Electronic properties of a methane-water solution

Electronic properties of a methane-water solution were investigated by a sequential quantum mechanical/molecular dynamics approach. Upon hydration methane acquires an induced dipole moment of similar to 0.5 +/- 0.2 D. This is related to polarisation effects and to weak methane-water hydrogen bond interactions. From gas phase to solution, the first vertical excitation and ionisation energies of methane are red-shifted by 0.45 +/- 0.25 and 0.87 +/- 0.40 eV, respectively. We also report results for the dynamic polarisability of methane in water. In comparison with water, no difference was found for the average monomeric dipole moment of water molecules in close interaction with methane. (c) 2011 Elsevier B.V. All rights reserved.

Spin current in the Mobius cyclacene belts

We report on density functional theory studies of the electronic structure and magnetic properties of Mobius-[n]cyclacenes. The geometry of Mobius bands presents a modulation of bond lengths that is needed to accommodate the twist. This modulation takes the form of bond alternation defects analogous to those of solitons in polyacetylene. The ground state of all Mobius bands is a triplet, with a spin density distribution that follows the bond length modulation. A molecular dynamics simulation of the Mobius cyclacene at 300 K shows that the twist travels around the belt inducing a magnetic current. (C) 2009 Elsevier B. V. All rights reserved.

Theoretical investigation of Hf and Zr defects in c-Ge

The introduction of high-permittivity gate dielectric materials into complementary metal oxide semiconductor technology has reopened the interest in Ge as a channel material mainly due to its high hole mobility. Since HfO(2) and ZrO(2) are two of the most promising dielectric candidates, it is important to investigate if Hf and Zr may diffuse into the Ge channel. Therefore, using ab initio density functional theory calculations, we have studied substitutional and interstitial Hf and Zr impurities in c-Ge, looking for neutral defects. We find that (i) substitutional Zr and Hf defects are energetically more favorable than interstitial defects; (ii) under oxygen-rich conditions, neither Zr nor Hf migration towards the channel is likely to occur; (iii) either under Hf- or Zr-rich conditions it is very likely, particularly for Zr, that defects will be incorporated in the channel.

Ground-state energy of a classical artificial molecule

We study the ground-state energy of a classical artificial molecule formed by two-dimensional clusters (artificial atoms) of N/2 charged particles separated by a distance d. For the small molecules of N = 2 and 4, we obtain analytical expressions for this energy. For the larger ones, we calculate the ground-state energy using molecular dynamics simulation for N up to 128. From our numerical results, we are able to find out a function to approximate the ground-state energy of the molecules covering the range from atoms to molecules for any inter-atom distance d and for particle number from N = 8 to 128 within a difference less than one percent from the MD data.

Adsorption Behavior of 5-Fluorouracil on Au(111): An In Situ STM Study

The biological effects of chemical substitution of DNA bases triggered several investigations of their physicochemical properties This paper studies the adsorption behavior of a halogenated uracil, 5-fluorouracil (5FU). at the electrochemical interface of Au(111) and sulfuric acid solution. Upon modulation of the electric field across the interface, four distinct phases could be inferred by means of cyclic voltammetry (CV) At negative potentials relative to the SCE electrode, limited by the threshold of hydrogen evolution, no molecular species could be detected by scanning tunneling microscopy (STM) at the reconstructed Au(111)-(23 x root 3) surface, indicating that any physisorbed molecules are randomly distributed Incursion into more positive potentials increases the surface population but doer not form any two-dimensional (2D) physisorbed ordered structure Instead, we observed metastable structures that are only detectable. on surfaces with high defect density At sufficiently high positive potentials. limited by gold oxidation, the molecules are chemisorbed in a (3 x 2 root 3) ordered structure. with the aromatic ring perpendicular to the surface We report the densest chemisorbed monolayer for pyrimidine-derivative molecules (area per molecule 0 14 +/- 0 04 nm(2)). A comparison of the adsorption behavior of uracil derivatives has been made based on recent results of chemical substitution and solvent effects. We propose that pi-stacking is enhanced when halogens are incorporated in the uracil structure, in a similar fashion to what is observed in then crystal structure

Catalytic mechanism of inulinase from Arthrobacter sp S37

Detailed catalytic roles of the conserved Glu323, Asp460, and Glu519 of Arthrobacter sp. S37 inulinase (EnIA), a member of the glycoside hydrolase family 32, were investigated by site-directed mutagenesis and pH-dependence studies of the enzyme efficiency and homology modeling were carried out for EnIA and for D460E mutant. The enzyme efficiency (k(cat)/K-m) of the E323A and E519A mutants was significantly lower than that of the wild-type due to a substantial decrease in k(cat), but not due to variations in K-m, consistent with their putative roles as nucleophile and acid/base catalyst, respectively. The D460A mutant was totally inactive, whereas the D460E and D460N mutants were active to some extent, revealing Asp460 as a catalytic residue and demonstrating that the presence of a carboxylate group in this position is a prerequisite for catalysis. The pH-dependence studies indicated that the pK(a) of the acid/base catalyst decreased from 9.2 for the wild-type enzyme to 7.0 for the D460E mutant, implicating Asp460 as the residue that interacts with the acid/base catalyst Glu519 and elevates its pK(a). Homology modeling and molecular dynamics simulation of the wild-type enzyme and the D460E mutant shed light on the structural roles of Glu323, Asp460, and Glu519 in the catalytic activity of the enzyme. (C) 2008 Elsevier Inc. All rights reserved.

The Mobility of Chondroitin Sulfate in Articular and Artificial Cartilage Characterized by (13)C Magic-Angle Spinning NMR Spectroscopy

We have studied the molecular dynamics of one of the major macromolecules in articular cartilage, chondroitin sulfate. Applying (13)C high-resolution magic-angle spinning NMR techniques, the NMR signals of all rigid macromolecules in cartilage can be suppressed, allowing the exclusive detection of the highly mobile chondroitin sulfate. The technique is also used to detect the chondroitin sulfate in artificial tissue-engineered cartilage. The tissue-engineered material that is based on matrix producing chondrocytes cultured in a collagen gel should provide properties as close as possible to those of the natural cartilage. Nuclear relaxation times of the chondroitin sulfate were determined for both tissues. Although T(1) relaxation times are rather similar, the T(2) relaxation in tissue-engineered cartilage is significantly shorter. This suggests that the motions of chondroitin sulfate in data:rat and artificial cartilage different. The nuclear relaxation times of chondroitin sulfate in natural and tissue-engineered cartilage were modeled using a broad distribution function for the motional correlation times. Although the description of the microscopic molecular dynamics of the chondroitin sulfate in natural and artificial cartilage required the identical broad distribution functions for the correlation times of motion, significant differences in the correlation times of motion that are extracted from the model indicate that the artificial tissue does not fully meet the standards of the natural ideal. This could also be confirmed by macroscopic biomechanical elasticity measurements. Nevertheless, these results suggest that NMR is a useful tool for the investigation of the quality of artificially engineered tissue. (C) 2010 Wiley Periodicals, Inc. Biopolymers 93: 520-532, 2010.

Orthogonal packing of identical rectangles within isotropic convex regions

A mixed integer continuous nonlinear model and a solution method for the problem of orthogonally packing identical rectangles within an arbitrary convex region are introduced in the present work. The convex region is assumed to be made of an isotropic material in such a way that arbitrary rotations of the items, preserving the orthogonality constraint, are allowed. The solution method is based on a combination of branch and bound and active-set strategies for bound-constrained minimization of smooth functions. Numerical results show the reliability of the presented approach. (C) 2010 Elsevier Ltd. All rights reserved.

Minimizing the object dimensions in circle and sphere packing problems

Given a fixed set of identical or different-sized circular items, the problem we deal with consists on finding the smallest object within which the items can be packed. Circular, triangular, squared, rectangular and also strip objects are considered. Moreover, 2D and 3D problems are treated. Twice-differentiable models for all these problems are presented. A strategy to reduce the complexity of evaluating the models is employed and, as a consequence, instances with a large number of items can be considered. Numerical experiments show the flexibility and reliability of the new unified approach. (C) 2007 Elsevier Ltd. All rights reserved.

Effect of the Y(2)O(3)-ZrO(2) support composition on nickel catalyst evaluated in dry reforming of methane

Nickel catalysts with a load of 5 wt.% Ni, supported on pure ZrO(2) and ZrO(2) stabilized with 4 mol%, 8 mol% and 12 mol% of Y(2)O(3), were prepared by the polymerization method. The samples were characterized by X-ray diffraction (XRD), temperature-programmed reduction with hydrogen (TPR-H(2)), specific surface area (BET) and electronic paramagnetic resonance (EPR) and tested as catalysts for carbon dioxide reforming of methane. The XRD patterns showed the presence of the oxide precursor (NiO) and the tetragonal phase of a Y(2)O(3)-ZrO(2) solid solution. According to the TPR-H(2) analysis, the reduction of various NiO species was influenced by the composition of the support. Catalytic tests were conducted at 800 degrees C for 6 h, and the composition of the gaseous products and the catalytic conversion rate depended on the composition of the Y(2)O(3)-ZrO(2) solid solution and its influence on the supported NiO species. A direct relation was observed between the variation in the support, the nickel species supported on it and the performance in the catalytic tests. (C) 2008 Elsevier B.V. All rights reserved.

Local Structure and Photochromic Response in Ormosils Containing Dodecatungstophosphoric Acid

Two hybrid materials based on dodecatungstophosphoric acid (HPW) dispersed in ormosils modified with 3-aminopropiltrietoxysilane (APTS) or with N-(3-(trimethoxysilyl)-propyl)-ethylene-diamine (TSPEN) show reversible photochromic response induced by irradiation in the 200-390 nm UV range. A set of solid-state nuclear magnetic resonance (NMR) techniques was used to analyze the structural properties of the main components of these hybrids (the HPW polyanion, the inorganic matrix, and the organic functionalities). For the ormosils, the use of (29)Si NMR, {(1)H}-(29)Si cross-polarization, and {(1)H}-(29)Si HETCOR revealed a homogeneous distribution of silicon species Q ``, T(2), and T(3) for the APTS hybrid, contrasting with the separation of T(3) species in the TSPEN hybrid. The combination of (31)P NMR, {(1)H}-(31)P cross-polarization and (31)P-{(1)H} spin-echo double resonance (SEDOR) revealed the dispersion of the HPW ions in the ormosil, occupying sites with a high number of close protons (>50). Differences in the molecular dynamics at room temperature, inferred from SEDOR experiments, indicate a state of restricted mobility of the HPW ion and the surrounding molecular groups in the TSPEN hybrid. This behavior is consistent with the presence of more amino groups in the TSPEN, acting as chelating groups to the HPW ion. This hybrid, with the strong chelate interaction of the diamine group, shows the most intense photochromic response, in agreement with the charge transfer models proposed to explain the photochromic effect. Electronic reflectance spectroscopy in irradiated samples revealed the presence of one-electron and two-electron reduced polyanions. The one-electron reduced species could be detected also by (31)P NMR spectroscopy immediately after UV irradiation.

Influência de parâmetros moleculares em funções de correlação temporal na dinâmica de solvatação mecânica

No presente trabalho descrevemos nossos resultados relativos à investigação da dinâmica de solvatação mecânica por meio de simulações por dinâmica molecular, respeitando o regime da resposta linear, em sistemas-modelo de argônio líquido com um soluto monoatômico ou diatômico dissolvido. Estudamos sistematicamente a influência dos parâmetros moleculares dos solutos (tamanho, polarizabilidade) e da densidade frente a vários modelos de solvatação. Funções de Correlação Temporal da Energia de Solvatação foram calculadas com relação à correlações de n-corpos (n = 2; 3) distinguindo interações repulsivas e atrativas para ambos os sistemas líquidos. Também obtivemos segundas derivadas temporais dessas funções referindo-se à parcelas translacionais, rotacionais e roto-translacionais na solução do diatômico. Encontramos que funções de correlação temporal coletivas podem ser razoavelmente bem aproximadas por correlações binárias a densidades baixas e, a densidades altas, correlações ternárias tornam-se mais importantes produzindo um descorrelacionamento mais rápido das funções coletivas devido a efeitos de cancelamento parciais. As funções de correlação para interações repulsivas e atrativas exibem comportamentos dinâmicos independentes do modelo de solvatação devido a fatores de escalonamento linear que afetam apenas as amplitudes das dessas funções de correlação temporal. Em geral, os sistemas com grau de liberdade rotacional apresentam tempos de correlação mais curtos para a dinâmica coletiva e tempos de correlação mais longos para as funções binárias e ternárias. Finalmente, esse estudo mostra que os sistemas contendo o diatômico relaxam-se predominantemente por mecanismos translacionais binários em modelos de solvatação envolvendo alterações apenas na polarizabilidade do soluto, e por mecanismos rotacionais atrativos binários em modelos envolvendo alterações no comprimento de ligação.

Otimização de alavancagem e gestão de risco em estratégias long-short

Alavancagem em hedge funds tem preocupado investidores e estudiosos nos últimos anos. Exemplos recentes de estratégias desse tipo se mostraram vantajosos em períodos de pouca incerteza na economia, porém desastrosos em épocas de crise. No campo das finanças quantitativas, tem-se procurado encontrar o nível de alavancagem que otimize o retorno de um investimento dado o risco que se corre. Na literatura, os estudos têm se mostrado mais qualitativos do que quantitativos e pouco se tem usado de métodos computacionais para encontrar uma solução. Uma forma de avaliar se alguma estratégia de alavancagem aufere ganhos superiores do que outra é definir uma função objetivo que relacione risco e retorno para cada estratégia, encontrar as restrições do problema e resolvê-lo numericamente por meio de simulações de Monte Carlo. A presente dissertação adotou esta abordagem para tratar o investimento em uma estratégia long-short em um fundo de investimento de ações em diferentes cenários: diferentes formas de alavancagem, dinâmicas de preço das ações e níveis de correlação entre esses preços. Foram feitas simulações da dinâmica do capital investido em função das mudanças dos preços das ações ao longo do tempo. Considerou-se alguns critérios de garantia de crédito, assim como a possibilidade de compra e venda de ações durante o período de investimento e o perfil de risco do investidor. Finalmente, estudou-se a distribuição do retorno do investimento para diferentes níveis de alavancagem e foi possível quantificar qual desses níveis é mais vantajoso para a estratégia de investimento dadas as restrições de risco.

Effect of combined hormonal and insulin therapy on the steroid hormone receptors and growth factors signalling in diabetic mice prostate

Fundação de Amparo à Pesquisa do Estado de São Paulo (FAPESP)

Structural and functional properties of Bp-LAAO, a new L-amino acid oxidase isolated from Bothrops pauloensis snake venom

Fundação de Amparo à Pesquisa do Estado de São Paulo (FAPESP)