On color critical graphs

It is shown that the minimal number of edges which have to be omitted from a (k + 1)-critical graph on n vertices in order to make it bipartite is at least ^k₂ for n large enough. This bound is best possible. Various related questions are considered.     

On inequalities for products of power sums

In the present note we investigate several inequalities concerning the product of power sums . The only restriction which is used is ₁ + ... + _k = 0. The conditions obtained are necessary and sufficient everywhere.     

Maximum bipartite subgraphs of Kneser graphs

We investigate the maximum number of edges in a bipartite subgraph of the Kneser graphK(n, r). The exact solution is given for eitherr arbitrary andn (4.3 + o(1))r, orr = 2 andn arbitrary. The problem is in connection with the study of the bipartite subgraph polytope of a graph.Research supported in part by the AKA Research Fund of the Hungarian Academy of Sciences     

Association of Cytochrome P450 Enzymes is a Determining Factor in their Catalytic Activity

Previously, our laboratory demonstrated that one cytochrome P450 isoenzyme can influence the catalytic properties of another P450 isoenzyme when combined in a reconstituted system. Moreover, our data and that of other investigators indicate that P450 interaction is required for catalytic activity even when one isoenzyme is present. The goal of the current study was to examine the possible mechanism of these interactions in more detail. Analyzing recently published X-ray data of microsomal P450 enzymes and protein docking studies, four types of dimer formations of P450 enzymes were examined in more detail. In case of two dimer types, the aggregating partner was shown to contribute to NADPH cytochrome P450 reductase (CPR) binding-a flavoprotein whose interaction with P450 is required for expressing P450 functional activity of the neighboring P450 moiety. Thus, it was shown that dimerization of P450 enzymes might result in an altered affinity towards the CPR. Two dimer types were shown to exist only in the presence of a substrate, while the other two types exist also without a substrate present. The molecular basis was established for the fact that the presence of a substrate and other P450 enzymes simultaneously determine the catalytic activity. Furthermore, a kinetic model was improved describing the catalytic activity of P450 enzymes as a function of CPR concentration based on equilibrium between different supramolecular organizations of P450 enzymes. This model was successfully applied in order to explain our experimental data and that of other investigators.Eszter Hazai and Zsolt Bikádi contributed equally to this workDavid Kupfer-Deceased     

Determination of plutonium and americium in environmental samples by inductively coupled plasma sector field mass spectrometry and alpha spectrometry

Application of inductively coupled plasma sector field mass spectrometry (ICP-SFMS) for the determination of americium and plutonium in environmental samples is described in comparison with alpha spectrometry. A sequential sample preparation method was employed using a co-precipitation step for pre-concentration and a separation step applying extraction chromatographic resins. The average recovery of sample preparation calculated from the concentration of the tracer before and after sample treatment was better than 80%. The method development focused on the elimination of possible interferences in mass spectrometric analysis caused by molecular ions (e.g. ²⁰⁰Hg⁴⁰Ar⁺, ²⁰⁴Pb³⁶Ar⁺, ²⁰⁸Pb¹⁶O₂⁺or ²³⁸U¹H⁺) employing matrix separation and desolvation prior to ICP-SFMS analysis. The effect of N₂ gas on signal intensity and oxide ratio was investigated. A two-fold signal improvement was obtained by adding 5 ml min^− 1 N₂ to the sample gas after the desolvation system. For ²³⁹Pu, ²⁴⁰Pu, ²⁴¹Pu and ²⁴¹Am limit of detection (LOD) of 15, 9.2, 14 and 104 fg g^− 1 was achieved, respectively. Calculation of LOD was based on three times standard deviation of the method blank solution. Absolute detection limit was calculated to be 10-25 fg. For all investigated actinides the precision of the analysis was in the range 0.8-3% relative standard deviation. Results from the analysis of certified reference materials (IAEA-384 and IAEA-385) showed good agreement with recommended values and data available in the literature. The method was applied for analysis of environmental samples originated from Chernobyl and from Mayak region. The possibility of the determination of the origin and date of pollution was demonstrated using isotopic data obtained by ICP-SFMS and alpha spectrometry.     

Synthesis of bicyclic carbamates as precursors of Sedum alkaloid derivatives

Synthesis of a N-Boc-protected piperidin-2-yl phosphine oxide starting from piperidine in three steps, followed by olefination using a variety of α,β-unsaturated aldehydes resulted in tert-butyl 2-(2′-alkenylidene)piperidine-1-carboxylates in high yields. A novel type of intramolecular cyclization of these enamides furnished a new family of 3-alkyl-4,6,7,8-tetrahydro-3H-pyrido[1,2-c][1,3]oxazin-1-ones as useful substrates for further elaboration. Subsequent reduction of these unsaturated bicyclic carbamates using NaCNBH₃ or NaBH₄ afforded the corresponding 3-alkylhexahydropyrido[1,2-c][1,3]oxazin-1-ones in a highly stereoselective way. Reductive ring opening of two representatives furnished the corresponding Sedum alkaloid derivatives in good yields.     

Possibilities and limits of the simultaneous determination of As, Bi, Ge, Sb, Se, and Sn by flow injection-hydride generation-inductively coupled plasma-time-of-flight mass spectrometry (FI-HG-ICP-TOFMS)

A simultaneous multi-elemental measurement of As, Bi, Ge, Sb, Se, and Sn was performed in this study by flow injection-hydride generation-inductively coupled plasma-time-of-flight mass spectrometry (FI-HG-ICP-TOFMS). An off-line pre-reduction treatment using a solution of 5% (m/v) KI and ascorbic acid for 15 min at 80 °C is described by presenting its advantages and disadvantages and compared with the results achieved without pre-reduction. Using optimised conditions the following figures of merit were achieved: limits of detection in the 0.08-0.54 ng ml⁻¹ range and relative standard deviations (RSD) of 1.7-6.7%, respectively. Applying the presented method, two certified reference materials (NIST 1643d freshwater and PACS-2 marine sediment) were analysed to demonstrate the suitability of the method for analysis of real samples. Results obtained from treated samples showed good agreement with certified values while the untreated ones considerably departed from them.     

Hydrogenation of biological membranes using a polymer-anchored colloidal palladium catalyst

Colloidal palladium anchored onto the surface of insoluble polyvinyl-polypirrolidone (Pd-PVPP) is anactive, non-permeable catalyst for surface selective hydrogenation of model or isolated biomembranes.

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Preparation,homogeneity and stability studies of a candidate LRM for Se speciation

A laboratory reference material (LRM) was prepared from Brazil nuts (Bertholletia excelsa) for quality control (QC) purposes of selenium speciation. The preparation of this LRM led through the usual operation steps applied during routine reference material production from biota samples-preparation of the raw material, homogenisation, storage design, checking of homogeneity, microbiological status and possible irradiation effects, and monitoring the species stability vs time at different storage temperatures. The selenium speciation studies to check species stability were carried out on a HPLC-UV-HG-AFS measurement set-up. Special attention was paid to the correct identification of selenium species by applying independent HPLC separation techniques (ion-pairing and anion-exchange chromatography). The concentration of selenomethionine (SeMet) and total Se content were quantified (79.9 microg g(-1) (calculated as Se) and 82.9 microg g(-1), respectively). The homogeneity and stability of this candidate reference material passed the relevant tests recommended by Bureau Communautaire de Référence (BCR).