Lithium bromide in acetonitrile: Thermodynamics,theory, and simulation

A variety of methods has been used for the study of lithium bromide solutions in acetonitrile yielding by their combination reliable information on different levels of approximation. Osmotic coefficients based on precise vapor pressure measurements are reproduced by CM (chemical model) and HNC (hypernetted chain) calculations and by BD (brownian dynamics) simulations. The results of neutron scattering experiments are treated with the help of HNC and BD methods. Hartree-Fock calculations on isolated LiBr pairs and solvated lithium ions yield reliable particle distances and reveal the geometry of the lithium solvation sphere.     

Rhodium and palladium β-diketonate determination with on-line supercritical fluid extraction-high performance liquid chromatography via solid phase extraction

 An on-line system of supercritical fluid extraction (SFE) and high performance liquid chromatography (HPLC) via solid phase extraction (SPE) is described for the determination of palladium and rhodium 2,2,6,6-tetramethyl-3,5-heptanedione-(thd) as well as rhodium-acetylacetonate-(acac) and benzylacetonate-(bzac) chelates. The chelates were extracted with supercritical CO₂ from sand and humic acid, concentrated on SPE cartridges and analysed with HPLC. Two cartridge materials were tested and compared to off-line trapping. The percentage of the breakthrough and cartridge retained material were measured in liquid dichloromethane. The SFE conditions could be optimized to separate metal chelates during the extraction. The supercritical fluid (SF) behaviour of different ligands on rhodium were investigated. Received: 19 July 1996/Revised: 11 December 1996/Accepted: 14 December 1996     

Dielectric relaxation of electrolyte solutions in acetonitrile

Complex permittivity spectra in the frequency range 0.95v (GHz)89 for acetonitrile and its solutions of LiBr, NaI, NaClO₄, and Bu₄NBr at 25°C show one Debye equation for the neat solvent whereas the superposition of a Debye process for the solute and a Cole-Cole distribution for the solvent is necessary to account for the dielectric relaxation behavior of the solutions. The reorientation of bulk acetonitrile is diffusive and only weakly coupled to viscosity. The number of solvent molecules irrotationally bound to the electrolyte is in good agreement with conventional solvation numbers for all electrolytes, when kinetic depolarization is assumed to be negligible. The solute relaxation process is dominated by the formation kinetics and reorientation of contact ion pairs. There is evidence for solvent-shared ion pairs in dilute NaClO₄ solutions.     

Enantioselective Preparation of 2‐Aminomethyl Carboxylic Acid Derivatives: Solving the β2‐Amino Acid Problem with the Chiral Auxiliary 4‐Isopropyl‐5,5‐diphenyloxazolidin‐2‐one (DIOZ). Preliminary Communication

Multigram amounts of suitably protected β²‐amino acids with 17 of the 20 proteinogenic side chains are prepared by diastereoselective reactions of Li, B, or Ti enolates of the corresponding 3‐acyl‐4‐isopropyl‐5,5‐diphenyloxazolidin‐2‐ones (acyl‐DIOZ; 1 ) with appropriate electrophiles (amidomethylation, hydroxyalkylation, (benzyloxycarbonyl)methylation) in yields of 55–90% and with diastereoselectivities of 80 to >97% (Scheme). The primary products 2 – 8 thus obtained are converted to protected β²‐amino acids by standard procedures (Table 1). Many of the DIOZ derivatives are highly crystalline compounds (31 X‐ray crystal structures in Table 2). The chiral auxiliary DIOZ, readily prepared in either enantiomeric form, is recovered with high yield.     

Conductivity Studies of Dilute Aqueous Solutions of Oxalic Acid and Neutral Oxalates of Sodium, Potassium, Cesium, and Ammonium from 5 to 35°C

Conductivity measurements of oxalic acid and neutral oxalates (disodium oxalate, dipotassium oxalate, dicesium, and diammonium oxalate) were performed on dilute aqueous solutions, c < 3 × 10^–3 mol-dm^–3, from 5 to 35°C. These data and those available from the literature were analyzed in terms of dissociation steps of oxalic acid, the Onsager conductivity equation for neutral oxalates, the Quint–Viallard conductivity equation for the acid, and the Debye–Hückel equation for activity coefficients, to give the limiting equivalent conductances of bioxalate anion ;(HC₂O₄ ^–) and oxalate anion (1/2C₂O₄ ^2–) and the corresponding dissociation constants K ₁ and K ₂.     

Calibration of conductance cells at various temperatures

Precise conductance measurements on aqueous potassium chloride solutions at 0, 10, 18, and 25°C have been made under various conditions over a concentration range 10^?4?2 mole-dm^?3, yielding the conductance equations $$\begin{gathered} 25^\circ C:\Lambda = 149.873 - 95.01\sqrt c + 38.48c log c + 183.1c - 176.4c^{3/2} \hfill \\ 18^\circ C:\Lambda = 129.497 - 80.38\sqrt c + 32.87c log c + 154.3c - 143.0c^{3/2} \hfill \\ 10^\circ C:\Lambda = 107.359 - 64.98\sqrt c + 27.07c log c + 125.4c - 110.3c^{3/2} \hfill \\ 0^\circ C:\Lambda = 81.700 - 47.80\sqrt c + 20.60c log c + 93.8c - 79.3c^{3/2} \hfill \\ \end{gathered} $$ which are proposed for calibration of conductance cells.     

Tetraalkylammonium bromides in methanol: Small angle neutron scattering and vapor pressure measurements

Vapor pressure measurements and small angle scattering (SANS) experiments are reported and discussed for bispiperidinium (BP) bromide and n-tetrapentylammonium bromide solutions in methanol at 25°C. The BP⁺ ion which may be considered as a tetraalkylammonium (TAA) ion with pairwise connected alkyl chains, is used for the study of the effects due to flexible TAA alkyl chains. SANS intensities are calibrated with the help of the precise osmotic coefficients from the vapor pressure measurements.     

Combinatorial diversity of fission yeast SCF ubiquitin ligases by homo- and heterooligomeric assemblies of the F-box proteins Pop1p and Pop2p

Background  

SCF ubiquitin ligases share the core subunits cullin 1, SKP1, and HRT1/RBX1/ROC1, which associate with different F-box proteins. F-box proteins bind substrates following their phosphorylation upon stimulation of various signaling pathways. Ubiquitin-mediated destruction of the fission yeast cyclin-dependent kinase inhibitor Rum1p depends on two heterooligomerizing F-box proteins, Pop1p and Pop2p. Both proteins interact with the cullin Pcu1p when overexpressed, but it is unknown whether this reflects their co-assembly into bona fide SCF complexes.