Proton-coupled electron transfer of flavodoxin immobilized on nanostructured tin dioxide electrodes: thermodynamics versus kinetics control of protein redox function

In this paper, we report a spectroelectrochemical investigation of proton-coupled electron transfer in flavodoxin D. vulgaris Hildenborough (Fld). Poly-L-lysine is used to promote the binding of Fld to the nanocrystalline, mesoporous SnO(2) electrodes. Two reversible redox couples of the immobilized Fld are observed electrochemically and are assigned by spectroelectrochemistry to the quinone/semiquinone and semiquinone/hydroquinone couples of the protein's flavin mononucleotide (FMN) redox cofactor. Comparison with control data for free FMN indicates no contamination of the Fld data by dissociated FMN. The quinone/semiquinone and semiquinone/hydroquinone midpoint potentials (E(q/sq) and E(sq/hq)) at pH 7 were determined to be -340 and -585 mV vs Ag/AgCl, in good agreement with the literature. E(q/sq) exhibited a pH dependence of 51 mV/pH. The kinetics of these redox couples were studied using cyclic voltammetry, cyclic voltabsorptometry, and chronoabsorptometry. The semiquinone/quinone reoxidation is found to exhibit slow, potential-independent but pH-sensitive kinetics with a reoxidation rate constant varying from 1.56 s(-)(1) at pH 10 to 0.0074 s(-)(1) at pH 5. The slow kinetics are discussed in terms of a simple kinetics model and are assigned to the reoxidation process being rate limited by semiquinone deprotonation. It is proposed that this slow deprotonation step has the physiological benefit of preventing the undesirable loss of reducing equivalents which results from semiquinone oxidation to quinone.     

<Emphasis Type="Italic">Brønsted</Emphasis> Acids in Ionic Liquids: Fundamentals,Organic Reactions,and Comparisons

Summary. A background for studying acids in various solvents is developed, emphasizing the importance of knowing to what extent a solvent conducts electricity and is therefore ionized, the dissociation equilibria of common molecular solvents and the acidic and basic species generated by solvent leveling. Acidity measurements in the atypical solvent water are discussed and the common method of expressing acidity in other systems – by Hammett values – is introduced. Representative examples of reactions involving Br?nsted acids in ionic liquids are presented and attention paid to the questions of speciation and acidity values. It is found that the gas phase proton affinity of a base is often a better guide to the acidity of its conjugate acid in an ionic liquid than is the dissociation constant of the said acid in water.     

Nitrofuran antibiotic residues in pork: The FoodBRAND retail survey

Use of nitrofuran drugs in food-producing animals has been prohibited within the EU because they may represent a public health risk. Monitoring compliance with the ban has focused on the detection of protein-bound nitrofuran metabolites which, in contrast to the parent compounds, are stable and persist in animal tissues. As part of the “FoodBRAND” project, an extensive survey of pork was undertaken across 15 European countries. Samples (n = 1500) purchased at retail outlets were analysed for the nitrofuran metabolites AOZ, AMOZ, AHD and SEM using LC–MS/MS determination of nitrobenzaldehyde derivatives. Limits of quantification for the method were 0.1 μg/kg (AOZ, AMOZ), 0.2 μg/kg (SEM) and 0.5 μg/kg (AHD). Of the 1500 samples tested, measurable residues of nitrofuran metabolites were confirmed in 12 samples (0.8% incidence overall) of which 10 samples were purchased in Portugal (AOZ, 0.3 μg/kg; AMOZ, 0.2–0.6 μg/kg) and one sample each in Italy (AMOZ, 1.0 μg/kg) and Greece (AOZ, 3.0 μg/kg).     

Conformational studies by dynamic NMR. 97. Structure, conformation, stereodynamics and enantioseparation of aryl substituted norbornanes

The structure of a 1,7,7-triaryl norbornane (compound 3) has been determined by X-ray diffraction and was found essentially equal to that predicted by molecular mechanics calculations. Restricted rotation of the aryl groups also has been observed by dynamic NMR spectroscopy in this compound and in a number of analogously substituted norbornanes. The aryl-norbornane bond rotation barriers were measured by line shape analysis of the (13)C NMR spectra obtained at temperatures lower than -100 degrees C and were found to cover the range 6.0 to 7.9 kcal mol(-1). An exception was the rotation involving the o-anisyl group in compound 5, which occurs near ambient temperature since the corresponding barrier is much higher (14.4 kcal mol(-1)). In one case (compound 4) configurational enantiomers could be separated by chiral HPLC and the corresponding CD spectra recorded.     

Use of headspace capillary GC to study the development of volatile compounds in fresh fruit

The composition of volatile compounds produced by fruit during growth and post-harvest storage and ripening has been studied and the different headspace methods compared. Static and dynamic headspace sampling have been compared and evaluated according to their capacity to collect and concentrate volatiles from the atmosphere surrounding the fruits, and FID, MSD, and organoleptic detection have been compared. The results emphasize that the headspace sampling procedure selected is crucial to the performance of subsequent analysis.     

Theoretical modeling of vibroelectronic quantum states in complex molecular systems: solvated carbon monoxide, a test case

In this paper we extend the perturbed matrix method by explicitly including the nuclear degrees of freedom, in order to treat quantum vibrational states in a perturbed molecule. In a previous paper we showed how to include, in a simple way, nuclear degrees of freedom for the calculation of molecular polarizability. In the present work we extend and generalize this approach to model vibroelectronic transitions, requiring a more sophisticated treatment.     

Partial Synthesis and Characterization of the Mono-and Diepoxides of β-Cryptoxanthin

β-Cryptoxanthin ( 1 ) was acetylated and then epoxidized with monoperoxyphthalic acid. After hydrolysis, repeated chromatography, and crystallization, (3S,5R,6S)-5,6-epoxy-β-cryptoxanthin ( 3 ), (3S,5S,6R)-5,6-epoxy-β-cryptoxanthin ( 4 ), (3R,5′R,6′R)-5′,6′-epoxy-β-cryptoxanthin ( 5 ), (3S,5R,6S,5′R,6′S)-5,6:5′,6′-diepoxy-β-cryp-toxanthin ( 6 ), and (3S,5S,6R,5′S,6′R)-5,6:5′,6′-diepoxy-β-cryptoxanthin ( 7 ) were isolated as main products and characterized by their UV/VIS, CD, ¹H- and ¹³C-NMR, and mass spectra. The comparison of the carotenoid isolated from yellow, tomato-shaped paprika (Capsicum annuum var. lycopersiciforme flavum) with 3–5 strongly supports the structure of 3 for the natural product.     

Photophysical properties of methyl triazone included in MCM-41

Methyl triazone (4,4',4'-[1,3,5-triazine-2,4,6-triyltriimino]tris-trimethyl benzoate) has been included in mesoporous MCM-41 (Mobil's composition of matter-41) silica, and its fluorescence emission has been compared in solution and in the solid state. Although inclusion does not affect significantly the absorption properties, a fluorescence emission shift and a behavior similar to the solid state has been observed by increasing the loading. It is believed that this observation reflects molecular aggregation inside the MCM-41 channels. The potential of formulations of this type for sunscreen applications is discussed.     

Determination of the Insect Repellent Bayrepel® in Pool and Lake Water by Gas Chromatography after Preconcentration with Solid-Phase Extraction and Stir-Bar-Sorptive Extraction

A method for the determination of the new insect repellent Bayrepel in bathing lakes and public pools is presented. Sample preconcentration is performed either by solid-phase extraction (SPE) or stir bar sorptive extraction (SBSE). Analysis of the enriched analyte using gas chromatography-mass spectrometry (GC-MS) led to almost identical limits of detection (LOD) of 25ngL^–1 for Bayrepel with both extraction methods. Because of the slightly reduced handling effort involved, the method based on preconcentration by SBSE was finally chosen for the analysis of real samples.     

Probing the Conformational Changes in the Enzymatic Hydrolysis of 2‐Acetamido‐2‐deoxy‐β‐D‐glucopyranosides

The isoquinuclidines 7 and 8 were synthesised and tested as inhibitors of hexosaminidases from jack beans and from bovine kidney. These isoquinuclidines mimick the ^1,4B‐conformer of a N‐acetyl‐glucosamine‐derived β‐d‐ glucopyranoside; they are competitive inhibitors with K_i values from 0.014 to 0.30 μM . The strong inhibition of these enzymes agrees with the hypothesis that the enzymatic hydrolysis of 2‐acetamido‐2‐deoxy‐β‐d‐ glucopyranosides proceeds via a boat‐like conformer with a pseudo‐axial scissile glycosidic bond and a pseudo‐axial acetamido substituent optimally oriented to effect an intramolecular substitution of the aglycon.