Separation and determination of arsenic(V) and arsenic(III) in sea-water by solvent extraction and atomic-absorption spectrophotometry by the hydride-generation technique

Arsenic(V) and arsenic(III) in sea-water have been separated by complexing the arsenic(III) with ammonium pyrrolidinedithiocarbamate (APDC) in the range 4.0-4.5 and extracting the complex with chloroform. The organic phase is then wet-ashed with a 1:1 mixture of concentrated nitric acid and perchloric acid to get rid of all organics, and the arsenic(III) is determined by hydride generation and atomic-absorption spectrophotometry. Total arsenic is determined by first reducing arsenic(V) to arsenic(III) with potassium iodide and then applying the method used for arsenic(III). The arsenic(V) content is determined by difference. The low detection limit of 0.031 ng ml and the high sensitivity and precision make the method suitable for analysis of open ocean waters.     

EPR study of the mechanism of interaction of NO and NO2 molecules with zeolite surfaces

The adsorption of the paramagnetic molecules of NO and NO₂ by zeolites in the alkali and alkaline earth cationic forms has been studied by EPR and reflectance spectroscopic methods. The change in the EPR spectra of adsorbed nitric oxide with increase in the degree of covering of the surface of the alkali cationic form of the zeolites, and also the nature of the change in the spectra when oxygen is adsorbed on zeolites on which NO has previously been adsorbed, indicate the existence of two types of adsorption center. At low degrees of covering of the surface, on the order of 10¹⁸ g^–1, as can be judged from the EPR spectra, the adsorbed NO molecule is strongly polarized and the unpaired electron is almost completely localized on the oxygen atom. At high degrees of covering, for an appreciable proportion of the NO molecules, the bond with the surface is weaker. In this case, the EPR spectra show a hyperfine structure (HFS) with a constant which changes with change in the cation in the order Li⁺ Na⁺ K⁺. The replacement of the singly charged Na⁺ by the doubly charged Ca²⁺ produces a marked change in the adsorption properties of the zeolite. The adsorption of NO on CaA leads not only to polarization of the adsorbed molecule but also to transfer of the electron from the nitrogen atom to the atoms of the adsorbent; this is recorded in the EPR spectrum in the form of an F-center. On further adsorption, the NO molecules are adsorbed both on the nitrogen atom and on the oxygen atom of the first molecule; thus, NO₂ and N₂O are formed.     

Yeast protein farnesyltransferase. Binding of S-alkyl peptides and related analogues

[reaction: see text] Protein farnesyltransferase (PFTase) catalyzes alkylation of cysteine residues by farnesyl diphosphate (FPP). The dissociation constants for the PFTase-peptide analogue complexes for the series of analogues fl-RTRC(X)VIA (X = H, methyl, dodecyl, farnesyl) were measured by fluorescence anisotropy. The results indicate that an ionizable sulfhydryl moiety is important for substrate binding and the farnesyl group in the product facilitates binding.     

The mechanism of bacteriorhodopsin functioning. I. The light-induced proton throw-over by retinal

The path of proton motion in protein bacteriorhodopsin is constructed: Tyr 43 → Tyr 26 → Asp 212 → Tyr 83 → Tyr 57 → Tyr 79 → Glu 204 → Tyr 64 → Glu 74. From the position of quantum theory the light-induced isomerization of retinal, the process carrying out the proton and throw-over from the inlet H-bond chain (Tyr 43 → Tyr 26 → Lys 216) to the outlet one (Lys 216 → Asp 212 → … → Glu 74) are investigated. The kinetics of the transition of the excitation from the π-electron subsystem to the vibrational one is studied; the function of the nonequilibrial excitation distribution of the collective intramolecular retinal vibrations is found. The mechanism of deprotonation of Schiff base (transmission of proton to the outlet channel) is considered. The correspondence between the theoretically estimated states and spectroscopically identified forms of bacteriorhodosin (initial form bR, intermediates J, K) is proposed.     

Use of azolium ylides in the synthesis of con-densed heterocyclic systems from 2-quinox-alylacetonitriles

The reaction of -substituted 2-quinoxalylacetonitriles with 1-alkyl(aryl)imidazoles, -benzimidazoles, -1,2,4-triazoles and 5,6-dihydroimidazo[i,j]quinoline was studied. It was found that during the course of the reaction an unusually easy dealkylation of the azole ring takes place, while the aryl substituent is not split off. A reaction mechanism has been proposed including the formation of an ylide intermediate, followed by subsequent electrophilic attack on the C₍₂₎ position of the azolium ring. The applicability boundaries of the reaction studied and the spectral characteristics of the synthesized compounds were investigated.Translated from Khimiya Geterotsiklicheskikh Soedinenii, No. 2, pp. 224–229, February, 1993.     

Analytical applications of crown ethers : III. Extraction and determination of nitrophenol isomers

The oxidation of aldehydes by dicyclohexyl-18-crown-6-potassium permangante complex is presented. Results obtained by this procedure showed that the presence of crown ether (DCC) and the catalyst play an important role in the oxidation of the aldehydes. The stoichiometry found corresponds to 2 μeq of permanganate per 1 μmol of aldehydes in all cases. Alcohols and ketones are not oxidized. Finally, a method for quantitative evaluation of aldehydes is proposed.     

Crystallization of filled nylon 6 III. Non-isothermal crystallization

Summary DSC data on crystallization kinetics from the melt at different cooling rates of nylon 6 containing various amonts of untreated and surface-treated fillers, were analyzed in terms of a modified Kolmogorov-Avrami equation. It was established that mechanism of crystallinity development in molten nylon 6 does not change appreciably in presence of aminosilane-treated glass beads and small amounts of untreated glass beads, whereas time exponentn was found to decrease with increasing filler content in samples containing untreated glass beads and Aerosil. On the other hand, dependence of temperature of the onset of crystal nucleation on cooling rate obeyedm = 2 law for pure nylon 6 and samples containing surface-treated filler, whilem = 4 law seemed to hold for samples containing large amounts of untreated fillers at low cooling rates (m is the exponent at degree of supercooling). It was concluded that although isothermal conditions of crystallization should be preferred for further quantitative investigations of polymer-filler interactions in highly filled polymer melts, the above results qualitatively are consistent with trends discovered in isothermal crystallization experiments.     

Enthalpimetric and thermometric determination of some nitrogen containing bases and some physiologically active alkaloids

Summary The enthalpimetric determination of a series of bases by using the reaction of the bases and sodium tetraphenylboron is described. The accuracy of the method is analytically acceptable, over the range 3×10^–5 moles to 7.5×10^–4 moles. The upper limit is determined by the bulk of the precipitate.Dosage forms of some physiologically active alkaloids contain more than the upper limit of analyte. Yet, preliminary investigations using silicotungstic acid indicate that this is suitable for thermometric titrations.

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Enthalpimetrische und thermometrische Bestimmung stickstoffhaltiger Basen und physiologisch aktiver Alkaloide

Zusammenfassung Die enthalpimetrische Bestimmung einiger N-haltiger Basen erfolgte aufgrund der Reaktion mit Natriumtetraphenyloborat. Die Genauigkeit ist im Bereich von 3 · 10^–5-7,5 · 10^–4 Mol befriedigend. Eine obere Grenze der Anwendbarkeit ergibt sich aus der Menge des anfallenden Niederschlags. Diese wird bei der Bestimmung von Formulierungen einiger physiologisch aktiver Alkaloide überschritten. Jedoch ist in diesem Fall eine thermometrische Titration mit Hilfe von Silicowolframsäure möglich.

Lecture presented at Euroanalysis I Conference, 28. 8.–1. 9. 1972 in Heidelberg, Germany.     

Über Hexagonale Perowskite mit Kationenfehlstellen. I. Verbindungen vom Typ Ba2B□2/3ReVIIO6

On Hexagonal Perovskites with Cationic Vacancies. I. Compounds of the Type Ba₂B □_2/3Re^VIIO₆ Compounds of Type Ba₂B□_2/3Re^VIIO₆ are formed with B^III = Sm? Gd Ho? Lu, Y, Sc, In (yellow); Tb (black-brown); Dy (yellow-orange). They crystallize with B^III = Sm? Lu, Y and Sc in a rhombohedral layer structure of 12 L-type (space group R3 m; sequence: cchhcchhcchh) with 6 formula units in the unit cell.     

Automation of the gas analysis of14C-labelled compounds utilizing a piston-type counter tube

In this paper an automatic apparatus designed for the radioactivity measurement of¹⁴C-labelled organic compounds in the gaseous phase is described. The labelled organic compounds are combusted in a mixture of argon and oxygen. After combustion the oxygen content of the gas is eliminated by passing it through a copper packing. The water and heteroelements present are also removed and the radioactive carbon dioxide gas is swept by argon carrier gas into a piston-type counter tube. In the counter tube the piston forming a dividing wall moves forward in accordance with the rate of combustion and sweeping, and thus sucks the gases leaving the combustion tube into the effective tube volume. The anode wire is carried by a reel located in the piston and a spring device ensures its stretched state. At the end of the sweeping period methane is fed into the counter tube and the activity of the argon—methane—carbon dioxide mixture is measured in the limited proportional region. Manual and automatic operation is possible. The piston-type counter tube provides possibility for strandardization by means of extrapolation and for measurement of absolute activities.