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991.
A new parameterization of the SINDO model is introduced in the recently developed MSINDO version for third‐row main group elements Ga, Ge, As, Se, and Br. It is shown that the accuracy achieved for structure and stability as well as ionization potential and dipole moment of compounds containing these elements is good enough to make the extension a suitable tool for the study of larger systems. © 2000 John Wiley & Sons, Inc. J Comput Chem 21: 974–987, 2000 相似文献
992.
Karl‐Heinz Diener 《Mathematical Logic Quarterly》2000,46(4):563-568
We will prove that some so‐called union theorems (see [2]) are equivalent in ZF0 to statements about the transitive closure of relations. The special case of “bounded” union theorems dealing with κ‐hereditary sets yields equivalents to statements about the transitive closure of κ‐narrow relations. The instance κ = ω1 (i. e., hereditarily countable sets) yields an equivalent to Howard‐Rubin's Form 172 (the transitive closure Tc(x) of every hereditarily countable set x is countable). In particular, the countable union theorem (Howard‐Rubin's Form 31) and, a fortiori, the axiom of countable choice imply Form 172. 相似文献
993.
Modifying the Steric Properties in the Second Coordination Sphere of Designed Peptides Leads to Enhancement of Nitrite Reductase Activity 下载免费PDF全文
Dr. Karl J. Koebke Dr. Fangting Yu Elvin Salerno Casey Van Stappen Dr. Alison G. Tebo Prof. James E. Penner‐Hahn Prof. Vincent L. Pecoraro 《Angewandte Chemie (International ed. in English)》2018,57(15):3954-3957
Protein design is a useful strategy to interrogate the protein structure‐function relationship. We demonstrate using a highly modular 3‐stranded coiled coil (TRI‐peptide system) that a functional type 2 copper center exhibiting copper nitrite reductase (NiR) activity exhibits the highest homogeneous catalytic efficiency under aqueous conditions for the reduction of nitrite to NO and H2O. Modification of the amino acids in the second coordination sphere of the copper center increases the nitrite reductase activity up to 75‐fold compared to previously reported systems. We find also that steric bulk can be used to enforce a three‐coordinate CuI in a site, which tends toward two‐coordination with decreased steric bulk. This study demonstrates the importance of the second coordination sphere environment both for controlling metal‐center ligation and enhancing the catalytic efficiency of metalloenzymes and their analogues. 相似文献
994.
Rosemary Anibogwu Karl De Jesus Samjhana Pradhan Srinath Pashikanti Sameena Mateen Kavita Sharma 《Molecules (Basel, Switzerland)》2021,26(22)
Diverse medicinal plants such as those from the genus Artemisia have been employed globally for centuries by individuals belonging to different cultures. Universally, Artemisia species have been used to remedy various maladies that range from simple fevers to malaria. A survey conducted by the World Health Organization (WHO) demonstrated that 80% of the global population is highly reliant on herbal medicine for their primary healthcare. WHO recommends artemisinin-based combination therapies (ACT) for the treatment of global diseases such as malaria. Artemisinin is a bioactive compound derived from Artemisia annua leaves. It is a sesquiterpene endoperoxide with potent antimalarial properties. This review strives to instill natural products to chemists and others in diverse fields with a heterogeneous set of knowledge compiled from multifaceted researchers and organizations in literature. In particular, the various Artemisia species and effective extraction, isolation, and characterization methodologies are discussed in detail. An in-depth investigation into the literature reveals that divergent species of Artemisia exhibit a vast array of biological activities such as antimalarial, antitumor, and anti-inflammatory activities. There is substantial potential for bioactive compounds from Artemisia to provide significant relief from differing human ailments, but more meticulous research in this field is needed. 相似文献
995.
996.
Catherine Pale-Grosdemange Corinna Feil Michel Rohmer Karl Poralla 《Angewandte Chemie (International ed. in English)》1998,37(16):2237-2240
Tetracyclic triterpenes (for example, 3 ) are formed as minor products of the cyclization of squalene ( 1 ) by squalene/hopene cyclase; they have also been found in vivo. The identification of these side products offers new insight into the complex biosynthesis of the hopane skeleton, and proves that the enzyme has no absolute control of the conformation of the acyclic substrate squalene and that tetra- and pentacyclic carbocationic intermediates are involved (for example, 2 ) in the cyclization process. 相似文献
997.
998.
Heiko Kalpen Wolfgang Hnle Mehmet Somer Ulrich Schwarz Karl Peters Hans Georg von Schnering Roger Blachnik 《无机化学与普通化学杂志》1998,624(7):1137-1147
Bismuth(II) Chalcogenometallates(III) Bi2M4X8, Compounds with Bi24+ Dumbbells (M = Al, Ga and X = S, Se) The ternary bismuth(II) chalcogenometallates(III) Bi2M4X8 (with M = Al, Ga and X = S, Se) were synthesized from the binary chalcogenides M2X3 and Bi2X3 and elementary bismuth. All compounds are diamagnetic semiconductors with Eg (opt.) = 1.8–2.7 eV. The phases (except Bi2Al4Se8) are thermodynamically stable and decompose peritectically above 965–1020 K. Bi2Al4Se8 is metastable below 825 K and is obtained only by rapid quenching from T > 825 K. The isotypic compounds crystallize in a new tetragonal tP28 structure type (P4/nnc). The characteristic unit is the hitherto unknown clustercation Bi24+, with the mean bond length d(Bi–Bi) = 314.2 pm, the Raman frequency 102 cm–1 ≤ νs ≤ 108 cm–1, and the mean force constant of f = 0.68 N · cm–1. The Electron Localization Function, ELF, shows the covalent Bi–Bi bond, the lone electron pairs of the ψ-octahedrally coordinated Bi(II) cations, and the polar character of the Bi–X bonds. 相似文献
999.
Amido Ligands for the Synthesis of Polynuclear Lanthanoid Complexes By 1 : 1 reaction of LnBr3 with NaNHPh in THF the Rare-Earth Complexes [Ln2Br4(μ2-NHPh)2(thf)5] (Ln = Sm ( 1 ), Ln = Gd ( 2 )) with two bridging anilido ligands are obtained. In the system LnBr3/NaNHPh/(Me2SiO)3 the tetranuclear compounds [Ln4(μ4-O)(NHPh)3(OSiMe2NPh)6Na5(thf)7] · THF (Ln = Gd ( 3 ), Ln = Yb ( 4 )) can be built up. They have a central μ4-oxygene atom in the Ln4-tetrahedron. It has an oxa-dimethylsilyl-N-phenylamido ligand over all edges and an anilido ligand on three vertexes. By this reaction small amount of [Na4(thf)6Yb2(OSiMe2NPhSiMe2O)2(OSiMe2NPh)2(NHPh)2] ( 5 ) with a O- and N-bridged Yb–Na polyhedron and N-phenyl-bis(dimethylsilanolato)-ligands coordinating μ2-η2 with its oxygen atoms are obtained. Reaction of Lanthanideshalides with LiNHtBu leads to dimeric complexes. The formation of bridging oxasilylamido ligands is also observed. The compound [Li2Ln(OSiMe2NtBu)2(HNtBu)(thf)]2 (Ln=Sm ( 6 ), Gd ( 7 ) and Yb ( 8 )) contains now an O- and N-bridged Ln–Li polyhedron. (Crystal Data of 1–8 see ‘‘Inhaltsverzeichnis”︁”︁). 相似文献
1000.
The reaction of W6Br12, NaBr, and WO2Br2 in the presence of Br2 in a sealed silica tube yields Na[W2O2Br6] together with WOBr4 and WO2Br2 in the low temperature zone (temperature gradient 1030/870 K). Na[W2O2Br6] crystallizes orthorhombically in the space group Immm (no. 71) with a = 3.775 Å, b = 10.400 Å, c = 13.005 Å and Z = 2. Pairs of condensed trans-[WO2Br4] octahedra with a common Br2 edge form along [100] double chains [W2O4/2Br6]1– via the oxygen atoms. The mixed valent tungsten atoms are bonded to W2 pairs with a 2 c–3 e bond (d(W–W) = 2.946 Å, d(W–O) = 1.888 Å, d(W–Brb) = 2.537 Å, d(W–Brt) = 2.535 Å, ∢O–W–O = 177.4°, ∢Brb–W–Brb (endocyclic) = 109.0°). The Na+ cations connect the anionic double chains to form two-dimensional layers parallel (001), which interact by van der Waals forces. The cations are eightfold coordinated by a cube of the terminal Brt ligands of the polymeric anions (d(Na–Br) = 3.138 Å). Na[W2O2Br6] may be discussed as an intercalation compound of the oxide bromide WOBr3. 相似文献