Layer-block dendrimers with alternating thienylenevinylene and phenylenevinylene units

A series of new hybrid, layer-block π-conjugated dendrons and dendrimers with alternating thienylenevinylene and phenylenevinylene units has been prepared by means of an orthogonal and convergent-growth methodology that made use of the Horner-Wadsworth-Emmons (HWE) reaction. The placement of the thiophene and benzene rings can be accurately controlled to afford a large variety of dendritic structures, although access to compounds of high generation proved difficult. The optical properties of the synthesized dendrimers were determined by UV/vis and fluorescence spectroscopy, and the influence of the generation and nature of the core on the behavior of these materials was evaluated.     

Derivative spectrophotometric resolution of mixtures of the food colourants Tartrazine, Amaranth and Curcumin in a micellar medium

A study of the absorption spectra of the food colourants Tartrazine (E-102), Amaranth (E-123) and Curcumin (E-100) in various organic solvents and pH media, have been carried out. In addition, the spectral behaviour in aqueous micellar systems of anionic, cationic and non-ionic surfactants, as well as beta-cyclodextrin, was investigated. Analytical methods are proposed for the determination of the three dyes in mixtures without any prior separation step, by the application of the derivative technique to the normal absorption spectra, based on the use of cetyltrimethylammonium bromide (CTAB) in 0.1 M NaOH, over the concentration range 2-50 mug ml(-1). The methods are applied for determining the three dyes in commercial food products.     

Pyrrolodiazines. 2. Structure and Chemistry of Pyrrolo[1,2-a]pyrazine and 1,3-Dipolar Cycloaddition of Its Azomethine Ylides

A new synthesis of the pyrrolo[1,2-a]pyrazine system from pyrrole is described. In light of the ab initio calculations carried out on this heterocyclic system some of its basic chemistry was investigated and included electrophilic substitution, addition of organolithium reagents, metalation with lithium diisopropylamide and subsequent reaction with electrophiles, and formation of salts by quaternization of the nonbridgehead nitrogen. N-ylides obtained from these salts undergo 1,3-dipolar cycloaddition with suitable dipolarophiles to give dipyrrolo[1,2-a]pyrazines, pyrazolo[1,5-a]-pyrrolo[2,1-c]pyrazines, and heterobetaines. Examples of intramolecular 1,3-dipolar cycloadditions are also reported.     

Apparent molar volumes,heat capacities,and molecular excluded volumes of methylated,hydroxy and methoxy derivatives of 1-methylcytosine in aqueous solutions at 25°C

Densities and specific heat capacities of aqueous solutions: 1-methylcytosine; 1-methyl-N⁴-hydroxycytosine; 1,5-dimethylcytosine; I,N⁴-dimethylcytosine; 1,5-dimethyl-N⁴-hydroxycytosine; 1-methyl-N⁴-methoxycytosine; 1,N⁴,N⁴-trimethylcytosine, 1,5-dimethyl-N⁴-methoxycytosine were determined using flow calorimetry and flow densimetry at 25°C. Apparent molar volumes and heat capacities were then determined. Molecular excluded volumes were evaluated. A relationship was found between the values of the increments in partial molar values and the kind of groups substituted. Four types of contributions were distinguished: substitution of hydrogen on C, N, and O (in OH group on N4) atoms by CH₃ group and replacement of hydrogen on N4 atom by OH group. The correlation between the experimental partial molar volumes and calculated molecular excluded volumes was also elaborated.     

Ag(I) complexes with alkylidene-bis(2-aminopyrimidines) as building units for discrete metallomacrocyclic frames. A structural and solution study

Alkylidene-bis(2-aminopyrimidines) (pyr2Cx, x = 2-5) are useful ligands to interact with Ag(I) yielding discrete metallocycles. Crystal structures of the [(pyr2C2)Ag(NO3)]2 and [(H-pyr2C4)Ag(NO3)2]2 have been isolated where each macrocyclic moiety interacts with their surroundings through weak interactions, yielding 3D discrete structures, On the other hand, the solution study shows that the equilibrium constants for the formation of Ag(pyr2Cx)+ complexes are higher than the literature values for Ag(I) complexes with single pyrimidines, although the differences could be explained by invoking the solid-state structures of the Ag(I)-pyr2Cx complexes.     

Investigation of the negative thermal expansion mechanism of zeolite chabazite using the pair distribution function method

We have used the pair distribution function (PDF) method to gain insight into the mechanism of contraction of zeolite chabazite. Using this method we followed how the interatomic distances of the local structure changed with temperature. By optimization of the structure by free energy minimization and using the Reverse Monte Carlo technique we were able to find structural models at low and at high temperatures that agreed quantitatively with our experimental PDFs. From these models we conclude that the mechanism of contraction with temperature cannot involve rocking of the tetrahedra as rigid unit modes as there are distortions of the tetrahedra with temperature (indicating internal vibrations) and also that the mechanism of contraction probably involves a mode that translates along the Si-O3-Si-O4-Si linkages inside of the D6R of zeolite chabazite.     

Three-state 2',7'-difluorofluorescein excited-state proton transfer reactions in moderately acidic and very acidic media

2',7'-Difluorofluorescein (Oregon Green 488, OG488) is a novel fluorescein dye derivative which presents important advantages for improving the fluorimetric applications in the biomedical and biochemical sciences. In aqueous solution it displays four prototropic forms, namely cation (C), neutral (N), monoanion (M), and dianion (D). In previous works, we found (J. Phys. Chem. A 2005, 109, 734-747, 2840-2846) that OG488 undergoes excited-state proton transfer reactions, which may affect the results from applications using this dye. We established that the excited-state proton transfer (ESPT) reactions between neutral, monoanionic, and dianionic forms of OG488 are promoted by acetate buffer, and we characterized the ground and excited species involved. We also solved the kinetics of the prototropic reactions using global compartmental analysis. In the present paper, we extend our study on the ESPT reactions of OG488 to acidic media, in which only the three prototropic species cation, neutral, and monoanion coexist. We have solved the kinetics of the three-state ESPT reaction by means of global three-compartmental analysis of a fluorescence decay surface in moderately acidic media (pH between 1.1 and 3.0), recovering the kinetic and spectral parameters of this three-state system. This system is one of the most complex solved to date, due to the strong overlap of the absorption and emission spectra of the neutral and monoanionic forms of OG488. We also found that the cation behaves as "super" photoacid, showing a very high deprotonation rate constant (1.04 x 10(11) s(-1)) and an enhanced acidity. Therefore, we also carried out experiments at very high perchloric acid concentrations, dealing with some other effects which become noteworthy at these [H(+)]. The presence of xanthylium cation quenching due to "free" water molecules, and the reduction in the amount of water clusters acting as proton acceptors, are processes which alter notably the time course of the excited-species in this high [H(+)] range.     

Capillary electrophoretic determination of triamterene, methotrexate, and creatinine in human urine

A capillary zone electrophoresis (CZE) method using a fused-silica capillary (60.2 cm x 75 microm ID) was investigated for the determination of triamterene (TRI), methotrexate (MTX), and creatinine (CREA) in human urine. The separation was performed using a hydrodynamic injection time of 7 s (0.5 psi), a voltage of 25 kV, a capillary temperature of 30 degrees C, and 40 mM phosphoric acid adjusted to pH 2.25 by addition of triethanolamine as separation electrolyte. Under these conditions, analysis takes about 15 min. A linear response over the 0.5-15.0 mg L(-1) concentration range was found for TRI and MTX, and 0.5-80.0 mg L(-1) for CREA. Dilution of the sample (water:urine, 1:1 for TRI and MTX, and 1:25 for CREA determination) was the only step necessary prior to analysis by electrophoresis. The developed method is easy, rapid, and sensitive and has been applied to determine triamterene,methotrexate, and creatinine in urine samples with satisfactory results.     

A new procedure for bovine milk digestion in a focused microwave oven: gradual sample addition to pre-heated acid

Milk samples can be efficiently digested using a focused microwave oven, however the conventional procedure of addition of concentrated acids to the liquid sample leads to digestates with elevated acidity and residual carbon concentrations. In this work a focused microwave oven was applied for acid digestion of bovine milk samples using a conventional and an alternative procedure based on gradual sample addition to hot and concentrated acids. A two-level 2³ full factorial design experiment with eight runs was carried out to evaluate the optimum experimental conditions for reducing both the residual carbon and the final acidity of digestates. The three studied parameters were: temperature of the digestion medium for sample addition, addition of sulfuric acid before the sample or during the first step, and number of aliquots of the sample gradually added. The best conditions were attained by adding small aliquots of milk (ten-fold a volume of 0.5 ml added during 5.0 min) to a digestion mixture containing 3.0 ml nitric acid plus 1.0 ml sulfuric acid heated at 105 °C. It was demonstrated that the digestion efficiency of the alternative procedure was better than the conventional procedure, i.e. 98 and 80%, respectively. The alternative procedure was applied for determination of Ba, Ca, Cu, K, Mg, Na, P, and Zn in whole and non-fat bovine milk. The accuracy was proved using two certified reference materials (whole and non-fat milk powder).     

Mineral content and botanical origin of Spanish honeys

Eleven elements (Zn, P, B, Mn, Mg, Cu, Ca, Ba, Sr, Na and K) were determined by inductively plasma coupled spectrometry in 40 honey samples from different places of Spain and four different botanical origins: Eucalyptus (Eucalyptus sp.), Heather (Erica sp.), Orange-blossom (Citrus sinensis) and Rosemary (Rosmarinus officinalis). K, Ca and P show the higher levels with average concentrations ranged between 434.1-1935 mg kg⁻¹ for K; 42.59-341.0 mg kg⁻¹ for Ca and 51.17-154.3 mg kg⁻¹ for P. Levels of Cu (0.531-2.117 mg kg⁻¹), Ba (0.106-1.264 mg kg⁻¹) and Sr (0.257-1.462 mg kg⁻¹) are the lowest in all honey samples. Zn (1.332-7.825 mg kg⁻¹), Mn (0.133-9.471 mg kg⁻¹), Mg (13.26-74.38 mg kg⁻¹) and Na (11.69-218.5 mg kg⁻¹) concentrations were found strongly dependent on the kind of botanical origin.Results were submitted to pattern recognition procedures, unsupervised methods such as cluster and principal components analysis and supervised learning methods like linear discriminant analysis in order to evaluate the existence of data patterns and the possibility of differentiation of Spanish honeys from different botanical origins according to their mineral content. Cluster analysis shows four clusters corresponding to the four botanical origins of honey and PCA explained 71% of the variance with the first two PC variables. The best-grouped honeys were those from heather; eucalyptus honeys formed a more dispersed group and finally orange-blossom and rosemary honeys formed a less distinguishable group.