Publications

Abstract The synthesis of carbon nanoparticles obtained by direct laser ablation [UV pulsed laser irradiation (248 nm, KrF)] of carbon targets immersed in water is described. Laser ablation features were optimized to produce carbon nanoparticles with dimensions up to about 100 nm. After functionalization with NH2-polyethylene-glycol (PEG(200)) and N-acetyl-L-cysteine (NAC) the carbon nanoparticles become fluorescent with excitation and emission wavelengths at 340 and 450 nm, respectively. The fluorescence decay time was complex and a three-component decay time model originated a good fit (chi = 1.09) with the following lifetimes: tau(1) = 0.35 ns; tau(2) = 1.8 ns; and tau(3) = 4.39 ns. The fluorescence of the carbon dots is sensitive to pH with an apparent PKa = 4.2. The carbon dots were characterized by H-1 NMR and HSQC and the results show an interaction between PEG(200) and the carbon surface as well as a dependence of the chemical shift with the reaction time. The fluorescence intensity of the nanoparticles is quenched by the presence of Hg(II) and Cu(II) ions with a Stern-Volmer constant (pH = 6.8) of 1.3 x 10(5) and 5.6 x 10(4) M-1, respectively. As such the synthesis and application of a novel biocompatible nanosensor for measuring Hg(II) is presented.

Abstract The standard (p(o) = 0.1 MPa) molar enthalpies of formation in the crystalline state of the 2-,3- and 4-hydroxymethylphenols, Delta(f)H(m)(o)(cr) = -(377.7 +/- 1.4) kJ mol(-1), Delta(f)H(m)(o)(cr) = -(383.0 +/- 1.4) kJ mol(-1) and Delta(f)H(m)(o)(cr) = -(382.7 +/- 1.4) kJ mol(-1), respectively, were derived from the standardmolar energies of combustion, in oxygen, to yield CO(2)(g) and H(2)O(l), at T = 298.15 K, measured by static bomb combustion calorimetry. The Knudsen mass-loss effusion technique was used to measure the dependence of the vapour pressure of the solid isomers of hydroxymethylphenol with the temperature, from which the standard molar enthalpies of sublimation were derived using the Clausius-Clapeyron equation. The results were as follows: Delta(g)(cr)H(m)(o) = (99.5 +/- 1.5) kJ mol(-1), Delta(g)(cr)H(m)(o) = (116.0 +/- 3.7) kJ mol(-1) and D(cr)(g)H(m)(o) = (129.3 +/- 4.7) kJ mol(-1), for 2-, 3- and 4-hydroxymethylphenol, respectively. From these values, the standard molar enthalpies of formation of the title compounds in their gaseous phases, at T = 298.15 K, were derived and interpreted in terms of molecular structure. Moreover, using estimated values for the heat capacity differences between the gas and the crystal phases, the standard (p(o) = 0.1 MPa) molar enthalpies, entropies and Gibbs energies of sublimation, at T = 298.15 K, were derived for the three hydroxymethylphenols.

Abstract The Knudsen mass-loss effusion technique was used to measure the vapor pressures at different temperatures of the following aminomethoxybenzoic acids: 2-amino-3-methoxybenzoic acid, between (349.12 and 369.18) K; 2-amino-5-methoxybenzoic acid, between (353.13 and 375.17) K; 3-amino-4-methoxybenzoic acid, between (379.67 and 399.18) K; 3-amino-5-methoxybenzoic acid, between (380.16 and 400.18) K; and 4-amino-3-methoxybenzoic acid, between (373.14 and 395.17) K. From the temperature dependences of the vapor pressure of the crystalline compounds, the standard (p degrees = 10(5) Pa) molar enthalpies and Gibbs energies Of Sublimation, at T = 298.15 K, were derived. The results allowed the estimation of these two thermodynamic properties for two isomers not Studied in this work-2-amino-6-methoxybenzoic acid and 3-amino-2-methoxybenzoic acid. Differential scanning calorimetry was Used to measure the temperature and molar enthalpy of fusion of the studied isomers.

Abstract A series of 6-halo-3-hydroxyphenylcoumarins (resveratrol-coumarins hybrid derivatives) was synthesized in good yields by a Perkin reaction followed by hydrolysis. The new compounds were evaluated for their vasorelaxant activity in intact rat aorta rings pre-contracted with phenylephrine (PE), as well as for their inhibitory effects on platelet aggregation induced by thrombin in washed human platelets. These compounds concentration-dependently relaxed vascular smooth muscle and some of them showed a platelet antiaggregatory activity that was up to thirty times higher than that shown by trans-resveratrol and some other previously synthesized derivatives.

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.