Publications

Abstract Microcalorimetric techniques, combined with turbidity measurements, were used to study the thermodynamics of self-assembling of hydrophobically modified cationic polysaccharides and their mixtures with oppositely charged surfactants in aqueous solution. The studied polyelectrolytes were a series of polymers based on dextran having pendant N-(2-hydroxypropyl)-N,N-dimethyl-N-alkylammonium chloride groups randomly distributed along the polymer backbone. The parameters for their micellization process are evaluated from the results of the observed dilution enthalpy curves and compared with those of the related cationic surfactants (DTAC and CTAC). The microcalorimetric results for the mixed systems (polyelectrolytes with oppositely charged surfactants) are used along with turbidity measurements to characterize systematically the thermodynamics of their interaction. The phase behavior is described and the interaction enthalpies are derived from the differences between the observed enthalpy curves with and without polyelectrolyte. Therefore, we discuss in detail the effect of changing the alkyl chain length of polyelectrolyte pendant groups, the molecular weight of the dextran backbone, and the temperature of the measurements on the interactions between polyelectrolyte and surfactant.

Abstract We have used a precision isothermal titration microcalorimeter (ITC) to measure the enthalpy curves for the interaction of a hydrophobically modified polyelectrolyte (D40OCT30) with oppositely charged surfactants (SCnS) in aqueous solution. D40OCT30 is a newly synthesized polymer based on dextran having pendant N-(2-hydroxypropyl)-N,N-dimethyl-N-octylammonium chloride groups randomly distributed along the polymer backbone with degree of substitution of 28.1%. The employed anionic surfactants are sodium octyl sulfate (SC8S) and sodium tetradecyl sulfate (SC14S). Microcalorimetric results along with turbidity and kinematic viscosity measurements demonstrate systematically the thermodynamic characterization of the interaction of D40OCT30/SCnS. A three-dimensional diagram with the derived phase boundaries is drawn to describe the effect of the alkyl chain length of surfactant and of the ratio between surfactant and pendant groups on the interaction. A more complete picture of the interaction mechanism for D40OCT30/SCnS systems is proposed here.

Abstract The interaction of two hybrid peptides of cecropin A and melittin [CA(1-8)M(1-18) and CA(1-7)M(2-9)] with liposomes was studied by differential scanning calorimetry (DSC), circular dichroism (CD), and quasi-elastic light scattering (QELS). The study was carried out with large unilamellar vesicles (LUVs) of three different lipid compositions: 1,2-dimyristoil-sn-glycero-3-phosphocholine (DMPG), 1,2-dimyristoylsn-glycero-3-phospho-rac-(1-glycerol) (DMPG) and a binary mixture of DMPC/DMPG, in a wide range of peptide-to-lipid (P:L) molar ratios (0 to 1:7). DSC results indicate that, for both peptides, the interaction depends on membrane composition, with very different behavior for zwitterionic and anionic membranes. CD data show that, although the two peptides have different secondary structures in buffer (random coil for CA(1-7)M(2-9) and predominantly beta-sheet for CA(1-8)M(1-18)), they both adopt an alpha-helical structure in the presence of the membranes. Overall, results are compatible with a model involving a strong electrostatic surface interaction between the peptides and the negatively charged liposomes, which gives place to aggregation in the gel phase and precipitation after a threshold peptide concentration. In the case of zwitterionic membranes, a progressive surface coverage with peptide molecules destabilizes the membrane, eventually leading to membrane disruption. Moreover, delicate modulations in behavior were observed depending on the peptide.

Abstract Microcalorimetric techniques, combined with turbidity measurements, were used to study the thermodynamics of self-assembling of hydrophobically modified cationic polysaccharides and their mixtures with oppositely charged surfactants in aqueous solution. The studied polyelectrolytes were a series of polymers based on dextran having pendant N-(2-hydroxypropyl)-N,N-dimethyl-N-alkylammonium chloride groups randomly distributed along the polymer backbone. The parameters for their micellization process are evaluated from the results of the observed dilution enthalpy curves and compared with those of the related cationic surfactants (DTAC and CTAC). The microcalorimetric results for the mixed systems (polyelectrolytes with oppositely charged surfactants) are used along with turbidity measurements to characterize systematically the thermodynamics of their interaction. The phase behavior is described and the interaction enthalpies are derived from the differences between the observed enthalpy curves with and without polyelectrolyte. Therefore, we discuss in detail the effect of changing the alkyl chain length of polyelectrolyte pendant groups, the molecular weight of the dextran backbone, and the temperature of the measurements on the interactions between polyelectrolyte and surfactant. © 2005 American Chemical Society.

Abstract The standard (p degrees = 0.1 MPa) molar enthalpy of formation for gaseous 2-phenylbenzimidazole (2-PhBIM) and 2-benzylbenzimidazole (2-BzBIM) were derived from the standard molar enthalpies of combustion, at T = 298.15 K, measured by static bomb calorimetry, and the standard molar enthalpy of sublimation, at T = 298.15 K, measured by Calvet microcalorimetry in the case of 2-phenylbenzimidazole or derived from the variation of the vapour pressures, determined by the Knudsen effusion technique, with temperatures between (393 and 412) K for 2-benzylbenzimidazole. Heat capacities, in the temperature ranges from (268.15 to 322.10) K for 2-phenylbenzimidazole and (270.15 to 316.02) K for 2-benzylbenzimidazole, were also measured with a differential scanning calorimeter.

Abstract Ferrochelatase (EC 4.99.1.1), the terminal enzyme of the haem biosynthetic pathway, catalyses the chelation of Fe(II) into the protoporphyrin IX ring. The energetics of the binding between murine ferrochelatase and mesoporphyrin were determined using isothermal titration calorimetry, which revealed a stoichiometry of one molecule of mesoporphyrin bound per protein monomer. The binding is strongly exothermic, with a large intrinsic enthalpy (Delta H = -97.1 kJ(.)mol(-1)), and is associated with the uptake of two protons from the buffer. This proton transfer suggests that hydrogen bonding between ferrochelatase and mesoporphyrin is a key factor in the thermodynamics of the binding reaction. Differential scanning calorimetry thermograms indicated a co-operative two-state denaturation process with a single transition temperature of 56 degrees C for wild-type murine ferrochelatase. An increase in the thermal stability of ferrochelatase is dependent upon mesoporphyrin binding. Similarly, murine ferrochelatase variants, in which the active site Glu-289 was replaced by either glutamine or alanine and, when purified, contained specifically-bound protoporphyrin, exhibited enhanced protein stability when compared with wild-type ferrochelatase. However, in contrast with the wildtype enzyme, the thermal denaturation of ferrochelatase variants was best described as a non-co-operative denaturation process.

Abstract An electrochemical method for the detection of enzymatically created anions is described that uses a thin-film electrode with decamethylferrocene as an electroactive redox probe. The enzymatic oxidation of glucose with enzyme glucose oxidase produces gluconic acid as a final product. The oxidation of decamethylferrocene dissolved in the thin-nitrobenzene film, that is spread on the working graphite electrode and submerged in the aqueous solution containing glucose and glucose oxidase, is followed by the up-take of gluconate anions from the aqueous phase to nitrobenzene. The peak currents of the square-wave voltammetric responses of that system are a linear function of the glucose concentration in the milimolar range from 0.1 mmol/L to 0.7 mmol/L (R-2 = 0.994).

Abstract A new sol-gel application for the development of SPME fibers is described. Phenyltrimethoxysilane (PTMOS) and methyltrimethoxysilane (MTMOS) were the sol-gel precursors used at different proportions, together with different water contents, catalyst and reaction time. It was observed that obtaining a good film quality was determinant for a good extracting fiber performance. The film thickness ranged 0.2-1 mu m and could not be increased by multi-coating processes. Apparently, a dense, non-porous microstructure was obtained. These coatings exhibited a strong hydrophobic character, as shown by the capability of extraction of long chain and apolar aromatic compounds, which, was comparable to that of the 100 mu m polydimethylsiloxane (PDMS) and 65 mu m carbowax-divinylbenzene (CW-DVB). The developed fiber has shown high thermal (350 degrees C) and organic solvent stability (ethanol, toluene and dichloromethane), thus bearing adequate characteristics to be associated to GC and potentialities that nay also envisage suitability for HPLC. The new fibers may be useful for the microextraction of non-polar compounds. although at trace levels and in simple matrixes only, due to the susceptibility to competition.

Abstract The ion transfer of acetylcholine (AcH+) ions across the unmodified and phospholipid-modified water 1,2-dichloroethane (DCE) interface has been studied by means of square-wave and cyclic voltammetry, as well as by electrochemical impedance spectroscopy. After being transferred in the organic phase, the AcH+ ions undergo chemical reactions with the phospholipids. The overall behavior of the experimental system studied in the presence of phospholipids has been compared with the theoretical results of an ECrev reaction. The kinetic parameters of the chemical interactions between AcH+ and the phospholipids have been determined from the voltammetric and impedance measurements. Additional characterization of those interactions has been made by using the surface tension measurements.