National Autonomous University of Mexico

About: National Autonomous University of Mexico is a education organization based out in Mexico City, Distrito Federal, Mexico. It is known for research contribution in the topics: Population & Galaxy. The organization has 72868 authors who have published 127797 publications receiving 2285543 citations. The organization is also known as: UNAM & Universidad Nacional Autónoma de México.

Molecular Cloud Evolution. II. From Cloud Formation to the Early Stages of Star Formation in Decaying Conditions

Abstract: We study the formation of giant dense cloud complexes and of stars within them using SPH numerical simulations of the collision of gas streams (‘‘inflows’’) in the WNM at moderately supersonic velocities. The collisions cause compression,cooling,andturbulencegenerationinthegas,formingacloudthatthenbecomesself-gravitatingandbeginsto collapse globally. Simultaneously, the turbulent, nonlinear density fluctuations induce fast, local collapse events. The simulationsshowthat(1)Thecloudsarenotinastateofequilibrium.Instead,theyundergosecularevolution.Duringits early stages, the cloud’s mass and gravitational energy jEgj increase steadily, while the turbulent energy Ek reaches a plateau.(2)When jEgjbecomescomparabletoEk,globalcollapsebegins,causingasimultaneousincreasein jEgjandEk that maintains a near-equipartition condition jEg j� 2Ek. (3) Longer inflow durations delay the onset of global and local collapsebymaintainingahigherturbulentvelocitydispersioninthecloudoverlongertimes.(4)Thestarformationrate islargefrom the beginning,without any periodofslow and acceleratingstar formation.(5) The column densities of the local star-forming clumps closely resemble reported values of the column density required for molecule formation, suggesting that locally molecular gas and star formation occur nearly simultaneously. The MC formation mechanism discussedherenaturallyexplainstheapparent‘‘virialized’’stateofMCsandtheubiquityofHihalosaroundthem.Also, within their assumptions, our simulations support the scenario of rapid star formation after MCs are formed, although long (k15 Myr) accumulation periods do occur during which the clouds build up their gravitational energy, and which are expected to be spent in the atomic phase.

Multiple successional pathways in human-modified tropical landscapes: New insights from forest succession, forest fragmentation and landscape ecology research

Abstract: Old-growth tropical forests are being extensively deforested and fragmented worldwide. Yet forest recovery through succession has led to an expansion of secondary forests in human-modified tropical landscapes (HMTLs). Secondary forests thus emerge as a potential repository for tropical biodiversity, and also as a source of essential ecosystem functions and services in HMTLs. Such critical roles are controversial, however, as they depend on successional, landscape and socio-economic dynamics, which can vary widely within and across landscapes and regions. Understanding the main drivers of successional pathways of disturbed tropical forests is critically needed for improving management, conservation, and restoration strategies. Here, we combine emerging knowledge from tropical forest succession, forest fragmentation and landscape ecology research to identify the main driving forces shaping successional pathways at different spatial scales. We also explore causal connections between land-use dynamics and the level of predictability of successional pathways, and examine potential implications of such connections to determine the importance of secondary forests for biodiversity conservation in HMTLs. We show that secondary succession (SS) in tropical landscapes is a multifactorial phenomenon affected by a myriad of forces operating at multiple spatio-temporal scales. SS is relatively fast and more predictable in recently modified landscapes and where well-preserved biodiversity-rich native forests are still present in the landscape. Yet the increasing variation in landscape spatial configuration and matrix heterogeneity in landscapes with intermediate levels of disturbance increases the uncertainty of successional pathways. In landscapes that have suffered extensive and intensive human disturbances, however, succession can be slow or arrested, with impoverished assemblages and reduced potential to deliver ecosystem functions and services. We conclude that: (i) succession must be examined using more comprehensive explanatory models, providing information about the forces affecting not only the presence but also the persistence of species and ecological groups, particularly of those taxa expected to be extirpated from HMTLs; (ii) SS research should integrate new aspects from forest fragmentation and landscape ecology research to address accurately the potential of secondary forests to serve as biodiversity repositories; and (iii) secondary forest stands, as a dynamic component of HMTLs, must be incorporated as key elements of conservation planning; i.e. secondary forest stands must be actively managed (e.g. using assisted forest restoration) according to conservation goals at broad spatial scales.

Impact of fishing on size composition and diversity of demersal fish communities

Abstract: By analysing data sets from different world regions we add evidence to documented changes in demersal fish community structure that may be related to fishing. Changes are analysed by community properties that might be expected to capture relevant overall changes – size spectra slopes and intercepts, Shannon-Wiener diversity, and dominance. Cross-system differences in the shape of the integrated community size spectra appear to be related to ecosystem productivity. The slope of size spectra appears to respond in a consistent way to changes in exploitation levels. In most areas studied, but particularly in high-latitude regions, we observe a decreasing trend in the slope, reflecting changes in size composition toward a relative decline in larger fish. The results from tropical regions are less conclusive, partly owing to the difficulty in obtaining consistent data series, but probably also because the generally higher growth rates of the constituent species make the slope less sensitive to changes in fishing. No evidence was found of any decline in species richness, while changes in diversity (richness and evenness) were caused either by changes in patterns of dominance or by changes in the number of species identified resulting from improved survey protocols.

The search of DNA-intercalators as antitumoral drugs: What it worked and what did not work

Abstract: The discovery of new compounds with antitumoral activity has become one of the most important goals in medicinal chemistry. One interesting group of chemotherapeutic agents used in cancer therapy comprises molecules that interact with DNA. Research in this area has revealed a range of DNA recognizing molecules that act as antitumoral agents, including groove binders, alkylating and intercalator compounds. DNA intercalators (molecules that intercalate between DNA base pairs) have attracted particular attention due to their antitumoral activity. For example, a number of acridine and anthracycline derivatives are excellent DNA intercalators that are now on the market as chemotherapeutic agents. Commercially available acridine and anthracycline derivatives have been widely studied from a variety of viewpoints, such as physicochemical properties, structural requirements, synthesis and biological activity. However, the clinical application of these and other compounds of the same class has encountered problems such as multidrug resistance (MRD), and secondary and/or collateral effects. These shortcomings have motivated the search for new compounds to be used either in place of, or in conjunction with, the existing compounds. Unfortunately, the results of this search have not met expectations. The vast majority of candidate intercalator compounds tested for use as anticancer agents have shown little or no biological activity. Research in this area has not been without benefits, however, for it has produced much information on the synthesis and antitumoral properties of hundreds of compounds, which have been tested on diverse tumoral cell lines. This review considers the structural and biological considerations relevant to the use of DNA intercalators and bis-intercalators as antitumoral agents, with an emphasis on the relationship between structure and activity, produced in last decade.

Chitin characterization by SEM, FTIR, XRD, and 13C cross polarization/mass angle spinning NMR

Abstract: The full characterization of chitin obtained from squid, shrimp, prawn, lobsters, and king crab is reported. Elemental analysis, including metals such as Ca, Mg, Zn, Cd, Hg, Cr, Mn, Cu, and Pb, was performed, which is quite relevant because the skeleton composition is slightly different for each species. The morphology was studied by means of TEM and their compositions were determined by energy-dispersive X-ray analysis. 13C cross polarization/magic angle spinning NMR was applied to determine the chemical shift of all the carbons and the difference between them. Chitin was isolated by using chemical methods, alternating hydrochloric acid and sodium hydroxide. The α-chitin from shrimp, prawn, lobsters, and king crabs showed two signals at 73.7 and 75.6 ppm. Meanwhile, the β-chitin from squid exhibited one signal at 75.2 ppm. FTIR studies were used to analyze α-chitin from shrimp and β-chitin from squid. The α-chitin exhibited amide I vibration modes at 1660 and 1627 cm−1, whereas the β-chitin showed one band at 1656 cm−1. X-ray diffraction showed that α-chitin is orthorhombic (a = 4.74 A, b = 18.86 A, and c = 10.32 A) and β-chitin had a monoclinic dihydrated form (a = 4.80 A, b = 10.40 A, c = 11.10 A, and β = 97°). © 2004 Wiley Periodicals, Inc. J Appl Polym Sci 93: 1876–1885, 2004