The effect of the sixth sulfur ligand in the catalytic mechanism of periplasmic nitrate reductase

The catalytic mechanism of nitrate reduction by periplasmic nitrate reductases has been investigated using theoretical and computational means. We have found that the nitrate molecule binds to the active site with the Mo ion in the +6 oxidation state. Electron transfer to the active site occurs only in the proton-electron transfer stage, where the MoV species plays an important role in catalysis. The presence of the sulfur atom in the molybdenum coordination sphere creates a pseudo-dithiolene ligand that protects it from any direct attack from the solvent. Upon the nitrate binding there is a conformational rearrangement of this ring that allows the direct contact of the nitrate with MoVI ion. This rearrangement is stabilized by the conserved methionines Met141 and Met308. The reduction of nitrate into nitrite occurs in the second step of the mechanism where the two dimethyl-dithiolene ligands have a key role in spreading the excess of negative charge near the Mo atom to make it available for the chemical reaction. The reaction involves the oxidation of the sulfur atoms and not of the molybdenum as previously suggested. The mechanism involves a molybdenum and sulfur-based redox chemistry instead of the currently accepted redox chemistry based only on the Mo ion. The second part of the mechanism involves two protonation steps that are promoted by the presence of MoV species. MoVI intermediates might also be present in this stage depending on the availability of protons and electrons. Once the water molecule is generated only the MoVI species allow water molecule dissociation, and, the concomitant enzymatic turnover.Fil: Cerqueira, N.M.F.S.A.. Faculdade de Ciências E Tecnologia, Universidade Nova de Lisboa; PortugalFil: Gonzalez, P.J.. Faculdade de Ciências E Tecnologia, Universidade Nova de Lisboa; PortugalFil: Brondino, Carlos Dante. Universidad Nacional del Litoral; ArgentinaFil: Romão, M.J.. Faculdade de Ciências E Tecnologia, Universidade Nova de Lisboa; PortugalFil: Romão, C.C.. Instituto de Tecnologia Qu&Fil: Moura, I.. Faculdade de Ciências E Tecnologia, Universidade Nova de Lisboa; PortugalFil: Moura, J.J.G.. Faculdade de Ciências E Tecnologia, Universidade Nova de Lisboa; Portuga

Anomalous diffusion on crumpled wires in two dimensions

It is investigated the statistical properties of random walks evolving on real configurations of a crumpled wire rigidly jammed in two dimensions. These crumpled hierarchical structures with complex topology are obtained from a metallic wire injected at a constant rate into a transparent planar cell of 20cm of diameter. The observed diffusion is anomalous with an exponent very close to that obtained at the threshold of two dimensional percolation. A comparison of the system studied in this paper with other systems of physical interest is also made, and an experimental consequence of our results is discussed.Comment: 10 pages including 4 figures. Physica A (2006). Preprin

How to Characterize Cylindrical Magnetic Nanowires

Cylindrical magnetic nanowires made through the help of nanoporous alumina templates are being fabricated and characterized since the beginning of 2000. They are still actively investigated nowadays, mainly due to their various promising applications, ranging from high-density magnetic recording to high-frequency devices, passing by sensors, and biomedical applications. They also represent suitable systems in order to study the dimensionality effects on a given material. With time, the development in fabrication techniques allowed to increase the obtained nanowire complexity (controlled crystallinity, modulated composition and/or geometry, range of materials, etc.), while the improvements in nanomanipulation permitted to fabricate system based either on arrays or on single nanowires. On the other side, their increased complexity requires specific physical characterization methods, due to their particular features such as high anisotropy, small magnetic volume, dipolar interaction field between them, and interesting electronic properties. The aim of this chapter was to offer an ample overview of the magnetic, electric, and physical characterization techniques that are suitable for cylindrical magnetic nanowire investigation, of what is the specific care that one needs to take into account and which information will be extracted, with typical and varied examples

Teste De Caminhada De Seis Minutos: Desempenho De CrianÇas Com Excesso De Peso

The six-minute walk test (6MWT) has been used to evaluate exercise functional capacity in children. Objective: To compare the performance in six-minute walk test in schoolchildren aged 8 to 10 years, according to nutritional classification based on two different criteria. Methods: The sample of 226 children aged 8 to 10 years was divided into three groups (eutrophic, overweight, and obese), according to the criteria of nutritional classification proposed by the World Health Organization (WHO) and by the International Obesity Task Force (IOTF) and assessed by the six-minute walk test. Results: The prevalence of overweight and obesity in the sample was 42.48% according to the WHO criteria and 38.94% according to the IOTF criteria. There were no statistically significant differences between the groups eutrophic, overweight, and obese, when the results of the 6MWT were compared, both in the distance covered and in the percentage predicted for children, according to the reference values, and accordin to the two criteria of nutritional classification. The final heart rate of the test was statistically higher in the overweight or obese groups, both by the WHO criteria (p=0.0015) and by the IOTF criteria (p=0.0160). Conclusions: In the studied sample, there was no difference in the performance of the 6MWT among eutrophic, overweight, and obese children. However, the final heart rate test was significantly higher among overweight children. The results emphasize that prevention, monitoring and treatment measures are necessary, promoting healthy habits. © 2017, Redprint Editora Ltda. All rights reserved.23214214

Mechanical Properties and Microstructural Characterization of Aged Nickel-based Alloy 625 Weld Metal

The aim of this work was to evaluate the different phases formed during solidification and after thermal aging of the as-welded 625 nickel-based alloy, as well as the influence of microstructural changes on the mechanical properties. The experiments addressed aging temperatures of 650 and 950 A degrees C for 10, 100, and 200 hours. The samples were analyzed by electron microscopy, microanalysis, and X-ray diffraction in order to identify the secondary phases. Mechanical tests such as hardness, microhardness, and Charpy-V impact test were performed. Nondestructive ultrasonic inspection was also conducted to correlate the acquired signals with mechanical and microstructural properties. The results show that the alloy under study experienced microstructural changes when aged at 650 A degrees C. The aging was responsible by the dissolution of the Laves phase formed during the solidification and the appearance of gamma aEuro(3) phase within interdendritic region and fine carbides along the solidification grain boundaries. However, when it was aged at 950 A degrees C, the Laves phase was continuously dissolved and the excess Nb caused the precipitation of the delta-phase (Ni3Nb), which was intensified at 10 hours of aging, with subsequent dissolution for longer periods such as 200 hours. Even when subjected to significant microstructural changes, the mechanical properties, especially toughness, were not sensitive to the dissolution and/or precipitation of the secondary phases