Publications

Abstract This paper reports an experimental and theoretical study of the gas phase standard (p(o) = 0.1 MPa) molar enthalpies of formation, at T = 298.15 K, of alpha-alanine (DL) and beta-alanine. The standard (p(o) = 0.1 MPa) molar enthalpies of formation of crystalline alpha-alanine (DL) and beta-alanine were calculated from the standard molar energies of combustion, in oxygen, to yield CO2(g), N-2(g), and H2O(l), measured by static-bomb combustion calorimetry at T = 298.15 K. The vapor pressures of both amino acids were measured as function of temperature by the Knudsen effusion mass-loss technique. The standard molar enthalpies of sublimation at T = 298.15 K was derived from the Clausius-Clapeyron equation. The experimental values were used to calculate the standard (p(o) = 0.1 MPa) enthalpy of formation of alpha-alanine (DL) and beta-alanine in the gaseous phase, Delta H-f(m)o(g), as -426.3 +/- 2.9 and -421.2 +/- 1.9 kJ.mol(-1), respectively. Standard ab initio molecular orbital calculations at the G3 level were performed. Enthalpies of formation, using atomization reactions, were calculated and compared with experimental data. Detailed inspections of the molecular and electronic structures of the compounds studied were carried out.

Abstract Gemini surfactants possess interesting interfacial and aggregation properties that have prompted comprehensive studies and successful applications in a wide variety of fields. However, a systematic study on the effect of gemini tail and spacer length upon the organization of lipid membranes has not been presented so far. In this study, we analyze the action of dicationic alkylammonium bromide gemini surfactants on DPPC liposomes, the latter employed as a model of lipid membranes. Differential scanning calorimetry results indicate that the surfactants presenting shorter tails (12 carbons) induce a decrease in the overall order of the bilayer, while those with longer tails (16 and 18 carbons) lead to the formation of more ordered structures. The respective influence on the degree of lipid order transverse to the bilayer was additionally studied resorting to a detailed fluorescence anisotropy study. In this case, it is observed that among the shorter tail surfactants, those with longer spacers (6 and 10 carbons) are responsible for a more pronounced disrupting effect upon the membrane, especially close to the lipid polar heads. Molecular dynamics simulation supports the most important findings and provides insight into the mechanism that governs this interaction. Accordingly, the interplay between tail and spacer length accounts for the differential vertical positioning of the gemini molecules and atom-density in the core of the bilayer, that provide a rationale for the experimental observations.

Abstract The vapour pressures of six para-substituted benzoic acids were measured using the Knudsen effusion method within the pressure range (0.1-1 Pa) in the following temperature intervals: 4-hydroxybenzoic acid (365.09-387.28) K; 4-cyanobenzoic acid (355.14-373.28) K; 4-(methylamino)benzoic acid (359.12-381.29) K; 4-(dimethylamino)benzoic acid (369.29-391.01) K; 4-(acetylamino)benzoic acid (423.10-443.12) K; 4-acetoxybenzoic acid (351.28-373.27) K. From the temperature dependence of the vapour pressure, the standard molar enthalpy, entropy and Gibbs energy of sublimation, at the temperature 298.15 K, were derived for each of the studied compounds using estimated values of the heat capacity differences between the gaseous and the crystalline phases. Equations for estimating the vapour pressure of para substituted benzoic acids at the temperature of 298.15 K are proposed.

Abstract The binding mode of several substrate analogues, (2R)-2-benzyl-3- dehydroquinic acids 4, which are potent reversible competitive inhibitors of type II dehydroquinase (DHQ2), the third enzyme of the shikimic acid pathway, has been investigated by structural and computational studies. The crystal structures of Mycobacterium tuberculosis and Helicobacter pylori DHQ2 in complex with one of the most potent inhibitor, p-methoxybenzyl derivative 4a, have been solved at 2.40 Å and 2.75 Å, respectively. This has allowed the resolution of the M. tuberculosis DHQ2 loop containing residues 20-25 for the first time. These structures show the key interactions of the aromatic ring in the active site of both enzymes and additionally reveal an important change in the conformation and flexibility of the loop that closes over substrate binding. The loop conformation and the binding mode of compounds 4b-d has been also studied by molecular dynamics simulations, which suggest that the benzyl group of inhibitors 4 prevent appropriate orientation of the catalytic tyrosine of the loop for proton abstraction and disrupts its basicity. © 2010 Wiley-VCH Verlag GmbH & Co. KGaA.

Abstract The molar heat capacity and the standard (p(0) = 0.1 MPa) molar enthalpies of formation of the crystalline of bis(glycinate) lead(II), Pb(gly)(2); bis(DL-alaninate) lead(II), Pb(DL-ala)(2); bis(DL-valinate) lead(II), Pb(DL-val)(2); bis(DL-valinate) cadmium(II), Cd(DL-val)(2) and bis(DL-valinate) zinc(II), Zn(DL-val)(2), were determined, at T = 298.15 K, by differential scanning calorimetry, and high precision solution-reaction calorimetry, respectively. The standard molar enthalpies of formation of the complexes in the gaseous state, the mean molar metal-ligand dissociation enthalpies, M(II)-amino acid, < D(m)>(M-L), were derived and compared with analogous copper(II)-ligand and nickel(II)-ligand. [GRAPHICS] .

Abstract Salt-free ion-paired catanionic amphiphiles of the C m +C n - type, with a high solubility mismatch (n ≫ m or m≫) display a remarkable phase behavior in water. A temperature-driven vesicle-to-micelle transition in the dilute side together with a coexistence of two lamellar phases on the concentrated side is one of the peculiar effects that have been reported for the hexadecyltrimethylammonium octylsulfonate surfactant, C 16C 8 or TA 16So 8 (extensive to C 14C 8 and C 12C 8). In this work, with TA 16So 8 as a reference, the cationic trimethylammonium (TA +) and pyridinium (P +) headgroups are combined with the anionic sulfate (S -) and sulfonate (So -) headgroups to yield other C 16C 8 compounds: hexadecyltrimethylammonium octylsulfate (TA 16S 8), 1-hexadecylpyridinium octylsulfonate (P 16S 08), and 1-hexadecylpyridinium octylsulfate (P 16S 8). We show that, if the asymmetry of the chain lengths is kept constant at C 16C 8 and the headgroup chemistry is changed, most of the unusual self-assembly properties are still observed, indicating that they are not system-specific but extensive to other combinations of headgroups and mainly dictated by the ion-pair solubility mismatch. Thus, all the compounds in water quite remarkably show a lamellar-lamellar phase coexistence and spontaneously form vesicles upon solubilization. Moreover, P 16So 8 undergoes a temperature-driven vesicle-to-micelle transition that involves an intermediate planar lamellar state, similar to TA 16So 8. Some interesting effects on the global, phase behavior, however, do arise when the headgroups are changed. Geometric packing effects are shown to be important, but the differences in phase behavior seem to be mainly dictated by (i) the charge density of the headgroups, which tunes the solubility mismatch of the ion-pair, and (ii) specific interactions between headgroups, which affect the short-range repulsive force that controls the swelling of the concentrated lamellar phase. © 2009 American Chemical Society.

Abstract Four typical soils of the Porto (Portugal) area were characterized and used to study the decomposition of buried pieces of pork meat under controlled laboratory experiments (an 8 month experiment with a relatively high soilmoisture and a 1 month experiment with relatively low soilmoisture). The soils types were: organic, sandy, gravel and clay-gravel soils. Soils were characterized for their grain size distribution, pH, water content, organicmatter percentage and mineral composition. Four free fatty acids (myristic, palmitic, oleic and stearic) were analysed (using a methodology based on an extraction step followed by a derivatization reaction and high performance liquid chromatography analysis) in soil samples as a sign of adipocere formation. The direct sensorial analysis of the buried sample residues and the free fatty acids profiles of the sampled soils showed that sandy and clay-gravel soils (in a low moisture environment) slowed the normal decomposition process promoting the formation of adipocere. Nevertheless, this apparent soil effect is indirect and a consequence of the different water retention and permeability of the soils. Thus, the water content of the soils is a crucial factor for adipocere formation.

Abstract The standard (p degrees = 0.1 MPa) molar enthalpies of formation. of the 2-, 3-, and 4-monobromonitrobenzene isomers, in the crystalline phase, at T = 298.15 K, were derived from the standard massic energies of combustion, in oxygen, at T = 298.15 K. measured by rotating bomb combustion calorimetry From the temperature dependence of the vapour pressures of these compounds, measured by the Knudsen effusion technique, their standard molar enthalpies of sublimation, at T = 298.15 K, were derived using the Clausius-Clapeyron equation. [GRAPHICS] The combination of the values of the standard molar enthalpies of formation in the crystalline phase, and of the standard molar enthalpies of sublimation. yielded the standard molar enthalpies of formation in the gaseous phase. The results were interpreted in terms of enthalpic increments, and compared with estimated Values applying the empirical scheme developed by Cox.

Abstract The standard gas-phase enthalpies of formation of difluoro-substituted phenols have been determined experimentally, and have also been predicted by means of computational and empirical estimative methods. Combustion calorimetric studies were used to determine the standard molar enthalpies of formation of the six difluorophenol isomers, whereas their standard molar enthalpies of sublimation or vaporization, at T = 298.15 K was measured by the Calvet drop calorimetric technique. [GRAPHICS] From the values obtained for Delta(f)H(m)degrees (cr, I) and Delta(g)(cr,I)H(m)degrees(298.15 K), the standard molar enthalpies of formation in the gas-phase, at T = 298.15 K, were derived for the six isomers. Moreover, the standard gas-phase enthalpies of formation of difluorophenols have been predicted based on two different methodologies: one using the empirical scheme developed by Cox and the other one based on density functional theory calculations performed at the B3LYP/6-311++G(d,p) level of theory coupled with suitable homodesmic reactions and were compared to the experimental values.

Abstract The present work is part of a research program on the energetics of the linear 4-n-alkoxybenzoic acids, aiming the study of the enthalpic effect of the introduction of an alkoxy chain in the position 4- of the benzoic acid ring. In this work, we present the results of the thermochemical research on 4-n-alkoxybenzoic acids with the alkoxy chain length n = 2, 4, and 8. The standard (p degrees = 0.1 MPa) molar enthalpy of formation of crystalline 4-ethoxybenzoic acid, 4-butoxybenzoic acid, and 4-(octyloxy)benzoic acid was measured, at T = 298.15 K, by static-bomb calorimetry. These values, combined with the values of standard molar enthalpies of sublimation, were used to derive the standard molar enthalpies of formation in the gaseous phase. [GRAPHICS] From those experimental values, the standard molar enthalpies of formation, in the gaseous phase, at T = 298.15 K, are interpreted in terms of structural contributions to the energetics of the substituted benzoic acids and compared with the same parameters estimated from the Benson's Group Method.