A simple differential titrator for automatic potentiometric titration at zero current, with two identical indicator electrodes

A simple differential titrator for automatic potentiometric titration at zero current, with two identical indicator electrodes, is described. The autoburette is a normal microburette connected to one tube of a peristaltic pump, and the differentiating system is a closed-flow circuit in a second tube of the pump, this tube containing two electrodes 5 cm apart. The apparatus has been applied to the determination of halides and sulphide-disulphide mixtures, with electrodes coated with an appropriate silver salt.     

Tungsten-mannitol and sorbitol complexes: Structural characterization by IR and carbon-13 nuclear magnetic resonance spectroscopy

Polarimetric and spectrophotometric studies of the mannitol-tungsten(IV) system show the formation of three table complexes. Their stoichiometries, degrees of condensation and interconversion equilibria have been studied. The structure of these compounds has been investigated by ¹³C NMR spectroscopy in aqueous solution and by IR spectroscopy in the solid state for the two species stable at pH < 7.     

Partial filling multiple injection affinity capillary electrophoresis (PFMIACE) to estimate binding constants of receptors to ligands

Partial filling multiple injection affinity capillary electrophoresis (PFMIACE) is used to determine binding constants between vancomycin (Van) from Streptomyces orientalis, teicoplanin (Teic) from Actinoplanes teicomyceticus and ristocetin (Rist) from Nocardia lurida to d-Ala-d-Ala terminus peptides and carbonic anhydrase B (CAB, E.C.4.2.1.1) to arylsulfonamides. Two variations of PFMIACE are described herein. In the first technique, the capillary is partially filled with ligand at increasing concentrations, a non-interacting standard, three or four separate plugs of receptor each separated by small plugs of buffer, a plug containing a second non-interacting standard, and then electrophoresed in buffer. Upon continued electrophoresis, equilibrium is established between the ligand and receptors causing a shift in the migration time of the receptors with respect to the non-interacting standards. This change in migration time is utilized for estimating multiple binding constants (K_b) for the same interaction. In the second technique, separate plugs of sample containing non-interacting standards, peptide one, buffer, and peptide two, were injected into the capillary column. The capillary is partially filled with a series of buffers containing an antibiotic at increasing concentrations and electrophoresed. Peptides migrate through the column at similar electrophoretic mobilities since their charge-to-mass ratios are approximately the same but remain as distinct zones due to the buffer plug between peptides. Upon electrophoresis, the plug of antibiotic flows into the peptide plugs affecting a shift in the migration time of the peptides with respect to the non-interacting standards occurs due to formation of the of the antibiotic-peptide complex. The shift in the migration time of the peptides upon binding to the antibiotic is used for the Scatchard analysis and measurement of a K_b. The PFMIACE technique expands the functionality and potential of ACE as an analytical tool to examine receptor-ligand interactions. In PFMIACE, a smaller amount of sample is required in the assay compared to both conventional ACE and MIACE. Furthermore, a wide array of data is obtained from a single experiment, thus, expediting the assay of biological species.     

Multinuclear NMR studies and ab initio structure calculations of hindered phencyclone diels‐alder adducts from symmetrical cyclic dienophiles: Cyclohexene,vinylene carbonate,vinylene trithiocarbonate and two N‐aryl maleimides

A series of hindered Diels‐Alder adducts have been prepared from phencyclone, 1 , with various unusual symmetrical cyclic dienophiles, including cyclohexene, 2a ; vinylene carbonate, 2b ; vinylene trithiocarbonate, 2c ; and the N‐aryl maleimides: N‐(4‐dimethylamino‐3,5‐dinitrophenyl)maleimide (“Tuppy's maleimide”), 2d ; and N‐[3,5‐bis(trifluoromethyl)phenyl]maleimide, 2e . The highly hindered adducts, 3a‐e , respectively, were extensively characterized by one‐ and two‐dimensional NMR methods, observing proton, carbon‐13 and fluorine‐19. High resolution COSY45 spectra permitted rigorous proton NMR assignments. The 2D heteronuclear C‐H chemical shift correlation spectra (HETCOR, XHCORR) were obtained for adducts 3a‐d , allowing specific assignments for protonated carbons. Corrections to earlier proton NMR assignments for the vinylene carbonate adduct are given; results of the gated decoupling ¹³C NMR experiment for this adduct supported endo adduct stereochemistry. Relative proton chemical shifts for bridgehead phenyls of adduct 3c appeared anomalous relative to other adducts, suggesting possible special anisotropic interactions (with endocyclic sulfur or other anisotropic groups in the product) due to the unusual calculated orientation of the phenyls. The unsubstituted bridgehead phenyls in all adducts were shown to exhibit slow exchange limit (SEL) ¹H and ¹³C spectra on the NMR timescales at ambient temperatures (7 tesla) showing slow rotations about the C(sp³)‐C(aryl sp²) bonds. The rapid rotation of the N‐aryl rings of the maleimide adducts was indicated by fast exchange limit spectra, suggesting that ortho substitution of the N‐aryl ring may be necessary to slow this rotation to the SEL regime. Ab initio geometry optimizations at the Hartree‐Fock level were carried out for each adduct, with the 6‐31G* basis sets. Appreciable geometry differences were seen in calculated structures, and significant NMR chemical shift differences were experimentally observed, depending on the nature of the groups attached to the (Z)‐HC=CH moiety of the dienophiles.     

Compounds with the YbFe2O4Structure Type: Frustrated Magnetism and Spin-Glass Behavior

The YbFe₂O₄structure type consists of triangular layers of lanthanide oxygen octahedra stacked with triangular double layers of transition metal oxygen triangular bipyramids. The crystallographic structures determined by neutron diffraction powder profile analysis at 300 and 11 K for new members of this structural family are reported. The compounds are found to be magnetically frustrated, by both lattice geometry and disorder. The magnetic properties of YbCuGaO₄, LuCuGaO₄, LuZnFeO₄, LuCoGaO₄, and LuCuFeO₄reveal the effects of total spin, spin mixing, and interaction between spins on different sublattices on the magnetic frustration. The magnetism is increasingly frustrated as the spin on the magnetic ions is decreased.     

Phototoxic effects of Hypericum extract in cultures of human keratinocytes compared with those of psoralen

Extracts of Hypericum perforatum (St. John's wort) are used in the treatment of depression. They contain the plant pigment hypericin and hypericin derivates. These compounds have light-dependent activities. In order to estimate the potential risk of phototoxic skin damage during antidepressive therapy, we investigated the phototoxic activity of hypericin extract using cultures of human keratinocytes and compared it with the effect of the well-known phototoxic agent psoralen. The absorbance spectrum of our Hypericum extract revealed maxima in the whole UV range and in parts of the visible range. We cultivated human keratinocytes in the presence of different Hypericum concentrations and irradiated the cells with 150 mJ/cm2 UVB, 1 J/cm2 UVA or 3 h with a white light of photon flux density 2.6 mumol m-2 s-1. The determination of the bromodeoxyuridine incorporation rate showed a concentration- and light-dependent decrease in DNA synthesis with high hypericin concentrations (> or = 50 micrograms/mL) combined with UVA or visible light radiation. In the case of UVB irradiation a clear phototoxic cell reaction was not detected. We found phototoxic effects even with 10 ng/mL psoralen using UVA with the same study design as in the case of the Hypericum extract. These results confirm the phototoxic activity of Hypericum extract on human keratinocytes. However, the blood levels that are to be expected during antidepressive therapy are presumably too low to induce phototoxic skin reactions.     

Density functional theory of solvation and its relation to implicit solvent models

We describe a density functional theory approach to solvation in molecular solvents. The solvation free energy of a complex solute can be obtained by direct minimization of a density functional, instead of the thermodynamic integration scheme necessary when using atomistic simulations. In the homogeneous reference fluid approximation, the expression of the free-energy functional relies on the knowledge of the direct correlation function of the pure solvent. After discussing general molecular solvents, we present a generic density functional describing a dipolar solvent and we show how it can be reduced to the conventional implicit solvent models when the solvent microscopic structure is neglected. With respect to those models, the functional includes additional effects such as the microscopic structure of the solvent, the dipolar saturation effect, and the nonlocal character of the dielectric constant. We also show how this functional can be minimized numerically on a three-dimensional grid around a solute of complex shape to provide, in a single shot, both the average solvent structure and the absolute solvation free energy.     

Single‐electron‐transfer/degenerative‐chain‐transfer mediated living radical polymerization of vinyl chloride catalyzed by thiourea dioxide/octyl viologen in water/tetrahydrofuran at 25 °C

The single‐electron‐transfer/degenerative‐chain‐transfer mediated living radical polymerization (SET–DTLRP) of vinyl chloride (VC) in H₂O/tetrahydrofuran at 25 °C catalyzed by thiourea dioxide [(NH₂)₂C?SO₂] is reported. This polymerization occurs only in the presence of a basic sodium bicarbonate (NaHCO₃) buffer and the electron‐transfer cocatalyst octyl viologen. The resulting poly(vinyl chloride) (PVC) has a number‐average molecular weight of 1500–7000 and a weight‐average molecular weight/number‐average molecular weight ratio of 1.5. This PVC does not contain detectable amounts of structural defects and has both active chloroiodomethyl and inactive chloromethyl chain ends. Because of possible side reactions caused by the primary sulfoxylate anion (SO), the catalytic activity of (NH₂)₂C?SO₂ in the SET–DTLRP of VC is lower than that of the single‐electron‐transfer agent sodium dithionite. © 2004 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 43: 287–295, 2005     

Synthesis and spectral properties of new complexes between glycine and titanium(III), vanadium(III), chromium(III), iron(III), cobalt(II), nickel(II) and copper(II)

Summary Complexes of formula [MCl₃(glyH)₃] (M=Ti, V and Fe) [CrCl₃(glyH)₂H₂O], [MCl₂(glyH)₂(H₂O)₂] (M=Co and Cu) and [NiCl₂(glyH)₃H₂O] have been prepared and characterized by potentiometric curves, chemical analysis, magnetic properties, i.r. and electronic spectral data.