RESUMEN
The synthesis and structure of the title compound, (C(24)H(20)P)(2)[Ge(C(2)O(4))(3)], are reported. The PPh(4)(+) cations in the structure form infinite zigzag chains in which the P.P distances alternate between 6.229 (1) and 7.118 (1) A, and the P.P.P angle is 131.4 (1) A. The shorter P.P distance is associated with a sixfold phenyl embrace. However, the longer P.P distance is associated with both phenyl-phenyl interactions and interactions between the cations and a twofold symmetric [Ge(C(2)O(4))(3)](2-) anion. In the cation-anion interactions, the P.O distance is 4.444 (2) A, the O.P-C(distal) angle is 175.0 (1) degrees and the shortest H.O distances are 2.74 and 3.09 A.
RESUMEN
Higher plants, algae and some yeasts respond to potentially toxic heavy metals such as cadmium by synthesizing phytochelatins and related cysteine-rich polypeptides. We have used X-ray absorption spectroscopy to study the nature of cadmium binding in such peptides isolated from maize (Zea mays) exposed to low levels of cadmium, and in two synthetic cadmium-peptide complexes, Cd-(gamma-Glu-Cys)3Gly and Cd-(alpha-Glu-Cys)3Gly. We have used the synthetic ions [Cd(SPh)4]2-, [Cd4(SPh)10]2- and [S4Cd10(SPh)16]4-as crystallographically defined models for the cadmium site. The Cd K-edge extended X-ray absorption fine structure (EXAFS) data, together with the Cd K, LI, LII and LIII near-edge spectra, reveal a predominantly tetrahedral coordination of cadmium by sulfur in both the phytochelatin and synthetic peptide complexes. In particular, the Cd LIII-edge lacks a peak at 3534.9 e V which was found to be prominent for oxygen- or nitrogen-coordinated species. The Cd-S distance in the phytochelatin complex is 2.54 A. The Cd K-edge EXAFS does not show any isolated, well-defined Cd-Cd interactions; however, contrary to the conclusion of previous work, their absence is not necessarily indicative of isolated cadmium-thiolate ligation. Evidence from other studies suggests that high static disorder, combined with a large vibrational component, serve to effectively wash out this contribution to the EXAFS. The sulfur K-edge, moreover, shows a low-energy feature both in the phytochelatin and in the synthetic cadmium-peptide complexes which is consistent with sulfide bound in a cluster with cadmium as found for [S4Cd10(SPh)16]4-. This feature strongly suggests the presence of a polynuclear cadmium cluster in maize phytochelatin.
Asunto(s)
Cadmio/química , Metaloproteínas/química , Compuestos Organometálicos/química , Proteínas de Plantas/química , Glutatión , Metaloproteínas/aislamiento & purificación , Modelos Moleculares , Fitoquelatinas , Proteínas de Plantas/aislamiento & purificación , Espectrometría por Rayos X , Compuestos de Sulfhidrilo/química , Sulfuros/química , Zea maysRESUMEN
Evidence is provided to show that Technegas has structure compatible with the Buckminsterfullerene model C60 in which 99Tcm atoms are trapped.
Asunto(s)
Grafito , Pertecnetato de Sodio Tc 99m/química , Humanos , Pulmón/diagnóstico por imagen , Espectrometría de Masas/métodos , Generadores de Radionúclidos , CintigrafíaRESUMEN
Solid-state 13C nuclear magnetic resonance (NMR) spectra of a number of inclusion compounds of 2,6-dimethyl-bicyclo[3.3.1]nonane-exo-2-exo-6-diol (host) with small organic small molecules (guests) have been studied. With 3,4-dichloro-1,2,5-thiadiazole and tetrachloroethylene as guests, line splittings of the host resonances were observed due to the location of the guest in the host lattice. The cross-polarization (CP) dynamics of these inclusion compounds have been studied and shown to be indicative of weakly coupled systems. As expected, the proton spin lattice relaxation times in the rotating frame (T1pH) of the host are increased by the presence of rapidly moving guest because the efficiency of homonuclear dipolar relaxation in the rotating frame is reduced. However, strong transient oscillations were also observed for the guest molecules during the Hartmann-Hahn transfer of magnetisation from the more abundant 1H spins to the 13C spins during spin lattice rotating frame relaxation. These oscillations were found to be greatest for carbons with largest chemical shift anisotropies.