Publications

Abstract Electrochemical impedance spectroscopy, Fourier transform infrared reflection-absorption spectroscopy, and cyclic voltammetry were employed to characterize polyelectrolyte multilayers (PEMs) fabricated with poly-(styrenesulfonate) as the polyanion and the polypeptides poly-L-histidine, poly-L-lysine, and poly-L-arginine as polycations. The layer-by-layer electrostatic assembly was produced onto alkanethiol-modified gold surfaces. The frequency response reveals that the effect of the number of layers seems to be related to a progressive reduction in the active area of the PEM-modified electrodes. The active area after the deposition of seven layers can be lower than 10% of its original value. The film surface is then inhomogeneous with respect to the transport of the electroactive species and has spots through which transport is quite favored. These structural features of the PEM have been taken into account in the theoretical model of ion transport and very good agreement with the experimental impedance results has been found.

Abstract The firefly luciferase reaction intermediate luciferyl adenylate was detected by RP-HPLC analysis when the luciferase reaction was performed under a nitrogen atmosphere. Although this compound is always specified as an intermediate in the light-production reaction, this is the first report of its identification by HPLC in a luciferase assay medium. Under a low-oxygen atmosphere, luciferase can catalyze the synthesis of luciferyl coenzyme A from luciferin, ATP, and coenzyme A, but in air dehydroluciferyl coenzyme A was produced. The luciferase-catalyzed synthesis of these coenzyme A derivatives may be a consequence of the postulated recent evolutionary origin of firefly luciferases from an ancestral acyl-coenzyme A synthetase.

Abstract The standard enthalpy of formation of the 2-amino-3-quinoxalinecarbonitrile-1,4-dioxide compound in the gas-phase was derived from the enthalpies of combustion of the crystalline solid measured by static bomb combustion calorimetry and its enthalpy of sublimation determined by Knudsen mass-loss effusion at T=298.15 K. This value is (383.8+/-5.4) kJ mol(-1) and was subsequently combined with the experimental gas-phase enthalpy of formation of atomic oxygen and with the computed gas-phase enthalpy of formation of 2-amino-3-quinoxalinecarbonitrile, (382.0+/-6.3) kJ mol(-1), in order to estimate the mean (N-O) bond dissociation enthalpy in the gas-phase of 2-amino-3-quinoxalinecarbonitrile-1,4-dioxide. The result obtained is (248.3+/-8.3) kJ mol(-1), which is in excellent agreement with the B3LYP/6-311+G(2d,2p)//B3LYP/631G(d) computed value.

Abstract The standard (pdegrees = 0.1 MPa) molar enthalpy of formation of liquid anthranil was measured at T = 298.15 K by static bomb calorimetry and the standard molar enthalpy of vaporization at T = 298.15 K was obtained using Calvet microcalorimetry. These values were used to derive the standard molar enthalpy of formation of anthranil in the gaseous phase. Thermochemical and quantum chemical comparisons were made to interrelate anthranil and its isomers, 1,2-benzisoxazole, benzoxazole and 2-cyanophenol, and the monocyclic heterocycles, isoxazole and oxazole. Comparisons with benzofurazan and isobenzofuran were also made. Additionally nucleus-independent chemical shifts were used as an aromaticity index. (C) Wiley-VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2004.

Abstract The standard (p(0) = 0.1 MPa) molar enthalpies of formation, at T= 298.15 K, of crystalline Ni(II), Cu(II) and Zn(II) complexes with N,N'-bis(salicylaldehydo)ethylenediamine (H(2)salen) were determined by solution-reaction calorimetry measurements as, respectively. Delta(f)H(m)(-) {[Ni(salen)], cr}=-(226.1 +/- 3.9)kJmol(-1), DeltafH(m)(o) {[Cu(salen)], cr}=-(139.0 +/- 3.9)kJmol(-1) and Delta(f)H(m)(o) {[Zn(salen)], cr}=-(281.3 +/- 4.6)kJmol(-1). The standard molar enthalpies of sublimation of the same metal complexes, at T=298.15K, were obtained by effusion methods as Delta(cr)(g)H(m)(o), [Ni(salen)]=(163.9+/-3.2)kJmol(-1), Delta(cr)(g)H(m)(o) [Cu(salen)]=(175.3 +/- 2.7)kJmol(-1) and Delta(g)(cr)H(m)(o) [Zn(salen)]=(179.6 +/- 3.7)kJmol(-1). The differences between the mean metal-ligand and hydrogen-ligand bond dissociation enthalpies were derived and discussed in terms of structure, in comparison with identical parameters for complexes of the same metals with other tetradentate Schiff bases involving a N2O2 donor set.

Abstract The standard (pdegrees = 0.1 MPa) molar enthalpies of formation for liquid 2-chloropyrazine and crystalline 2,6-dichloropyrazine, 2,3-dichloroquinoxaline and 2,3,6,7-tetrachloroquinoxatine were derived from the standard molar enthalpies of combustion, in oxygen, to yield CO(2)(g), N(2)(g), and HCl . 600H(2)O(l), at the temperature T = 298.15 K, measured by rotating-bomb combustion calorimetry. The standard molar enthalpies of vaporization or of sublimation, at T = 298.15 K, were measured by Calvet microcalorimetry.

Abstract The mean (N-O) bond dissociation enthalpies were derived for three 2-methyl-3-(R)-quinoxaline 1,4-dioxide (1) derivatives, with R = methyl (1a), ethoxycarbonyl (1b), and benzyl (1c). The standard molar enthalpies of formation in the gaseous state at T = 298.15 K for the three 1 derivatives were determined from the enthalpies of combustion of the crystalline solids and their enthalpies of sublimation. In parallel, accurate density functional theory-based calculations were carried out in order to estimate the gas-phase enthalpies of formation for the corresponding quinoxaline derivatives. Also, theoretical calculations were used to obtain the first and second N-O dissociation enthalpies. These dissociation enthalpies are in excellent agreement with the experimental results herewith reported.

Abstract In this paper we present the calorimetric determination of the standard molar enthalpies of combustion, sublimation, and formation of three methoxynitrophenol isomers: 2-methoxy-4-nitrophenol, 2-methoxy-5-nitrophenol and 4-methoxy-2-nitrophenol. In addition, density functional theory calculations with the B3LYP functional and two different atomic basis sets: 6-31G* and 6-311G** allowed the estimation of the standard molar enthalpies of formation in the gaseous phase, for all possible methoxynitrophenol isomers. The theoretical estimations are in good agreement with the experimental determined standard molar enthalpies of formation.

Abstract Oxygen reduction on platinum single crystals has been studied in perchloric and sulfuric acid media using a hanging meniscus rotating disk electrode configuration. The surfaces studied belong to the [01 (1) over bar] zone and can be classified in two different series, i.e., surfaces with terraces with (111) symmetry and (100) steps and surfaces with (100) terraces and (111) steps. In sulfuric acid media, the highest catalytic activity is found for the Pt(211) electrode whereas the Pt(111) electrode exhibits an anomalously low j(0) value. Conversely, the extrapolated value for the catalytic activity of Pt(111) obtained from the series of stepped surfaces having (111) terraces is much higher than that obtained experimentally. The comparison with the results obtained in perchloric acid solutions indicates that the anomalously low catalytic activity of the Pt(111) electrode in sulfuric acid solutions is related to the formation of an ordered adlayer of specifically adsorbed (bi)sulfate anions. For practical purposes, the effect of the step density and the (bi)sulfate specific adsorption is small, except for the Pt(111) electrode.