Publications

Abstract The present work reports an experimental thermochemical study supported by state of the art calculations of two heterocyclic compounds containing oxygen in the ring: xanthone and tetrahydro-gamma-pyrone. The standard (pA(0)A = 0.1 MPa) molar enthalpies of formation in the condensed phase, at T = 298.15 K, were derived from the measurements of the standard molar energies of combustion in oxygen atmosphere, using a static bomb calorimeter. The standard molar enthalpies of sublimation or vaporization, at T = 298.15 K, of the title compounds were obtained from Calvet microcalorimetry measurements. These values were used to derive the standard enthalpies of formation of the compounds in the gas-phase at the same temperature, which were compared with estimated data from G3(MP2)//B3LYP computations.

Abstract The molecular stability of thioxanthene, a key species from which very important compounds with industrial relevance are derived, has been studied by a combination of several experimental techniques and computational approaches, The standard (p(o) = 0.1 MPa) molar enthalpy of formation of crystalline thioxanthene (117.4 +/- 4.1 kJ . mol(-1)) was determined from the experimental standard molar energy of combustion, in oxygen, measured by rotating-bomb combustion calorimetry at T = 298.15 K. The enthalpy of sublimation was determined by a direct method, using the vacuum drop microcalorimetric technique, and also by an indirect method, using a static apparatus, where the vapor pressures at different temperatures were measured. The latter technique was used for both crystalline and undercooled liquid samples, and the phase diagram of thioxanthene near the triple point was obtained (triple point coordinates T = 402.71 K and p = 144.7 Pa). From the two methods, a mean value for the standard (p(o) = 0.1 MPa) molar enthalpy of sublimation, at T = 298.15 K (101.3 +/- 0.8 kJ . mol(-1)), was derived. From the latter value and from the enthalpy of formation of the solid, the standard (p(o) = 0.1 MPa) enthalpy of formation of gaseous thioxanthene was calculated as 218.7 +/- 4.2 kJ . mol(-1). Standard ab initio molecular orbital calculations were performed using the G3(MP2)//B3LYP composite procedure and several homodesmotic reactions in order to derive the standard molar enthalpy of formation of thioxanthene. The ab initio results are in excellent agreement with the experimental data.

Abstract The structure and temperature behaviour of the DNA+dipalmitoylphosphatidylcholine (DPPC) bilayer as a function of ZnCl(2) concentration were examined using differential scanning calorimetry (DSC), small-angle neutron scattering (SANS) and small-angle X-ray diffraction (SAXD). Experiments revealed the coexistence of two lamellar phases in the mixture: the L(PC) phase, formed due to Zn(2+) binding to the DPPC bilayers, and the condensed lamellar phase L(DNA+PC) with DNA strands packed between the DPPC bilayers. With increasing concentration of zinc, the temperature of the gel - liquid-crystal phase transition of DPPC increases in both phases, and the volume fraction of LDNA+PC phase decreases. In the gel state (at 20 degrees C), the repeat distance of LDNA+PC phase is constant, d(DNA+PC) similar to 8.3 nm, up to 20 mmol/l of ZnCl(2), and increases for higher concentrations of the salt. The periodicity of the L(PC) lamellar phase decreases substantially with the increasing concentration of the salt in the mixture. In the liquid-crystalline state, concentrations above 20 mmol/l ZnCl(2) promote the dissolution of the LDNA+PC phase into DPPC + Zn(2+) unilamellar vesicles and DNA is neutralized by Zn(2+) ions. The screening of Zn(2+) charge and the formation of a diffuse double layer due to increasing ionic strength of solution are responsible for the observed changes.

Abstract One of the possible approaches for the development of novel solid-phase microextraction (SPME)fibers is the physical deposition of porous materials Oil to a support using high-temperature epoxy glue. However, a major drawback arises from decomposition of epoxy glue at temperatures below 300 degrees C and instability in some organic solvents. This limitation motivated us to explore the possibility of replacing the epoxy glue with a sol-gel film, thermally more stable and resistant to organic solvents. We found that functionalised silica particles could be successfully attached to a robust Ni-Ti wire by using a UV-curable sol-gel film. The particles were found to be more important than the sol-gel layer during the microextraction process, as shown by competitive extraction trials and by the different extraction profiles observed with differently functionalised particles. If a quality control microscopic-check aiming at the rejection of fibers exhibiting unacceptably low particle load was conducted, acceptable (6-14%) reproducibility of preparation of C(18)-silica fibers was observed, and a strong indication of the durability of the fibers was also obtained. A cyclohexyldiol-silica fiber was used, as a simple example of applicability, for the successful determination of benzaldehyde. acetophenone and dimethyl phenol at trace level in spiked tap water. Recoveries: 95-109%; limits of detection: 2-7 mu g/L; no competition effects within the studied range (<= 125 mu g/L).

Abstract Marine organisms are known to accumulate arsenic (As) in The range of 1-100 mg kg(-1) from their environment and food sources. The majority of arsenic is present as organoarsenic compounds, metabolized from inorganic arsenic present in seawater or accumulaled from food sources such as algae or other fish. The aim of This study was to evaluate total arsenic content in The muscle of eight fish species, three cephalopods, and one crustacean consumed in Portugal. The measurement of total arsenic (10 individuals of each species) was carried out by EDXRF (Energy Dispersive X-Ray Fluorescence Method). The highest concentrations were found in Norway lobster (30.3 +/- 8.7 mg kg(-1) wet weight), followed by common octopus (25.9 +/- 8.4 mg kg(-1) wet weight). Within fish species, wild gilthead sea bream, anglerfish, and megrim species presented values around 12 mg kg,(-1) and in the others the average was lower than 10 mg kg(-1). Taking into consideration the tolerable daily intake recommended by the World Health Organization (WHO; 0.05 mg As kg(-1) body weight) the obtained results of total arsenic in The studied species do not represent a bazard for buman consumption.

Abstract A challenge that faces the footwear market is the improvement of casual shoe comfort, being shock absorption one of the most significant properties for comfort. However, the methodologies available for the quantification of the shock absorption properties in materials for footwear applications are limited. With this work, an evaluation methodology of the shock absorption properties of rubber materials is proposed and the relationship between different test methods is established. Eight different commercial compact heeled rubber materials at six different thicknesses were prepared and characterized according to their physical and impact absorption properties. From the experimental values, the following correlations were defined: energy compression/hardness and energy compression/maximum deceleration. Shock absorption prediction curves were also determined from the experimental maximum deceleration values. Results present an excellent tool for industrial applications since they allow correlation of shock absorption properties and heel thickness, and also to predict the contribution of rubber material composition to the final product shock absorption properties.

Abstract This study intends to present some further research in polymer-dispersed liquid crystal (PDLC) systems [1]. These PDLC type cells were produced based on a low-cost commercial polymer (PVC) and liquid crystal E48 (Merck), using a modified solvent preparation method (SIPS). These cells were characterized electro-optically and by low frequency dielectric relaxation (20 Hz to 1 MHz) experiments. Results are encouraging; the prepared composites show regular microstructures and optical transmittance changes induced by the electric field, introducing an alternative way of producing low-cost PDLC electro-optical cells.

Abstract Electrochemical properties of Au electrodes sequentially modified by self-assembled 1,6 hexanedithiol (1,6HDT) and gold nanorods (AuNRs) are investigated by cyclic voltammetry, square-wave voltammetry, and electrochemical impedance spectroscopy, using [Fe(CN)6](3-/4-) as redox probes. The nanorods stabilized by cetyltrimetylammonium bromide (CTAB) with aspect ratios of 2.20, 2.80, and 3.77 were grown by a seed-mediated procedure and chemically bonded to the 1,6HDT-coated electrodes by a place exchange reaction at a 27 degrees C solution temperature. Topographic tapping mode atomic force microscopy measurements revealed an end bonding of the 2.20 aspect ratio rods and a side surface bonding of the 2.80 and 2.77 aspect ratio rods. Analysis of the electrochemical responses as a function of the sizes and surface orientations of the rods revealed that the electron transfer is faster at electrodes modified with smaller and vertically aligned nanorods than those modified with larger and randomly attached nanorods (side surface bonding). A progressive increase in the charge transfer resistance RCT from bilayers composed of 1,6HDT and 2.20 aspect ratio rods to bilayers composed of 1,6HDT and rods of 2.80 or 3.77 aspect ratios was described by a tunneling parameter of beta = 1.07 per thiol chain unit, This behavior suggests that the electron transfer kinetics is controlled by coherent electron tunneling across the 1,6HDT monolayer. In addition, the several orders of magnitude changes of the apparent charge transfer resistance upon nanorod adsorption suggest a charging of the rods by. the redox probes in solution and electron transfer across them. It is concluded that the electron transfer proceeds via a three-step process: charging of the rods by the redox probes in solution, electron transport across the rods, and electron tunneling across the 1,6HDT-SAM toward the underlying An substrates.

Abstract Luciferase is a general term for enzymes catalyzing visible light emission by living organisms (bioluminescence). The studies carried out with Photinus pyralis (firefly) luciferase allowed the discovery of the reaction leading to light production. It can be regarded as a two-step process: the first corresponds to the reaction of luciferase's substrate, luciferin (LH(2)), with ATP-Mg(2+) generating inorganic pyrophosphate and an intermediate luciferyl-adenylate (LH(2)-AMP); the second is the oxidation and decarboxylation of LH(2)-AMP to oxyluciferin, the light emitter, producing CO(2), AMP, and photons of yellow-green light (550-570 nm). In a dark reaction LH(2)-AMP is oxidized to dehydroluciferyl-adenylate (L-AMP). Luciferase also shows acyl-coenzyme A synthetase activity, which leads to the formation of dehydroluciferyl-coenzyme A (L-CoA), luciferyl-coenzyme A (LH(2)-CoA), and fatty acyl-CoAs. Moreover luciferase catalyzes the synthesis of dinucleoside polyphosphates from nucleosides with at least a 3'-phosphate chain plus an intact terminal pyrophosphate moiety. The LH(2) stereospecificity is a particular feature of the bioluminescent reaction where each isomer, D-LH(2) or L-LH(2), has a specific function. Practical applications of the luciferase system, either in its native form or with engineered proteins, encloses the analytical assay of metabolites like ATP and molecular biology studies with luc as a reporter gene, including the most recent and increasing field of bioimaging. (C) 2008 IUBMB