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Cotton FA Gruhn NE Gu J Huang P Lichtenberger DL Murillo CA Van Dorn LO Wilkinson CC 《Science (New York, N.Y.)》2002,298(5600):1971-1974
We report a class of molecules with extremely low ionization enthalpies, one member of which has been determined to have a gas-phase ionization energy (onset, 3.51 electron volts) lower than that of the cesium atom (which has the lowest gas-phase ionization energy of the elements) or of any other known closed-shell molecule or neutral transient species reported. The molecules are dimetal complexes with the general formula M2(hpp)4 (where M is Cr, Mo, or W, and hpp is the anion of 1,3,4,6,7,8-hexahydro-2H-pyrimido[1,2-a]pyrimidine), structurally characterized in the solid state, spectroscopically characterized in the gas phase, and modeled with theoretical computations. The low-energy ionization of each molecule corresponds to the removal of an electron from the delta bonding orbital of the quadruple metal-metal bond, and a strong interaction of this orbital with a filled orbital on the hpp ligands largely accounts for the low ionization energies. 相似文献
143.
In the 1960s and 1970s, communicable disease seemed a minor threat, but since then the emergence of new infections and the reemergence of old diseases has provoked a renewed focus on European communicable disease surveillance and control. A "network approach" among European countries has been successful in detecting some international outbreaks, but management and funding aspects remain unresolved. Surveillance outside the European Union has faced new challenges as a result of economic and political change following the collapse of communism. Subsequently, innovative international surveillance schemes are currently being implemented in the countries of Central and Eastern Europe and the former Soviet Union. The challenge for surveillance in Europe is to ensure that it has the capacity to meet both the needs of today and the diseases of the future. 相似文献
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Helmuth L 《Science (New York, N.Y.)》2002,296(5576):2131-2133
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Alfalfa mosaic virus genomic RNAs are infectious only when the viral coat protein binds to the RNA 3' termini. The crystal structure of an alfalfa mosaic virus RNA-peptide complex reveals that conserved AUGC repeats and Pro-Thr-x-Arg-Ser-x-x-Tyr coat protein amino acids cofold upon interacting. Alternating AUGC residues have opposite orientation, and they base pair in different adjacent duplexes. Localized RNA backbone reversals stabilized by arginine-guanine interactions place the adenosines and guanines in reverse order in the duplex. The results suggest that a uniform, organized 3' conformation, similar to that found on viral RNAs with transfer RNA-like ends, may be essential for replication. 相似文献
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