Empirical parameters for solvent acidity, basicity, dipolarity, and polarizability of the ionic liquids [BMIM][BF4] and [BMIM][PF6].

The empirical solvent scales for polarizability (SP), dipolarity (SdP), acidity (SA), and basicity (SB) have been successfully used to interpret the solvatochromism of compounds dissolved in organic solvents and their solvent mixtures. Providing that the published solvatochromic parameters for the ionic liquids 1-(1-butyl)-3-methylimidazolium tetrafluoroborate, [BMIM][BF4] and 1-(1-butyl)-3-methylimidazolium hexafluorophosphate, [BMIM][PF6], are excessively widespread, their SP, SdP, SA, and SB values are measured herein at temperatures from 293 to 353 K. Four key points are emphasized herein: (i) the origin of the solvatochromic solvent scales--the gas phase, that is the absence of any medium perturbation--; (ii) the separation of the polarizability and dipolarity effects; (iii) the simplification of the probing process in order to obtain the solvatochromic parameters; and (iv) the SP, SdP, SA, and SB solvent scales can probe the polarizability, dipolarity, acidity, and basicity of ionic liquids as well as of organic solvents and water-organic solvent mixtures. From the multiparameter approach using the four pure solvent scales one can draw the conclusion that (a) the solvent influence of [BMIM][BF4] parallels that of formamide at 293 K, both of them miscible with water; (b) [BMIM][PF6] shows a set of solvatochromic parameters similar to that of chloroacetonitrile, both of them water insoluble; and (c) that the corresponding solvent acidity and basicity of the ionic liquids can be explained to a great extent from the cation species by comparing the empirical parameters of [BMIM](+) with those of the solvent 1-methylimidazole. The insolubility of [BMIM][PF6] in water as compared to [BMIM][BF4] is tentatively connected to some extent to the larger molar volume of the anion [PF6](-), and to the difference in basicity of [PF6](-) and [BF4](-).

Interpenetrated PNIPAM-polythiophene microgels for nitro aromatic compound detection.

In this work, we present a facile and reproducible method to obtain thermally responsive, monodisperse, fluorescent microgels with diameters smaller than 700 nm based on poly(N-isopropyl acrylamide) (PNIPAM) interpenetrated with poly(thiophene-ethyl buthyl sulfonate) (PTEBS). Changing the temperature and inducing the microgel volume phase transition, it is possible to modify the photoluminescence (PL) properties of the microgels. Thus, when the temperature was below the low critical solution temperature (LCST) of PNIPAM, the PL intensity was higher than that above the LCST. Time-resolved fluorescence measurements indicate that, in the swollen state, the increment of cross-linking increases the fluorescence decay time of PTEBS. By contrast, in the collapsed state, variations in the decay time were attributed to higher rigidity of the PNIPAM-PTEBS system, which was confirmed by neutron scattering measurements. Moreover, the shift in the wavelength of the fluorescence emission peak observed above the LCST indicates that the collapsed PNIPAM matrix was able to interact with the PTEBS chains hindering the formation of pi-pi interactions. This property is envisaged for developing a picric acid microsensor based on the formation of pi-pi interactions with the pi-conjugated polymer, thus quenching its PL emission. Above the LCST of PNIPAM-PTEBS microgels, the interactions would be broken and the initial PL emission would be recovered. This property could render reusable microsensors for detection of nitro aromatic compounds.

Characterization of binary solvent mixtures of DMSO with water and other cosolvents.

Binary mixtures of DMSO with nine different cosolvents were characterized in light of the pure solvent scales, using suitable probe/homomorph couples. Various physical (vapor pressure, surface tension, viscosity, and enthalpy of mixing) and spectroscopic (IR and NMR) properties of the DMSO/water mixtures are described in terms of their polarity, acidity, and basicity, and the descriptions are examined with a view to establishing their potential physical significance.

Trombo intraventricular y angioma cavernoso en cerebelo: disyuntiva de terapéutica hipocoagulante / Left ventricular thrombus and cavernous angioma of the cerebellum: controversy of the hypocoagulant therapy

No disponible

Effects of medium on decarboxylation kinetics: 3-carboxybenzisoxazoles and their potential use as environmental probes in biochemistry.

The decarboxylation rate of the tetramethylguanidinium salt of 3-carboxy-6-nitrobenzisoxazole in 24 pure solvents and 36 dimethyl sulfoxide binary mixtures with diglyme, acetonitrile, benzene, dichloromethane, chloroform, and methanol was analyzed in the light of the SPP, SA, and SB pure solvent scales. The results allow one to rationalize the high sensitivity of this kinetics to the reaction medium and to assess the potential use of this compound as a probe in biochemical environments. The natural environment for comparison of this kinetics was found to be the gas phase rather than the aqueous medium. In the latter, the process is much faster owing to such high polarity, which, however, is strongly diminished by the high acidity of the medium. Based on our calculations, the rate constant for the decarboxylation kinetics in the gas phase must be in the region of 2 x 10(-10) s(-1) (i.e., 3 orders of magnitude smaller than in water).

Intrinsic viscosity calculated out of single point measurements for chondroitin-4-sulfate and chondroitin-6-sulfate solutions.

The differences in the structure of polymer chain between the chondroitin-4-sulfate (C4-S) and the chondroitin-6-sulfate (C6-S) are reflected in the intrinsic viscosity values calculated starting from the traditional methods of Huggins, Kraemer, Mead and Fouss, and Martin, and by the method for a single-point determination of intrinsic viscosity [eta]. For the range between 0.4 and 0.5% (w/v) concentrations, we get an agreement in the intrinsic viscosity values. The chains of both polymers present a flexible structure and are not ramified between 0.28 and 1.00% (w/v) concentrations.

[Leopard syndrome and pulmonary subvalvular stenosis]. / Síndrome de Leopard y stenosis pulmonar infundibular.

Pulmonary valve stenosis is the most frequent congenital heart disease associated with Leopard syndrome, an inheritable condition with a low incidence that we should be aware of and diagnose early. We describe the evolution of a patient with Leopard Syndrome and pulmonary subvalvular stenosis, from birth to the surgical treatment.

Reversible denaturation of myo-inositol monophosphatase. The stability of the metal-binding loop.

The unfolding of bovine brain myo-inositol monophosphatase by guanidine. HCl (Gdn. HCl) has been investigated. The recovery of circular dichroism, emission spectra, and catalytic activity after dilution of Gdn.HCl-treated samples indicate that the overall process is reversible. The steepness of the spectroscopic changes between 3 M and 5 M Gdn.HCl, and the lack of any discernible plateau suggest that unfolding of the protein is a cooperative process. The sensitized luminescence of bound Tb(III) was used as a probe of conformational changes of the metal-binding loop. Denaturation of the enzyme by Gdn.HCl does not abolish sensitized luminescence. A 50% decrease in sensitized luminescence was observed in 5 M Gdn.HCl. Under this set of experimental conditions, the protein binds terbium with an association constant of 1 x 10(6)M-1. It is suggested that a residual structure of denatured myo-inositol monophosphatase is responsible for the binding of terbium ions. The kinetics of unfolding and refolding as a function of Gdn.HCl concentration were monitored by protein fluorescence in a stopped-flow instrument. The monophosphatase unfolded in a single kinetic phase with rate constants in the range 80-65 s-1 at 25 degrees C. The refolding kinetics fit monoexponential functions with rate constants in the range 120-65 s-1 depending on the Gdn.HCl concentration. Substantial refolding of the protein occurs within the dead time of mixing.

Study of the interaction between xylazine and bovine serum albumin by fluorescence quenching measurements.

The binding of xylazine to bovine werum albumin (BSA) was studied by fluoresence quenching, as a function of temperature. The experimental data could be fitted to both the Stern-Volmer equation and the Stern-Volmer equation modified by Lehrer. The temperature dependence of the Stern-Volmer constant, Ksv suggests that the mechanism of the quenching process is mainly dynamic in origin. The thermodynamic parameters were estimated based on such temperature dependence. The positive values found for the enthalpy and entropy changes seem to indicate that the hydrophobic contribution is the predominant intermolecular force stabilizing the xylazine-BSA complex. Fluorescence quenching was also used to calculate the binding constants by the Scatchard procedure. The values of these constants are of the same order of magnitude as the Stern-Volmer constants. These results, together with the spectral changes in the fluorescence emission spectra of BSA induced by xylazine, suggest that the interaction may take place in subdomains IIA and IIIA since these subdomains have been proposed to bind drugs and other hydrophobic materials.

Photophysical study of the Schiff bases of 5'-deoxypyridoxal and n-hexylamine in cationic micelles.

The absorption and fluorescence spectra of the Schiff bases formed between 5'-deoxypyridoxal and n-hexylamine in aqueous media containing different concentrations of the cationic surfactant hexadecyltrimethylammonium bromide were recorded at 25 degrees C. The quantum yields of fluorescence of the different zwitterionic and enol forms of the chemical species of the Schiff bases occurring in media of pH 4.5-8.5 were determined. Also, the fluorescence quenching resulting from the presence of the surfactant and that of iodide ion were analyzed. From the results obtained it follows that the zwitterionic forms do not interact with the cationic surfactant, whereas the enol forms do interact with it.

Alkaline hydrolysis of cefotaxime. A HPLC and 1H NMR study.

A kinetic study on the alkaline hydrolysis of cefotaxime at pH 10.5 and 37 degrees C has been carried out by using HPLC and 1H NMR. The main resulting degradation products have been isolated and identified. These include, apart from the well-known deacetylcefotaxime, the exocyclic methylene derivative, the 7-epimer of cefotaxime and the 7-epimer of deacetylcefotaxime. The kinetic constants involved in the process have been determined and according to the experimental results the attack of the hydroxyl group on the ester function bonded to the 3'-carbon is the fastest step in the proposed kinetic scheme. It should be emphasized that the base-catalyzed epimerization of the hydrogen at the 7 position clearly depends on the presence of a good electron-withdrawing group at C(3'). On the other hand, no hydrolysis of the amide at position 7 was detected.

Quantitative description of the absorption spectra of the coenzyme in glycogen phosphorylases based on log-normal distribution curves.

The absorption spectra of the coenzyme [pyridoxal 5'-phosphate (PLP)] in glycogen phosphorylase a (GPha), glycogen phosphorylase b (GPhb) and of the latter bound to various effectors and substrates were analysed on the basis of log-normal distribution curves. The results obtained showed that the ionization state of the PLP and GPha environment differs from that of GPhb. This divergence was interpreted in terms of tautomeric equilibria between some forms of the Schiff base of PLP and enzymic Lys-679. The ionic forms are slightly more predominant in GPha than they are in GPhb, so ionic and/or hydrogen-bonding interactions between the aromatic ring of PLP and GPha must be stronger than with GPhb. This confirms the purely structural role of the aromatic ring of the coenzyme. Binding of GPhb to AMP and Mg2+ results in the coenzyme adopting a similar state as in GPha. On the other hand, binding to IMP gives rise to no detectable changes in the tautomeric equilibrium of the coenzyme.

Stability of Schiff bases of amino acids and pyridoxal-5'-phosphate.

Stability of Schiff bases from Pyridoxal-5'-phosphate andα- and nonα-amino acids and amines have been studied in a wide range of pH. Furthermore the transamination process for the PLP-serine Schiff base and the cyclization reaction of PLP-histidine Schiff base have also been studied.Results show that theα-position on carboxyl group of amino acids plays an important role on the mechanism of hydrolysis of imine bond. Absence of ionic groups in the surroundings of that bond seems to be an important fact of stability.In the transamination reaction, the rate-determining step is the isomerization of the Schiff base to ketoimine, since the rate constants for disappearance of Schiff base coincide with the rate constants for PMP formation. This process is catalyzed by the OH(-)/H2O system and the monoprotonated amino acid.

Spectroscopic study of the Schiff bases of dodecylamine with pyridoxal 5'-phosphate and 5'-deoxypyridoxal. A model for the Schiff bases of pyridoxal 5'-phosphate in biological systems.

We recorded the absorption spectra of the Schiff bases of pyridoxal 5'-phosphate (PLP) and 5'-deoxypyridoxal (DPL) with dodecylamine (DOD) at different pH values. By applying deconvolution techniques to the spectra and analysing their different components we found that the above-mentioned Schiff bases in aqueous solutions of pH 7 adopted a conformation in which the pyridine ring is embedded in a very hydrophobic medium from which water is virtually completely excluded. This conformation in the same as that adopted by PLP when it acts as coenzyme for some enzymes such as glycogen phosphorylase. The experimental results obtained also show such a conformation to be highly favoured but sensitive to the protonation of the pyridine nitrogen, which makes the aromatic ring more readily accessible to the solvent.

Thermodynamic analysis of the interaction of the antibiotic teicoplanin and its aglycone with cell-wall peptides.

The thermodynamics of the interaction of the glycopeptidic antibiotic teicoplanin and its peptidic moiety with analogues of bacterial cell-wall peptides were studied by means of calorimetric and spectrophotometric techniques. The analysis of the thermodynamic data has allowed us to evaluate the contributions of the different peptide groups to the binding process. The nature of the primary binding forces is also discussed for each interacting group, on the basis of their enthalpic and entropic contribution and in connection with the detailed structural information available for these antibiotics from n.m.r. data. Similar analyses for the case of vancomycin and ristocetin are also reported.

Cyclic and linear sweep voltammetry of cefazolin and cefmetazole: electroanalytical applications.

A study of the variables influencing the voltammetric behaviour of cephalosporins and cefamycins bearing substituents of the S-R' type (R' = non-electroactive group) has been carried out in order to determine the most suitable conditions for obtaining reproducible peaks for the determination of both types of antibiotic and the calculation of kinetic and thermodynamic parameters. The voltammetric behaviour of cefazolin and cefmetazole has also been studied. The voltammetric peak yielded by these two antibiotics at high sweep rates and low concentrations is due solely to the reduction of adsorbed molecules as the contribution of those molecules moving to the electrode through diffusion to the current is negligible. The peak current of cefazolin varies linearly with its concentration in the range 10(-9)-10(-7) M, whereas the linear range for cefmetazole is between 10(-8) and 5 X 10(-7) M, which allows the electroanalytical determination of the two antibiotics at these concentrations.

Study of the hydrolysis and ionization constants of Schiff base from pyridoxal 5'-phosphate and n-hexylamine in partially aqueous solvents. An application to phosphorylase b.

Formation and hydrolysis rate constants as well as equilibrium constants of the Schiff base derived from pyridoxal 5'-phosphate and n-hexylamine were determined between pH 3.5 and 7.5 in ethanol/water mixtures (3:17, v/v, and 49:1, v/v). The results indicate that solvent polarity scarcely alters the values of these constants but that they are dependent on the pH. Spectrophotometric titration of this Schiff base was also carried out. We found that a pKa value of 6.1, attributed in high-polarity media to protonation of the pyridine nitrogen atom, is independent of solvent polarity, whereas the pKa of the monoprotonated form of the imine falls from 12.5 in ethanol/water (3:17) to 11.3 in ethanol/water (49:1). Fitting of the experimental results for the hydrolysis to a theoretical model indicates the existence of a group with a pKa value of 6.1 that is crucial in the variation of kinetic constant of hydrolysis with pH. Studies of the reactivity of the coenzyme (pyridoxal 5'-phosphate) of glycogen phosphorylase b with hydroxylamine show that this reaction only occurs when the pH value of solution is below 6.5 and the hydrolysis of imine bond has started. We propose that the decrease in activity of phosphorylase b when the pH value is less than 6.2 must be caused by the cleavage of enzyme-coenzyme binding and that this may be related with protonation of the pyridine nitrogen atom of pyridoxal 5'-phosphate.

Comparative study of the interactions AMP-phosphorylase b and AMP analogues-phosphorylase b.

The different effects induced by AMP and its analogues on the tertiary structure and the coenzyme environment of phosphorylase b were studied by titration of the slowly reacting thiol groups and by quenching of the coenzyme fluorescence, respectively, to determine the possible differences that activate phosphorylase b. The following results were obtained: The coenzyme environment depends upon the nucleotide studied. AMP, when bound to its first site, opens the coenzyme pocket. The slow cysteines were shielded by the nucleotides against their DTNB titration depending on the nucleotide studied. The enzyme difference spectra in presence of the nucleotide showed that the negative band of 260 nm is similar for all nucleotides possessing the same base, but the positive band obtained in the presence of AMP was diminished when other nucleotides were present.