Spectrophotometric study of the cobalt-3-nitroso-2,6-pyridinediol reaction

Summary A spectrophotometric study has been made on the cobalt-3-nitroso-2, 6-pyridinediol reaction. The yellow complex formed has an absorption maximum at 411 nm and a molar absorptivity of 3.4·10⁴ l mole^–1 cm^–1. The system follows Beer's law.

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Zusammenfassung Die Reaktion zwischen Kobalt und 3-Nitroso-2,6-Pyridindiol wurde spektrophotometrisch untersucht. Die maximale Absorption des sich bildenden gelben Komplexes liegt bei 411 nm, seine molare Extinktion beträgt 3,4·10⁴ l Mol^–1 cm^–1. Die Farbreaktion entspricht dem Beerschen Gesetz.

     

CLIP: similarity searching of 3D databases using clique detection

This paper describes a program for 3D similarity searching, called CLIP (for Candidate Ligand Identification Program), that uses the Bron-Kerbosch clique detection algorithm to find those structures in a file that have large structures in common with a target structure. Structures are characterized by the geometric arrangement of pharmacophore points and the similarity between two structures calculated using modifications of the Simpson and Tanimoto association coefficients. This modification takes into account the fact that a distance tolerance is required to ensure that pairs of interatomic distances can be regarded as equivalent during the clique-construction stage of the matching algorithm. Experiments with HIV assay data demonstrate the effectiveness and the efficiency of this approach to virtual screening.     

Impedance measurements using ionic contacts on purified and doped silver bromide crystals

Impedance measurements are made on purified, vacancy-doped and interstitial-doped silver bromide crystals of thicknesses from 0.061 cm to 0.260 cm using constant ionic strength solution contacts at temperatures from 20°C to 35°C. Results characterize the macroscopic transport properties from 316 kHz to 0.01 Hz and at d.c. In all cases the high frequency limit resistive component of the impedance, R_∞, is equal to the d.c. resistance R₀. This result indicates rapid ion exchange at the surfaces and neither Warburg diffusion nor surface kinetic-limitation of transport. As expected, R_∞ is a function of charge carrier concentration, which depends on extrinsic dopant concentration over the temperature range measured. For pure crystals the resistance, R_∞, is linearly dependent on crystal thickness, δ, while thickness correlation could not be tested for the doped crystals as dopant level was not uniform. The geometric capacitance for all crystals, C_g, is linear with δ^?1. The dimensionless dielectric constant, κ, calculated from C_g and crystal dimentsions, is 13.9±0.7. The electric relaxation time, τ_el, defined as the product R_∞C_g, at 25°C is found to be 50±3 μs for pure crystals, 1 to 4 μs for the Cd²⁺-doped crystals and 12 to 66 μs for the S^2?-doped crystals. Temperature dependance of R_∞ allows determination of crystal transport activation energies. For pure and Cd²⁺-doped crystals a value of 0.33±0.02 eV is found. For the S²⁺-doped crystals values from 0.48 to 0.28 eV are found. The parameter α, in the non-ideal Cole-Cole representation of impedance plane arcs, is 0.96±0.01 for purified crystals, 0.93±0.03 for Cd²⁺-doped, and 0.92±0.02 for S^2?-doped crystals.     

The Effect of a Hydrogen Bonding Environment (Dimethyl Sulfoxide) on the Ionisation and Redox Properties of the Thiol Group in Cysteine and a Protein Disulfide Isomerase Mimic (Vectrase)

Increasing enrichment of dimethyl sulfoxide, DMSO, in DMSO-water mixtures causes a reversal in the thermodynamic dissociation constants, pK _as, and has a marked effect on the redox potentails of the thiolic and amino groups in cysteine and the protein disulfide isomerase (PDI) mimic BMC, Vectrase. This paper illustrates the effect of a hydrogen-bonding environment on the ionisation and redox properties of thiol groups in amino acids. A combination of potentiometry and Raman spectroscopy was applied to rationalise the observations. Intracellular environments are full of hydrophobic, hydrogen-bonding environments. The results illustrate the profound effects of the local environment on the thiol group.     

Determination of the enantiomers of fenoldopam in human plasma by reversed-phase high-performance liquid chromatography after chiral derivatization.

Fenoldopam, a selective agonist at peripheral dopaminergic (DA-1) receptors, is administered as a racemic mixture and, consequently, an indirect stereospecific high-performance liquid chromatographic assay was developed to study the disposition of the individual enantiomers in human subjects. Fenoldopam enantiomers were extracted from alkalinized plasma into ethyl acetate prior to precolumn derivatization with the chiral reagent 2,3,4,6-tetra-O-acetyl-beta-D-glucopyranosyl isothiocyanate (GITC). The resulting diastereomers were separated on a reversed-phase butylsilica column and determined using triple-electrode coulometric detection. The limits of determination and detection for the S- and R-enantiomers of fenoldopam were 0.5 and 0.25 ng/ml, respectively. A linear response was observed for (S)- and (R)-fenoldopam concentrations ranging from 0.5 to 50 ng/ml in plasma. The intra-day relative standard deviations (R.S.D.s) for the plasma assay at nominal concentrations of 0.5, 5 and 50 ng/ml were 17.4, 5.2 and 6.9%, respectively, for (S)-fenoldopam and 9.9, 6.2 and 7.4%, respectively, for (R)-fenoldopam. The inter-day R.S.D.s of the method at these concentrations were 9.3, 7.7 and 7.4%, respectively, for (S)-fenoldopam and 9.5, 1.9 and 7.3%, respectively, for (R)-fenoldopam. The mean accuracy of the method at concentrations of 0.5, 5 and 50 ng/ml in plasma was found to be 106.4, 111.8 and 108.9%, respectively, for (S)-fenoldopam and 116.2, 104.2 and 111.2%, respectively, for (R)-fenoldopam. The assay developed was sufficiently sensitive, accurate and precise to support pharmacokinetic studies in human subjects.     

Microanalysis of perfluoro compounds

Summary Because of the extremely high thermal and chemical stability of perfluoro organic compounds, special methods of analysis are required for carbon, hydrogen and fluorine. Sulfur, phosphorus, and halogens can be determined by conventionalCarius digestion and subsequent gravimetric methods. Nitrogen is determined by theDumas method. Oxygen cannot be determined by the usualUnterzaucher method.Carbon and hydrogen are determined by combustion in oxygen with a tube filling consisting of (Ag; 3 MgO · Al₂O₃; Ag) at 950° C; (Ag; Pb₂O₃) at 450° C; (Ag;PbO₂;Ag) at 180° C. The sample burner is operated at 950° to 1000° C.Fluorine is determined by reaction with sodium in a conventional sodium peroxide bomb with lead or copper gasket. The lower part of the bomb is held at red heat with a torch for about 20 seconds. The sodium fluoride from the reaction is titrated with thorium nitrate.

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Zusammenfassung Für die Bestimmung von Kohlenstoff, Wasserstoff und Fluor in organischen perfluorierten Verbindungen sind wegen deren außerordentlich hoher thermischer und chemischer Stabilität spezielle analytische Methoden notwendig. Schwefel, Phosphor und die Halogene können nachCarius bzw. mit den üblichen gravimetrischen Methoden bestimmt werden. Sauerstoff kann nicht nach derUnterzaucher- Methode bestimmt werden.Kohlenstoff und Wasserstoff sind bestimmbar bei Verbrennung mit einer Rohrfüllung aus (Ag; 3 MgO · Al₂O₃; Ag) bei 900°; (Ag; Pb₂O₃) bei 450°: (Ag; PbO₂; Ag) bei 180°. Der bewegliche Brenner wird auf einer Temperatur von 950° bis 1000° gehalten.Fluor kann durch Umsetzung mit Natrium in einer gewöhnlichen Peroxyd-Bombe mit Blei- oder Kupfer-Dichtung bestimmt werden. Der untere Teil der Bombe wird mit einem Gebläse für 20 Sekunden auf Rotglut erhitzt. Das gebildete Natriumfluorid wird in der gewöhnlichen Weise mit Thoriumnitrat titriert.

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Résumé La grande stabilité thermique et chimique des composés organiques perfluorés impose l'emploi de méthodes spéciales pour le dosage du carbone, de l'hydrogène et du fluor. Il est possible de déterminer le soufre, le phosphore, et les halogènes par digestion classique deCarius suivie d'un dosage gravimétrique approprié. L'azote est dosé par la méthode deDumas; quant à l'oxygène, il ne peut être déterminé par la méthode habituelle deUnterzaucher.Le carbone et l'hydrogène sont déterminés par combustion dans l'oxygène à l'intérieur d'un tube comportant un remplissage constitué par (Ag; 3 MgO · Al₂O₃; Ag) à 950°, (Ag; Pb₂O₃) à 450° (Ag; PbO₂; Ag) à 180° C. Le brûleur mobile permet d'obtenir une température de 950 à 1000° C.Le fluor est dosé après minéralisation par le sodium dans une bombe courante à peroxyde de sodium, comportant un joint de plomb ou de cuivre. La partie inférieure de la bombe est portée au rouge à l'aide d'un chalumeau pendant environ 20 secondes. Le fluorure de sodium obtenu est titré à l'aide de nitrate de thorium.

     

Solution Polymerization of Acrylonitrile Catalyzed by Sodium Triethylthioisopropoxyaluminate: A Polyacrylonitrile with High Structural Regularity

A new catalyst for the polymerization of acrylonitrile has been found by reacting sodium aluminum tetraethyl with a stoichiometric amount of oxygen, alcohols, or mercaptans, etc. The catalyst prepared by reacting NaAlEt₄ with RSH remains active at temperatures as low as ?78°C., polymerizing acrylonitrile into high molecular weight polymers with high conversions. At ?78°C., the propagating chain remains active over a period of days, and the chain length increases with time or conversion. At high conversions the molecular weight of the polymer is directly proportional to the concentration of the monomer, the catalyst concentration being kept constant. The efficiency of the catalyst expressed as the ratio of the number of the polymer molecules produced to the number of the sulfur atoms used is in excess of 80%. The weight-average molecular weight of the polymer measured by light scattering is roughly equal to the number-average molecular weight determined by sulfur analysis assuming only onesulfur atom in each polymer molecule. Although the data given here are only qualitative in nature, it is nevertheless evident that this system bears great resemblances to anionic polymerizations resulting in “living polymers.” The polymer obtained with the NaAlEt₃S(i-Pr) catalyst at ?78°C. differs from free-radical polyacrylonitrile in exhibiting substantially lower solubility, higher melting point, and higher rate of crystallization than that for the free-radical polymer. The polymer is also free from structural mpurities; it does not cause fluorescence in dilute solutions and has no absorption peak at 265 mμ; both these effects are produced by impurities in free-radical polyacrylonitrile. It is concluded that the polymer reported here is more regular in structure than free-radical polyacrylonitrile.     

Impedance measurements on purified silver chloride crystals using ionic vs. electronic contacts

Impedance measurements are made on purified silver chloride crystals covering thicknesses from 0.064 cm to 0.400 cm at temperatures from 293 K to 310 K using constant ionic strength solution contacts with varying silver ion concentrations. Results characterize the macroscopic transport properties from 10 kHz to 0.01 Hz and at d.c. Electrical relaxation times, τ_el, are independent of crystal thickness and bathing solution composition, but dependent on temperature. The resistive component of the impedance, R_∞, is linear with crystal thickness, δ, and the geometric capacitance, C_g, linear with δ^?1. In agreement with Macdonald's theory, R_∞ is equal to the d.c. resistance, R₀. This indicates rapid ion exchange at the surface and no Warburg diffusion. The dimensionless dielectric constant, calculated from C_g and the crystal dimensions, is 15.4. An average activation energy of 0.298 eV, indicative of vacancy transport is found. The parameter α in the equivalent non-ideal Cole-Cole representation is 0.96 indicating a narrow distribution of time constants. These results are compared with the electronic contact measurements typical to solid state physics. Higher values of resistivity and lower values of C_g and conductivity are found with graphite and vapor deposited silver. These differences were as large as 10%. Calculated dielectric constants at 10 kHz were 14.8 and 13.5, respectively, while the most recent published value is 11.14. These differences suggest that ionic solution contact measurements may give more reliable values because surface coverage is more nearly uniform on the molecular level.     

Reactions of fused dihydro-1,2,4-thiadiazoles with isocyanates and isothiocyanates to give 6aλ4-thia-1,3,4,6-tetraazapentalene derivatives

3-Methyl-6,7-dihydro-5H-1,2,4-thiadiazolo[4,5-a]pyrimidine 4a reacted with isocyanates and isothiocyanates with elimination of acetonitrile and concomitant addition of two molecules of the heterocumulene to give the 2,3-disubstituted-6,7-dihydro-5H-2aλ⁴-thia-2, 3, 4a, 7a-tetraazacyclopent[cd]indene-1 (2H), 4(3H)-diones 8a–8e and the corresponding dithiones 9a–9h , respectively. 3-Methyl-5, 10-dihydrobenzo[e]-1,2,4-thiadiazolo[4,5-a][1,3]diazepine 5a likewise reacted with isocyanates and isothiocyanates to give the 2,3-disubstituted-5,10-dihydro-2aλ⁴-thia-2,3, 4a, 10a-tetraazapentaleno[3, 3a, 4-gh]benzocyclopheptene-1, 4-diones 10a–10f and the corresponding dithiones 11a–11f . The base 4a reacted with phenyl isocyanate, methyl isothiocyanate, and phenyl isothiocyanate in toluene at room temperature to give the zwitterions 14a , 14b , and 14c , respectively, and the diazepine 5a reacted with phenyl isothiocyanate to give the zwitterion 17 .     

Modeling of plasma behavior in a plasma electrode Pockels cell

We present three interrelated models of plasma behavior in a plasma electrode Pockels cell (PEPC). In a PEPC, plasma discharges are formed on both sides of a thin, large-aperture electro-optic crystal (typically KDP). The plasmas act as optically transparent, highly conductive electrodes, allowing uniform application of a longitudinal field to induce birefringence in the crystal. First, we model the plasma in the thin direction, perpendicular to the crystal, via a one-dimensional fluid model. This yields the electron temperature and the density and velocity profiles in this direction as functions of the neutral pressure, the plasma channel width, and the discharge current density. Next, me model the temporal response of the crystal to the charging process, combining a circuit model with a model of the sheath which forms near the crystal boundary. This model gives the time-dependent voltage drop across the sheath as a function of electron density at the sheath entrance. Finally, we develop a two dimensional MHD model of the planar plasma, in order to calculate the response of the plasma to magnetic fields. We show how the plasma uniformity is affected by the design of the current return, by the longitudinal field from the cathode magnetron, and by fields from other sources. This model also gives the plasma sensitivity to the boundary potential at which the top and bottom of the discharge are held. We validate these models by showing how they explain observations in three large Pockels cells built at Lawrence Livermore National Laboratory