Publications

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.

Abstract The analysis of opiates is of vital interest in drug abuse monitoring and research. This review presents a general overview of the electrochemical methods used for detection and quantification of opiates in a variety of matrices. Emphasis has been placed on the voltammetric methods used for study and determination of morphine, codeine, and heroin. Specific issues that need to be solved and better explained as well as future trends in the use of electrochemical methods in the examination of opiates are also discussed.

Abstract A complete conformational analysis of beta-nitrostyrene and beta-methyl-beta-nitrostyrene derivatives was carried out by Raman spectroscopy coupled to ab initio MO calculations. Apart from the optimised geometrical parameters of the most stable conformers of the molecules under study, the corresponding harmonic vibrational frequencies were calculated, as well as potential-energy profiles for several internal rotations within the molecules. At the light of these results, a complete assignment of the Raman spectra of the solid samples was performed. The conformational behaviour of this kind of systems was found to be mainly determined by the stabilising effect of pi-electron delocalisation.

Abstract A conformational analysis of 3-(3,4,5-trihydroxyphenyl)-2-propenoic acid (3,4,5-trihydroxycinnamic acid, THPPE), a trihydroxylated cinnamic acid analogous to caffeic acid (a natural compound often present in diet), was carried out by Raman spectroscopy coupled to Ab initio MO calculations. Apart from the optimised geometrical parameters for the most stable conformers of this compound, and for one of its dimeric species, the corresponding harmonic vibrational frequencies, as well as potential-energy profiles for rotation around several bonds within the molecule, were obtained. Twenty one distinct conformers were found for THPPE, the lowest energy ones-THPPE 1 and THPPE 2-displaying a completely planar geometry. The conformational preferences of this system were thus found to be mainly ruled by the stabilising effect of pi-electron delocalisation. At the light of these results, a complete assignment of the corresponding solid state Raman spectra was performed.