Ba3ZnTa2-xNbxO9 and Ba3MgT2-xNbxO9: synthesis, structural and dielectric studies

Oxides of the families Ba3ZnTa2-xNbxO9 and Ba3MgTa2-xNbxO9 were obtained by the solid state reaction route at 1573 K and were found to crystallize in the disordered (cubic) perovskite structure. In Ba3ZnTa2-xNbXO9 and Ba3MgTa2-xNbxO9 the entire range (0 less than or equal to x less than or equal to 1) of solid solutions could be synthesized. The dielectric constant decreases with increase in frequency for all compositions in the range 40 Hz to 100 kHz (epsilon (r) varies from 16 to 22). The dielectric loss (D) shows a broad maximum for both Ba3ZnTa2-xNbxO9 and Ba3MgTa2-xNbxO9. The maxima is centered around 2 kHz in the former and near 10 kHz in the latter. (C) 2001 Elsevier Science Ltd. All sights reserved.

Structural alteration from non-B to B-form could reflect DNase I hypersensitivity

Preferential cleavage of active genes by DNase I has been correlated with a structurally altered conformation of DNA at the hypersensitive site in chromatin. To have a better understanding of the structural requirements for gene activation as probed by DNase I action, digestability by DNase I of synthetic polynucleotides having the ability to adopt B and non-B conformation (like Z-form) was studied which indicated a marked higher digestability of the B-form of DNA. Left handed Z form present within a natural sequence in supercoiled plasmid also showed marked resistance towards DNase I digestion. We show that alternating purine-pyrimidine sequences adopting Z-conformation exhibit DNAse I foot printing even in a protein free system. The logical deductions from the results indicate that 1) altered structure like Z-DNA is not a favourable substrate for DNase I, 2) both the ends of the alternating purine-pyrimidine insert showed hypersensitivity, 3) B-form with a minor groove of 12-13 A is a more favourable substrate for DNase I than an altered structure, 4) any structure of DNA deviating largely from B form with a capacity to flip over to the B-form are potential targets for the DNase I enzymic probes in naked DNA.