Separation of nucleoside 5′-phosphordiamidates by thin-layer chromatography with strong cation-exchange resin containing layers

Summary The application of the commercially available cation-exchange thin-layer chromatoplate (Fixion 50-X8) for the separation of nucleoside 577/xxlarge8242.gif" alt="prime" align="BASELINE" BORDER="0">-phosphordiamidates is described.Publication No. 12 in the series dealing with the separation of nucleic acid bases, nucleosides and nucleotides on thin layers consisting of strong cation-exchange resins.     

Nonlinear approximation theory on compact groups

In this article we extend to the setting of band-limited functions on compact groups previous results bounding from below the percentage of energy, contained in the low frequency portion of the spectrum of a positive function defined on a cyclic group. Connections to signal recovery for positive functions, as well as partial spectral analysis, are also discussed.     

Stereochemical study of the sterically crowded phenylselanylalkenes by means of 77Se?1H spin–spin coupling constants

Stereochemical study of five sterically crowded phenylselanylalkenes obtained via the hydroselenation of either terminal or internal alkynes with benzeneselenol catalyzed by the nanosized Ni complexes has been carried out based on the experimental HMBC measurements and theoretical second order palarization propagator approach (SOPPA) calculations of their ⁷⁷Se? ¹H spin–spin coupling constants across double bond in combination with the energy‐based theoretical conformational analysis performed at the MP2/6‐311G** level. It has been found that studied phenylselanylalkenes adopt mainly skewed s‐cis conformation with the noticeable out‐of‐plane deviations of the phenylselanyl and phenyl groups. Copyright © 2011 John Wiley & Sons, Ltd.     

Stereochemical behavior of geminal and vicinal 77Se–13C spin–spin coupling constants studied at the SOPPA(CC2) level taking into account relativistic corrections

A systematic theoretical study of geminal and vicinal ⁷⁷Se–¹³C spin–spin coupling constants in the series of the open‐chain selenides and selenium‐containing heterocycles revealed that relativistic effects play an essential role in the selenium–carbon coupling mechanism, especially when the coupling pathway includes a triple bond, contributing to about 10–15% of their total values and noticeably improving the agreement of the calculated couplings with experiment. Both geminal and vicinal ⁷⁷Se–¹³C spin–spin coupling constants show marked stereochemical behavior as documented by their calculated dihedral angle dependence that could be used as a practical guide in stereochemical studies of organoselenium compounds. Copyright © 2014 John Wiley & Sons, Ltd.     

An asymmetric synthesis of (+)-monomorine I

The synthesis of (+)-monomorine I, an indolizidine alkaloid isolated from Monomorium pharaonis, has been achieved. The 2,6-cis-piperidine ring moiety of (+)-monomorine I was constructed using diastereoselective aminopalladation. Chain elongation via cross-metathesis using Hoveyda-Grubbs 2nd catalyst followed by deprotection of the Cbz group and cyclic reductive hydroamination afforded (+)-monomorine I.     

A 13C, 15N and 77Se NMR Study of Some Diseleno Sulfonamides

¹³C, ¹⁵N and ⁷⁷Se NMR data are reported for ten title compounds. Some linear correlations of selenium, nitrogen and carbon chemical shifts values are described. A number of one- and two- bond ⁷⁷Se-¹³C coupling constants values are also given.     

Isotope dilution quantification of ultratrace gamma-glutamyl-Se-methylselenocysteine species using HPLC with enhanced ICP–MS detection by ultrasonic nebulisation or carbon-loaded plasma

A method for the accurate determination of ultratrace selenium species of relevance to cancer research, such as gamma-glutamyl-Se-methylselenocysteine (γ-glutamyl-SeMC), using species-specific double isotope dilution analysis (IDA) with HPLC–ICP–MS is reported for the first time. The ⁷⁷Se-enriched γ-glutamyl-SeMC spike was produced in-house by collecting the fraction at the retention time of the γ-glutamyl-SeMC peak from a chromatographed aqueous extract of ⁷⁷Se-enriched yeast, pooling the collected fractions and freeze-drying the homogenate. The Se content of this spike was characterised using reverse isotope dilution mass spectrometry (IDMS) and the isotopic composition of this spike was checked prior to quantification of the natural abundance dipeptide species in garlic using speciated IDA. The extraction of the γ-glutamyl-SeMC species in water was performed in a sonication bath for 2 h after adding an appropriate quantity of ⁷⁷Se-enriched γ-glutamyl-SeMC to 50 mg of garlic to give optimal ⁷⁸Se/⁷⁷Se and ⁸²Se/⁷⁷Se ratios of 1.5 and 0.6, respectively. The effect of ultrasonic nebulisation, in comparison with the loading of the ICP with carbon (through the addition of methane gas on-line), on the detection of Se associated with γ-glutamyl-SeMC using collision/reaction cell ICP–MS with hydrogen as collision gas was investigated. Sensitivity enhancements of approximately fourfold and twofold were achieved using USN and methane mixed plasma, respectively, in comparison with conventional nebulisation and conventional Ar ICP–MS. However, an approximately twofold improvement in the detection limit was achieved using both approaches (42 ng kg⁻¹ for ⁷⁸Se using peak height measurements). The use of species-specific IDMS enabled quantification of the dipeptide species at ng g⁻¹ levels (603 ng g⁻¹ Se) in the complex food matrix with a relative standard deviation (RSD, n = 4) of 4.5%, which was approximately half that obtained using standard addition as a confirmatory technique. Furthermore, good agreement was found between the γ-glutamyl-SeMC species concentrations obtained using both calibration methods.     

Acyclic tetrachalcogenoether ligands tethered with bulky substituents: Their syntheses and coordination chemistry

New o-phenylene-bridged tetrachalcogenoether ligands tethered with an extremely bulky substituent, 2,4,6-tris[bis(trimethylsilyl)methyl]phenyl (Tbt) group, [TbtE(o-phenylene)Se]₂(o-phenylene) (3: E = S, 4: E = Se), were synthesized. Complexation reactions of 3 and 4 with Na₂PdCl₄ gave the corresponding dichloropalladium(II) complexes 7 and 8. X-ray structural analysis of 7 and 8 indicated that the central palladium metals are in a distorted octahedral environment, where the two inner selenium atoms and the two chlorine atoms form a square planar arrangement around the palladium metal and the two terminal chalcogen atoms weakly coordinate to the palladium center at the axial positions. The AIM calculations also supported the existence of the interaction between the palladium and the terminal chalcogen atoms in the crystalline state. On the other hand, the ⁷⁷Se NMR spectra suggested that there are no or very weak interactions between the palladium and the terminal chalcogen atoms in solution. The UV/Vis spectra of 7, 8, and related compounds imply the possibility of weak interaction between the terminal selenium atoms and the palladium center in solution.     

Relaxor behavior of (Sr,Ba,Bi)TiO3 ferroelectric ceramic

The dielectric behavior of (Sr_0.4Ba_0.6)_0.925Bi_0.05TiO₃ (SBBT) ceramic was investigated in the temperature range from 100 K to 450 K. Broad dielectric maxima, which shift to higher temperature with increasing frequency, and the value of the relaxation parameter γ=1.6-2 estimated from the linear fit of the modified Curie-Weiss law, indicated the relaxor nature of the SBBT ceramic. The dielectric relaxation which follows the Vogel-Fulcher relationship with  K, E_a=0.0392 eV, and ν_o=2.98×10¹¹ Hz, further supports such a relaxor nature. The P-E hysteresis loop at different temperatures and ‘butterfly’ shape dc bias field dependence of permittivity at T>T_m (the temperature of permittivity maximum) signifies the occurrence of nanopolar clusters, which is the typical characteristic of relaxor ferroelectrics. At 300 K and 10 kHz, the dielectric constant and loss tgδ are ∼2210 and 0.00118, respectively. The tunability (28%) and figure of merit (237) at room temperature show that the SBBT ceramic could be a promising candidate for tunable capacitor applications.     

Conformational analysis and diastereotopic assignments in the series of selenium-containing heterocycles by means of 77Se-1H spin-spin coupling constants: a combined theoretical and experimental study

A combined theoretical and experimental study on the stereochemical behavior of (77)Se-(1)H spin-spin coupling constants has been performed at the second-order polarization propagator approach level together with heteronuclear multiple-bond correlation technique in the series of selenium-containing four-, five- and six-membered heterocycles including the derivatives of thiaselenetane, selenasilole, thiaselenole, thiaselenolane and dihydrothiaselenine. Geminal and vicinal (77)Se-(1)H spin-spin couplings were shown to have the pronounced stereochemical dependences in respect with the topology of the coupling pathway, internal rotation of the side-chain substituents and ring inversion providing a straightforward tool for the conformational analysis and diastereotopic assignments in the chiral organoselenium compounds.