Concentration of sodium ion as the determining factor for the association of dansyl amino acids with the teicoplanin molecule in reversed-phase liquid chromatography

The mechanism of the binding of D,L dansyl amino acids to teicoplanin was investigated. Na+ was used as an indicator of the interactions between the solutes and teicoplanin. The number (n) of sodium ions, Na+, excluded from the solute-teicoplanin interface when analyte transfer occurred was determined. A thermodynamic study and enthalpy-entropy compensation were performed to further explore the interaction mechanism. From these results, it was shown that teicoplanin was balanced between 2 conformational states characterized by distinct enantioselective properties. This approach indicates that liquid chromatography (LC) is a useful tool to extract physicochemical and molecular information from retention data. Thus, LC can be used as a complementary technique with the conventional techniques of molecular interaction analysis.     

LiVGe2O6, an anomalous quasi-1D, S = 1 system, as revealed by NMR

We report the results of 7Li nuclear magnetic resonance (NMR) studies of LiVGe2O6, a quasi-one-dimensional spin S = 1 model system, at low temperatures. Our data, including NMR spectra and the temperature dependence of the spin-lattice relaxation rate T-11, indicate that a first-order phase transition occurs at T(c) approximately 23 K. The NMR response of LiVGe2O6 below T(c) suggests that the ordered phase is antiferromagnetic and has unusual features. Possible reasons for this unexpected behavior are discussed.     

Theoretical and experimental study of differential group delay and polarization dependent loss of Bragg gratings written in birefringent fiber

In this paper, we completely study the wavelength dependency of differential group delay (DGD) and polarization dependent loss (PDL) for Bragg gratings written in birefringent fibers. Based on the coupled mode theory, we present analytical expressions for the evolution with wavelength of the transmission coefficient, the DGD and the PDL. The wavelength dependencies of these evolutions on the birefringence are then discussed. Experimental results are finally presented for an apodized FBG written in a bow-tie fiber. A very good agreement between theory and experience is reported, confirming the validity of the theoretical analysis.     

Diversity-orientated synthesis of 3,5-bis(arylamino)pyrazoles

The synthesis of differentially-substituted 3,5-bis(arylamino)pyrazoles has not yet been documented. During our investigation, we managed to develop a novel, entirely combinatorial synthesis of 3,5-bis(arylamino)pyrazoles relying on a simple one-pot two-step operation.     

Poly(4-vinylpyridine) catalyzed hydrolysis of methyl bromide to methanol and dimethyl ether

The hydrolysis of methyl bromide with neutral water is performed in the presence and absence of various amines in a batch reactor at different temperatures (50–125 °C). Screening of poly(4-vinylpyridine) as a potential reusable solid amine catalyst showed maximum efficiency. This significant enhancement in efficiency is due to the capture of HBr by solid PVP and remains phase-separated driving the reaction forward. The major advantage of this process is that the polymer can be easily regenerated and reused without loss of activity making it a very effective catalyst for the conversion of methyl halides to methanol and dimethyl ether.     

Syntheses and conformations of some cyclic hydroxamates. X-Ray crystal structure of 2-(p-nitrobenzoyl)tetrahydro-2H-1,2-oxazine

Alkylation of potassium p-nitrobenzohydroxamate with 1,4-dibromobutane gave 2-(p-nitrobenzoyl)tetrahydro-2H-1,2-oxazine (3). The X-ray crystal structure of 3 has been determined. The crystals are monoclinic, space group P2₁/n with a = 6.749(1), b = 7.644(1), c = 21.557(2)Å, β = 98.89(1), V = 1098.8(2)ų and Z = 4. The structure, which was refined to R = 0.039 using 1340 observed reflections, shows the oxazine and carbonyl oxygen atoms trans to each other. Alkylation of potassium benzohydroxamate with 1,3-dibromobutane gave a mixture of 3-methyl-2-benzoyloxazolidine (4) and 5-methyl-2-benzoyloxazolidine (5). The ¹H and ¹³C nmr spectra of the mixture of 4 and 5 indicates that these cyclic hydroxamates exist predominantly in the s-trans conformation.     

Large deviations for the Boussinesq equations under random influences

A Boussinesq model for the Bénard convection under random influences is considered as a system of stochastic partial differential equations. This is a coupled system of stochastic Navier–Stokes equations and the transport equation for temperature. Large deviations are proved, using a weak convergence approach based on a variational representation of functionals of infinite-dimensional Brownian motion.     

Encapsulated clay particles in polystyrene by RAFT mediated miniemulsion polymerization

RAFT grafted montmorillonite (MMT) clays [i.e., N,N‐dimethyl‐N‐(4‐(((phenylcarbonothioyl)thio)methyl)benzyl)ethanammonium‐MMT (PCDBAB‐MMT) and N‐(4‐((((dodecylthio)carbonothioyl)thio)methyl)benzyl)‐N,N‐dimethylethanammo‐nium‐MMT (DCTBAB‐MMT)] of various loadings were dispersed in styrene (S) monomer and the resultant mixtures emulsified and sonicated in the presence of a hydrophobe (hexadecane) into miniemulsions. The stable miniemulsions thus obtained were polymerized to yield encapsulated polystyrene‐clay nanocomposites (PS‐CNs). The molar mass and polydispersity index (PDI) of the PS‐CNs depended on the amount of RAFT agent in the system, in accordance with the features of the RAFT process. The morphology of the PS‐CNs ranged from partially exfoliated to an intercalated morphology, depending on the percentage clay loading. The thermomechanical properties of the PS‐CNs were better than those of the neat PS polymer, and were dependent on the molar mass, PS‐CN morphology and clay loading. The similarities and differences of the PS‐CNs prepared here by miniemulsion polymerization were compared to those prepared using the same RAFT agents and polymer system by bulk polymerization (as reported by us in a previous article). © Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 46: 7114–7126, 2008     

Using apparent molecular weight from SEC in controlled/living polymerization and kinetics of polymerization

Apparent molecular weights from size exclusion chromatography, that is molecular weights relative to standards of a nature different to that of the polymer sample being studied, are frequently used. We use calculations corresponding to realistic cases to provide guidelines for situations when, and to what extent, apparent molecular weights (MWs) can be meaningful. In controlled polymerization, we show how, without due care, use of apparent MW, could lead to the incorrect conclusion that the reaction was not controlled, whereas the true MWs would be close to theoretical values. We show here that the quality of the eluent as a solvent for the standard and the polymer sample is a good indication of the accuracy and the significance of the apparent polydispersity index. Accurate Mark–Houwink–Sakurada parameters are of limited availability, but the data about solvent quality available in handbooks or available from static light scattering measurements. Apparent M_n is of no use in controlled polymerization if simple simulations as performed in this work do not validate their use. The determination of transfer constants by the Mayo plot can be performed using apparent M_n without introducing any significant error, contrary to apparent weight‐average molecular weight M_w or apparent ln number distribution. © 2007 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 46: 897–911, 2008