Stereochemical analysis of N-cyclohexylidene-N-(1-phenylethyl)amine derivatives

The configurational properties of a series of cyclohexylidene imines are discussed on the basis of their 1H, 13C and 15N NMR spectral data.     

Accurate intermolecular ground state potential of the Ne-N2 van der Waals complex

Ab initio ground state potential energy surfaces are obtained from interaction energies calculated with the coupled cluster singles and doubles model including connected triples corrections [CCSD(T)] and the aug-cc-pVXZ (X=5,Q,T,D) basis sets augmented with two different sets of midbond functions (denoted 33221 and 33211). The aug-cc-pV5Z-33221 surface is characterized by a T-shaped 49.5 cm(-1) minimum at Re=3.38 Angstroms and a linear saddle point at 3.95 Angstroms with De=36.6 cm(-1). These results agree well with the values provided by the accurate semiempirical potentials available. The rovibronic spectroscopic properties are determined and compared to the available experimental data and previous theoretical results. We study the basis set convergence of the intermolecular potentials and the rotational frequencies. The aug-cc-pVTZ basis sets provide reasonable binding parameters, but seem not to be converged enough for the evaluation of the microwave spectra. The aug-cc-pVQZ basis sets considerably improve the triple zeta results. The differences between the results obtained with the aug-cc-pVTZ-33221 basis set surface and those with the aug-cc-pVQZ-33221 are smaller than those of the corresponding bases with the set of 33211 midbond functions. The aug-cc-pVQZ surfaces are close to the aug-cc-pV5Z, that are expected to be close to convergence. With our best surfaces the errors in the frequencies with respect to the accurate experimental results go down to 0.6%.     

Crystal engineering of novel cocrystals of a triazole drug with 1,4-dicarboxylic acids

Cocrystals of the poorly soluble antifungal drug cis-itraconazole (1) with 1,4-dicarboxylic acids have been prepared. The crystal structure of the succinic acid cocrystal with 1 was determined to be a trimer by single-crystal X-ray. The trimer is comprised of two molecules of 1 oriented in antiparallel fashion to form a pocket with a triazole at either end. The extended succinic acid molecule fills the pocket, bridging the triazole groups through hydrogen-bonding interactions rather than interacting with the more basic piperazine nitrogens. The solubility and dissolution rate of some of the cocrystals are approximately the same as those of the amorphous drug in the commercial formulation and are much higher than those for the crystalline free base. The results suggest that cocrystals of drug molecules have the possibility of achieving the higher oral bioavailability common for amorphous forms of water-insoluble drugs while maintaining the long-term chemical and physical stability that crystal forms provide.     

Effect of oxidative stabilization on the electrochemical performance of carbon mesophases as electrode materials for lithium batteries

The effect of oxidative stabilization as a mean to modify the carbon texture was essayed in a group of mesophases previous to carbonization at 900 °C with the aim of evaluating the influence on electrochemical performance when used as electrode materials in lithium test cells. X-ray diffraction, optical microscopy and chemical analysis, Fourier-transform infrared spectroscopy have been used to describe the compositional and textural properties of the as-produced parent mesophases, the samples were further treated under air current to stabilize their microstructures and the corresponding carbonized samples at 900 °C. The electrochemical performance was determined by the galvanostatic method and further correlated to the physical–chemical properties and interface resistance of the materials. In all cases, the stabilization process has demonstrated a beneficial effect on the capacity retention in the measured range.     

Slow water diffusion in micellar solutions

Slowly diffusing water molecules were found by quasi-elastic neutron scattering (QENS) in a sodium dodecyl sulfate (SDS) micellar solution, and both their diffusion coefficient (4.33 x 10(-6) cm2 x s(-1)) and mole fraction (0.057) were determined. After successfully checking the mean slowing down of solvent molecules by the gradient compensated stimulated spin-echo (GCSTE) pulse sequence NMR method, a similar effect was observed with this technique in the solvent phase of dodecyl trimethylammonium bromide (DTAB) and differing chain length (X = 12, 20, 30, and 40) ethoxylated nonyl phenol (9NX) micellar systems. Following the literature, the experimental results are qualitatively explained by assuming that, apart from ionic hydration, H-bonds may form between the solvent molecules and the O or N atoms present in the hydrophilic (head)groups of the micelle-forming monomers.     

On-line filtration system for determining total chromium and chromium in the soluble fraction of industrial effluents by flow injection flame atomic absorption spectrometry

Two manifolds were assessed for the purpose of determining both the total chromium content and that present as a soluble form in industrial effluents by flow injection flame atomic absorption spectrometry (FI-FAAS). To determine the chromium content in the soluble fraction the samples were used without additional treatment, a 0.45 microm filter being included in the FI system. To determine the total chromium content, the samples were acidified with nitric acid 20% (v/v) and heated for 30 s in a microwave oven (temperatures of about 70 degrees C were reached). The problem posed by the very different concentration range in which total and soluble chromium are present was overcome by using programmed flow rate methodology and by only partially emptying the sample loop. A personal computer controlled both the rotation speed of a peristaltic pump and the volume of sample injected into the system, thus obtaining the dispersion degree required. Using the manifold proposed, the chromium content in the soluble fraction can be determined in the 0.5-20 microg mL(-1) range using a 10 microg mL(-1) single standard for calibration. To determine the total chromium content, a calibration line in the 20-200 microg mL(-1) range was obtained using a single 50 microg mL(-1) chromium standard solution. The reliability of the semi-automatic devices was verified by comparing the results obtained with those found by treating the samples and using both FAAS in a conventional way and a spectrophotometric method using diphenylcarbazide at the 95% confidence level (ANOVA test). The proposed procedures showed a RSD lower than +/-3%.     

On Transition Structures for Hydride Transfer Step in Enzyme Catalysis. A Comparative Study on Models of Glutathione Reductase Derived from Semiempirical, HF, and DFT Methods

As a model of the chemical reactions that take place in the active site of gluthatione reductase, the nature of the molecular mechanism for the hydride transfer step has been characterized by means of accurate quantum chemical characterizations of transition structures. The calculations have been carried out with analytical gradients at AM1 and PM3 semiempirical procedures, ab initio at HF level with 3-21G, 4-31G, 6-31G, and 6-31G basis sets and BP86 and BLYP as density functional methods. The results of this study suggest that the endo relative orientation on the substrate imposed by the active site is optimal in polarizing the C4-Ht bond and situating the system in the neighborhood of the quadratic region of the transition structure associated to the hydride transfer step on potential energy surface. The endo arrangement of the transition structure results in optimal frontier HOMO orbital interaction between NADH and FAD partners. The geometries of the transition structures and the corresponding transition vectors, that contain the fundamental information relating reactive fluctuation patterns, are model independent and weakly dependent on the level of theory used to determine them. A comparison between simple and complex molecular models shows that there is a minimal set of coordinates describing the essentials of hydride transfer step. The analysis of transition vector components suggests that the primary and secondary kinetic isotope effects can be strongly coupled, and this prompted the calculation of deuterium and tritium primary, secondary, and primary and secondary kinetic isotope effects. The results obtained agree well with experimental data and demonstrate this coupling.     

Electrodeposition of cobalt on stainless steel foils. Preparation of57Co sources for Mössbauer experiments

Conditions for the electrodeposition of⁵⁹Co and⁵⁷Co on stainless steel foils have been studied. A set of electrolytic parameters has been determined to get a reasonably good electrodeposited sample. The⁵⁷Co electrodeposited foil was then introduced in a quartz tube to be annealed in an argon flow. The final product was a 11 Ci Mössbauer source for college experiments which was succesfully tested by obtaining spectra of stainless steel and sodium nitroprusside.     

Determination of some trace elements in human tooth enamel

Determination of seven elements (Cu, Fe, Mg, Mn, Pb, Sr and Zn) in whole enamel and surface layers of extracted non-carious human teeth by FAAS, ETA AAS, ICP-AES and ICP-MS (Pb) is demonstrated. Techniques are described for obtaining whole enamel and its acid dissolution. Fifty microm width enamel layers from outer enamel surface to a 200 microm depth were successively etched in 1 mL of 3 M HClO4 for 3 min dissolution periods. Enamel samples were analyzed for populations under and over 20 years of age and enamel from Bronze Age teeth. Concentrations of microelements in the whole enamel and in the first surface layer (50 microm depth) were compared. With exception of Sr and Mg, all elements show significantly higher concentrations in the first layer than in whole enamel and higher concentration in teeth of individuals over 20 years, which demonstrate the cumulative effect of these elements. The Cu, Fe, Mn, Pb and Zn concentrations in four layers of erupted and non-erupted teeth decreased while Mg and Sr concentrations increased toward enamel-dentine junction. The concentrations of most elements were almost constant as they approached the 150 microm layer. This concentration gradient may result from interaction between saliva and teeth and supports the hypothesis that the surface de- and re-mineralization process is effective at most to 150 microm from the enamel surface.     

Inducible stable DNA replication (iSDR) and the uvr-dependent tolerance of pyrimidine dimers in UV-irradiated Escherichia coli are two uncoupled processes

Inducible stable DNA replication (iSDR) is dependent on recombination and is supposed to play a role in DNA repair of Escherichia coli. Our previous data suggested that iSDR may be involved in the tolerance of UV lesions, which remain unexcised in excision-proficient E. coli exposed to some UV pretreatments. Now, the tolerance of unexcised lesions has been followed in E. coli recB21 and in E. coli priA1 sup mutants, incapable of iSDR. The obtained data do not confirm the previous hypothesis about the involvement of iSDR in the putative uvr-dependent lesion tolerance. They rather suggest that iSDR and the uvr-dependent lesion tolerance are two uncoupled processes.