A neutron diffraction study of the R15Ge9C compounds (R = Ce, Pr, Nd)

In this work we report the results of the neutron diffraction investigation performed on the germanides R15Ge9C, for R = Ce, Pr and Nd (La15Ge9Fe-type, hP50, P63mc, Z = 2), to refine the crystal superstructure of these compounds and determine their magnetic structures. The interstitial carbon atoms occupy mainly the 2b Wyckoff site in the position (1/3 2/3 ∼1/2) and also, with a smaller occupancy rate, the Wyckoff site 2a at (0 0 ∼1/2). In the magnetic state, the three compounds display predominantly a ferromagnetic behavior with the propagation vector k = [0 0 0]. These results are in agreement with the magnetization measurements, with TC = 10, 30 and 80 K as Curie temperature of Ce15Ge9C, Pr15Ge9C and Nd15Ge9C, respectively. Ce15Ge9C and Nd15Ge9C present a ferromagnetic alignment of the R moments along the c-axis and an antiferromagnetic spin arrangement within the (a-b) plane. For Pr15Ge9C the ferromagnetic contribution is found within the (a-b) plane, as previously observed for the isotypic compound Tb15Si9C. The carbides crystal structure possesses four inequivalent rare earth sites carrying different magnetic moments, leading to mean values of 0.9 μB/Ce, 1.1 μB/Pr and 2.2 μB/Nd for Ce15Ge9C, Pr15Ge9C and Nd15Ge9C, respectively. The magnetic structures of these R15Ge9C compounds differ strongly from those of their parent R5Ge3 germanides, but present strong similarities with the structures of the R15Si9C compounds. The overall results indicate and confirm the drastic influence of carbon insertion in the rare earth environment

Crystal Chemistry of the New Families of Interstitial Compounds R6Mg23C (R = La, Ce, Pr, Nd, Sm, or Gd) and Ce6Mg23Z (Z = C, Si, Ge, Sn, Pb, P, As, or Sb)

The crystal chemical features of the new series of compounds R6Mg23C with R = La-Sm or Gd and Ce6Mg23Z with Z = C, Si, Ge, Sn, Pb, P, As, or Sb have been studied by means of single-crystal and powder X-ray diffraction techniques. All phases crystallize with the cubic Zr6Zn23Si prototype (cF120, space group Fm3m, Z = 4), a filled variant of the Th6Mn23 structure. While no Th6Mn23-type binary rare earth-magnesium compound is known to exist, the addition of a third element Z (only 3 atom %), located into the octahedral cavity of the Th6Mn23 cell (Wyckoff site 4a), stabilizes this structural arrangement and makes possible the formation of the ternary R6Mg23Z compounds. The results of both structural and topological analyses as well as of LMTO electronic structure calculations show that the interstitial element plays a crucial role in the stability of these phases, forming a strongly bonded [R6Z] octahedral moiety spaced by zeolite cage-like [Mg45] clusters. Considering these two building units, the crystal structure of these apparently complex intermetallics can be simplified to the NaCl-type topology. Moreover, a structural relationship between RMg3 and R6Mg23C compounds has been unveiled; the latter can be described as substitutional derivatives of the former. The geometrical distortions and the consequent symmetry reduction that accompany this transformation are explicitly described by means of the B\ue4rnighausen formalism within group theory

Magnetic ordering of novel La3NiGe2-type R3CoGe2 compounds (R = Pr, Nd, Sm, Gd-Dy)

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Crystal Chemistry of the New Families of Interstitial Compounds R₆Mg₂₃C (R = La, Ce, Pr, Nd, Sm, or Gd) and Ce₆Mg₂₃Z (Z = C, Si, Ge, Sn, Pb, P, As, or Sb)

The crystal chemical features of the new series of compounds R₆Mg₂₃C with R = La–Sm or Gd and Ce₆Mg₂₃Z with Z = C, Si, Ge, Sn, Pb, P, As, or Sb have been studied by means of single-crystal and powder X-ray diffraction techniques. All phases crystallize with the cubic Zr₆Zn₂₃Si prototype (<i>cF</i>120, space group <i>Fm</i>3̅<i>m</i>, <i>Z</i> = 4), a filled variant of the Th₆Mn₂₃ structure. While no Th₆Mn₂₃-type binary rare earth–magnesium compound is known to exist, the addition of a third element Z (only 3 atom %), located into the octahedral cavity of the Th₆Mn₂₃ cell (Wyckoff site 4<i>a</i>), stabilizes this structural arrangement and makes possible the formation of the ternary R₆Mg₂₃Z compounds. The results of both structural and topological analyses as well as of LMTO electronic structure calculations show that the interstitial element plays a crucial role in the stability of these phases, forming a strongly bonded [R₆Z] octahedral moiety spaced by zeolite cage-like [Mg₄₅] clusters. Considering these two building units, the crystal structure of these apparently complex intermetallics can be simplified to the NaCl-type topology. Moreover, a structural relationship between RMg₃ and R₆Mg₂₃C compounds has been unveiled; the latter can be described as substitutional derivatives of the former. The geometrical distortions and the consequent symmetry reduction that accompany this transformation are explicitly described by means of the Bärnighausen formalism within group theory

The active essential CFNS3d protein complex.

The NS2B-NS3 protease complex is essential for the replication of dengue virus, which is the etiologic agent of dengue and hemorrhagic fevers, diseases that are a burden for the tropical and subtropical areas of the world. The active form of the NS3 protease linked to the 40 residues of the NS2B cofactor shows highly flexible and disordered region(s) that are responsible for its high propensity to aggregate at the concentrations necessary for NMR spectroscopy studies or for crystallization. Limited proteolysis of this active form of the protease enabled us to obtain a folded and new essential form of the NS2B-NS3 protease complex. We found that the region from residues D50 to E80 of NS2B interacts directly and strongly with the NS3 protease domain. The proteolytic activity of the noncovalently binding complex was determined by a rapid and continuous fluorescence resonance energy transfer activity assay using a depsipeptide substrate. The new protein-cofactor complex obtained, encompassing the NS2B fragment (D50-E80) and the NS3 protease, shows proteolytic activity. The H-1-N-15-heteronuclear single quantum coherence spectrum of the isotopically enriched protein complex shows good cross-peak dispersion; this is indicative of a stable folded state. Our results significantly complement the X-ray structure of the NS2B-NS3pro complex published recently. Moreover, these results open the way to performing direct structural and interaction studies in solution on a new active NS2B-NS3pro complex with libraries of substrates and inhibitors in order to identify new drugs that prevent viral polyprotein processing