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Salcedo J.,Instituto Politcnico Nacional | Salcedo B.,Instituto Tecnolgico Autnomo Of Mxico | Snchez-Bernabe F.J.,University Autnoma Metropolitana Iztapalapa
Computers and Mathematics with Applications | Year: 2011

In this paper we introduce a method for the numerical solutions of initial value problems, that combines finite differences with Simpson's rule. The effectiveness of the method is proved by solving, in one spatial dimension, a stiff and convection-dominated transport problem. To solve the same problem in two spatial dimensions, the proposed method was used successfully in combination with Strang's operator decomposition method. © 2010 Elsevier Ltd. All rights reserved.

Crdenas C.,University of Chile | Crdenas C.,McMaster University | Ayers P.W.,McMaster University | Cedillo A.,University Autnoma Metropolitana Iztapalapa
Journal of Chemical Physics | Year: 2011

Density-functional-theory-based chemical reactivity indicators are formulated for degenerate and near-degenerate ground states. For degenerate states, the functional derivatives of the energy with respect to the external potential do not exist, and must be replaced by the weaker concept of functional variation. The resultant reactivity indicators depend on the specific perturbation. Because it is sometimes impractical to compute reactivity indicators for a specific perturbation, we consider two special cases: point-charge perturbations and Dirac delta function perturbations. The Dirac delta function perturbations provide upper bounds on the chemical reactivity. Reactivity indicators using the common used average of degenerate states approximation for degenerate states provide a lower bound on the chemical reactivity. Unfortunately, this lower bound is often extremely weak. Approximate formulas for the reactivity indicators within the frontier-molecular-orbital approximation and special cases (two or three degenerate spatial orbitals) are presented in the supplementary material. One remarkable feature that arises in the frontier molecular orbital approximation, and presumably also in the exact theory, is that removing electrons sometimes causes the electron density to increase at the location of a negative (attractive) Dirac delta function perturbation. That is, the energetic response to a reduction in the external potential can increase even when the number of electrons decreases. © 2011 American Institute of Physics.

Guevara-Chumacero L.M.,University Autnoma Metropolitana Iztapalapa | Lpez-Wilchis R.,University Autnoma Metropolitana Iztapalapa | Pedroche F.F.,University Autnoma Metropolitana Iztapalapa | Juste J.,Estacin Biolgica de Donana CSIC | And 2 more authors.
Journal of Mammalogy | Year: 2010

Pteronotus davyi, Davy's naked-backed bat, is a tropical bat whose current distribution in Mexico covers the watersheds of the Pacific and the Gulf coasts of Mexico, converges at the Isthmus of Tehuantepec, and continues into the Yucatan peninsula. We evaluated phylogeographic relationships among 18 populations in Mexico using data from the nucleotide sequence of the hypervariable II domain of the mitochondrial DNA control region from 105 specimens. Extant populations were distributed over 3 geographic regions (Pacific Coast, Gulf Coast, and Southeastern) that were delineated a priori according to floristic characteristics and biogeography. Coalescent simulations supported a phylogeographic model of 2 refugia situated in the south of Mexico (Arc and Soconusco refuges) during the late Pleistocene, followed by expansion into Pacific Coast, Gulf Coast, and Southeastern groups. The populations of the Southeast were genetically divergent from the Pacific and Gulf Coast populations, supporting the existence of 2 distinct lineages of P. davyi in Mexico, likely due to the Isthmus of Tehuantepec acting as a geographical barrier. Examination of our data revealed genetic differentiation of the Pacific and Gulf coastal groups but at a lower level relative to the Southeast. This pattern suggests that the Sierra Madre Occidental, Sierra Madre Oriental, and Sierra Madre del Sur act as physical barriers to dispersal for P. davyi. © 2010 American Society of Mammalogists.

Ramrez Trejo M.D.R.,University Autnoma Metropolitana Iztapalapa | Perez-Garca B.,University Autnoma Metropolitana Iztapalapa | Perez-Salicrup D.R.,University of Morelia | Orozco-Segovia A.,National University of Costa Rica
Journal of Tropical Ecology | Year: 2010

Pteridium caudatum is a fern that frequently invades burnt areas in the Yucatn Peninsula and other neotropical sites. While post-fire regeneration of this fern apparently occurs mainly by vegetative means, little is known about the role of its spores in post-fire regeneration and in colonization of newly invaded fields. Central to these questions is whether bracken fern spores maintain their viability after fires. Here we experimentally evaluate the effect of fire-induced temperatures on Pteridium caudatum spore germination. We used 1200-cm3 blocks containing a constant fuel load of 47.4 g of litter, in which we placed spores at three different depths. The blocks were then ignited, and temperatures at each depth were monitored at 1-min intervals for 2 h. One day after the experimental fires, spores were dug out and cultured at 25 C and 12-h light/dark cycles. Soil temperatures decreased significantly in relation to depth during fires. Spores on the surface were severely affected by fire, while those buried at 1 and 3 cm showed 77% germination. Germination in unburned controls was 86%. Our results suggest that during fires, Pteridium caudatum spores buried a few centimetres below the surface have a high percentage of viability, which could explain the rapid establishment of this species in burnt fields. Copyright © 2010 Cambridge University Press.

Vazquez-Luna A.,University Autnoma Metropolitana Iztapalapa | Rivera-Cabrera F.,University Autnoma Metropolitana Iztapalapa | Perez-Flores L.J.,University Autnoma Metropolitana Iztapalapa | Diaz-Sobac R.,University of Veracruz
Journal of Economic Entomology | Year: 2011

The effect of the use of rootstock Criollo on the susceptibility of Manila mango fruit to infestation by Anastrepha obliqua (Macquart) was determined in the present investigation. Growth, quality parameters (Soluble solids content (%), citric acid content (%), firmness, monoterpene volatiles (3-carene, limonene, α-pinene, and β-myrcene), main flavonoids content (milligrams/100 g fresh pulp), and larvicidal activity of methanolic extracts, as well as the degree of infestation during preharvest development of the fruits were analyzed. The results indicated that the rootstock did not have any significant effect on growth, soluble solids content, or citric acid content; although it increased firmness as well as 3-carene levels and main flavonoids content, resulting in a greater resistance to infestation by A. obliqua. The obtained results support the use of this rootstock because of the beneficial effects observed on the resistance of mango fruit to infestation by this fruit fly. © 2011 Entomological Society of America.

Gonzlez-Candela E.,University Autnoma Metropolitana Iztapalapa | Romero-Rochn V.,National University of Costa Rica | Del Ro F.,University Autnoma Metropolitana Iztapalapa
Journal of Chemical Physics | Year: 2011

Brownian ratchets have recently been considered as models to describe the ability of certain systems to locate very specific states in multidimensional configuration spaces. This directional process has particularly been proposed as an alternative explanation for the protein folding problem, in which the polypeptide is driven toward the native state by a multidimensional Brownian ratchet. Recognizing the relevance of robustness in biological systems, in this work we analyze such a property of Brownian ratchets by pushing to the limits all the properties considered essential to produce directed transport. Based on the results presented here, we can state that Brownian ratchets are able to deliver current and locate funnel structures under a wide range of conditions. As a result, they represent a simple model that solves the Levinthals paradox with great robustness and flexibility and without requiring any ad hoc biased transition probability. The behavior of Brownian ratchets shown in this article considerably enhances the plausibility of the model for at least part of the structural mechanism behind protein folding process. © 2011 American Institute of Physics.

Chapela G.A.,University Autnoma Metropolitana Iztapalapa | De Ro F.,University Autnoma Metropolitana Iztapalapa | Alejandre J.,University Autnoma Metropolitana Iztapalapa
Journal of Chemical Physics | Year: 2011

Molecular aggregates are formed by heteronuclear vibrating square-well dumbbells. In a recent article [G. A. Chapela and J. Alejandre, J. Chem. Phys., 132(10), 104704 (2010)], it is shown that heteronuclear vibrating square-well dumbbells with a diameter ratio between particles of 1/2 and interacting potential ratio of 4 form micelles of different sizes and shapes which manifest themselves in both the liquid and vapor phases, up to and above the critical point. This means that micellization and phase separation are present simultaneously in this simple model. These systems present a maximum in the critical temperature when plotted against the potential well depth of the second particle 2. In the same publication, it was speculated that the formation of micelles was responsible for the appearance of the maximum. A thorough study on this phenomena is presented here and it is found that there is a threshold on the size of the second particle and its corresponding depth of interaction potential, where the micelles are formed. If the diameter and well depth of the second particle are small enough for the first and deep enough for the second, micelles are formed. For 21 between 0.25 and 0.65 and 21 larger than 5.7, micelles are formed up to and above the critical temperature. Outside these ranges micelles appear only at temperatures lower than the critical point. There is a strong temperature dependence on the formation and persistence of the aggregates. For the deepest wells and large enough second particles, a gel interconnected aggregate is obtained. In this work, the micelles are formed at temperatures as low as the triple point and as high as the critical point and, in some cases, persist well above it. The presence of these maxima in critical temperatures Tc when plotted against 2 as follows. At lower values of 2, an increase of Tc is obtained as is expected by the increase of the attractive volume as indicated by the principle of corresponding states. As 2 increases further, the formation of molecular aggregates produce a saturation effect of the deepening of the potential well by encapsulating the particles of the second kind inside the micelles, so the resulting Tc represents a new poly disperse system of molecular aggregates and not the original heteronuclear vibrating square-well dumbbells. The surface tension is also analyzed for these systems, and it is shown that decreases with increasing attraction due to the formation of molecular aggregates. © 2011 American Institute of Physics.

Valdes-Parada F.J.,University Autnoma Metropolitana Iztapalapa | Alvarez-Ramrez J.,University Autnoma Metropolitana Iztapalapa
Journal of Chemical Physics | Year: 2011

Asymmetric diffusion has been observed in different contexts, from transport in stratified and fractured porous media to diffusion of ions and macromolecular solutes through channels in biological membranes. Experimental and numerical observations have shown that diffusion is facilitated in the direction of positive void fraction (i.e., porosity) gradients. This work uses the method of volume averaging in order to obtain effective medium equations for systems with void fraction gradients for passive and diffusive mass transport processes. The effective diffusivity is computed from the solution of an associated closure problem in representative unit cells that allow considering porosity gradients. In this way, the results in this work corroborate previous findings showing that the effective diffusivity exhibits important directional asymmetries for geometries with void fraction gradients. Numerical examples for simple geometries (a section with an obstacle and a channel with varying cross section) show that the diffusion asymmetry depends strongly on the system configuration. The magnitude of this dependence can be quantified from the results in this work. © 2011 American Institute of Physics.

Aguirre-Hernandez B.,University Autnoma Metropolitana Iztapalapa | Lpez-Renteria J.-A.,University Autnoma Metropolitana Iztapalapa | Verduzco F.,University of Sonora
Mathematical Problems in Engineering | Year: 2011

The boundary crossing theorem and the zero exclusion principle are very useful tools in the study of the stability of family of polynomials. Although both of these theorem seem intuitively obvious, they can be used for proving important results. In this paper, we give generalizations of these two theorems and we apply such generalizations for finding the maximal stability interval. © 2011 Jorge-Antonio Lpez-Renteria et al.

PubMed | University Autnoma Metropolitana Iztapalapa
Type: Journal Article | Journal: Environmental technology | Year: 2010

In a previous paper, the authors showed that a slight aeration of a methanogenic reactor treating wastewater from the manufacture of polymeric resins could improve its performance, by increasing or allowing the removal of some of its contaminants, including vinyl acetate (VA). This paper reports the isolation under aerobic conditions of a VA-biodegrading axenic culture (strain C1) retrieved from the sludge of a slightly aerated methanogenic reactor at 1 mg L(-1) d(-1) of dissolved oxygen (DO). The axenic culture obtained was phenotypically (morphology, biochemical properties, VA consumption kinetics) and phylogenetically characterized. It formed white colonies with a branched and flat morphology on solid medium. The cell morphology of the isolate was bacillus with round endings and flagellate. The cells could form chains and were stained Gram-negative. The isolate required simple nutritional elements and had a growth rate of 0.024 h(-1). The phylogenetical analysis showed that the aerobic bacterium was identified as Brevibacillus agri, with 99.3% similarity. The VA consumption kinetics in the methanogenic sludge were: volumetric consumption rate (rVA) of 1.74 +/- 0.2 mg L(-1) h(-1), maximum specific consumption rate (qVAmax) of 3.98 mg g(-1) volatile suspended solids (VSS) h(-1) and affinity constant (Ks) of 457.1 mg L(-1). The same parameters in the axenic culture were 1.69 +/- 0.04 mg L(-1) (h-1), 4.09 mg g(-1) dry weight h(-1) and 421.9 mg L(-1), respectively. These results show evidence that the aerobic isolated bacterium, identified as Brevibacillus agri, carried out the VA hydrolysis in the slightly aerated methanogenic sludge, which is the limiting step in the degradation of this compound.

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