Publications

Abstract Excitation-emission matrices (EEM) of fluorescence of aqueous solutions of humic substances (HS), and sets of EEM acquired as function of the HS concentration, were analysed by multivariate curve resolution alternating least squares (MCR-ALS). Three types of HS samples were studied: one commercial humic acid; two samples of fulvic acid (FA) extracted from a pinewood soil; two samples of FA extracted from recycled wastes. The fluorescence measurements were carried out at HS concentration between 5 and 100 mg/L and at pH 6. The application of MCR-ALS algorithm on each individual EEM, as well as on column-wise augmented matrices, allows the identification of three major fluorophores in all HS samples analysed. The emission and excitation spectra of these fluorophores were recovered and are characteristic of each sample. Moreover, the variation of the fluorescence intensities of each fluorophore with HS concentration shows deviations from linearity at HS concentration higher than 30 mg/L, depending on the fluorophore and/or sample. This behaviour reveals the existence of inner filter effects that affect the proportionally between the fluorescent signal and concentration but do not provoke measurable distortions on the fluorescence spectra of the detected fluorophores.

Abstract Radical anions of o-, m-, and p-benzoquinone were produced in a Fourier transform mass spectrometer by low energy electron attachment or collision-induced dissociation and were differentiated. Classical derivatization experiments also were carried out to authenticate the ortho and meta anions. Gas-phase techniques were used to measure the proton affinities of all three radical anions and the electron affinities of o- and m-benzoquinone. By combining these results in thermodynamic cycles, we derived heats of hydrogenation of o-, m-, and p-benzoquinone (Delta(hyd)H degrees(1o, 1m, and 1p) = 42.8 +/- 4.1, 74.8 +/- 4.1, and 38.5 +/- 3.0 kcal mol(-1), respectively) and their heats of formation (Delta(t)H degrees(1o, 1m, and 1p) = -23.1 +/- 4.1, 6.8 +/- 4.1, and -27.7 +/- 3.0 kcal mol-1, respectively). Good accord with the literature value for the para derivative was obtained. Combustion calorimetry and heats of sublimation also were measured for benzil and 3,5-di-tert-butyl-o-benzoquinone. The former heat of formation agreed with previous determinations, while the latter result (Delta(f)H degrees (g) = -73.09 +/- 0.87 kcal mol(-1)) was transformed to Delta(f)H degrees(1o) = -18.9 +/- 2.2 kcal mol(-1) by removing the effect of the tert-butyl groups via isodesmic reactions. This led to a final value of Delta(f)H degrees) = -21.0 +/- 3.1 kcal mol(-1). Additivity was found to work well for m-benzoquinone, but BDE1 and BDE2 for 1,2- and 1,4-dihydroxybenzene differed by a remarkably small 14.1 +/- 4.2 and 23.5 +/- 3.7 kcal mol(-1), respectively, indicating that o- and p-benzoquinone should be excellent radical traps.

Abstract The standard (pdegrees = 0.1MPa) molar enthalpies of formation for 2-, 3- and 4-cyanophenol in the gaseous phase were derived from the standard molar enthalpies of combustion in oxygen at T D 298: 15 K, measured by static bomb combustion calorimetry, and the standard molar enthalpies of sublimation at 298.15 K, measured by Calvet microcalorimetry: 2-cyanophenol, (32.8 +/- 2.1) kJ-mol(-1); 3-cyanophenol, (37.8 +/- 2.2) kJ-mol(-1); 4-cyanophenol, (35.1 +/- 2.5)-kJ-mol(-1). Ab initio geometry optimizations of the three cyanophenols and respective phenoxyl radicals and phenoxide anions were performed using the 6-31G* basis sets. Single-point MP2 and DFT energy calculations allowed the estimation of the enthalpies of formation in the gaseous phase, the O-H bond dissociation energies, and the gas-phase acidities of the three cyanophenols. The theoretical results are generally in good agreement with the experimental findings.

Abstract The permeability of a self-assembled polyelectrolyte multilayer to small ions under the influence of an applied potential difference is studied as a function of the number of layers and the nature of the supporting electrolyte. The multilayer is described as a series of homogeneously charged membranes with alternating sign of their fixed charge. Ion transport is described on the basis of the diffusion equation and the assumption of (Donnan) electrochemical equilibrium at the boundaries between layers. The calculated steady-state current-voltage curves are found to be in good agreement with experimental linear sweep voltammograms (at low sweep rate). The permeability of polystyrenesulfonate/polyallylamine multilayers to ferro/ferricyanide ions is found to decrease monotonically with increasing number of layers when sodium perchlorate is used as the supporting electrolyte. However, an interesting non-monotonic dependence on the number of layers is observed when the supporting electrolyte is barium perchlorate. Both types of behaviour are accounted for satisfactorily by the theoretical model.

Abstract The standard (pdegrees = 0.1 MPa) molar enthalpy of formation of crystalline 2H-1,3-benzoxazine-2,4(3H)dione was measured, at T = 298.15 K, by static bomb calorimetry and the standard molar enthalpy of sublimation, at T = 298.15 K, was obtained using Calvet microcalorimetry. These values were used to derive the standard molar enthalpy of formation in the gaseous phase, T = 298.15 K, of -(401.0 +/- 3.5) kJ mol(-1). The standard molar enthalpy of sublimation of isatoic anhydride was recalculated, and our recommended experimental value for the standard molar enthalpy of formation in the gaseous phase, T = 298.15 K, is -(406.2 +/- 3.4) kJ mol(-1). Density functional calculations for the two isomers 2H-1,3-benzoxazine-2,4(3H)dione and isatoic anhydride, in which the ring nitrogen and oxygen have been transposed, confirm the experimental evidence of nearly identical thermochemical stability for these isomers.

Abstract The assisted transfer of silver ion, Ag+, by complexation of Ag+ with the tetrakis(4-chlorophenyl)borate (TPBCl-) anion was studied at the interface between two immiscible electrolyte solutions (ITIES). The elucidation of the interfacial mechanisms employed the cyclic voltammetry technique applied to macro and micro liquid/liquid interfaces. This procedure allowed to identify Ag+ as the species which is transferred at the liquid/liquid interface and also the formation of an 1:2 metal:ligand complex when TPBCl- is in excess. The value for the formal transfer potential of silver(I) from aqueous to 1,2-dichloroethane was evaluated and the association constant for the [Ag(TPBCl)(2)](-) complex was determined.

Abstract The standard (pdegrees = 0.1 Mpa) molar enthalpy of formation for liquid sulpholane was derived from the standard molar enthalpy of combustion, in oxygen at T = 298.15 K, measured by rotating bomb combustion calorimetry. The standard molar enthalpy of vaporization, at T = 298.15 K, was measured by Calvet microcalorimetry. The results are: Delta(f)H(m)degrees(l) = -(425.7 +/- 1.7)kJmol(-1;) Delta(1)(g) H(m)degrees = 69.1 +/- 1.4 kJ mol(-1); and Delta(f)H(m)degrees (g) = -(356.6 +/- 2.2)kJmol(-1). Additionally, high-level density functional theory calculations using the B3LYP hybrid exchange-correlation energy functional have been performed for sulpholane in order to obtain its geometry, enthalpy of formation in the gaseous phase and strain energy.

Abstract Interfacial tension was used to study the electrochemical properties of the interface between LiCl in water and tetraalkylammonium tetraphenylborate in 1,2-dichloroethane. The interfacial tension decrease with decreasing of the size of the cation of the organic supporting electrolyte on the order TBA(+) < THA(+) < THpA(+) < TOA(+). The decrease of interfacial tension is also observed at the electrocapillary maximum the potential of which (potential of zero charge) shift towards more positive values in the order TBA(+) > THA(+) > THpA(+) > TOA(+). This fact maybe an evidence that there is an increase of adsorption when the alkyl chain length of the organic cation is reduced.

Abstract Dehydroluciferyl-coenzyme A (L-CoA) was chemically synthesized and characterized by MS, UV-vis spectrometry and RP-HPLC. The identity of the chemically synthesized compound with the one that was produced by firefly luciferase was confirmed. Moreover, the reversibility of the enzymatic conversion of dehydroluciferin reversible arrow dehydroluciferyl-adenylate reversible arrow L-CoA was also confirmed. The chemical synthesis of L-CoA, described here, may help the clarification of the activator effect of CoA on luciferase bioluminescent assays, in which the enzyme catalyzed formation of L-CoA and the consequent destruction of L-AMP is one of the possible explanations for that effect.

Abstract The antiproliferative and cytotoxic properties of polyphenolic acid derivatives, structurally related with the natural models caffeic and gallic acids. have been tested in human cervix adenocarcinoma cells (HeLa). Simultaneous structural information was obtained for these Compounds through theoretical ab initio methods. This study was conducted for the following esters: methyl caffeate (MC, 1), propyl caffeate (PC, 2), octyl caffeate (OC, 3), methyl gallate (MG, 4), propyl gallate (PG, 5) and octyl gallate (OG, 6). A significant growth-inhibition effect was assessed for some of these compounds, clearly dependent on their structural characteristics. Marked structure-activity relationships (SARs)-namely the number of hydroxyl ring substituents-were found to rule the biological effect of such systems.