Simultaneous determination of arsenic and antimony in environmental samples by radiochemical neutron activation analysis

A radiochemical separation procedure using an inorganic exchanger, tin dioxide (TDO), for the separation of arsenic from antimony is reported here. This separation avoids the interference of 564 keV gamma-ray of¹²²Sb in the measurement of the 559 keV gamma-ray of⁷⁶As in neutron activation analysis. Environmental samples, after neutron irradiation and digestion, are taken up in 1M HCl–0.1M HF and passed through a TDO column which selectively retains arsenic. The effluent from the TDO column, after proper conditioning, is passed through an anion exchange column for quantitative retention of antimony. The procedure has been utilized for arsenic and antimony determination in NBS Orchard Leaves and NBS Albacore Tuna.     

Preparation and Crystal Structure of (Ph4P)2Te4 · 2 CH3OH

The Zintl phase K₄SnTe₄ reacts with tetraphenylphosphonium bromide in methanol to give the polytelluride (Ph₄P)₂Te₄ · 2 CH₃OH. The compound crystallizes in the monoclinic space group C2/c with lattice constants a = 2329.6(11) pm, b = 1472.0(6) pm, and c = 1461.9(6) pm with ß = 111.34(3)°. Significant structural units are tetratelluride(2?) anions with CH₃OH molecules hydrogen bonded to both ends of the anion.     

Hydrothermal Syntheses and Structural Characterization of Layered Vanadium Oxides Incorporating Organic Cations: alpha-, beta-(H(3)N(CH(2))(2)NH(3))[V(4)O(10)] and alpha-, beta-(H(2)N(C(2)H(4))(2)NH(2))[V(4)O(10)

Four new layered mixed-valence vanadium oxides, which contain interlamellar organic cations, alpha-(H(3)N(CH(2))(2)NH(3))[V(4)O(10)] (1a), beta-(H(3)N(CH(2))(2)NH(3))[V(4)O(10)] (1b), alpha-(H(2)N(C(2)H(4))(2)NH(2))[V(4)O(10)] (2a), and beta-(H(2)N(C(2)H(4))(2)NH(2))[V(4)O(10)] (2b), have been prepared under hydrothermal conditions and their single-crystal structures determined: 1a, triclinic, space group P&onemacr;, a = 6.602(2) ?, b = 7.638(2) ?, c = 5.984(2) ?, alpha = 109.55(3) degrees, beta = 104.749(2) degrees, gamma = 82.31(3) degrees, Z = 1; 1b, triclinic, P&onemacr;, a = 6.387(1) ?, b = 7.456(2) ?, c = 6.244(2) ?, alpha = 99.89(2) degrees, beta = 102.91(2) degrees, gamma = 78.74(2) degrees, Z = 1; 2a, triclinic, P&onemacr;, a = 6.3958(5) ?, b = 8.182(1) ?, c = 6.3715(7) ?, alpha = 105.913(9) degrees, beta = 104.030(8) degrees, gamma = 94.495(8) degrees, Z = 1; 2b, monoclinic, space group P2(1)/n, a = 9.360(2) ?, b = 6.425(3) ?, c = 10.391(2) ?, beta = 105.83(1) degrees, Z = 2. All four of the compounds contain mixed-valence V(5+)/V(4+) vanadium oxide layers constructed from V(5+)O(4) tetrahedra and pairs of edge-sharing V(4+)O(5) square pyramids with protonated organic amines occupying the interlayer space.     

Characterization of partially transesterified poly(beta-hydroxyalkanoate)s using matrix-assisted laser desorption/ionization time-of-flight mass spectrometry

Matrix-assisted laser desorption/ionization time-of-flight mass spectrometry (MALDI-TOFMS) was used for the characterization of a partially transesterified poly(beta-hydroxyalkanoate), PHA, polymer produced by the bacterial strain Alcaligenes eutrophus using saponified vegetable oils as the sole carbon sources. The transesterification was carried out separately under acidic and basic conditions to obtain PHA oligomers weighing less than 10 kDa. The intact oligomers were detected in their cationized [M + Na](+) and [M + K](+) forms by MALDI-TOFMS. A composition analysis, using the MALDI-TOF spectra, indicate that the oligomers obtained via acid catalysis were terminated with a methyl 3-hydroxybutyrate end group, and those obtained by base catalysis had a methyl crotonate (olefinic) termination. In addition to HB (hydroxy butyrate), the oligomers were found to contain a small percentage of HV (hydroxy valerate). This was independently confirmed using gas chromatography/mass spectrometry (GC/MS). In comparison, the analysis of a commercial PHA polymer, transesterified under identical conditions, only showed the presence of HB, i.e. a pure PHB homopolymer. Copyright 1999 John Wiley & Sons, Ltd.     

Direct visualization of a DNA glycosylase searching for damage

DNA glycosylases preserve the integrity of genetic information by recognizing damaged bases in the genome and catalyzing their excision. It is unknown how DNA glycosylases locate covalently modified bases hidden in the DNA helix amongst vast numbers of normal bases. Here we employ atomic-force microscopy (AFM) with carbon nanotube probes to image search intermediates of human 8-oxoguanine DNA glycosylase (hOGG1) scanning DNA. We show that hOGG1 interrogates DNA at undamaged sites by inducing drastic kinks. The sharp DNA bending angle of these non-lesion-specific search intermediates closely matches that observed in the specific complex of 8-oxoguanine-containing DNA bound to hOGG1. These findings indicate that hOGG1 actively distorts DNA while searching for damaged bases.     

More efficient Brownian dynamics algorithms

We examine the relative efficiencies of three- algorithms for performing Brownian Dynamics simulations without many-body hydrodynamics. We compare the conventional Brownian Dynamics algorithm of Ermak (CBD), Smart Monte Carlo (SMC) which incorporates Boltzmann sampling into essentially a CBD procedure, and the Stochastic Runge Kutta (SRK) method. We show, using the repulsive potential φ(r) = ε(σ/r) ⁿ , where n = 36 and 72, that the SRK algorithm gives the most accurate short-time dynamics for the mean-square displacements. The SRK algorithm static and dynamical properties converge better with a reducing time step to the exact values, than those generated by the CBD algorithm; giving efficiency gains typically of a factor of 3–4. Both CBD and SMC have the incorrect sign for the first correction term to the mean square displacement in a time step, whereas the SRK algorithm gives essentially the exact solution to order Δt ², where Δt is the simulation time step. In fact, these correction terms are almost equal and opposite in sign. Expressions for these terms were derived in terms of the average interaction energy per particle. The force, shear and bulk stress autocorrelation functions were calculated. The average energy per particle and time correlation functions at short time have values in excess of the exact values, while the corresponding quantities for SRK are below this. This difference in behaviour can be traced back to the extent of compliance of the particle trajectories with the exact expansion of the Smoluchowski equation. The accuracy, at a given value of the time step, of the stochastic algorithms can significantly depend on the form of the interaction potential between particles. It is also demonstrated that the long time limits of various correlation functions are fairly insensitive to a particular scheme (SRK or CBD) used in the simulations. All the correlation functions have a stretched exponential region at intermediate to long times, and the values of the exponents on density and force law steepness have been determined.