Publications

Abstract Catanionic surfactants formed by the pairing of two ionic amphiphilic chains of opposite charge are now recognized as an important class of amphiphiles. Many aspects of their phase behavior have yet to be explored. In this work, two homologous series of catanionic surfactants were synthesized, based on the cationic headgroups trimethylammonium and pyridinium. Within each series, the headgroup and chain length of the cationic counterpart remains constant while for the anionic counterpart the headgroup is varied, while its alkyl chain length is also kept constant. Thus, one can directly monitor the influence of headgroup chemistry on the thermal behavior of these compounds. Differential scanning calorimetry (DSC) and polarizing light microscopy show that these compounds bear a rich and often complex thermotropic behavior, with the headgroup chemistry in some instances having a rather dramatic influence on phase behavior. Several liquid crystalline phases appear between the solid crystalline phase and the isotropic liquid phase. A qualitative correlation between the observed thermotropic behavior and the chemical nature of headgroup, is presented.

Abstract The standard (p degrees = 0.1 MPa) molar enthalpies of formation for gaseous pyridine-2,5-dicarboxylic acid, pyridine-2,6-dicarboxylic acid, and dimethylpyridine-2,6-dicarboxylate were derived from the standard molar enthalpies of combustion, in oxygen, at T = 298.15 K, measured by static bomb combustion calorimetry, and the standard molar enthalpies of sublimation, at 298.15 K, measured by Calvet microcalorimetry: pyridine-2,5-dicarboxylic acid, -(580.6 +/- 5.0) kJ-mol(-1); pyridine-2,6-dicarboxylic acid, -(608.0 +/- 6.1) kJ-mol(-1); dimethylpyridine-2,6-dicarboxylate, -(562.4 +/- 4.2) kJ-mol(-1). The enthalpy of formation for crystalline pyridine-2,3-dicarboxylic acid was also derived from combustion calorimetry measurements as -(733.1 +/- 2.0) kJ-mol(-1). In addition, theoretical calculations using the density functional theory and the B3LYP/6-311G** hybrid exchange-correlation energy functional have been performed for these molecules in order to obtain the most stable geometries and to access their relative stabilities. The good correlation obtained between experimental and theoretical results allowed the estimation of the enthalpy of sublimation for pyridine-2,3-dicarboxylic acid.

Abstract The standard (pdegrees = 0.1 MPa) molar enthalpies of formation of liquid pyrazinecarbonitrile and of crystalline pyrazinecarboxylic acid and pyrazinamide were measured, at T = 298.15 K, by static bomb calorimetry and the standard molar enthalpies of vaporization or of sublimation, at T = 298.15 K, were obtained using Calvet microcalorimetry. These values were used to derive the respective standard molar enthalpies of formation in gaseous phase.

Abstract The Knudsen mass-loss effusion technique was used to measure the vapour pressures at different temperatures of the following halobenzoic acids: 2-chlorobenzoic acid, between T = (320.16 and 339.15) K, 3-chlorobenzoic acid, between T = (320.13 and 340.13) K, 4-chlorobenzoic acid, between T= (333.15 and 356.14) K, 2-bromobenzoic acid, between T= (327.56 and 347.68) K, 3-bromobenzoic acid, between T = (327.67 and 347.60) K and 4-bromobenzoic acid, between T = (348.67 and 366.65) K. From the temperature dependence of the vapour pressure of the crystalline compounds, the standard, pdegrees = 10(5) Pa, molar enthalpies, entropies and Gibbs energies of sublimation at T= 298.15 K, were derived. A d.s.c. study of the fusion of the crystals is also presented.

Abstract The effect of CoA on the characteristic light decay of the firefly luciferase catalysed bioluminescence reaction was studied. At least part of the light decay is due to the luciferase catalysed formation of dehydroluciferyl-adenylate (L-AMP), a by-product that results from oxidation of luciferyl-adenylate (LH2-AMP), and is a powerful inhibitor of the bioluminescence reaction (IC50 = 6 nm). We have shown that the CoA induced stabilization of light emission does not result from an allosteric effect but is due to the thiolytic reaction between CoA and L-AMP, which gives rise to dehydroluciferyl-CoA (L-CoA), a much less powerful inhibitor (IC50 = 5 mu m). Moreover, the V-max for L-CoA formation was determined as 160 min(-1), which is one order of magnitude higher than the V-max of the bioluminescence reaction. Results obtained with CoA analogues also support the thiolytic reaction mechanism: CoA analogues without the thiol group (dethio-CoA and acetyl-CoA) do not react with L-AMP and do not antagonize its inhibitor effect; CoA and dephospho-CoA have free thiol groups, both react with L-AMP and both antagonize its effect. In the case of dephospho-CoA, it was shown that it reacts with L-AMP forming dehydroluciferyl-dephospho-CoA. Its slower reactivity towards L-AMP explains its lower potency as antagonist of the inhibitory effect of L-AMP on the light reaction. Moreover, our results support the conjecture that, in the bioluminescence reaction, the fraction of LH2-AMP that is oxidized into L-AMP, relative to other inhibitory products or intermediates, increases when the concentrations of the substrates ATP and luciferin increases.

Abstract The Knudsen mass-loss effusion technique was used to measure the vapor pressures, between 0.1 Pa and 1 Pa, of the three monomethyl benzenedicarboxylates over the following temperature intervals: monomethyl phthalate, between 335.28 K and 355.28 K; monomethyl isophthalate, between 359.26 K and 379.12 K; monomethyl terephthalate, between 363.16 K and 381.51 K. From the variation of the vapor pressures with temperature, the standard molar enthalpies, entropies, and Gibbs energies of sublimation at T = 298.15 K were calculated. The volatility of the studied compounds is compared with the volatilities of the benzene dicarboxylic acids and of the dimethyl benzenedicarboxylates.

Abstract Coumarins, also known as benzopyrones, are present in remarkable amounts in plants, although their presence has also been detected in microorganisms and animal sources. The structural diversity found in this family of compounds led to the division into different categories, from simple coumarins to many other kinds of policyclic coumarins, such as furocoumarins and pyranocoumarins. Simple coumarins and analogues are a large class of compounds that have attracted their interest for a long time due to their biological activities: they have shown to be useful as antitumoural, anti-HIV agents and as CNS-active compounds. Furthermore, they have been reported to have multiple biological activities (anticoagulant, anti-inflammatory), although all these properties have not been evaluated systematically. In addition, their enzyme inhibition properties, antimicrobial and antioxidant activities are other foremost topics of this field of research. The present work is to survey the information published or abstracted from 1990 till 2003, which is mainly related to the occurrence, synthesis and biological importance of simple coumarins and some analogues, such as biscoumarins and triscoumarins. Data are also highlighted, concerning the development of new synthetic strategies that could help in drug design and in the work on SAR or QSAR.

Abstract Dissociation enthalpies of terminal (N-O) bonds, DH degrees(N-O), in amine N-oxides, nitrile N-oxides, pyridine N-oxides, quinoxaline 1,4-dioxides, furoxans, nitrones, azoxy-derivatives, azo-N,N-dioxides, nitro compounds, nitramines, and alkyl nitrates are calculated from published enthalpy of formation, enthalpy of sublimation, and enthalpy of vaporization data. For each class of organic compounds, the calculated DH degrees(N-O) values are critically evaluated. The derived DH degrees(N-O) values can be used to estimate enthalpies of formation of other molecules in each of these classes of organic compounds. (C) 2005 American Institute of Physics.

Abstract A scheme for the simultaneous determination of temperature and analyte concentration for application in luminescence-based chemical sensors is proposed. This scheme is applied to an optical oxygen sensor, which is based on the quenching of the fluorescence of a ruthenium complex. Temperature measurement is performed using the excitation radiation and an absorption long-pass filter. Preliminary results are presented that show the viability of an oxygen measurement that is independent of temperature and optical power level. The possibility of self-referenced temperature measurements with semiconductor nanoparticles is also investigated. In order to optimize the sensor design, several different optical fiber probe geometries for oxygen sensing are tested and compared, including different methods of coupling radiation into the optical fiber system. Polyvinyl alcohol (PVA) and polyacrylamide membranes are tested as supports for sensor immobilization in fiber-optical pH sensing devices in aqueous solution. Some results are presented that show the feasibility of using fiber-optical pH indicators for remote monitoring.

Abstract The formation thermodynamic properties of crystalline and aqueous inosine have been determined by using a combination of calorimetric techniques. Oxygen bomb calorimetric measurements on crystalline inosine yielded a standard molar enthalpy of combustion of Delta(c)H degrees(m) = -(4802.2 +/- 4.5) kJ center dot mol(-1). From this, a value for the standard molar enthalpy of formation Delta(f)H(m)degrees of -(847.9 +/- 4.7) kJ mol-1 was obtained. The standard molar heat capacity of crystalline inosine has been measured over the temperature interval 11 <= T/K <= 325 by using an adiabatic calorimeter. The heat capacities were fit to a series of polynomials from which smoothed values for the standard thermal properties were calculated for 20 <= T/K <= 320. The standard molar entropy Delta(0)(T)S degrees(m) (inosine, cr) at T = 298.15 K is (288.0 +/- 0.6) J center dot K-1 center dot mol(-1) and the standard molar entropy of formation Delta(f)S degrees(m) is -(1449.6 +/- 0.6) J center dot K-1 center dot mol(-1). The standard Gibbs free energy of formation Delta(f)G degrees(m) (inosine, cr) = -(415.7 +/- 4.7) kJ center dot mol(-1). By using literature values of the standard molar enthalpy of solution and the saturation molality of inosine(cr), the standard molar thermodynamic properties of aqueous inosine at T=298.15 K are found to be Delta(f)H degrees(m)(inosine, aq) = -(819.8 +/- 4.7) kJ center dot mol(-1); Delta(f)G degrees(m) (inosine, aq)= -(409.2 +/- 4.8) kJ center dot mol(-1); and Delta(0)(T)S degrees(m)(inosine, aq) = (360.1 +/- 1.8) J center dot K-1 center dot mol(-1). These results are used to calculate standard thermodynamic properties of various aqueous species involving inosine.