Development of acrylate-based monolithic stationary phases for electrochromatographic separations

Organic monolithic stationary phases were synthesized in fused-silica capillaries. They were prepared by in situ polymerization under UV irradiation of various alkyl acrylates, 1,3-butanediol diacrylate, and 2-acrylamido-2-methyl-1-propanesulfonic acid in a ternary porogenic solvent. The resulting stationary phases were tested in CEC. The influence of UV irradiation energy on the resulting separative performances of the monoliths was studied. It was thus demonstrated that the use of hexyl acrylate rather than butyl acrylate and lauryl methacrylate gives highly efficient monoliths (more than 300 000 plates per meter) with optimized EOF. It was also confirmed that the mobile phase ionic strength may affect significantly the separation efficiency. The influence of the nature of the mobile phase organic modifier (ACN or methanol) on EOF, retention, efficiency, and selectivity was studied and differences were observed. Finally, the performances of monolithic stationary phases developed and optimized for CEC separations were evaluated in nanoLC.     

Towards accurate all-electron quantum Monte Carlo calculations of transition-metal systems: spectroscopy of the copper atom

In this work we present all-electron fixed-node diffusion Monte Carlo (FN-DMC) calculations of the low-lying electronic states of the copper atom and its cation. The states considered are those which are the most relevant for the organometallic chemistry of copper-containing systems, namely, the (2)S, (2)D, and (2)P electronic states of Cu and the (1)S ground state of Cu(+). We systematically compare our FN-DMC results to CCSD(T) calculations using very large atomic-natural-orbital-type all-electron basis sets. The FN-DMC results presented in this work provide, to the best of our knowledge, the most accurate nonrelativistic all-electron correlation energies for the lowest-lying states of copper and its cation. To compare our results to experimental data we include the relativistic contributions for all states through numerical Dirac-Fock calculations, which for copper (Z=29) provide almost the entire relativistic effects. It is found that the fixed-node errors using Hartree-Fock nodes for the lowest transition energies of copper and the first ionization potential of the atom cancel out within statistical fluctuations. The overall accuracy achieved with quantum Monte Carlo for the nonrelativistic correlation energy (statistical fluctuations of about 1600 cm(-1) and near cancelation of fixed-node errors) is good enough to reproduce the experimental spectrum when relativistic effects are included. These results illustrate that, despite the presence of the large statistical fluctuations associated with core electrons, accurate all-electron FN-DMC calculations for transition metals are nowadays feasible using extensive but accessible computer resources.     

New data on the solubility of amorphous silica in organic compound-water solutions and a new model for the thermodynamic behavior of aqueous silica in aqueous complex solutions

Experimental solubilities of amorphous silica in several aqueous electrolyte solutions and in aqueous solutions of organic compounds, and theoretical considerations concerning cavity formation, electrostriction collapse, ion solvation, and long- and short-range interaction of the solvated ions with one another⁽¹⁾ permit the calculation of the partial excess free energies and the activity coefficients of aqueous silica. It is shown that, in the case of non-dissociated aqueous organic solutions, the variation of log m (SiO₂) with the reciprocal of the dielectric constant of the solution is described by a single linear equation independent of the nature of the organic compound. For aqueous electrolyte solutions, a specific linear relationship between log m (SiO₂) and the reciprocal of the dielectric constant occurs for each electrolyte. The success of the equation in reproducing the experimental solubilities of amorphous silica in aqueous solutions of electrolytes and organic compounds supports previous evidence indicating a polar charge distribution in the solvated SiO₂ molecule. Our data permit the calculation of the effective local charge of dissolved SiO₂ molecules and of the short-range interaction parameters between SiO₂ and various ions. The proposed equation of state can be used to calculate the affinity of reactions among SiO₂ minerals and complex aqueous solutions.     

Inhibition of mitosis by glycopeptide dendrimer conjugates of colchicine

Glycopeptide dendrimers have been prepared bearing four or eight identical glycoside moieties at their surface (beta-glucose, alpha-galactose, alpha-N-acetyl-galactose, or lactose), natural amino acids within the branches (Ser, Thr, His, Asp, Glu, Leu, Val, Phe), 2,3-diaminopropionic acid as the branching unit, and a cysteine residue at the core. These dendrimers have been used as drug-delivery devices for colchicine. Colchicine was attached to the dendrimers at the cysteine thiol group through a disulfide or thioether linkage. The biological activities of the glycopeptide dendrimer conjugates were evaluated in HeLa tumor cells and non-transformed mouse embryonic fibroblasts (MEFs). The concentrations of glycopeptide dendrimer drug conjugates required to achieve inhibition of cell proliferation by interference with the tubulin system were found to be higher (IC50 > 1 microM) compared to the required colchicine concentration. On the other hand, the glycopeptide dendrimer conjugates inhibited the proliferation of HeLa cells 20-100 times more effectively than the proliferation of MEFs. In comparison, non-glycosylated dendrimers and colchicine itself showed a selectivity of 10-fold or less for HeLa cells.     

Electrochemical insertion of lithium into the ramsdellite-type oxide Li2Ti3O7: influence of the Li2Ti3O7 particle size

The effect of a milling process on the electrochemical performance of Li₂Ti₃O₇ electrodes has been investigated by the galvanostatic intermittent titration technique (GITT) and AC impedance spectroscopy. The insertion ratio is slightly increased by the milling treatment and a value of x _Li=1.25 per mol Li₂Ti₃O₇ has been determined. The average potential during insertion is close to 1.5 V/Li. The analysis of impedance data obtained at equilibrium during insertion and deinsertion shows two relaxation processes and a diffusion phenomenon at low frequency according to the Frumkin-Melik-Gayakazian model. Cycling experiments of batteries using this material were performed with unmilled and milled particles. Composite electrodes containing different amounts of electroactive material added to a binder and a conductive additive have also been prepared in order to check the effect of grinding on the cyclability of the compound. Interesting electrochemical performances have been determined with such electrodes: lithium uptake up to 1.25 Li per Li₂Ti₃O₇, low irreversible capacity loss between the first and the following cycles, good stability upon cycling even after 50 cycles. However, the milled process has not improved significantly the electrochemical performance of the Li₂Ti₃O₇ electrodes. Electronic Publication     

Strategies for developing NADH detectors based on Meldola Blue and screen-printed electrodes: a comparative study

Three strategies have been compared to produce screen-printed amperometric detectors for NADH: mixing Meldola Blue (MB) in the screen-printing ink, incorporation of MB-Reinecke salt (MBRS) in the graphite ink and electrodeposition of films of MB-derived polymer (poly (MB)) on electrode surface. Following modification of graphite electrodes the mediators displayed values of the formal potential E°′ from −0.129 to −0.160 V vs. Ag/AgCl and pK_as of 5.09-6.02. A second redox couple with E°′=−0.450 V vs. Ag/AgCl was observed in cyclic voltammetry experiments with poly (MB) sensors or with old electrodes obtained according to the other two strategies. Electropolymerisation of MB allowed to achieve the best operational stability and best detection limit, 2×10⁻⁶ M, for amperometric detection of NADH, while the most extended linear range, 1×10⁻⁵-7.5×10⁻⁴ M, corresponds to sensors with MBRS. MB and MBRS electrodes were compared with a similar NADH detector produced by Gwent Electronic Materials, England. Several characteristics of the modified-electrodes induced by the fabrication by screen-printing were also highlighted.     

Selective recognition of G-qQuadruplex telomeric DNA by a bis(quinacridine) macrocycle

The interaction of G-quadruplex DNA with the macrocyclic compound BOQ1, which possesses two dibenzophenanthroline (quinacridine) subunits, has been investigated by a variety of methods. The oligonucleotide 5'-A(GGGT(2)A)(3)G(3), which mimics the human telomeric repeat sequence and forms an intramolecular quadruplex, was used as one model system. Equilibrium binding constants measured by biosensor surface plasmon resonance (SPR) methods indicate a high affinity of the macrocycle for the quadruplex conformation (K > 1 x 10(7) M(-)(1)) with two equivalent binding sites. The affinity of BOQ1 for DNA duplexes is at least 1 order of magnitude lower. In addition, the macrocycle is more selective than the monomeric control compound (MOQ2), which is not able to discriminate between the two DNA structures (K(duplex) approximately K(quadruplex) approximately 10(6) M(-)(1)). Strong binding of BOQ1 to G4 DNA sequences was confirmed by fluorometric titrations with a tetraplex-forming oligonucleotide. Competition dialysis experiments with a panel of different DNA structures, from single strands to quadruplexes, clearly established the quadruplex binding specificity of BOQ1. Fluorescence resonance energy transfer (FRET) T(m) experiments with a doubly labeled oligonucleotide also revealed a strong stabilization of the G4 conformation in the presence of BOQ1 (DeltaT(m) = +28 degrees C). This DeltaT(m) value is one of the highest values measured for a G-quadruplex ligand and is significantly higher than observed for the monomer control compounds (DeltaT(m) = +10-12 degrees C). Gel mobility shift assays indicated that the macrocycle efficiently induces the formation of G-tetraplexes. Strong inhibition of telomerase was observed in the submicromolar range (IC(50) = 0.13 microM). These results indicate that macrocycles represent an exciting new development opportunity for targeting DNA quadruplexes.     

Immobilized respiratory chain activities fromEscherichia coli utilized to measure d and l-lactate,succinate, l-malate, 3-glycerophosphate,pyruvate, or NAD(P)H

The respiratory chain (membranous, multienzymatic system) from Escherichia coli, was coimmobilized with gelatin and insolubilized in film form by tanning with glutaraldehyde. The film was fixed onto an oxygen sensor. The enzyme electrode can be used for measuring NAD(P)H, D- and L-lactate, succinate, L-malate, 3-glycerophosphate, or pyruvate. The range of metabolites concentrations was from 1 to 50 mM. It was possible to discriminate between the different metabolites (if mixed): By inducing during bacterial growth the specific flavoproteins necessary for L-lactate, succinate, L-malate, and 3-glycerophosphate respirations. The constitutive activities are unaltered on glucose or glycerol, namely D-lactate, NAD(P)H, and pyruvate respiration. When intact bacteria were immobilized (with or without induction), D- and L-lactate, succinate, 3-glycerophosphate, and L-malate respiration were measured, no activities of pyruvate and NAD(P)H respiration were obtained. For these last activities, French press breakage (see section on Membrane Preparations) of bacteria prior to immobilization was necessary. Products of reactions can be used as enzyme inhibitors: Pyruvate inhibits D- and L-lactate; fumarate inhibits succinate, and oxaloacetate inhibits L-malate respirations. Heat denaturation of the bacteria at 55 degrees C for 1 h maintains full activity of succinate and pyruvate respiration. On the other hand, no activity of D- and L-lactate, L-malate, or NAD(P)H respiration was measurable. These enzyme electrodes have many applications in basic and applied research.     

Proton NMR spectroscopy and magnetic properties of a solution-stable dicopper(II) complex bearing a single mu-hydroxo bridge

The reaction of copper(II) perchlorate with the macrocyclic ligand [22]py4pz in the presence of base leads to formation of a dinuclear complex [Cu(2)([22]py4pz)(mu-OH)](ClO(4))(3)xH(2)O, in which two copper ions are bridged by a single mu-hydroxo bridge. Each copper ion is further surrounded by four nitrogen atoms of the ligand. The mu-hydroxo bridge mediates a strong antiferromagnetic coupling (2J = -691(35) cm(-1)) between the metal centers, leading to relatively sharp and well-resolved resonances in the (1)H NMR spectrum of the complex in solution. We herein report the crystal structure, the magnetic properties, and the full assignment of the hyperfine-shifted resonances in the NMR spectrum of the complex, as well as the determination of the exchange coupling constant in solution through temperature-dependent NMR studies.     

Electrochromatography in poly(dimethyl)siloxane microchips using organic monolithic stationary phases

This paper shows the in situ synthesis of an hexyl acrylate monolith in PDMS microfluidic devices and its subsequent use as stationary phase for electrochromatography on chip. To overcome the ability of PDMS material to absorb organic monomers, surface modification of the enclosed channels was realized by UV-mediated graft polymerization. This grafting procedure is based on the preliminary adsorption of a photoinitiator onto the PDMS surface and polymerization of charged monomers. Next, hexyl acrylate monoliths were cast in situ using photopolymerization process. The chromatographic behavior of the monolithic column was confirmed by the successful separation of derivatized catecholamines in the PDMS device using a 30 mm effective separation length (100 microm x 100 microm section). Efficiencies reached up to 200,000 plates per meter.