Local Structure of Iridate Pyrochlores from Hydrothermal Synthesis

Iridate pyrochlores of general formula M₂Ir₂O₇ have potential applications in catalysis [1]. They also often exhibit unusual magnetic and electronic properties caused by spin-orbit coupling and geometric frustration [2]. A detailed understanding of structure is necessary to enable these properties to be understood and exploited. Because of the propensity of the pyrochlore structure to accommodate structural disorder, we have chosen to utilise the technique of total scattering to examine the structure of M₂Ir₂O₇ (M = Bi, Nd). The sensitivity of our measurements to all the constituent elements is maximised by the combination of both neutron and X-ray total scattering. We find no evidence for magnetic ordering in our samples of Nd₂Ir₂O₇, in contrast to literature reports [3]. By comparing the local structure of our samples with that of one reported to exhibit magnetic ordering, we explore the possibility of a structural origin for the differences in magnetic behaviour. We have found that synthesis method can directly influence the structure of these iridate pyrochlores. Local structural analysis provides evidences of A-site cation deficiency and partial oxidation of Ir(IV) to Ir(V) in samples produced by hydrothermal techniques. Irreversible changes to the lattice parameter upon heating these samples at 400 - 900 ⁰C further support the inference that the cation content is somewhat variable. We report the results of reverse Monte Carlo (RMC) refinements using the program RMCProfile, which is capable of simultaneously fitting to X-ray and neutron data, and therefore provides structural models of the greatest possible accuracy. We also report the results of in situ X-ray total scattering measurements which provide local-scale insight into the interesting thermal behaviour and apparent flexible cation content of these materials.

Crystal structure of the complex of GLPO with an anti-thyroid drug PTU

Lactoperoxidase is a member of mammalian heme peroxidase superfamily that consists of lactoperoxidase (LPO), myloperoxidase (MPO), thyroid peroxidase (TPO), eosinophil peroxidase (EPO). So far crystal structures of only two mammalian peroxidases LPO and MPO have been determined. The structures of LPO have been determined from the samples obtained from bovine, buffalo, goat and sheep. Their complexes have also been determined with inhibitors and substrate analogues. However, the structures of EPO and TPO have not been obtained so far. Due to significant sequence identity between LPO and TPO, the structures of these proteins are expected to be similar. Similarly their substrates will also have similarity as well as the substrate and enzyme interactions will be similar. Therefore, a complex of LPO was prepared with propylthiouracil (PTU), a compound used as a drug in thyroid ailments. The complex was crystallized using co-crystallization. The crystals of native LPO were also soaked in the solution containing PTU. The crystals belonged to monoclinic space group with cell dimensions a = 80.2 Å, b = 82.5, c = 95.0, β = 73.7° There were four molecules of LPO in the asymmetric unit. The structure determination of the complex revealed that PTU binds to the LPO at the distal heme site. It is held at this site through several hydrogen bonds and van der Waals contacts. The mode of binding and number of interactions suggest that a similar mode of binding may occur with TPO. Based on the information of interactions and the missing potential interactions, the modifications in the structure of PTU are suggested so that the improved design of the TPO inhibitor is obtained.