Light and heavy dansyl reporter groups in food chemistry: amino acid assay in beverages

5‐Dimethylamino‐1‐sulfonyl naphthalene (DNS, commonly referred as dansyl) is a functionality, bearing well‐established properties in directing the fragmentation, by mass spectrometry (MS), of the corresponding ionized sulfonylated derivatives. This property is shared also by its labeled analogs. The use of d₀/d₆ DNS derivatives is now exploited in the application of the well‐established isotope dilution mass spectrometric approach in the assay of complex mixtures. A new method for the quantitation of amino acids (AAs) in beverages is therefore presented, which relies on liquid chromatographic separation of their N‐dansylated derivatives followed by comparative electrospray tandem MS/MS of the d₀/d₆ isobaric mixtures. Labeled and unlabeled DNS derivatives of the selected AAs are readily available by microwave‐assisted synthetic protocols. The novelty of the method is represented by the use of heavy and light DNS‐isotopologue providing suitable reporter groups. Multiple‐reaction monitoring has been applied in the assay of AAs in wine, pineapple juice and bergamot juice with good‐to‐excellent results as proved by both relative standard deviation, lower than 15%, and by the accuracy values in the range 90–110%. Copyright © 2012 John Wiley & Sons, Ltd.     

Electronic structure and optical properties of an alternated fluorene-benzothiadiazole copolymer: interplay between experimental and theoretical data

The donor-acceptor copolymer containing benzothiadiazole (electron acceptor), linked to functionalized fluorene (electron donor), [poly[9,9-bis(3'-(tert-butyl propanoate))fluorene-co-4,7-(2,1,3-benzothiadiazole)] (LaPPS40), was synthesized through the Suzuki route. The polymer was characterized by scanning electron microscopy, gel permeation chromatography, NMR, thermal analysis, cyclic voltammetry, X-ray photoelectron spectroscopy, UV-vis spectrometry, and photophysical measurements. Theoretical calculations (density functional theory and semiempirical methodologies) used to simulate the geometry of some oligomers and the dipole moments of molecular orbitals involved were in excellent agreement with experimental results. Using such data, the higher energy absorption band was attributed to the π-π* (S(0) → S(4)) transition of the fluorene units and the lower lying band was attributed to the intramolecular (ICT) (S(0) → S(1)) charge transfer between acceptor (benzothiadiazole) and donor groups (fluorene) (D-A structure). The ICT character of this band was confirmed by its solvatochromic properties using solvents with different dielectric properties, and this behavior could be well described by the Lippert-Mataga equation. To explain the solvatochromic behavior, both the magnitude and orientation of the dipole moments in the electronic ground state and in the excited state were analyzed using the theoretical data. According to these data, the change in magnitude of the dipole moments was very small for both transitions but the spatial orientation changed remarkably for the lower energy band ascribed to the ICT band.     

An alternative method by pressurized DSC to evaluate biodiesel antioxidants efficiency

The purpose of this article is to present an alternative method developed using pressurized differential scanning calorimetry (P-DSC) to evaluate the oxidative behavior of biodiesel obtained from soybean oil, produced by ethylic route (BSET), with and without the addition of synthetic antioxidants, as well as to compare the results with those obtained by Rancimat Method, which were carried out according to the European Official Standard Method EN 14112. BSET samples were analyzed using four different types of synthetic antioxidants at concentrations varying between 250, 500, and 1000 mg kg⁻¹. The measurements done by P-DSC were performed using static air at 80 psi and isothermal conditions at 110 °C, the same operating temperature than the Rancimat method. The experimental results of the oxidative stability measured from both methods have shown good correlations, which depend on the antioxidant, showing a gradual improvement in the oxidative stability of BSET with increasing added concentration of antioxidants. Opposite result behavior occurred for the measurements done in the presence of one of the antioxidants, which was composed by a significant content of components volatile below 110 °C, which accelerate the induction of the biodiesel oxidation in P-DSC method, but apparently do not interfere in the electrical conductivity measurement method (Rancimat). The advantages of the developed method using P-DSC technique with respect to Rancimat method is that it allows one to use a smaller sample, increases sensitivity to antioxidant action, reduces the analysis time for the evaluation of the antioxidant efficiency and detects any occurring oxidative process, independently of the volatility of the formed products, which may form oxidated products that remain in the liquid phase and that are non detectable by Rancimat method.     

Structure and Substituent Effects on Retention and Chiral Resolution of Ketones and Alcohols on Microcrystalline Cellulose Triacetate Plates

This paper reports a number of original thin layer chromatography enantioseparations of closely related ketones and alcohols such as tetralones, indanones, and benzhydrols carried out by elution with aqueous-alcoholic mixtures at different ratios. In order to investigate the structural and substituent effects on chiral recognition of microcrystalline cellulose triacetate, the results were compared with those obtained in previous papers for analogous compounds in similar experimental conditions. Even though the inclusion model of retention of analytes on this chiral stationary phase is confirmed, different and unexpected results were obtained for compounds having very favourable characteristics for resolution.     

SDS (sodium dodecyl sulfate) effect on the autoxidation of the Glossoscolex paulistus giant extracellular hemoglobin: kinetic studies at pH 7.0 and 9.0

The effect of the anionic surfactant sodium dodecyl sulfate (SDS) on the autoxidation process of the giant extracellular hemoglobin of Glossoscolex paulistus (HbGp) is addressed in the present work. The complex oligomeric assembly of hemoglobin subunits may influence the autoxidation rate and the exponential decay behavior. Kinetic studies were developed using UV-vis measurements at 415 nm. These spectroscopic measurements are analyzed at two pH values, 7.0 and 9.0, where the hemoglobin presents different oligomeric assembly. At pH 7.0 a high stability of the native form of the oxy-hemoglobin is observed, while at pH 9.0 an intense dissociation of the oligomer is promoted by alkalization. This difference is evident by comparison of the rate constants in the absence of surfactant: at pH 7.0 the kinetics presents a mono-exponential behavior with a rate constant of 0.27 x 10(-4)s(-1) while at pH 9.0 a bi-exponential behavior was observed with rate constant increase to 7 x 10(-4)s(-1) (fast process) and 1 x 10(-4)s(-1) (slow process). In the autoxidation induced by SDS two factors affect significantly the process rate, namely, the oligomeric arrangement of the hemoglobin and the strength of the interaction between SDS and HbGp. At pH 7.0, for SDS concentrations up to 0.3mM, a mono-exponential behavior was observed, showing rate constants around 0.4 x 10(-4)s(-1), which suggest that the hemoglobin still maintains the more compact structure observed at this pH for the native protein. In the SDS concentration range 0.75-1.0mM, the mono-exponential process changes into a bi-exponential behavior with rate constants varying from 48 x 10(-4) up to 99 x 10(-4)s(-1) for the fast process and from 1.7 x 10(-4) up to 3.7 x 10(-4)s(-1) for the slow process, suggesting hemoglobin dissociation. At pH 9.0, a bi-exponential decay is observed for all studied SDS concentration range, presenting rate constants from 11.0 x 10(-4) up to 179 x 10(-4)s(-1) for the fast process and from 1.0 x 10(-4) up to 8 x 10(4)s(-1) for the slow process probably due to hemoglobin dissociation, which is already present in the absence of surfactant. At pH 7.0, the highly packed native protein structure should inhibit the autoxidation process, but the SDS/HbGp interaction is more intense as compared to pH 9.0, due to the acid pI value, promoting oligomeric dissociation. So, the autoxidation process is regulated at pH 7.0 by the interaction with SDS, which triggers oligomeric dissociation and increase of autoxidation rate. At pH 9.0, the autoxidation process should be very fast, probably due to the oligomeric dissociation, which is already present in the absence of surfactant. At alkaline pH, the interaction with SDS seems be weaker than at pH 7.0. This behavior at pH 7.0 can be observed through the higher autoxidation rate for the faster chains and it is associated to the acid pI of the giant extracellular hemoglobins.     

Selective catalytic oxidation of organosulfur compounds with tert-butyl hydroperoxide

Rates and selectivities for the oxidation of various organosulfur compounds with tert-butyl hydroperoxide were measured on CoAPO-5 (APO = aluminophosphate; Co/P = 0.05), Co/H-Y (Co/Al = 0.15), and MoO(x)/Al2O3 (15 % wt MoO3). Rates increased with increasing electron density at the sulfur atom (methyl phenyl sulfide>diphenyl sulfide>4-methyldibenzothiophene>2,5-dimethyl thiophene). Rates (per metal atom) were significantly higher on CoAPO-5 than on Co/H-Y, MoO(x)/Al2O3, or homogeneous Co acetate catalysts. Small amounts of sulfoxides (1-oxide) were detected on all catalysts at low reactant conversions, together with their corresponding sulfones; at higher conversions, only sulfones (1,1-dioxide) were detected, indicating that the oxidation of sulfoxides is much faster than for organosulfur reactants in the sequential oxidation pathways prevalent on these catalysts. Framework Co cations were not leached from CoAPO-5 during the oxidation of 4-methyldibenzothiophene, but most exchanged Co cations in H-Y and >20 % of Mo cations in MoO(x)/Al2O3 were extracted during these reactions. The fraction of redox-active Co cations in CoAPO-5 and Co/H-Y was measured by reduction-oxidation cycles using H2 and O2 and by UV-visible spectroscopy. This fraction was much larger in CoAPO-5 (0.35) than in Co/H-Y (0.01), consistent with the higher oxidation rates measured on CoAPO-5 and with the involvement of redox-active species in kinetically-relevant steps in catalytic oxidation sequences. Redox-active Co cations at framework positions within accessible channels are required for catalytic activity and structural stability during oxidative desulfurization, whether hydroperoxides are used as reactants or as intermediates (when O2 is used as the oxidant).     

ATP recognition and sensing with a phenanthroline-containing polyammonium receptor

A new polyammonium receptor is able to selectively recognise and sense ATP among triphosphate nucleotides, thanks to ATP-induced quantitative quenching of its fluorescence emission.     

Fourier expansions of functions with bounded variation of several variables

In the first part of the paper we establish the pointwise convergence as for convolution operators under the assumptions that has integrable derivatives up to an order and that with . We also estimate the Hausdorff dimension of the set where divergence may occur. In particular, when the kernel is the Fourier transform of a bounded set in the plane, we recover a two-dimensional analog of the Dirichlet theorem on the convergence of Fourier series of functions with bounded variation. In the second part of the paper we prove an equiconvergence result between Fourier integrals on euclidean spaces and expansions in eigenfunctions of elliptic operators on manifolds, which allows us to transfer some of the results proved for Fourier integrals to eigenfunction expansions. Finally, we present some examples of different behaviors between Fourier integrals, Fourier series and spherical harmonic expansions.

     

Modeling heme proteins using atomistic simulations

Heme proteins are found in all living organisms, and perform a wide variety of tasks ranging from electron transport, to the oxidation of organic compounds, to the sensing and transport of small molecules. In this work we review the application of classical and quantum-mechanical atomistic simulation tools to the investigation of several relevant issues in heme proteins chemistry: (i) conformational analysis, ligand migration, and solvation effects studied using classical molecular dynamics simulations; (ii) electronic structure and spin state energetics of the active sites explored using quantum-mechanics (QM) methods; (iii) the interaction of heme proteins with small ligands studied through hybrid quantum mechanics-molecular mechanics (QM-MM) techniques; (iv) and finally chemical reactivity and catalysis tackled by a combination of quantum and classical tools.     

Isolation and synthesis of (-)-(5S)-2-imino-1-methylpyrrolidine-5- carboxylic acid from Cliona tenuis: structure revision of pyrostatins

[reaction: see text] (-)-(5S)-2-Imino-1-methylpyrrolidine-5-carboxylic acid (1), previously reported as the N-acetyl-beta-d-glucosaminidase inhibitor pyrostatin B, has been isolated from the organic extracts of the burrowing sponge Cliona tenuis. The structure of 1, including its absolute stereochemistry, was characterized from its spectral data and chemical transformations and confirmed by total synthesis. The synthesis of 1 reveals that the structure of pyrostatin B has been incorrectly assigned. Comparison of NMR spectral data strongly suggests that pyrostatins A and B are identical to 5-hydroxyectoine and ectoine, respectively.