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Mn(II) and Fe(II) compounds derived from azide and the zwitterionic l-carboxylatomethylpyridinium-4-carbox)date ligand are isomorphous three-dimensional metal--organic frameworks (MOFs) with the sra net, in which the metal ions are connected into anionic chains by mixed (μ-l, l-azide)bis(μ-carboxylate) triple bridges and the chains are cross-linked by the cationic backbones of the zwitterionic ligands. The Mn(ll) MOFs display typical one-dimensional antiferromagnetic behavior. In contrast, with one more d electron per metal center, the Fe(n) counterpart shows intrachain ferromagnetic interactions and slow relaxation of magnetization attributable to the single-chain components. The activation energies for magnetization reversal in the infinite- and finite-chain regimes are Δτ1, = 154 K and Δτ2 = 124 K, respectively. Taking advantage of the isomorphism between the Mn(II) and Fe(II) MOFs, we have prepared a series of mixedmetal Mn1-xIIFexII MOFs with x = 0.41, 0.63, and 0.76, which intrinsically feature random isotropic/anisotropic sites and competing antiferromagnetic--ferromagnetic interactions. The materials show a gradual antiferromagnetic-to-ferromagnetic evolution in overall behaviors as the Fe(II) content increases, and the Fe-rich materials show complex relaxation processes that may arise for mixed SCM and spin-glass mechanisms. A general trend is that the activation energy and the blocking temperature increase with the Fe(II) content, emphasizing the importance of anisotropy for slow relaxation of magnetization. [ABSTRACT FROM AUTHOR]

The effect of monovalent ions on both the reactivity and global folding of the 8-17 DNAzyme is investigated, and the results are compared with those of the hammerhead ribozyme, which has similar size and secondary structure. In contrast to the hammerhead ribozyme, the 8-17 DNAzyme activity is not detectable in the presence of 4 M K+, Rb+, or Cs+ or in the presence of 80 mM, [Co(NH3)6]3+. Only 4 M Li+, NH4+ and, to a lesser extent, Na+ conferred detectable activity. The observed rate constants (Kobs ≈ 10-3 min-1 for Li+ and NH4+) are ~1000-fold lower than that in the presence of 10mM Mg2+, and ~200,000- fold slower than that in the presence of 100 μM Pb2+. Since the hammerhead ribozyme displays monovalent ion-dependent activity that is often within ~10-fold of divalent metal ion-dependent activity, these results suggest that the 8-17 DNAzyme, obtained by in vitro selections, has evolved to have a more stringent divalent metal ion requirement for high activity as compared to the naturally occurring ribozymes, making the 8-17 DNAzyme an excellent choice as a Pb2+ sensor with high selectivity. In contrast to the activity data, folding was observed in the presence of all the monovalent ions investigated, although those monovalent ions that do not support DNAzyme activity have weaker binding affinity (Kd ~ 0.35 M for Rb+ and Cs+), while those that confer DNAzyme activity possess stronger affinity (Kd 0.22 M for Li+, Na+ and NH4+). In addition, a correlation between metal ion charge density, binding affinity and enzyme activity was found among mono- and divalent metal ions except Pb2+ higher charge density resulted in stronger affinity and higher activity, suggesting that the observed folding and activity is at least partially due to electrostatic interactions between ions and the DNAzyme. Finally, circular dichroism (CD) study has revealed Z-DNA formation with the monovalent metal ions, Zn2+ and Mg2+ the Kd values obtained using CD were in the same range as those obtained from folding studies using FRET. However, Z-DNA formation was not observed with Pb2+. These results indicate that Pb2+-dependent function follows a different mechanism from the monovalent metal ions and other divalent metal ions; in the presence of latter metal ions, metal- ion dependent folding and structural changes, including formation of Z-DNA, play an important role in the catalytic function of the 8-17 DNAzyme. [ABSTRACT FROM AUTHOR]