Small-angle x-ray scattering study of flow alignment of a thermotropic liquid crystal in the nematic and smectic phases.

The capillary flow alignment of the thermotropic liquid crystal 4-n-octyl-4'-cyanobiphenyl in the nematic and smectic phases is investigated using time-resolved synchrotron small-angle x-ray scattering. Samples were cooled from the isotropic phase to erase prior orientation. Upon cooling through the nematic phase under Poiseuille flow in a circular capillary, a transition from the alignment of mesogens along the flow direction to the alignment of layers along the flow direction (mesogens perpendicular to flow) appears to occur continuously at the cooling rate applied. The transition is centered on a temperature at which the Leslie viscosity coefficient alpha3 changes sign. The configuration with layers aligned along the flow direction is also observed in the smectic phase. The transition in the nematic phase on cooling has previously been ascribed to an aligning-nonaligning or tumbling transition. At high flow rates there is evidence for tumbling around an average alignment of layers along the flow direction. At lower flow rates this orientation is more clearly defined. The layer alignment is ascribed to surface-induced ordering propagating into the bulk of the capillary, an observation supported by the parallel alignment of layers observed for a static sample at low temperatures in the nematic phase.

Nanostructure formation in poly(γ-benzyl-l-glutamate)-poly(ethylene glycol)-poly(γ-benzyl-l-glutamate) triblock copolymers in the solid state.

The morphology in the solid state of a series of triblock copolymers comprising a poly(ethylene glycol)(PEG) midblock and symmetric poly(γ-benzyl--glutamate)(PBLG) end blocks has been studied using X-ray scattering and microscopy techniques. Transmission electron microscopy (TEM) on samples selectively stained with uranyl acetate provided clear assignment of morphologies for as-cast and annealed samples. The thickness of both PEG and PBLG domains was in good agreement with calculations based on the conformations of the respective chains, allowing for the crystal or amorphous state of PEG and the α-helical or ß-sheet structure of the PBLG. Atomic force microscopy provided complementary information on surface morphology for several samples that was in good agreement with the structure observed by TEM. A morphology diagram was constructed. Cylindrical structures were observed for ordered samples with low , whereas at higher there was evidence for broken lamellar and "hockey puck" nanostructures. Regular lamellae were observed for intermediate compositions.

Ordering on multiple lengthscales in a series of side group liquid crystal block copolymers containing a cholesteryl-based mesogen.

Hierarchical ordering in a side group liquid crystal block copolymer is investigated by differential scanning calorimetry, polarized optical microscopy, small-angle X-ray and neutron scattering (SAXS and SANS) and transmission electron microscopy (TEM). A series of block copolymers with a range of compositions was prepared by atom transfer radical polymerization, comprising a polystyrene block and a poly(methyl methacrylate) block bearing chiral cholesteryl mesogens. Smectic ordering is observed as well as microphase separation of the block copolymer. Lamellar structures were observed for far larger volume fractions than for coil-coil copolymers (up to a volume fraction of liquid crystal block, = 0.8). A sample with = 0.86 exhibited a hexagonal-packed cylinder morphology, as confirmed by SAXS and TEM. The matrix comprised the liquid crystal block, with the mesogens forming smectic layers. For the liquid crystal homopolymer and samples with high , a smectic-smectic phase transition was observed below the clearing point. At low temperature, the smectic phase comprises coexisting domains with monolayer S coexisting with interdigitated S domains. At high temperature a S phase is observed. This is the only structure observed for samples with lower . These unprecedented results point to the influence of block copolymer microphase separation on the smectic ordering.

Oxidative effects induced by dediazoniation of the p-hydroxybenzenediazonium ion in a neutral aqueous medium.

The toxicity of arenediazonium ions is believed to result from the appearance of very reactive compounds during the dediazoniation process. In the case of the p-hydroxybenzenediazonium ion (PDQ), radical species generated during dediazoniation could potentially initiate lipid peroxidation. The data obtained in spectrophotometric experiments suggest that an interaction between PDQ and linoleic acid (LA) gives rise to the characteristic absorption of oxidized products deriving from LA, both in the presence and absence of a mixed micellar medium containing the surfactant Tween 20 (Tw20). Spectroscopic evidence also clearly points to the interference of these processes in the dediazoniation of PDQ. Analysis by reverse-phase, high-pressure liquid chromatography (HPLC) confirms that the decomposition of PDQ in a mixed micellar medium induces the peroxidation of both LA and methyl linoleate (MEL), thus causing the appearance of peaks characteristic of dienic conjugated hydroperoxides. The same products are observed after interaction between LA and the water-soluble 2,2'-azobis (2-amidinopropane), a frequently used initiator of lipid peroxidation. The proportion of isomers produced during the peroxidation process agrees well with that reported for reactions mediated by free radicals. A further chromatographic analysis of the decomposition of PDQ in the presence of 2-methylcyclohexa-2,5-diene-1-carboxylic acid (CHD) shows that phenol and quinone are the main products of the reaction. These results are discussed on the understanding that aryl and peroxyl radicals abstract a hydrogen atom from CHD, in accordance with our general scheme for PDQ dediazoniation described in a previous publication.

Reaction of sodium amoxicillin with Cu(II) ion in a methanolic medium.

The present article considers several physicochemical aspects of the complexation reaction of sodium amoxicillin and Cu(II) ion in a methanolic medium. Analysis of spectrophotometric data demonstrated the formation of two complex species with amoxicillin:Cu(II) ion mole ratios of 1:1 and 2:1. Stability constants (beta) and molar absorptivities at 610 nm (epsilon) of the complexes (20 degrees C) in methanol were calculated simultaneously by a computer program on the basis of absorbance data obtained at 610 nm. The values thus calculated for the 1:1 complex were as follows: log beta 1 = 5.48 +/- 0.21 L.mol-1, epsilon 1 = 70 +/- 2 L.mol-1.cm-1. The values for the 2:1 complex were as follows: log beta 2 = 8.98 +/- 0.17 L2.mol-2 and epsilon 2 = 138 +/- 4 L.mol-1.cm-1. The amoxicillin:Cu(II) ion complexes were quite stable over time.

Degradation of ampicillin in the presence of cadmium (II) ions.

Cadmium (II) ion-catalyzed degradation of ampicillin in methanol at 20 degrees C has been studied. It has been observed that the rate values tend to saturate when the concentration of ampicillin or the metal ion is increased. The results obtained in the present study suggest that ampicillin degradation occurs through the formation of a 1:1 (SM) and 2:1 (S(2)M) ampicillin-metal complexes. These complexes decompose giving a single product (absorption maximum at 285 nm; ((p)=1.82x10(4) l mol(-1) cm(-1)) that has been isolated and identified (Cd(II) (L(2-))(2) (H(2)O)(4) Na(2)). The appearance of this product reflects a first order reaction with respect to the 1:1 complex, with a rate constant of 3.87x10(-2) min(-1) and the existence of an equilibrium between the 1:1 and 2:1 initial complexes. The equilibrium constant value, calculated from kinetic data, is 1.7x10(3) l mol(-1).

Beta-lactam degradation catalysed by Cd2+ ion in methanol.

Kinetic schemes are established for degradation catalysed by Cd2+ ions in methanolic medium for penicillin G, penicillin V and cephalothin, a cephalosporin. Methanolysis of penicillin V and cephalothin occurs with the formation of a single substrate-metal ion intermediate complex, SM, while degradation of penicillin G occurs with the initial formation of two complexes with different stoichiometry, SM and S2M. In each case. degradation is of first order with respect to SM with rate constant values equal to 0.079 min(-1), 0.120 min(-1) and 0.166 min(-1) at 20, 25 and 30 degrees C, respectively, for penicillin G; 0.061 min(-1) at 20 degrees C for penicillin V; and 2.0 x 10(-3) min(-1) at 20 degrees C for cephalothin. Activation energy for the decomposition process of the SM intermediate for penicillin G was calculated to be about 5.5 x 10(4) J/mol. Equilibrium constant values between SM compound and S2M at 20 degrees C (77.1 l/mol), 25 degrees C (45.3 l/mol) and at 30 degrees C (25.7 l/mol) were also calculated as well as the normal enthalpy of this equilibrium. With respect to the reaction products there is evidence that Cd2+ becomes part of their structure, forming complexes between Cd2+ and the product resulting from antibiotic methanolysis (L). Some characteristics of these complexes are discussed.

Determination of serum levels of dietary thiocyanate.

A spectrophotometric method was used to determine thiocyanate (SCN-) levels in human serum, in an attempt to relate serum levels of this ion to the dietary intake of vegetables of the genus Brassica. Thiocyanate ion was measured before and after a one-week period during which the subjects ate a diet rich in these vegetables. We also determined the serum increase in SCN- 2 h after the administration of 0.5 g KSCN.

Wormlike micelle formation and flow alignment of a pluronic block copolymer in aqueous solution.

The self-assembly into wormlike micelles of a poly(ethylene oxide)-b-poly(propylene oxide)-b-poly(ethylene oxide) triblock copolymer Pluronic P84 in aqueous salt solution (2 M NaCl) has been studied by rheology, small-angle X-ray and neutron scattering (SAXS/SANS), and light scattering. Measurements of the flow curves by controlled stress rheometry indicated phase separation under flow. SAXS on solutions subjected to capillary flow showed alignment of micelles at intermediate shear rates, although loss of alignment was observed for high shear rates. For dilute solutions, SAXS and static light scattering data on unaligned samples could be superposed over three decades in scattering vector, providing unique information on the wormlike micelle structure over several length scales. SANS data provided information on even shorter length scales, in particular, concerning "blob" scattering from the micelle corona. The data could be modeled based on a system of semiflexible self-avoiding cylinders with a circular cross-section, as described by the wormlike chain model with excluded volume interactions. The micelle structure was compared at two temperatures close to the cloud point (47 degrees C). The micellar radius was found not to vary with temperature in this region, although the contour length increased with increasing temperature, whereas the Kuhn length decreased. These variations result in an increase of the low-concentration radius of gyration with increasing temperature. This was consistent with dynamic light scattering results, and, applying theoretical results from the literature, this is in agreement with an increase in endcap energy due to changes in hydration of the poly(ethylene oxide) blocks as the temperature is increased.

[Evaluation of a new automated system for the determination of antifungal sensitivity]. / Evaluación de un nuevo sistema automatizado para la determinación de la sensibilidad a antifúngicos.

BACKGROUND: To evaluate a new semi-automatized system (ATB-Fungus, Bio Mérieux, Spain) for antifungal in vitro susceptibility testing in yeast. METHODS: We studied the susceptibility of 91 yeast strains (22 Candida albicans, 18 Candida parapsilopsis, 18 Candida krusei, 17 Candida tropicalis, 11 Candida glabrata, 5 Candida guillermondii) against amphotericin B, flucytosine, ketoconazole, miconazole, econazole and nystatin, by means of "ATB-fungus" and agar diffusion and only against systemic usage antifungal agents by agar-dilution method. RESULTS: The correlation between "ATB-fungus" and diffusion method and agar dilution was 100% for amphotericin B, except for Candida krusei and C. tropicalis strains. This correlation rate was lower for fluocytosine and ketoconazole, without major differences between species. There is an absolute concordance between the system under evaluation and agar diffusion method regarding nystatin. For the two azole drugs remaining (miconazole and econazole) the correlation found for Candida glabrata was 100% and for C. parapsilopsis 95% with agar diffusion method. Candida glabrata was the single species that showed better correlation with all three methods, while major differences were seen with C. krusei and C. tropicalis. CONCLUSIONS: Although "ATB-fungus" seems to be a nearly well standardized method for antifungal susceptibility testing, more studies are required for assessing its reproducibility and also to determine when, how and for which antifungal agents would be a valid procedure.

[Comparison of ATB STREP, AMS-Vitek GPS-TA, and disk diffusion for the detection of a high level of aminoglycoside resistance in Enterococcus spp]. / Comparación de ATB STREP, AMS-Vitek GPS-TA y difusión con discos en la detección de alto nivel de resistencia a aminoglucósidos en Enterococcus spp.

BACKGROUND: The absence of the synergic effect between beta-lactam and aminoglucoside versus Enterococcus is predicted by the presence of a high level of resistance to aminoglucoside. There are several commercial systems which include wells for the performance of this determination for streptomycin and gentamicin. METHODS: A comparative study of two commercial systems, Vitek GPS-TA and ATB STREP (Biomérieux) and disk diffusion versus the Mueller-Hinton agar dilution method used as a method of reference. Two hundred ninety strains of Enterococcus spp. from blood, wounds, pus, fluids from paracentesis and urine were studied. RESULTS: By the reference method, 17 presented high level of resistance only to gentamicin, 45 only to streptomycin, 46 to both and 182 were sensitive to the two antibiotics. The sensitivity of ATB STREP, Vitek GPS-TA and disk diffusion was respectively for high level of resistance to gentamicin of 96.8, 100 and 100% and specificity of 99.5, 99.1 and 96.5%. For high level of resistance to streptomycin sensitivity was 78.2, 100 and 100% and specificity 96, 79.4 and 72.4% respectively. CONCLUSIONS: The authors consider the three methods valid for the determination of high level of resistance to gentamicin, presenting excellent correlation between the results of the reference methods. However, for high level of resistance to streptomycin with ATB STREP 1/5 of the resistant strains would not be detected due to low sensitivity. With Vitek GPS-TA and disk diffusion, although excellent sensitivity was presented, a high percentage of false positives were obtained due to low specificity, thus the authors recommend that before a results of resistance be decided the same should be confirmed by another alternative method (agar dilution, macrodilution in tubes or death-time curve).

In vitro susceptibility of 245 yeast isolates to amphotericin B, 5-fluorocytosine, ketoconazole, fluconazole and itraconazole.

The in vitro activity of amphotericin B, 5-fluorocytosine, ketoconazole, fluconazole and itraconazole was tested against 245 yeast strains isolated from clinical specimens (68 Candida albicans, 74 Candida tropicalis, 43 Candida krusei, 28 Candida glabrata, 19 Candida parapsilosis, 8 Candida lusitaniae and 5 Candida guilliermondii). An agar dilution method was employed to carry out testing. Minimal inhibitory concentrations to restrain 90% of isolate growth (MIC90) ranged from 0.12 to 2 mg/l for amphotericin B and for 5-fluorocytosine, from 0.03 to 8 mg/l for ketoconazole, from 0.05 to 50 mg/l for itraconazole and from 0.1 to > 100 mg/l for fluconazole. Among the azole derivatives, the most active was ketoconazole, followed by itraconazole. Only 1 strain of C. albicans was resistant to amphotericin B (MIC > 4 mg/l). Both C. tropicalis and C. krusei responded poorly to fluconazole and the former to itraconazole as well. The species most susceptible to the antifungal agents tested was C. glabrata and the most resistant were C. tropicalis and C. krusei.

Estudio cinético de la fotodegradación del ión p-hidroxibencenodiazonio en medio polar / Kinetic study of the photodegradation of p-hydroxybenzenediazonium ion in polar media

Se ha realizado un estudio sobre la fotodescomposición del ión p-hidroxibencenodiazonio (PDQ) basado en los datos espectrofotométricos y cromatográfi cos obtenidos con disoluciones de PDQ expuestas a irradiación UV (254 nm) en medio de acetonitrilo y agua. Los resultados de HPLC y HPLC-masa (HPLC/MS) indican que el 4-acetamidofenol es el principal producto que se forma tras la irradiación de PDQ en acetonitrilo. Esto se explica como consecuencia de la formación inicial del catión arilo, que posteriormente participa en una reacción de Ritter. El análisis cinético de los datos espectrofotométricos revela que la fotodegradación de PDQ es más rápida en acetonitrilo (constante de velocidad observada, kobs, = 0,1442 s-1) que en acetonitrilo acidifi cado (kobs = 0,009 s-1), lo que indica una mayor fotoestabilidad de la especie protonada derivada de PDQ. La constante de segundo orden (0,062 M s-1) encontrada para la fotodescomposición de PDQ en tampón fosfato (pH 7) se justifi ca por el establecimiento de un equilibrio entre las especies protonada y no protonada procedentes de la disociación ácida de PDQ

A study on the photodecomposition of p-hydroxybenzenediazonium ion (PDQ) has been made using chromatographic and spectrophotometric data obtained from UV-irradiated (254 nm) PDQ solutions in acetonitrile and aqueous media. The HPLC and HPLC-mass results indicate that 4-acetamidophenol is the main product formed after the irradiation of PDQ in acetonitrile. This is explained as a consequence of the initial formation of the aryl cation which is later involved in a Ritter’s reaction. A kinetic analysis of the spectrophotometric data reveals that PDQ photodegradation is faster in acetonitrile (observed rate constant (kobs) = 0.1442 s-1) than in acidifi ed acetonitrile (kobs = 0.009 s-1) indicating a higher photostability of the protonated species derived from PDQ. The second order constant (0.062 M s-1) found for the PDQ photodecomposition in phosphate buffer (pH 7) is explained in term of the equilibrium between protonated and non-protonated species coming from the acid dissociation of PDQ

Signo de la arteria basilar hiperdensa / Hyperdense basilar artery sign

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