Fourier images in coherent convergent beam electron diffraction and atomic resolution scanning transmission electron microscopy

Sodalite-like rare-earth carbonates: a study of structural transformation and diluted magnetism

A series of novel rare earth carbonates, RE3(OH)6(CO3)Cl (RE = Dy, Er, Y), with sodalite-like (SOD-like) zeolite topologies have been successfully synthesized by introducing an appropriate amount of CO3(2-) from NaCO3 or atmospheric carbon dioxide as a template. Single-crystal X-ray diffraction reveals that the structure consists of the RE3(OH)6(3+) cationic framework with a SOD-like topology built from vertex-sharing [RE4(μ3-OH)4] cubes. The CO3(2-) anions seal the 6-ring opening and Cl(-) anions situated in the channels to achieve charge balance. After calcination at 370 °C, the compound RE3(OH)6(CO3)Cl in situ transforms into a new phase formulated as RE3O4Cl. Interestingly, the structure of RE3O4Cl represents a new SOD-like open framework, associated with the removal of the CO3(2-) from RE3(OH)6(CO3)Cl. The samples are characterized by thermogravimetric analysis (TGA), elemental analysis, X-ray photoelectron spectroscopy and magnetic studies. Furthermore, single-molecule magnet behaviours can be observed for the diluted samples of Dy0.0068Y2.9932(OH)6(CO3)Cl and Dy0.0068Y2.9932O4Cl with a Dy/Y molar ratio of up to 1/440 as well as Er0.19Y2.81(OH)6(CO3)Cl with an Er/Y ratio of 1/15, showing dominant single-ion effects.

Functionalization of Krebs-type polyoxometalates with N,O-chelating ligands: a systematic study

The first organic derivatives of 3d-metal-disubstituted Krebs-type polyoxometalates have been synthesized under mild bench conditions via straightforward replacement of labile aqua ligands with N,O-chelating planar anions on either preformed or in situ-generated precursors. Nine hybrid clusters containing carboxylate derivatives of five- or six-membered aromatic N-heterocycles as antenna ligands have been obtained as pure crystalline phases and characterized by elemental and thermal analyses, infrared spectroscopy, and single-crystal X-ray diffraction. They all show the general formula [{M(II)L(H2O)}2(WO2)2(B-β-XW9O33)2](n-) and can be classified as follows: 1-SbM, where L = 1H-imidazole-4-carboxylate (imc), X = Sb(III), n = 12, and M(II) = Mn, Co, Ni, Zn; 1-TeM, where L = imc, X = Te(IV), n = 10, and M(II) = Mn, Co; 2-SbNi, where L = 1H-pyrazole-3-carboxylate (pzc), X = Sb(III), n = 12, and M(II) = Ni; and 3-SbM, where L = pyrazine-2-carboxylate (pyzc), X =Sb(III), n = 12, and M(II) = Co, Zn. The 3d-metal-disubstituted tungstotellurate(IV) skeleton of compounds 1-TeM is unprecedented in polyoxometalate chemistry. The stability of these hybrid Krebs-type species in aqueous solution has been confirmed by (1)H NMR spectroscopy performed on the diamagnetic 1-SbZn and 3-SbZn derivatives. Our systematic study of the reactivity of disubtituted Krebs-type polyoxotungstates toward diazole-, pyridine-, and diazinecarboxylates demonstrates that organic derivatization is strongly dependent on the nature of the ligand, as follows: imc displays a "universal ligand" character, as functionalization takes place regardless of the external 3d metal and heteroatom; pzc and pyzc show selectivity toward specific 3d metals; pyridazine-3-carboxylate and pyrimidine-4-carboxylate promote partial decomposition of specific precursors, leading to [M(II)L2(H2O)2] complexes; and picolinate is inert under all conditions tested.

Reconstructing a missing link in the evolution of a recently diverged phosphotriesterase by active-site loop remodeling

Only decades after the introduction of organophosphate pesticides, bacterial phosphotriesterases (PTEs) have evolved to catalyze their degradation with remarkable efficiency. Their closest known relatives, lactonases, with promiscuous phosphotriasterase activity, dubbed PTE-like lactonases (PLLs), share only 30% sequence identity and also differ in the configuration of their active-site loops. PTE was therefore presumed to have evolved from a yet unknown PLL whose primary activity was the hydrolysis of quorum sensing homoserine lactones (HSLs) (Afriat et al. (2006) Biochemistry 45, 13677-13686). However, how PTEs diverged from this presumed PLL remains a mystery. In this study we investigated loop remodeling as a means of reconstructing a homoserine lactonase ancestor that relates to PTE by few mutational steps. Although, in nature, loop remodeling is a common mechanism of divergence of enzymatic functions, reproducing this process in the laboratory is a challenge. Structural and phylogenetic analyses enabled us to remodel one of PTE's active-site loops into a PLL-like configuration. A deletion in loop 7, combined with an adjacent, highly epistatic, point mutation led to the emergence of an HSLase activity that is undetectable in PTE (k(cat)/K(M) values of up to 2 × 10(4)). The appearance of the HSLase activity was accompanied by only a minor decrease in PTE's paraoxonase activity. This specificity change demonstrates the potential role of bifunctional intermediates in the divergence of new enzymatic functions and highlights the critical contribution of loop remodeling to the rapid divergence of new enzyme functions.