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701.
702.
Yang D Zhang YH Li B Zhang DW Chan JC Zhu NY Luo SW Wu YD 《Journal of the American Chemical Society》2004,126(22):6956-6966
We have investigated, using NMR, IR, and CD spectroscopy and X-ray crystallography, the conformational properties of peptides 1-10 of beta(3)-aminoxy acids (NH(2)OCHRCH(2)COOH) having different side chains on the beta carbon atom (e.g., R = Me, Et, COOBn, CH(2)CH(2)CH=CH(2), i-Bu, i-Pr). The beta N-O turns and beta N-O helices that involve a nine-membered-ring intramolecular hydrogen bond between NH(i)(+2) and CO(i), which have been found previously in peptides of beta(2,2)-aminoxy acids (NH(2)OCH(2)CMe(2)COOH), are also present in those beta(3)-aminoxy peptides. X-ray crystal structures and NMR spectral analysis reveal that, in the beta N-O turns and beta N-O helices induced by beta(3)-aminoxy acids, the N-O bond could be either anti or gauche to the C(alpha)-C(beta) bond depending on the size of the side chain; in contrast, only the anti conformation was found in beta(2,2)-aminoxy peptides. Both diamide 1 and triamide 9 exist in different conformations in solution and in the solid state: parallel sheet structures in the solid state and predominantly beta N-O turn and beta N-O helix conformations in nonpolar solvents. Theoretical studies on a series of model diamides rationalize very well the experimentally observed conformational features of these beta(3)-aminoxy peptides. 相似文献
703.
Summary Polyvinylchloride liquid membrane electrodes based on valinomycin lead to low readings in the determination of K+ in undiluted urine because of anion interference. This interference is eliminated by instead incorporating valinomycin into silicone rubber. When K+-concentrations, as determined in undiluted urine by ion selective electrodes, are correlated with those obtained for the same samples by flame photometry, the residual standard deviation is found to be ±1.5 mM over the 20 to 170 mM K+ range.
Bestimmung von K+ in verdünntem und unverdünntem Harn mit ionen-selektiven Elektroden
Zusammenfassung Bestimmungen von K+ in unverdünntem Harn mit auf Valinomycin beruhenden Polyvinylchlorid-Flüssigmembranelektroden ergeben systematisch zu tiefe Werte. Diese Abweichungen sind durch Anioneninterferenz bedingt und können durch den Einsatz von Silikongummi als Membranmatrix eliminiert werden. Die Korrelation der mit K+-selektiven Silikongummimembranen in unverdünntem Harn ermittelten Meßwerte mit Ergebnissen der Flammenphotometrie ergibt eine Standardabweichung um die Regressionsgerade von ±1,5 mM über den K+-Konzentrationsbereich von 20 bis 170 mM.相似文献
704.
Jenny B. Waern 《Journal of organometallic chemistry》2004,689(25):4655-4668
The study of molybdocene dichloride and related metallocenes has been dominated by their remarkable catalytic properties in organic synthesis and polymer chemistry. Interest in the aqueous, bioorganometallic chemistry of metallocene dihalides has stemmed from the potent antitumor properties of titanocene dichloride, including results from human clinical trials. This review will summarize key results reported in the last decade on the biological chemistry of molybdocene dichloride. The effect of concentration, pH and ionic strength on the rates of hydrolysis of both the cyclopentadienyl and halide ligands have established that the positively charged monoaquated species Cp2Mo(OH)(OH2)+, in equilibrium with the dipositively charged dimer Cp2Mo(μ-OH)2MoCp2, is present under physiological conditions. Systematic studies of the coordination chemistry of Cp2MoCl2 with nucleobases, nucleotides, single-stranded and double-stranded oligonucleotides, and calf-thymus DNA have shown that while simultaneous phosphate(O) and heterocyclic(N) adducts are formed with nucleotides, negligible interaction with DNA occurs under physiological conditions. In contrast, Cp2MoCl2 forms strong, non-labile complexes with deprotonated thiols in amino acids. Molybdocene dichloride is able to catalyse the hydrolysis of activated phosphate esters under physiological conditions, but hydrolysis of unactivated phosphodiesters is only significant at pH 4. Limited antitumor activity results, inhibition studies with protein kinase C and topoisomerase II, structure-activity and cell-uptake studies have provided some insight into possible mechanisms of antitumor action. 相似文献
705.
Interactions are shown to take place between ammonium polyphosphate and pentaerythritol in thermally degrading mixtures. Up to 500°C, gaseous products are evolved on programmed heating above 200°C in two steps with maximum rate temperatures differing by about 100–150°C. Swelling of the degrading mass seems, however, to be mainly due to gases evolved in the step occurring at the higher temperature. A strong dependence of the degradation process on the experimental conditions is observed. 相似文献
706.
In this paper the effect of aluminum trihydroxide (Al(OH)3—ATH) on the surface protection from fire for a styrene butadiene rubber (SBR) provided by ammonium polyphosphate ([NH4PO3]n—APP) is studied.The combustion behaviour is studied by means of Cone Calorimetric tests: a maximum fire retardant effect is observed with 10-12 wt.% of APP. ATH, less effective as fire retardant, has been added at 60 wt.% to reach comparable results as measured by cone calorimetry for 12 wt.% APP.Replacement of ATH in the best performing SBR + 12 wt.% of APP shows an antagonistic effect with as little as 1 wt.% of ATH.The combustion behaviour is explained with the mechanism of interaction between SBR, APP and ATH, in which formation of aluminum phosphates negatively affects the surface protection provided by the ultraphosphate surface coating formed on heating APP in SBR. 相似文献
707.
The thermal degradation under vacuum of copolymers of methyl methacrylate and 2,6-dimethoxycarbonyl-1,6-heptadiene of different compositions has been investigated. It has been found that the presence in the polymer chains of small amounts of cyclic structural units from the diene monomer considerably reduces the amount of degradation which occurs in poly(methyl methacrylate) at temperatures lower than 300°C. On the basis of the results of the analysis of the degradation products, a mechanism is suggested which accounts for this effect. 相似文献
708.
Addition Reactions of 2-Thiazolin-5-thiones with Acetylenes The reaction of the 2-thiazolin-5-thiones 1 and 5 , respectively, with acetylen carboxylates yields mainly 1, 4-dithiafulvenes of type 3. In the presence of the acetylenic compounds, the 1, 4-dithiafulvenes undergo an isomerization to give the corresponding 2, 3-dihydrothiophene-2-thiones 4. 相似文献
709.
Jean-Pierre Obrech Peter Schnholzer Christain Johannes Jenny Roland Prewo Heinz Heimgartner 《Helvetica chimica acta》1988,71(5):1319-1327
Reaction of 2,2-dialkyl-3-(dimethylamino)-2H-azirines 1a and 1b with 2,3-pyridinedicarboximide ( 4 ) in MeCN or DMF at room temperature yielded two regioisomeric tricyclic 1:1 adducts, the azacyclols 11/12 and 16/17 , respectively (Schemes 3 and 4). The structure of 12 was established by X-ray crystallography. Methanolysis of 11/12 and 16/17 led to mixtures of methyl [4, 4-dialkyl-5-(dimethylamino)-4H-imidazol-2-yl] pyridine carboxylates 13/14 and 18/19 , respectively. The structure of compound 14 is closely related to that of the powerful herbicide 9 (Scheme 9), i.e. the described reactions offer a new synthetic approach to this class of compounds. A mechanistic interpretation for the formation of regioisomeric 1:1 adducts as well as methyl (imidazol-2-yl) pyridine carboxylates is depicted in Scheme 5. 相似文献
710.
The symmetric rhenium(V) oxo Schiff base complexes trans-[ReO(OH2)(acac2en)]Cl and trans-[ReOCl(acac2pn)], where acac2en and acac2pn are the tetradentate Schiff base ligands N,N'-ethylenebis(acetylacetone) diimine and N,N'-propylenebis(acetylacetone) diimine, respectively, were reacted with monodentate phosphine ligands to yield one of two unique cationic phosphine complexes depending on the ligand backbone length (en vs pn) and the identity of the phosphine ligand. Reduction of the Re(V) oxo core to Re(III) resulted on reaction of trans-[ReO(OH2)(acac2en)]Cl with triphenylphosphine or diethylphenylphosphine to yield a single reduced, disubstituted product of the general type trans-[Re(III)(PR3)2(acac2en)]+. Rather unexpectedly, a similar reaction with the stronger reducing agent triethylphosphine yielded the intramolecularly rearranged, asymmetric cis-[Re(V)O(PEt3)(acac2en)]+ complex. Reactions of trans-[Re(V)O(acac2pn)Cl] with the same phosphine ligands yielded only the rearranged asymmetric cis-[Re(V)O(PR3)(acac2pn)]+ complexes in quantitative yield. The compounds were characterized using standard spectroscopic methods, elemental analyses, cyclic voltammetry, and single-crystal X-ray diffraction. The crystallographic data for the structures reported are as follows: trans-[Re(III)(PPh3)2(acac2en)]PF6 (H48C48N2O2P2Re.PF6), 1, triclinic (P), a = 18.8261(12) A, b = 16.2517(10) A, c = 15.4556(10) A, alpha = 95.522(1) degrees , beta = 97.130(1) degrees , gamma = 91.350(1) degrees , V = 4667.4(5) A(3), Z = 4; trans-[Re(III)(PEt2Ph)2(acac2en)]PF6 (H48C32N2O2P2Re.PF6), 2, orthorhombic (Pccn), a = 10.4753(6) A, b =18.4315(10) A, c = 18.9245(11) A, V = 3653.9(4) A3, Z = 4; cis-[Re(V)O(PEt3)(acac2en)]PF6 (H33C18N2O3PRe.1.25PF6, 3, monoclinic (C2/c), a = 39.8194(15) A, b = 13.6187(5) A, c = 20.1777(8) A, beta = 107.7730(10) degrees , V = 10419.9(7) A3, Z = 16; cis-[Re(V)O(PPh3)(acac2pn)]PF6 (H35C31N2O3PRe.PF6), 4, triclinic (P), a = 10.3094(10) A, b =12.1196(12) A, c = 14.8146(15) A, alpha = 105.939(2) degrees , beta = 105.383(2) degrees , gamma = 93.525(2) degrees , V = 1698.0(3) A3, Z = 2; cis-[Re(V)O(PEt2Ph)(acac2pn)]PF6 (H35C23N2O3PRe.PF6), 5, monoclinic (P2(1)/n), a = 18.1183(18) A, b = 11.580(1) A, c = 28.519(3) A, beta = 101.861(2) degrees , V = 5855.9(10) A(3), Z = 4. 相似文献