Publications

Abstract The esterification of hydrophilic phenolic antioxidants is an efficient approach to enhance their solubility in apolar media. Herein, structure property studies on the antiradical activity of a series of protocatechuic acid alkyl esters have been accomplished. The increase of the lipophilicity was shown to significantly improve the antioxidant activity of protocatechuic esters. Their efficiency as radical scavengers was evaluated using distinctive analytical methods, namely, 2,2-diphenyl-1-picrylhydrazyl (DPPH) UV/visible method, electrochemistry, and differential scanning calorimetry. All the new alkyl protocatechuate antioxidants studied possessed better radical-scavenging capacity than the natural antioxidant protocatechuic acid. This work has shown that the alkyl ester side chain markedly influences the lipophilicity of this type of phenolic system without disturbing the core of the molecule responsible for antioxidant activity. The data on the antioxidant activity obtained using the different analytical methods correlated well with each other and have revealed the interesting antioxidant potential of alkyl esters of protocatechuic acid.

Abstract The methodology for the prediction of properties of environmental relevance of polycyclic aromatic hydrocarbons based on the conductor-like screening model for real solvents (COSMO-RS/COSMOtherm) is presented and evaluated, with a special focus on the aqueous solubility of polycyclic aromatic hydrocarbons and related aromatic hydrocarbons (PAHs). It is shown that the solubility predictions as well as their temperature dependence obtained for a set of 12 polycyclic aromatic hydrocarbons and two related aromatic hydrocarbons are in good agreement with the experimental data. (Subcooled) Vapor pressures, Henry's law constants as well as octanol-water partition coefficients were also estimated and compared with experimental data showing the capability of the model to predict environmental related data with sufficient precision for practical purposes.

Abstract The partition of the amphiphile sodium dodecyl sulfate (SDS) between an aqueous solution and a 1-palmitoyl-2-oleoyl-sn-glycero-3-phosphocholine (POPC) bilayer was followed by isothermal titration calorimetry (ITC) as a function of the total concentration of SDS. It was found that the obtained partition coefficient is strongly affected by the ligand concentration, even after correction for the charge imposed in the bilayer by the bound SDS. The partition coefficient decreased as the total concentration of SDS increased, with this effect being significant for local concentrations of SDS in the lipid bilayer above 5 molar%. At those high local concentrations, the properties of the lipid bilayer are strongly affected, leading to nonideal behavior and concentration-dependent apparent partition coefficients. It is shown that with the modern ITC instruments available, the concentrations of SDS can be drastically reduced while maintaining a good signal-to-noise ratio. The intrinsic parameters of the interaction with unperturbed membranes can be obtained from the asymptotic behavior of the apparent parameters as a function of the ligand concentration for both nonionic and ionic solutes. A detailed analysis is performed, and a spreadsheet is provided to obtain the interaction parameters with and without correction for electrostatics.

Abstract This paper reports an experimental and theoretical study of the standard (p degrees = 0.1 MPa) molar enthalpies of formation at T = 298.15 K of the sulfur-containing amino acids L-cysteine [CAS 52-90-4] and L-cystine [CAS 56-89-3]. The standard (p degrees = 0.1 MPa) molar enthalpies of formation of crystalline L-cysteine and L-cystine were calculated from the standard molar energies of combustion, in oxygen, to yield CO(2)(g) and H(2)SO(4)center dot 115H(2)O, measured by rotating-bomb combustion calorimetry at T = 298.15 K. The vapor pressures of L-cysteine were measured as function of temperature by the Knudsen effusion mass-loss technique. The standard molar enthalpy of sublimation, at T = 298.15 K, was derived from the Clausius-Clapeyron equation. The experimental values were used to calculate the standard (p degrees = 0.1 MPa) enthalpy of formation of L-cysteine in the gaseous phase, Delta(f)H(m)degrees(g) = -382.6 +/- 1.8 kJ.mol(-1). Due to the low vapor pressures of L-cystine and since this compound decomposes at the temperature range required for a possible sublimation, it was not possible to determine its enthalpy of sublimation. Standard ab initio molecular orbital calculations at the G3(MP2)//B3LYP and/or G3 levels were performed. Enthalpies of formation, using atomization and isodesmic reactions, were calculated and compared with experimental data. A value of -755 +/- 10 kJ.mol(-1) was estimated for the enthalpy of formation of cystine. Detailed inspections of the molecular and electronic structures of the compounds studied were carried out. Finally, bond dissociation enthalpies (BDE) of S-H, S-S, and C-S bonds, and enthalpies of formation of L-cysteine-derived radicals, were also computed.

Abstract By employing H-1 NMR spectroscopy and molecular simulations, we provide an explanation for recent observations that the aqueous Solubilities of ionic liquids exhibit salting-out to salting-in regimes upon addition of distinct inorganic salt ions. Using a typical ionic liquid [1-butyl-3-methylimidazolium bis(trifluoromethylsulfonyl)imide], we observed the existence of preferential specific interactions between the low electrical charge density ("apolar moiety") parts of the ionic liquid cation and the inorganic salts. These a priori unexpected interactions become increasingly favorable as one moves from salting-out to salting-in effects. More specifically, this interpretation is validated by distinct aqueous solution H-1 NMR data shifts in the ionic liquid cation upon inorganic salt addition. These shifts, which are well noted in the terminal and preterminal hydrogens of the alkyl chain appended to the imidazolium ring, correlate quantitatively with solubility data, both for cases where the nature of inorganic salt is changed, at constant concentration, and for those where the concentration of a given inorganic salt is varied. Molecular simulations have also been performed permitting us to garner a broader picture of the underlying mechanism and structure of this complex solvation phenomenon. These findings can now be profitably used to anticipate solution behavior upon inorganic salt addition well beyond the specificity of the ionic liquid solutions, i.e., for a diversity of distinct solutes differing in chemical nature.

Abstract A qualitative and quantitative energetic and structural study of dibenzyl ketone (DBK) and benzyl ethyl ketone (BEK) was carried out in order to obtain insights into the type and magnitude of aromatic interactions that these systems present in their different phases. The crystal structure of DBK was obtained by X-ray crystallography, and it shows that the conformation adopted in the crystalline state is governed by the intermolecular interactions. The standard (p(0) = 10(5) Pa) molar enthalpy of formation in the gaseous state at T = 298.15 K was derived by Calvet and combustion calorimetry. Using a homodesmic reaction scheme, the first calorimetric evaluation of the interaction enthalpy between two stacked phenyl rings is presented. A stabilizing enthalpic effect of (12.9 +/- 4.9) kJ mol(-1) associated with the intramolecular pi-pi interaction in DBK was found. The gas phase intramolecular pi ... pi interaction in DBK is in agreement with quantum chemical calculations at B3LYP/6-311++ G(d, p) and MP2 with various basis-sets. An intramolecular pi ... pi interaction in DBK and a weak C-H ... pi interaction in BEK were found by variable-temperature (1)H-NMR spectroscopy in MeOD. These observations are consistent with a hindered rotor interpretation, supported by ab initio calculations for the gas phase at the MP2/cc-pVDZ level. The global results indicate a distinct molecular structure on going from crystalline DBK to liquid, gas, and solution phases, ruled by the overall contribution of the intra- and intermolecular interactions.

Abstract The energetic effects caused by replacing one of the methylene groups in the 9,10-dihydroanthracene by ether or ketone functional groups yielding xanthene and anthrone species, respectively, were determined from direct comparison of the standard (p degrees = 0 1 MPa) molar enthalpies of formation in the gaseous phase, at T = 298.15 K. of these compounds. The experimental static-bomb combustion calorimetry and Calvet microcalorimetry and the computational G3(MP2)//B3LYP method were used to get the standard molar gas-phase enthalpies of formation of xanthene. (41 8 +/- 3 5) kJ mol(-1), and anthrone, (31 4 +/- 3 2) kJ mol(-1) The enthalpic increments for the substitution of methylene by ether and ketone in the parent polycyclic compound (9,10-dihydroanthracene) are -(1179 +/- 5 5) kJ mol(-1) and -(1283 +/- 5.4) kJ mol(-1), respectively

Abstract In the title compound, C(21)H(18)N(4)O(4), there is an intramolecular N-H center dot center dot center dot O hydrogen bond between the amino H atom and an O atom of the 2-nitro group of the adjacent benzene ring. The central benzene ring forms dihedral angles of 79.98 (7) and 82.88 (7)degrees with the two phenyl rings. In the crystal structure, molecules are linked into a three-dimensional network by weak C-H center dot center dot center dot N, C-H center dot center dot center dot O and C-H center dot center dot center dot pi interactions.

Abstract The standard molar enthalpies of formation, at T = 298.15 K, of all possible single methylated derivatives of benzothiophene and dibenzothiophene were calculated by means of the G3(MP2)//B3LYP approach employing several different working reactions (homodesmotic and atomization). The most stable compounds are the 7-methylbenzothiophene and 4-methyldibenzothiophene while the least stable are the 5-methylbenzothiophene and 1-methyldibenzothiophene compounds. Calculated enthalpic increments for the reactions of methylation are in the range -29.5 and - 39.1 kJ mol(-1).