Determination of antimony in rain water at the nanogram level with surfactant and brilliant green

A new, simple, selective and sensitive method for the spectrophotometric determination of antimony in rain water is described. It includes preconcentrating Sb with surfactants (i.e. cetylpyridinium chloride (CPC) and Triton X-100 (TX-100)) into toluene and allowing the extract to react with a dye, i.e. brilliant green (BG). The value of apparent molar absorptivity is 5.55 × 10⁵ L-mol^–1· cm^–1 at λ_max = 620 nm; the detection limit is 3 ng/mL Sb in rain water at 3-fold preconcentration. Received: 6 June 1997 / Revised: 1 August 1997 / Accepted: 5 August 1997     

Small molecule matrix-assisted laser desorption/ionization time-of-flight mass spectrometry using a polymer matrix

We have employed a light-absorbing electrically conductive polymer as a matrix to determine the molecular mass of small organic molecules using matrix-assisted laser desorption/ionization time-of-flight (MALDI-TOF) mass spectrometry. This method, which is in contrast to the usual MALDI strategy for matrix selection in which a small molecule matrix is used with a high molecular mass analyte, addresses the problem of matrix interference which limits the usefulness of MALDI-TOF for small molecule analysis. Use of negative ion mode offers advantages for this application. Using this approach, we have obtained clean molecular ion mass spectra of small organic molecules in the mass range 100-300 Da.     

Mesoscale model of polymer melt structure: self-consistent mapping of molecular correlations to coarse-grained potentials

Development and application of coarse-graining methods to condensed phases of macromolecules is an active area of research. Multiscale modeling of polymeric systems using coarse-graining methods presents unique challenges. Here we apply a coarse-graining method that self-consistently maps structural correlations from detailed molecular dynamics (MD) simulations of alkane oligomers onto coarse-grained potentials using a combination of MD and inverse Monte Carlo methods. Once derived, the coarse-grained potentials allow computationally efficient sampling of ensemble of conformations of significantly longer polyethylene chains. Conformational properties derived from coarse-grained simulations are in excellent agreement with experiments. The level of coarse graining provides a control over the balance of computational efficiency and retention of chemical identity of the underlying polymeric system. Challenges to extension and application of this and similar structure-based coarse-graining methods to model dynamics and phase behavior in polymeric systems are briefly discussed.     

Erratum to: Identification of a new source of interference leached from polypropylene tubes in mass-selective analysis

An interference leached from polypropylene tubes was identified to be a sulfoxide oxidative product of didodecyl 3,3′-thiodipropionate (DDTDP) that is used to prevent oxidative degradation of synthetic polymers. A sulfone oxidative product of DDTDP leached from the polypropylene tubes was also observed. The interfering compounds were isolated by LC and characterized using time-of-flight mass spectrometry and NMR. Authentic sulfoxide and sulfone products of DDTDP were also prepared by reacting DDTDP with hydrogen peroxide reaching an unequivocal structural assignment. In conclusion, when analytes of interest are solubilized in predominantly organic solvents and kept in polypropylene containers, the possibility of contamination from leached chemicals should be taken into account.     

Identification of a new source of interference leached from polypropylene tubes in mass-selective analysis

An interference leached from polypropylene tubes was identified to be a sulfoxide oxidative product of didodecyl 3,3'-thiodipropionate (DDTDP) that is used to prevent oxidative degradation of synthetic polymers. A sulfone oxidative product of DDTDP leached from the polypropylene tubes was also observed. The interfering compounds were isolated by LC and characterized using time-of-flight mass spectrometry and NMR. Authentic sulfoxide and sulfone products of DDTDP were also prepared by reacting DDTDP with hydrogen peroxide reaching an unequivocal structural assignment. In conclusion, when analytes of interest are solubilized in predominantly organic solvents and kept in polypropylene containers, the possibility of contamination from leached chemicals should be taken into account.     

(3 + 1) Assembly of three human telomeric repeats into an asymmetric dimeric G-quadruplex

We present an NMR study on the structure of a DNA fragment of the human telomere containing three guanine-tracts, d(GGGTTAGGGTTAGGGT). This sequence forms in Na(+) solution a unique asymmetric dimeric quadruplex, in which the G-tetrad core involves all three G-tracts of one strand and only the last 3'-end G-tract of the other strand. We show that a three-repeat human telomeric sequence can also associate with a single-repeat human telomeric sequence into a structure with the same topology that we name (3 + 1) quadruplex assembly. In this G-quadruplex assembly, there are one syn.syn.syn.anti and two anti.anti.anti.syn G-tetrads, two edgewise loops, three G-tracts oriented in one direction and the fourth oriented in the opposite direction. We discuss the possible implications of the new folding topology for understanding the structure of telomeric DNA, including t-loop formation, and for targeting G-quadruplexes in the telomeres.     

E.S.R., magnetic, electronic and superoxide dismutase studies of imidazolate-bridged Cu(II)-Cu(II) complexes with ethylenediamine as capping ligand

X-band E.S.R., magnetic and electronic spectra of some imidazolate-bridged homometallic complexes [(en)2Cu-R-Im-Cu(en)2](ClO4)3 where en, ethylenediamine; R-ImH, R = H imidazole (ImH); if R = CH3, 2-methylimidazole (M-ImH) and if R = C2H5, 2-ethylimidazole (E-ImH), and mononuclear complexes [(en)Cu-dien](ClO4)2 and [(en)Cu-PMDT](ClO4)2 where dien, diethylenetriamine; PMDT, pentamethyldiethylenetriamine have been described. Superoxide dismutase (SOD) activity has also been measured and compared with earlier reported complexes. In frozen solution at 77 K, the spectra show axial symmetry with a d(x2-y2) ground state. Difference in lambda(max) between mononuclear and binuclear complexes was found to be approximately 65-75 nm. Magnetic susceptibility and E.S.R. spectral measurements for all these binuclear complexes revealed that the copper(II) ions are involved in antiferromagnetic exchange interactions propagated by the imidazolate bridge.     

Polychalcone derived from 8-acetoxyquinoline-5-aldehyde

Fries rearrangement of 8-acetoxyquinoline-5-aldehyde (aqa) in nitrobenzene using AlCl₃ as catalyst affords the polychalcone poly(8-hydroxy-5,7-quinolinylenecarbonylvinylene) (phqcv). Thephqcv samples were characterized by elemental analysis,ir spectroscopy, molecular weight, intrinsic viscosity andtga. Polymeric metal chelates of Cu²⁺, Fe³⁺, Co²⁺, Ni²⁺, Mn²⁺, Zn²⁺ and UO ₂ ²⁺ withphqcv polychalcone were prepared and characterized.     

Studies of copper(II) chelates ofo-hydroxyacetophenone and its substituted derivatives in dioxane-water mixtures

Summary The interactions of Cu^II witho-hydroxyacetophenone (OHA) and its halogeno- and nitro-substituted derivatives were investigated potentiometrically in dioxane-water mixtures by the Calvin-Bjerrum titration technique at 25° C and ionic strength =0.1 M (NaClO₄). All the ligands form 11 and 12 chelates with Cu^II.The variations of pK with mole fraction of dioxane and 1/D were almost linear. Linearity was observed in the plots of log K₁/logK₂ versus pK, and the magnitudes of slopes led to the conclusion that -donation is: (i) strong in aqueous medium and decreases with the increase in dioxane content in 11 chelates, and (ii) weak and does not change with dioxane, in 12 chelates.