Ein Pionier oder mehrere Pioniere? Die Entdeckung der Edelgas-Verbindungen

Für die breite Öffentlichkeit haben Wissenschaftsgeschichte und Speisekarten eines gemeinsam: Wie Pfirsich Melba, Sauce Soubise oder Tournedos Rossini wird auch manche wissenschaftliche Entdeckung einem einzelnen zugeschrieben. Durch solch eine über-Vereinfachung können wichtige oder entscheidende Beitrage anderer allzu leicht in Vergessenheit geraten. Die Entdeckungsgeschichte der Edelgas-Verbindungen paßt sich dieser stereotypen und mythischen Sichtweise an: Nach gängiger Sicht wurden sie von Neil S. Bartlett, seinerzeit an der University of British Columbia, vor etwa einem Vierteljahrhundert, am 23. März 1962, entdeckt. Eine umfassende und endgültige Entdeckungsgeschichte kann, schon aus Platzgründen, nicht geschrieben werden; dies wäre auch verfrüht. Stattdessen sollen die (theoretischen) Konzepte und die frühen Versuche zur Synthese von Edelgas-Verbindungen, die Bartletts Erfolg um drei Jahrzehnte vorausgingen, dokumentiert werden. Die Motivation hierzu ergibt sich aus den drei folgenden Fragen: 1. Welcher Art waren die Einsichten und Beiträge von Linus Pauling ? 2. Welche Gründe können für den Fehlschlag der Versuche von Yost und Kaye zur Synthese von Edelgasfluoriden in den dreißiger Jahren vermutet werden? 3. Gibt es andere Forscher, die die Reaktivität der Edelgase vorausgesehen haben?     

Post-translational modifications in prion proteins

Prions are a novel class of infectious pathogens that cause a group of fatal prion diseases in which the benign cellular form of the prion protein (PrP(C)) is transformed into the disease-related scrapie variant (PrP(SC)). The two PrP isoforms differ in their structure and resistance to degradation. The molecular mechanism by which the PrP(SC) is formed and causes infectivity or neurodegeneration is not known. In a compelling and emerging view, post-translational modifications (or the lack thereof) play roles in the transformation of PrP(C) to PrP(SC). Human PrP contains two consensus sites for N-linked glycosylation, at Asn181 and Asn197. From the functional standpoint, glycosylation can modify either the conformation of PrP(C), or the stability of PrP(SC) and, hence, the rate of PrP(SC) clearance. So far the NMR structures of only recombinant, non-glycosylated prions are known, while the structure of the glycosylated form is estimated by molecular modeling. A number of native amino acid mutations in PrP can be mapped near the glycosylation sites. Normal prion protein has been demonstrated to be a copper binding protein, and increasing evidence has shown correlation between the level of PrP expression and tolerance to oxidative stress. Moreover, histochemistry for nitrotyrosine is used for detection of neuronal labeling, a sign of a peroxynitrite-mediated neuronal degradation and a marker for nitrative stress in scrapie-infected mouse brains. It is an intriguing proposition that the post-translational modifications alone, or in combination with amino acid changes, play dominant roles in the pathogenic transformation of PrP(C) to PrP(SC).     

Synthesis and characterization of novel 99gTc(V) and Re(V) complexes with water-soluble tetraaza diamido dipyridino ligands: single-crystal X-ray structural investigations of mono- and dinuclear complexes

Rhenium and technetium are known for their useful applications in nuclear medicine with similar properties. In this study, new diamido dipyridino (N(4)) water-soluble ligands (2-C(5)H(4)NCH(2)NHCO)(2)CH(2), 1 (L(1)H2), (2-C(5)H(4)NNHNHCO)(2)CH(2), 2, and [2-C(5)H(4)N(+)(O)(-)CH(2)NHCO](2)CH(2), 3, were synthesized. Reaction of L(1)H2 with ReOCl(3)(PPh(3))(2) resulted in the novel six-coordinated rhenium(V) complex, trans-ReO(L(1))(OEt), 4. The complex was characterized by spectroscopic methods, and its X-ray crystallographic analysis revealed that rhenium is coordinated to four nitrogen atoms of the ligand and to two oxygen atoms from the deprotonated ethanol and the oxo group respectively in a distorted octahedral geometry. In solution, complex 4 was transformed to a new complex 5, which was proved to be the dinuclear complex mu-oxo [ReO(L(1))](2)O. Reaction of 1 with [n-Bu(4)N][ReOCl(4)] resulted in the neutral complex 6, trans-[ReO(L(1))]Cl. Similarly, when ligand 1 was reacted with [n-Bu(4)N][(99g)TcOCl(4)], the neutral trans-[(99)TcO(L(1))]Cl complex 7 was formed, which upon dissolution transformed into a cationic complex 8, trans-[(99)TcO(L(1))(OH(2))](+)Cl(-). The single-crystal X-ray structure of 8 reveals that the coordination sphere about technetium is a distorted octahedron with four nitrogen atoms in the equitorial plane, while doubly bonded oxygen and coordinated water occupy the apical positions. Further dissolution of 8 resulted in the formation of dinuclear mu-oxo [TcO(L(1))](2)O, 9. This study shows that Tc and Re have similar metal core structures in solution for diamido dipyridino systems, besides similarity in geometrical structure, proved by the X-ray structures on the same ligands.     

Deterministic chaos in the Belousov-Zhabotinsky reaction: Experiments and simulations

An account of the experimental discovery of complex dynamical behavior in the continuous-flow, stirred tank reactor (CSTR) Belousov-Zhabotinsky (BZ) reaction, as well as numerical simulations based on the BZ chemistry are given. The most recent four- and three-variable models that are deduced from the well-accepted, updated chemical mechanism of the BZ reaction and which exhibit robust chaotic states are summarized. Chaos has been observed in experiments and simulations embedded in the regions of complexities at both low and high flow rates. The deterministic nature of the observed aperiodicities at low flow rates is unequivocally established. However, controversy still remains in the interpretation of certain aperiodicities observed at high flow rates.     

Organic Diselenides and Ditellurides: Disproportionations,Synthesis of Stannyl Selenides,Reactions with Acetylenes

The disproportionation reaction of organic diselenides 1 and ditellurides 2, the synthesis of stannyl selenides 4, and the addition reaction (palladium-catalyzed, thermal) of dialkyl diselenides to acetylenes are discussed.     

Derivation of component aerosol direct radiative forcing at the top of atmosphere for clear-sky oceans

A two-step approach is proposed to derive component aerosol direct radiative forcing (ADRF) at the top of atmosphere (TOA) over global oceans from 60°S to 60°N for clear-sky condition by combining Terra CERES/MODIS-SSF shortwave (SW) flux and aerosol optical thickness (AOT) observations with the fractions of component AOTs from the GSFC/GOCART model. The derived global annual mean component ADRF is +0.08±0.17 W/m² for black carbon, −0.52±0.24 W/m² for organic carbon, −1.10±0.42 W/m² for sulfate, −0.99±0.37 W/m² for dust, −2.44±0.84 W/m² for sea salt, and −4.98±1.67 W/m² for total aerosols. The total ADRF has also been partitioned into anthropogenic and natural components with a value of −1.25±0.43 and −3.73±1.27 W/m², respectively. The major sources of error in the estimates have also been discussed. The analysis adds an alternative technique to narrow the large difference between current model-based and observation-based global estimates of component ADRF by combining the satellite measurement with the model simulation.