consequences of these differences are discussed.

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A nitrate reductase was solubilized with Triton X-100 from the membranes of Pseudomonas chlororaphis DSM 50135 grown microaerobically in the presence of nitrate. Like other membrane-bound nitrate reductases, it contains three subunits, of 129, 66 (64) and 24 kDa, referred to in the literature as alpha, beta and gamma, respectively. Electrocatalytic studies revealed that only the membrane-bound, not the solubilized form of the enzyme, can accept electrons from a menaquinone analog, menadione, whereas both forms can accept electrons from methylviologen. The isolated enzyme possesses several iron-sulfur clusters and a molybdopterin guanine dinucleotide active center. The iron-sulfur clusters can be grouped in two classes according to their redox properties, the high-potential and low-potential clusters. In the as-isolated enzyme, two forms of the molybdenum center, high- and low-pH, are detectable by electron paramagnetic resonance spectroscopy. The low-pH form shows a hyperfine splitting due to a proton, suggesting the presence of an -OHx ligand. Dithionite reduces the Mo(V) center to Mo(W) and subsequent reoxidization with nitrate originates a new Mo(V) signal, identical to the oxidized low-pH form but lacking its characteristic hyperfine splitting. The isolated preparation also contains heme c (in a sub-stoichiometric amount) with the ability to relay electrons to the molybdenum center, suggesting that this nitrate reductase may contain heme c instead of the heme b usually found in this class of enzymes. (c) 2005 Elsevier B.V. All rights reserved.

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Thiol-linked DNA-gold nanoparticles were used in a novel colorimetric method to detect the presence of specific mRNA from a total RNA extract of yeast cells. The method allowed detection of expression of the FSY1 gene that encodes a specific fructose/H+ symporter in Saccharomyces bayanus PYCC 4565. FSY1 is strongly expressed when the yeast is grown in fructose as the sole carbon source, while cells cultivated in glucose as the sole carbon source repress gene expression. The presence of FSY1 mRNA is detected based on color change of a sample containing total RNA extracted from the organism and gold nanoparticles derivatized with a 15-mer of complementary single stranded DNA upon addition of NaCl. If FSY1 mRNA is present, the solution remains pink, changing to blue-purple in the absence of FSY1 mRNA. Direct detection of specific expression was possible from only 0.3 microg of unamplified total RNA without any further enhancement. This novel method is inexpensive, very easy to perform as no amplification or signal enhancement steps are necessary and takes less than 15 min to develop after total RNA extraction. No temperature control is necessary and color change can be easily detected visually.

Ferredoxin (Fd) and ferredoxin-NADP(+)-reductase (FNR) are two terminal physiological partners of the photosynthetic electron transport chain. Based on a nuclear magnetic resonance (NMR)-restrained-docking approach, two alternative structural models of the Fd-FNR complex in the presence of NADP+ are proposed. The protein docking simulations were performed with the software BiGGER. NMR titration revealed a 1:1 stoichiometry for the complex and allowed the mapping of the interacting residues at the surface of Fd. The NMR chemical shifts were encoded into distance constraints and used with theoretically calculated electronic coupling between the redox cofactors to propose experimentally validated docked complexes.

A new fluorescent macrocyclic structure (0) bearing two naphthalene units at both ends of a cyclic polyaminic chain containing two phenanthroline units was investigated with potentiometric and fluorescence (steady-state and time-resolved) techniques. The fluorescence emission spectra show the simultaneous presence of three bands: a short wavelength emission band (naphthalene monomer), a middle emission band (phenanthroline emission), and a long-wavelength band. All three bands were found to be dependent on the protonation state of the macrocyclic unit (including the polyaminic and phenanthroline structures). The existence of the long-wavelength emission band is discussed and is shown to imply that a bending movement involving the two phenanthroline units leads to excimer formation. This is determined by comparison with the excimer emission formed by intermolecular association of 1,10-phenanthroline. With ligand L1, excimer formation occurs only at pH values above 4. At very acidic pH values, the protonation of the polyamine bridges is extensive leading to a rigidity of the system that precludes the bending movement. The interaction with metal cations Zn(II) and Cu(II) was also investigated. Excimer formation is, in these situations, increased with Zn(II) and decreased with Cu(II). The long-emission band is shown to present a different wavelength maximum, depending on the metal, which can be considered as a characteristic to validate the use of ligand L1 as a sensor for a given metal.

Flavylium compounds can conveniently be encapsulated in water permeable cross-linked poly(2-hydroxyethyl methacrylate) ( PHEMA) polymer hydrogel matrices. The polymer is also permeable to gases, for example ammonia and hydrogen chloride. The ability of the polymer to concentrate water from the surrounding atmosphere was demonstrated. The absorption and emission spectra of the encapsulated flavylium compounds are very dependent on the pH, and by consequence ammonia ( or hydrogen chloride) can be easily detected. The intensity of the fluorescence emission of the flavylium cation (AH+ species obtained in acidic media) is very dependent on the water content due to the efficient excited state proton transfer involving the AH(+)* excited state and water. The combination of flavylium emission sensitivity to water and ability of the polymer to concentrate water from the surrounding atmosphere, confers this material potential utility as a humidity sensor with high sensitivity.

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