Multi-analyte HPLC–DAD Method for Concurrent Analysis of Six Antimicrobials and Three Proton Pump Inhibitors Frequently used in Management of Helicobacter pylori Infection: Application to Simulated Intestinal Fluid Samples

Chromatographia - The present work deals with the optimization, validation and application of a versatile HPLC–DAD method for concurrent estimation of nine antimicrobials and proton pump...     

A recent approach for the synthesis of 1,2,4-triazine systems bearing and/or containing phosphorus atoms as donor-acceptor interaction and their biocidal affects: An overview

Abstract

This review reports the developed methods for the synthesis of polyfunctional 1,2,4-triazine systems bearing and/or containing phosphorus atoms as donor-acceptors interactions. Most of the synthetic methods depend on the interaction between an amino group of the side chain of 1,2,4-triazine with phosphorus reagents. The biological activities, i.e. their biocidal affects as antioxidants, antimicrobials, anti-inflammatory as well as molluscicidal agents against some snails are discussed.     

Evaluating formation and growth mechanisms of silica particles using fluorescence anisotropy decay analysis

At present, there is no direct experimental evidence that primary silica particles, which exist only transiently for a few seconds during the St?ber silica synthesis, can be stable in aqueous solutions. In the present work, we show that primary silica particles are formed spontaneously after the dissolution of diglycerylsilane (DGS) in aqueous solutions and remain stable for prolonged periods of time. By using time-resolved fluorescence anisotropy (TRFA), we demonstrate that this unique property of DGS is ascribed to the slow kinetics of silica particle growth in diluted sols at pH approximately 9.0. The anisotropy decay of the cationic dye rhodamine 6G (R6G), which strongly adsorbs to silica oligomers and nanoparticles in DGS sols, could be fit to three components: a fast (picosecond) scale component associated with free R6G, a slower (nanosecond) rotational component associated with R6G bound to primary silica particles, and a residual (nondecaying) anisotropy component associated with R6G that was bound to secondary or larger particles that were unable to rotate on the time scale of the R6G emission lifetime (4 ns). The data show that, under conditions where fast hydrolysis is obtained, the initial size of the nuclei depends on the silica concentration, with larger nuclei being present in more concentrated sols, while the rate of growth of primary particles depends on both silica concentration and solution pH. At low silica concentrations and high pHs, it was possible to observe the growth of stable, nonaggregating primary silica particles by a mechanism involving rapid nucleation followed by monomer addition. The presence of stable primary particles was confirmed by atomic force microscopy (AFM) imaging. At higher silica concentrations and lower pHs, there was an increase in the initial size of the nuclei formed, which subsequently grew to a larger radius (> 4.5 nm) or aggregated with time, and in such cases, nucleation and aggregation occurred simultaneously in the early stage of silica formation. The data clearly show the power of time-resolved fluorescence anisotropy decay measurements for probing the growth of silica colloids and show that this method is useful for elucidating the mechanism of particle formation and growth in situ.     

Chemo-enzymatic synthesis of 4-methylumbelliferyl beta-(1-->4)-D-xylooligosides: new substrates for beta-D-xylanase assays

Transglycosylation catalyzed by a beta-D-xylosidase from Aspergillus sp. was used to synthesize a set of 4-methylumbelliferyl (MU) beta-1-->4-D-xylooligosides having the common structure [beta-D-Xyl-(1-->4)]2-5-beta-D-Xyl-MU. MU xylobioside synthesized chemically by the condensation of protected MU beta-D-xylopyranoside with ethyl 2,3,4-tri-O-acetyl-1-thio-beta-D-xylopyranoside was used as a substrate for transglycosylation with the beta-D-xylosidase from Aspergillus sp. to produce higher MU xylooligosides. The structures of oligosaccharides obtained were established by 1H and 13C NMR spectroscopy and electrospray tandem mass spectrometry. MU beta-D-xylooligosides synthesized were tested as fluorogenic substrates for the GH-10 family beta-D-xylanase from Aspergillus orizae and the GH-11 family beta-D-xylanase I from Trichoderma reesei. Both xylanases released the aglycone from MU xylobioside and the corresponding trioside. With substrates having d.p. 4 and 5, the enzymes manifested endolytic activities, splitting off MU, MUX, and MUX2 primarily.     

Nets and generalized quadrangles

There is a new method of constructing generalized quadrangles (GQs) given by S. Löwe, which is based on covering of nets; all GQs with a regular point can be represented in this way. Here we give a method of constructing GQs with a regular point using the so-called content functions on nets. In the last part of the paper we lay the foundations for a research project aiming to use the more general notion of content to classify GQs and maybe to construct new ones.Both authors acknowledge the financial support by CRUI and DAAD in the frame of Programma Vigoni, which made this work possible.     

Bimodal proton transfer in acid-base reactions in water

We investigate one of the fundamental reactions in solutions, the neutralization of an acid by a base. We use a photoacid, 8-hydroxy-1,3,6-trisulfonate-pyrene (HPTS; pyranine), which upon photoexcitation reacts with acetate under transfer of a deuteron (solvent: deuterated water). We analyze in detail the resulting bimodal reaction dynamics between the photoacid and the base, the first report on which was recently published. We have ascribed the bimodal proton-transfer dynamics to contributions from preformed hydrogen bonding complexes and from initially uncomplexed acid and base. We report on the observation of an additional (6 ps)(-1) contribution to the reaction rate constant. As before, we analyze the slower part of the reaction within the framework of the diffusion model and the fastest part by a static, sub-150 fs reaction rate. Adding the second static term considerably improves the overall modeling of the experimental results. It also allows to connect experimentally the diffusion controlled bimolecular reaction models as defined by Eigen-Weller and by Collins-Kimball. Our findings are in agreement with a three-stage mechanism for liquid phase intermolecular proton transfer: mutual diffusion of acid and base to form a "loose" encounter complex, followed by reorganization of the solvent shells and by "tightening" of the acid-base encounter complex. These rearrangements last a few picoseconds and enable a prompt proton transfer along the reaction coordinate, which occurs faster than our time resolution of 150 fs. Alternative models for the explanation of the slower "on-contact" reaction time of the loose encounter complex in terms of proton transmission through a von Grotthuss mechanism are also discussed.     

Proton affinities of N-heterocyclic carbene super bases

The gas-phase proton affinity of the N-heterocyclic carbene, 1-ethyl-3-methylimidazol-2-ylidene, was determined to be 251.3 +/- 4 kcal/mol using the kinetic method, a value which makes the carbene one of the strongest bases reported thus far. Density functional theory calculations have been carried out at the B3LYP/6-31+G(d) level to compare the high experimental value with that estimated theoretically. Experimental results also show that two other N-heterocyclic carbenes with larger substituents have even higher proton affinities. [structure: see text]     

Solubility of Triethylamine in Tetraethylammonium Chloride Aqueous Solutions from 20 to 35°C

Solubilities of triethylamine in aqueous tetraethylammonium chloride solutions were measured at 20, 25, and 35°C. The molalities in Et₄NCl of the aqueous solvents ranged from 0.03 to 1 mol-kg^–1. The data were evaluated from density measurements using a vibrating-tube densimeter. At each temperature, least-squares method was used to fit experimental density data points to double polynomial equations of various degrees. Triethylamine molalities of the saturated aqueous phases were estimated by extrapolation from those equations. Experimental data were interpreted in terms of hydrophobic and electrostatic perturbed domains in the hydration shells of the noneleceory and of the cation of the salt, as a function of temperature and salt concentration. The conclusions obtained are consistent with previous volumetric studies.     

Functionalized mesoporous silica films as a matrix for anchoring electrochemically active guests

Mesoporous silica thin films were shown to be an appropriate matrix for immobilization of discrete electroactive moieties, yielding uniform transparent thin film electrodes with defined texture and enhanced electrochemical activity. The mesoporous silica films prepared on conducting FTO-coated glass substrate were postsynthetically functionalized. Alkoxysilanes were used as precursors for subsequent grafting via ionic or covalent bonds of representative electroactive species, such as polyoxometalate PMo12O(40)3-, hexacyanoferrate(III), and ferrocene. The electrochemically active concentration within the silica-based composite electrodes achieves 90, 260, and 60 micromol cm(-3) for polyoxometalate, hexacyanoferrate(III), and ferrocene, respectively. The amount of molecules involved in the charge-transfer sequence is proportional to the film thickness and comparable to the total amount of embedded guests. Thus, eventually the whole bulk volume of the modified silica films is electrochemically accessible. Immobilization in the chemically modified silica matrix alters the redox potential of the electroactive molecules. Electron exchange between the adjacent redox centers (electron hopping) is proposed as a possible charge propagation pathway through the insulating silica matrix, which is supported by the fact that the high charge uptake is observed also for the hybrid electrodes with the covalently anchored redox guests.