Publications

Abstract The self-aggregation of polyelectrolytes having N-alkyl-N,N-dimethyl-N-(2-hydroxypropyl)ammonium chloride pendant groups (alkyl = octyl, dodecyl, or cetyl) randomly distributed along a polysaccharide backbone (dextran) was studied by steady-state fluorescence techniques using several free fluorescent probes or pyrene-labeled polymers and by viscometry. The onset, offset, and highest values of the fluorescence response of N-phenylnaphthylamine (NPN), pyrene (Py), and 1,6-diphenyl-3,5,6-hexatriene (DPH) were corroborated with NPN and DPH anisotropy and quenching experiments to describe the dynamic of hydrophobic microdomain formation and microdomain characteristics. The start of the aggregation process (critical aggregation concentration, cac) and the microdomain characteristics such as polarity, microviscosity, size, and number strongly depend on the alkyl chain length and the degree of substitution with cationic pendant groups. Fluorescence experiments with pyrene-labeled polymers and viscosity data suggest that microdomains are mainly formed by intramolecular hydrophobic associations, except for the polymers carrying octyl groups, where some intermolecular associations were revealed.

Abstract The standard (p°=0.1 MPa) molar enthalpies of formation of the crystalline complexes bis[N-(N″,N″-diethylaminothiocarbonyl) benzamidinato]nickel(II), {Ni(datb)2}, and bis[N-(N″,N″- diethylaminothiocarbonyl)-N′-phenylbenzamidinato]nickel(II), {Ni(datpb)2}, were determined, at T=298.15 K, by high precision solution-reaction calorimetry. ΔfHm°(cr)/ (kJ·mol-1)Ni(datb)26.5 ± 7.9Ni(datpb) 2285.1 ± 11.3 From the obtained results, the metal-ligand exchange enthalpies in the crystalline phase were derived. The enthalpy of a hypothetical metal-ligand exchange reaction in the crystalline phase was derived, thus allowing a discussion of the energetics of complexation in comparison with known crystal-structural parameters.

Abstract The standard (pdegrees = 0.1 MPa) molar enthalpies of formation of crystalline thianthrene and monothiodibenzoylmethane, HDBMS, at T = 298.15 K were measured by rotating-bomb calorimetry and the standard molar enthalpies of sublimation at T = 298.15 K of dibenzoylmethane (HDBMO) and monothiodibenzoylmethane (HDBMS) were measured by microcalorimetry. [GRAPHIC] From the present results it could be shown that, in these molecules, the intramolecular hydrogen bond energy (O-H...O), is ca. 20 kJ . mol(-1) larger than for (S-H...O).

Abstract Quasielastic neutron scattering measurements were performed in hydrated samples of ds-DNA and ss-DNA. The samples were hydrated in a high relative humidity atmosphere, and their final water content was 0.559 g H2O/g ds-DNA and 0.434 g H2O/g ss-DNA. The measurements were performed at 8 and 5.2 Angstrom for the ds-DNA sample, and at 5.2 Angstrom for the ss-DNA sample. The temperature was in both cases 298 K. Analysis of the obtained data indicates that in the ds-DNA sample we can distinguish two types of protons-those belonging to water molecules strongly attached to the ds-DNA surface and another fraction belonging to water that diffuses isotropically in a sphere of radius 2.8 Angstrom, with a local diffusion coefficient of 2.2 x 10(-5) cm(2) s(-1). For ss-DNA, on the other hand, no indication was found of motionally restricted or confined water. Further, the fraction of protons strongly attached to the ds-DNA surface corresponds to 0.16 g H2O/g ds-DNA, which equals the amount of water that is released by ds-DNA upon thermal denaturation, as studied by one of us (G.M.) by differential scanning calorimetry. This value also equals the difference between the critical hydration values of ds-DNA and ss-DNA, also determined by DSC. These results represent, thus, a completely independent measurement of water characteristics and behavior in ds- and ss-DNA at critical hydration values, and therefore substantiate the previous suggestions/conclusions of the results obtained by calorimetry.

Abstract The standard (p(0) = 0.1 MPa) molar enthalpies of formation, at T = 298.15 K, in the gaseous phase of two Schiff bases, N,N'-bis(salicylaldehydo)ethylenediimine and N.N'-bis(salicylaldehydo)tetramethylenediimine, were determined from their enthalpies of combustion and sublimation, obtained by static bomb calorimetry in oxygen and by the Knudsen effusion technique, respectively. The enthalpies of fusion of both compounds have been determined by differential scanning calorimetry.

Abstract The state of the art of voltammetric and amperometric methods used in the study and determination of pesticides in crops, food, phytopharmaceutical products, and environmental samples is reviewed. The main structural groups of pesticides, i.e., triazines, organophosphates, organochlorides, nitrocompounds, carbamates, thiocarbamates, sulfonylureas, and bipyridinium compounds are considered with some degradation products. The advantages, drawbacks, and trends in the development of voltammetric and amperometric methods for study and determination of pesticides in these samples are discussed.

Abstract This paper reports the values of the standard (p(o) = 0. 1 MPa) molar enthalpy of formation in the condensed, at T = 298.15 K, for 2-R-benzimidazoles (R = propyl, isopropyl), derived from, the respective enthalpies of combustion in oxygen, measured by static bomb combustion calorimetry and the standard molar enthalpies of sublimation, at T = 298.15 K, obtained using Calvet micro-calorimetry in the case of 2-isopropylbenzimidazole and, by the variation of vapour pressures, determined by the Knudsen effusion technique, with temperatures between (344 and 365) K for 2-propylbenzimidazole. Heat capacities, in the temperature ranges from T = 268 K to near their respective melting temperatures, T = 421 K for 2-propylbenzimidazole and T = 464 K for 2-isopropylbenzimidazole, were measured with a differential scanning calorimeter. These values were used to derive the standard molar enthalpies of formation, of the two 2-benzimidazole derivatives, in gaseous phase.

Abstract The photophysics of the single tyrosine in bovine ubiquitin (UBQ) was studied by picosecond time-resolved fluorescence spectroscopy, as a function of pH and along thermal and chemical unfolding, with the following results: First, at room temperature (25degreesC) and below pH 1.5, native UBQ shows single-exponential decays. From pH 2 to 7, triple-exponential decays were observed and the three decay times were attributed to the presence of tyrosine, a tyrosine-carboxylate hydrogen-bonded complex, and excited-state tyrosinate. Second, at pH 1.5, the water-exposed tyrosine of either thermally or chemically unfolded UBQ decays as a sum of two exponentials. The double-exponential decays were interpreted and analyzed in terms of excited-state intramolecular electron transfer from the phenol to the amide moiety, occurring in one of the three rotamers of tyrosine in UBQ. The values of the rate constants indicate the presence of different unfolded states and an increase in the mobility of the tyrosine residue during unfolding. Finally, from the pre-exponential coefficients of the fluorescence decays, the unfolding equilibrium constants (K-U) were calculated, as a function of temperature or denaturant concentration. Despite the presence of different unfolded states, both thermal and chemical unfolding data of UBQ could be fitted to a two-state model. The thermodynamic parameters T-m=54.6degreesC, DeltaH(Tm)=56.5 kcal/mol, and DeltaC(p)=890 cal/mol//K, were determined from the unfolding equilibrium constants calculated accordingly, and compared to values obtained by differential scanning calorimetry also under the assumption of a two-state transition, T-m=57.0degreesC, DeltaH(m)=51.4 kcal/mol, and DeltaC(p)=730 cal/mol//K.

Abstract The biocompatibility of implant materials used for substitution of bone tissue depends on its ability to induce the deposition of a hydroxyapatite layer when in contact with body fluids. In previous work, some of the authors found that bovine serum albumin (BSA) promotes calcium phosphate deposition if preadsorbed on hydroxyapatite and retards precipitation if preadsorbed on titania. In the present study, we investigated the adsorption of BSA upon particles of titania and hydroxyapatite in order to understand the different role played by the protein on the mineralization of both biomaterials. The adsorption isotherms were determined and the structural changes induced by adsorption at different surface coverages were investigated by circular dichroism spectroscopy and differential scanning microcalorimetry. At low surface coverages, the adsorbed BSA molecules lost part of their a-helix content. However, at high surface coverages, corresponding to the plateau values of the adsorption isotherms, the BSA molecules did not undergo structural rearrangements upon adsorption. In the latter circumstances, the availability of BSA calcium binding sites, which should be responsible for inducing mineralization, depends on the electrostatic interactions between BSA and the sorbent surface. A possible explanation for the different mineralization behavior of hydroxyapatite and titania is advanced. (C) 2004 Wiley Periodicals, Inc.