Adenine versus guanine quartets in aqueous solution: dispersion-corrected DFT study on the differences in π-stacking and hydrogen-bonding behavior

We have investigated the performance of the dispersion-corrected density functionals (BLYP-D, BP86-D and PBE-D) and the widely used B3LYP functional for describing the hydrogen bonds and the stacking interactions in DNA base dimers. For the gas-phase situation, the bonding energies have been compared to the best ab initio results available in the literature. All dispersion-corrected functionals reproduce well the ab initio results, whereas B3LYP fails completely for the stacked systems. The use of the proper functional leads us to find minima for the adenine quartets, which are energetically and structurally very different from the C_4h structures, and might explain why adenine has to be sandwiched between guanine quartets to form planar adenine quartets.     

Color alteration in teeth subjected to different bleaching techniques

This study evaluated the color alteration of teeth subjected to the action of different bleaching agents and the influence of light sources commonly used in association with these products, In GI, the specimens remained immersed in artificial saliva. The specimens in GII were bleached with a 10% carbamide peroxide gel 4 hours/day during 3 weeks; the teeth in the other three groups were subjected to three sessions of three 10-min applications of 35% hydrogen peroxide gel at 7-day intervals. In GIII, no light was used, while in GIV and GV the gel was associated with a quartz-tungsten-halogen light and a LED/laser source, respectively. The teeth color was evaluated before and 7 days after the bleaching sessions by reflectance spectrophotometry. Data were analyzed statistically by ANOVA and Fisher’s test (α = 0.05), and showed that a significant color change was obtained in all treated groups. After the first week of treatment and at the end of it, the bleaching protocols showed similar results. The results of the present study indicate that association of a light source is not necessary to obtain the bleaching effect and that optimal bleaching can be achieved with all techniques tested.     

Characterization of Commercial Amylases for the Removal of Filter Cake on Petroleum Wells

Drilling fluid has many functions, such as carry cuttings from the hole permitting their separation at the surface, cool and clean the bit, reduce friction between the drill pipe and wellbore, maintain the stability of the wellbore, and prevent the inflow of fluids from the wellbore and form a thin, low-permeable filter cake. Filter cake removal is an important step concerning both production and injection in wells, mainly concerning horizontal completion. The drilling fluids are typically comprised of starch, the most important component of the filter cake. A common approach to remove this filter cake is the use of acid solutions. However, these are non-specific reactants. A possible alternative is the use of enzymatic preparations, like amylases, that are able to hydrolyze starch. Wells usually operate in drastic conditions for enzymatic preparations, such as high temperature, high salt concentration, and high pressure. Thus, the main objective of this work was to characterize four enzymatic preparations for filter cake removal under open hole conditions. The results showed that high salt concentrations (204,000 ppm NaCl) in completion fluid decreased amylolytic activity. All enzymatic preparations were able to catalyze starch hydrolysis at all temperatures tested (30, 65, 80, and 95 °C). An increase of amylolytic activity was observed with the increase of pressure (100, 500 and 1,000 psi) for one commercial amylase.     

Revisiting the Γ(K→eν)/Γ(K→μν) ratio in supersymmetric unified models

We present a generator for single top-quark production via flavour-changing neutral currents. The MEtop event generator allows for Next-to-Leading-Order direct top production pp??t and Leading-Order production of several other single top processes. A?few packages with definite sets of dimension six operators are available. We discuss how to improve the bounds on the effective operators and how well new physics can be probed with each set of independent dimension six operators.     

The diameter of the acyclic Birkhoff polytope

In this work we give an interpretation of vertices and edges of the acyclic Birkhoff polytope, T_n=Ω_n(T), where T is a tree with n vertices, in terms of graph theory. We generalize a recent result relatively to the diameter of the graph G(T_n).     

Synthesis of modified vermiculite by interaction with aromatic heterocyclic amines

Vermiculite of general formula [Si_6.85Al_1.15][Mg_4.68Al_0.51Fe_0.63]O₂₀(OH)₄Ca_0.128Na_0.032K_0.094 reacted with heteroaromatic amines α-, β-, and γ-picolines from aqueous solution. The products were characterized by elemental analysis, infrared spectroscopy, and X-ray diffraction. The intercalated nanocompounds maintained the crystallinity and changed the original interlayer distance of 1422 pm to 1474, 1456, and 1474 pm, for the sequence of the guest picoline molecules. Natural and intercalated vermiculite can remove copper at the solid/liquid interface; removal 0.40 mmol g⁻¹ was obtained for the original matrix, and 1.10, 0.92, and 1.33 mmol g⁻¹ for the intercalated forms. These values are near the capacity of cation exchange (CEC) of this clay mineral, which can be possibly used as source of copper removal from aqueous solution.     

Fractionation of HDPE on quench cooling

In a previous paper we discussed co-crystallization in a LDPE/HDPE blend using TREF and DSC. As part of that study it was observed that pure HDPE showed an unexpected fractionation behavior when quench crystallized in TREF. The overall peak broadened and two peaks appeared instead of the previously observed single peak for slow cooled HDPE.The development of two peaks was observed for all commercial HDPEs investigated, independent of their melting indices and densities. TREF and GPC were used in an attempt to evaluate the origin of the two HDPE components.The authors appreciate support from CAPES-BRAZIL (C.A.F.). Additional thanks go to Dr. G. W. Knight in Dow Chemical Company for kindly providing the polymer samples and performing the GPC analysis.     

Composition of higher fullerenes obtained by laser ablation of carboniferous materials

The compositions of higher fullerenes produced by ablation of various carboniferous materials are experimentally studied. The yield of fullerenes versus the type of carboniferous materials and laser radiation parameters is found.     

Explicit inverse of a tridiagonal k−Toeplitz matrix

Summary We obtain explicit formulas for the entries of the inverse of a nonsingular and irreducible tridiagonal k–Toeplitz matrix A. The proof is based on results from the theory of orthogonal polynomials and it is shown that the entries of the inverse of such a matrix are given in terms of Chebyshev polynomials of the second kind. We also compute the characteristic polynomial of A which enables us to state some conditions for the existence of A^–1. Our results also extend known results for the case when the residue mod k of the order of A is equal to 0 or k–1 (Numer. Math., 10 (1967), pp. 153–161.).The work was supported by CMUC (Centro de Matemática da Universidade de Coimbra) and by Acção Integrada Luso-Espanhola E-6/03