Patricia Guadarrama

Multiconfigurational character of the ground states of polycyclic aromatic hydrocarbons. A systematic study

AbstractA systematic study of the electronic structure of polycyclic hydrocarbons from naphthalene to a system containing 80 fused benzene has been carried out. Geometries were optimized for closed shell singlet, open shell singlet, triplet and multiplet states at B3LYP/cc-pVDZ level of theory, D1 (second order Møller–Plesset) and D1 (second-order approximate coupled-cluster) diagnostics have been calculated for studied molecules. Complete active space self-consistent field (10,10)/6-31G(d) single point energy calculations have been carried out for all optimized structures. Multireference character of the ground state becomes important when the number of atoms in the polycyclic hydrocarbon exceeds 40–50. At this point, D1 diagnostics reaches 0.04–0.05 and the squared configuration interaction expansion coefficient for dominant configuration drops to about 0.6. However, only for the three largest systems predominantly polyradicalic ground states have been detected. All other polycyclic hydrocarbons showing significant multiconfigurational character of singlet ground state have only two dominant configurations which are closed shell singlet and doubly excited singlet, respectively. Thus, small polycyclic hydrocarbons have mostly single reference singlet ground state, the medium size systems have notably multireference ground state (singlet or triplet) with only moderate polyradicalic character. The ground state of largest systems is singlet polyradical. FigureᅟGround states of polycyclic aromatic hydrocarbons

Solubilization and anticancer-activity enhancement of Methotrexate by novel dendrimeric nanodevices synthesized in one-step reaction

The one-step synthesis of nanodevices based on PAMAM framework for targeted cancer therapy is described. Four water-soluble nanodevices (named fractions F1 to F4) were rightly separated by size discrimination, and characterized. From biological assays of cell growth inhibition percentage, the anticancer activity of Methotrexate (chemotherapeutic drug) as part of a nanodevice, generally increases over cancer cell lines and notably, in case of human lymphocytes, the cell growth inhibition percentage decreases drastically (more than 80%), thus, the nanodevices exhibited a favorable discrimination between healthy and diseased cells. From the characterization it can be conclude that the synthesized nanodevices provide a dual scenario of drug transportation: encapsulation and conjugation.

Seedling growth of two pioneer tropical tree species in Competition: The role of arbuscular mycorrhizae

The effects of arbuscular mycorrhizae (AM) fungi on growth and survival of two pioneer tropical plant species were studied in a greenhouse experiment for 6 months. One, an early pioneer species (Heliocarpus appendiculatus Turcz., Tiliaceae) and the second, a late pioneer species (Stemmadenia donnell-smithii (Rose) Woodson, Apocynaceae). Three growing conditions were used as competition factors—no competition, with intraspecific and interspecific competitions—along with two different conditions of AM—with and without mycorrhizal inoculum. Mycorrhizal colonization of H. appendiculatus roots was seven-fold more than in S. donnell-smithii. With AM infection, H. appendiculatus did not increase in growth, but showed greater survival in the absence of competition. S. donnell-smithii grew better in the presence of AM fungi and the effect of competition was diminished. S. donnell-smithii with AM fungi infection showed better survival and increase in biomass, making it a better competitor than H. appendiculatus.

Tubular Aggregates of Cyclic Oligothiophenes. A Theoretical Study

The geometries of neutral, mono-, and dioxidized tubular aggregates of cyclo[8]thiophenes containing up to 5 repeating units were fully optimized at the MPWB1K/3-21G* level of theory. Calculated interplane distances between macrocycles were found to be close to 3.1 A for neutral and charged aggregates. The binding energies between macrocycles in neutral intermediates were in the range of 40-45 kcal/mol, increasing for monocations and dropping strongly for dicationic species due to electrostatic repulsion between polarons. It was established that there exists a noticeable interaction between pi-orbitals of individual macrocycles in tubular aggregates as follows from decreasing of the band gap with a number of repeating units in aggregates and the polaron delocalization toward tube axes in oxidized species. A polaron pair is the most stable dicationic state for all studied molecules according to the calculations. A singlet polaron pair is more stable than a triplet one. The energy difference between singlet and triplet states is growing smaller with the size of the system, becoming zero for the pentamer corresponding to a completely dissociated bipolaron.

Research on arbuscular mycorrhizae in Mexico: an historical synthesis and future prospects

This review analyzes the historical development and advances of the research on arbuscular mycorrhizal fungi (AMF) in Mexico, as well as the prospects for future research. AMF-research has been focused on studying both diversity and functionality in several ecosystems of Mexico, but mainly in the tropical dry and rainy ecosystems, and the agricultural systems. In Mexico, 95 species of AMF have been recorded, representing 41% of the known species worldwide. The functional effects of AMF colonization have been examined in approximately 10% of the known host plants, but greenhouse studies continue to dominate over those conducted under field conditions. Even though research to date has been at the organismic level, further effort is needed due to the high plant diversity in Mexico. Studies on AMF biomass under field conditions and more taxonomic determination are required based on morphological features, biochemical determinations (fatty acids) and molecular tools. In addition, ecophysiological and ecological in situ studies would help in understanding the relationships among AMF, soil fauna, nutrients, and host plants. The contribution of AMF to ecosystemic processes is a priority line of research that requires an integrated approach (inter- and multidisciplinary) in order to define the role of AM symbioses for biogeochemical models. The creation of a Mexican mycorrhizal research network has and will help to identify the main challenges. Generating similar research protocols, and sharing databases and experience will assist mycorrhizologists working under the diverse financial and ecological contexts that is to be found in Mexico and Latin America.

Construction of simplified models to simulate estrogenic disruptions by esters of 4-hydroxy benzoic acid (parabens)

Four parabens (methyl, n-butyl, benzyl and isobutylparaben) were theoretically studied in order to evaluate their estrogenic activity through simplified models. The experimental structure of the human estrogen receptor ligand-binding domain in complex with 17beta-estradiol was used as the starting point to construct the models. The complex between 17beta-estradiol and three fragments of the estrogenic receptor (Arg, Glu and His), resulted in a reasonable simplified model of interaction. The replacement of 17-beta-estradiol by parabens was evaluated by conformational analyses and interaction energy calculations at BHandHLYP/cc-PVTZ(-f)+ level of theory. According with the calculated interaction energies, methylparaben is the paraben with higher estrogenic activity, which is in agreement with experimental studies of extraction and quantification of parabens in tumors. The antibacterial activity of parabens was also explored considering the formation of potassium salts in the phenolic OH groups. From the obtained relative energy values, methylparaben is the most active preservative.

Macrocyclic vs. Dendrimeric Effect. A DFT Study

Macrocycles up to 15 members with different heteroatoms (N, O, and S) and dendrimeric functionalized branches were assembled, resulting in unique “collective” supramolecular hosts with several active sites for transition metal ions complexation. The nature of the interactions between these kinds of systems and metal ions of the first transition series (Fe, Ni, Cu, Zn) was evaluated by calculations of the binding energies at the B3LYP/LACVP* level of theory, resulting in a preference of metal ions for macrocyclic cavity in terms of complexation; however, there is a favorable contribution in energy due to the cooperative effect of dendrimeric branches (DBs) in the inclusion process by means of long‐range interactions between metal ions and the heteroatoms present in DBs. According to calculated binding energies, even when the complexation in the middle of DBs appears as a less favored situation, still competes with the complexation occurred in several known macrocycles traditionally used in the formation of inclusion complexes. The capability of macrocycles as host entities is related to some criteria like: (1) the compatibility in orbital symmetry between host and guest molecules; (2) the cavity dimensions and the negative charge inside; and (3) the hardness–softness affinity between host and guest molecules. When DBs are included in host systems, their flexibility seems to be very important, in addition to localized negative charge, which permits the occurrence of long‐range interactions.

Geometric distortions on a three-coordinated T1 Cu site model as a potential strategy to modulate redox potential. A theoretical study

A model of the three-coordinated T1 Cu site from Trametes versicolor was considered to evaluate the effect on redox potential of geometrical distortions in the copper coordination sphere. Systematic modifications of geometrical parameters (distances and angles) of the coordination sphere of the T1 Cu site were carried out within a density functional theory (DFT) framework, to evaluate their effects on electron affinity directly related to redox potential. The most promising result in terms of redox potential increment was distortion of the dihedral angle Cmethylthiolate–S–Cu–NImA (ω), which can be rationalized as a decrease in the overlap of imidazole orbitals in the redox-active molecular orbital (β-LUMO). This overlap is minimized when ω achieves the value of 10°, therefore, this conformation might have the highest redox potential. From the molecular orbital viewpoint, a parallelism was found between the effect caused by the presence of a fourth ligand and the distorted three-coordination, which could be extrapolated to spectroscopic properties. It was also found that solvation effects on the redox potentials during geometrical distortions produce a very similar tendency, independently of the polarity of the solvent.

Phosphorus Dependence in Seedlings of a Tropical Pioneer Tree: The Role of Arbuscular Mycorrhizae

Abstract In general, according to previous studies, pioneer species do not require arbuscular mycorrhizal fungi (AMF) to increase their growth and survival in tropical systems. The aim of this study was to determine the dependence response to AMF of Heliocarpus appendiculatus, a pioneer species, at different phosphorus (P) levels. In a greenhouse experiment, H. appendiculatus seedlings were grown in pots with a sterile vermiculite-sand mixture (1:1). Two sets of pots were set up: One set was inoculated (150 spores per pot) with indigenous AMF from a tropical rain forest at “Los Tuxtlas” (Veracruz, Mexico); the other set was not inoculated. To each set, 0, 0.02, 0.2, and 2 g L−1P was added. All pots were watered with 250 mL of nutrient solution. Mycorrhizal plants showed a higher total dry weight and relative growth rate in 0.02 g L−1P concentration, while nonmycorrhizal plants responded positively at 0.2 g L−1P; a decrease in plant responses at higher P levels was observed in both treatments. H. appendiculatus showed to have higher relative dependence at lower P concentration (≈50%). As levels of P increased, mycorrhizal colonization decreased. Successful growth of pioneer species during succession process may be improved if there is AMF content in soils, prior to disturbance.

Synthesis and characterization of well-defined fully conjugated hyperbranched oligomers of β, β-dibromo-4-ethynylstyrene

Abstract Well-defined dendritic oligomers of poly(β,β-dibromo-4-ethynylstyrene) of the first and second generation havebeen synthesized by a stepwise synthesis, and characterized. It was found from n.m.r. analysis and by theoretical calculations that free rotation around formal single bonds is hampered by conjugation. All oligomers were found to be blue emitters with their emission maxima correlating with the number of repeating units. All dendrimers except dendrimer 12 (β,β-bis[β′,β′-di(β″,β″-dibromostyryl-4″-ethynyl)styryl-4′-ethynyl]-4-ethynylstyrene) showed two maxima in the excitation spectra.