Cation-Dependent Self-assembly of Vanadium Polyoxoniobates

Reaction of Na7H[Nb6O19]·15H2O with NaVO3·2H2O at 220 °C in the presence of NaHCO3 gives new bicapped α-Keggin vanadododecaniobate [VNb12O40{NbO(CO3)}2]13–, isolated and structurally characterized as Na9H4[VNb12O40{NbO(CO3)}2]·37H2O (1). According to 51V NMR and ESI-MS data, this anion equilibrates in solution with [VNb12O40]15– and oligomeric species that result from dissociation of the {NbO(CO3)}+ fragments. In the presence of potassium, the same reaction gives [VxNb24O76]n− (x = 4, n = 12 (2a); x = 3, n = 17 (2b)). The anions with x = 3 and 4 cocrystallize together, but exist as separate entities both in solid and in solution according to 51V MAS NMR and ESI-MS data

Comparative Analysis of Exo- and Endonuclease Activities of APE1-like Enzymes

Apurinic/apyrimidinic (AP)-endonucleases are multifunctional enzymes that are required for cell viability. AP-endonucleases incise DNA 5′ to an AP-site; can recognize and process some damaged nucleosides; and possess 3′-phosphodiesterase, 3′-phosphatase, and endoribonuclease activities. To elucidate the mechanism of substrate cleavage in detail, we analyzed the effect of mono- and divalent metal ions on the exo- and endonuclease activities of four homologous APE1-like endonucleases (from an insect (Rrp1), amphibian (xAPE1), fish (zAPE1), and from humans (hAPE1)). It was found that the enzymes had similar patterns of dependence on metal ions’ concentrations in terms of AP-endonuclease activity, suggesting that the main biological function (AP-site cleavage) was highly conserved among evolutionarily distant species. The efficiency of the 3′-5′ exonuclease activity was the highest in hAPE1 among these enzymes. In contrast, the endoribonuclease activity of the enzymes could be ranked as hAPE1 ≈ zAPE1 ≤ xAPE1 ≤ Rrp1. Taken together, the results revealed that the tested enzymes differed significantly in their capacity for substrate cleavage, even though the most important catalytic and substrate-binding amino acid residues were conserved. It can be concluded that substrate specificity and cleavage efficiency were controlled by factors external to the catalytic site, e.g., the N-terminal domain of these enzymes