Estudio de las estructuras cristalinas y de las transiciones de fase en nuevos materiales de wolframio (SrM2+W6+O) y de antimonio (AM3+Sb5+O) con estructura de perovskita doble.

Los óxidos mixtos con estructura tipo perovskita doble A2BBO6 presentan gran interés desde el punto de vista científico y tecnológico debido a la gran variedad de propiedades que poseen: superconductoras, catalíticas, magnéticas y magnetorresistentes, por ejemplo. La temperatura es un variable que permite modificar la simetría de la estructura cristalina y, consecuentemente, las propiedades físicas del material. El trabajo describe la síntesis, caracterización estructural y de las transiciones de fase en nuevos materiales de dos familias de perovskitas dobles: la familia de wolframio (Sr2M2+W6+O6) y la familia de antimonio (A2M3+Sb5+O6). Se ha llevado a cabo la síntesis de 29 compuestos, 22 de ellos sintetizados por primera vez. Los compuestos se han caracterizado mediante técnicas de difracción de rayos X y de neutrones, determinando su estructura cristalina a temperatura ambiente, así como las posibles transiciones de fase a bajas y altas temperaturas, y en algunos casos, también las estructuras de altas y bajas temperaturas. Los materiales de la familia de wolframio estudiados en este trabajo presentan un ordenamiento total entre los cationes M2+ y W6+ en los sitios B y B de la perovskita doble (A2BBO6); y presentan, además, una única secuencia de transiciones de fase a altas temperaturas: P21/n -> I4/m -> Fm3m. Las temperaturas de las transiciones de fase observadas en estos compuestos en función del factor de tolerancia (t), muestran una tendencia general de disminución según t se aproxima a 1. En esta familia, se observa, también, que el rango de existencia de la fase tetragonal intermedia es más amplio para valores de t mayores. Con respecto de la familia de antimonio, el ordenamiento catiónico en los sitios A y B, de una parte, y en los sitios B y B de otra, depende del tamaño de los cationes. Los compuestos de esta familia presentan una gran variedad de grupos espaciales a temperatura ambiente: P2_1 /n, I2/m, I4/m, R-3 y Fm-3m. Además, dependiendo de la diferencia entre los tamaños de los cationes M^3+ y Sb^5+ , los compuestos presentan dos secuencias de transiciones de fase en todo el rango de temperatura: P21/n->I2/m->I4/m->Fm-3m, la misma que en la familia del wolframio pero con una simetría intermedia monoclínica I2/m (compuestos con cationes M^3+ de tamaños similares al del Sb^5+ ); y P21/n -> R-3 -> Fm-3m, con una simetría intermedia trigonal en vez de tetragonal, como en la familia del wolframio (compuestos con cationes M3+ de tamaños mayores que el del Sb5+ ). En esta familia, las temperaturas de las transiciones de fase disminuyen conforme aumenta t.

Coastal erosion as a natural resource management problem: An economic perspective

Unremitting waves and occasional storms bring dynamic forces to bear on the coast. Sediment flux results in various patterns of erosion and accretion, with an overwhelming majority (80 to 90 percent) of coastline in the eastern U.S. exhibiting net erosion in recent decades. Climate change threatens to increase the intensity of storms and raise sea level 18 to 59 centimeters over the next century. Following a lengthy tradition of economic models for natural resource management, this paper provides a dynamic optimization model for managing coastal erosion and explores the types of data necessary to employ the model for normative policy analysis. The model conceptualizes benefits of beach and dune sediments as service flows accruing to nearby residential property owners, local businesses, recreational beach users, and perhaps others. Benefits can also include improvements in habitat for beach- and dune-dependent plant and animal species. The costs of maintaining beach sediment in the presence of coastal erosion include expenditures on dredging, pumping, and placing sand on the beach to maintain width and height. Other costs can include negative impacts on the nearshore environment. Employing these constructs, an optimal control model is specified that provides a framework for identifying the conditions under which beach replenishment enhances economic welfare and an optimal schedule for replenishment can be derived under a constant sea level and erosion rate (short term) as well as an increasing sea level and erosion rate (long term). Under some simplifying assumptions, the conceptual framework can examine the time horizon of management responses under sea level rise, identifying the timing of shift to passive management (shoreline retreat) and exploring factors that influence this potential shift. (PDF contains 4 pages)

Design of tailored strategies for preservation and re-use of information about heritage

[EN] This paper is an outcome of the following dissertation:

Directed evolution of terpene synthases for non-natural substrates

Terpenes represent about half of known natural products, with terpene synthases catalyzing reactions to increase the complexity of substrates and generate cyclizations of the linear diphosphate substrates, therefore forming rings and stereocenters. With their diverse functionality, terpene synthases may be highly evolvable, with the ability to accept a wide range of non-natural compounds and with high product selectivity. Our hypothesis is that directed evolution of terpene synthases can be used to increase selectivity of the synthase on a specific substrate. In the first part of the work presented herein, three natural terpene synthases, Cop2, BcBOT2, and SSCG_02150, were tested for activity against the natural substrate and a non-natural substrate, called Surrogate 1, and the relative activities on both the natural and non-natural substrates were compared. In the second part of this work, a terpene synthase variant of BcBOT2 that has been evolved for thermostability, was used for directed evolution for increased activity and selectivity on the non-natural substrate referred to as Surrogate 2. Mutations for this evolution were introduced using random mutagenesis, with error prone polymerase chain reactions, and using site-specific saturation mutagenesis, in which an NNK library is designed with a specific active site amino acid targeted for mutation. The mutant enzymes were then screened and selected for enhancement of the desired functionality. Two neutral mutants, 19B7 W367F and 19B7 W118Q, were found to maintain activity on Surrogate 2, as measured by the screen.

Caprate: un sistema de interpretación de problemas en lenguaje natural

199 p.

The effects of brood stock size on the economy of catfish (Clarias anguillaris) fry production using the hormone induced natural breeding technique

89 ripe female brooders of the catfish, Clarias anguillaris (Body wt. Range 150g-1, 200g) were induced to spawn by hormone (Ovaprim) induced natural spawning technique over a period of 10 weeks. Matching ripe males were used for pairing the females at the ratio of two males to a female. Six ranges of brood stock body weights were considered as follows; <200g; 200g-399g; 400g-599g; 600-799g; 800g-999g; > 1000g and the number of fry produced by each female brooder was scored/recorded against the corresponding body weight range. The number of fry per unit quantity of hormone and the cost of production a fry based on the current price of Ovaprim (hormon) were determined so as to ascertain most economic size range. The best and most economic size range was between 400g-599g body weight with about 20,000 fry per ml of hormone and N0.028 per fry, while the females above 1000g gave the poorest results of 9,519 fry per ml of hormone and N0.059 per fry. For optimum production of Clarias anguillaris fry and maximum return on investment female brooders of body weights ranging between 400g-599g are recommended for hormone induced natural breeding exercises

The effects of brood stock size on the economy of catfish (Clarias anguillaris) fry production using the hormone induced natural breeding technique

89 ripe female brooders of the catfish, Clarias anguillaris (Body wt. Range 150g-1, 200g) were induced to spawn by hormone (Ovaprim) induced natural spawning technique over a period of 10 weeks. Matching ripe males were used for pairing the females at the ratio of two males to a female. Six ranges of brood stock body weights were considered as follows; <200g; 200g-399g; 400g-599g; 600-799g; 800g-999g; > 1000g and the number of fry produced by each female brooder was scored/recorded against the corresponding body weight range. The number of fry per unit quantity of hormone and the cost of production a fry based on the current price of Ovaprim (hormon) were determined so as to ascertain most economic size range. The best and most economic size range was between 400g-599g body weight with about 20,000 fry per ml of hormone and N0.028 per fry, while the females above 1000g gave the poorest results of 9,519 fry per ml of hormone and N0.059 per fry. For optimum production of Clarias anguillaris fry and maximum return on investment female brooders of body weights ranging between 400g-599g are recommended for hormone induced natural breeding exercises

New strategies for the total synthesis of aza-propellane natural products

The propellane alkaloids comprise a large class of natural products that possess varying degrees of structural complexity and biological activity. The earliest of these to be isolated was acutumine, a chlorinated alkaloid that has been shown to exhibit selective T-cell cytotoxicity and antiamnesic properties. Alternatively, the hasubanan family of natural products has garnered considerable attention from the synthetic community in part due to its structural similarities to morphine. While these alkaloids have been the subject of numerous synthetic studies over the last forty years, very few enantioselective total syntheses have been reported to date.

As part of a research program directed towards the synthesis of various alkaloid natural products, we have developed a unified strategy for the preparation of the hasubanan and acutumine alkaloids. Specifically, a highly diastereoselective 1,2-addition of organometallic reagents to benzoquinone-derived tert-butanesulfinimines was established, which provides access to enantioenriched 4-aminocyclohexadienone products. This methodology enabled the enantioselective construction of functionalized dihydroindolones, which were found to undergo intramolecular Friedel-Crafts conjugate additions to furnish the propellane cores of several hasubanan alkaloids. As a result of these studies, the first enantioselective total syntheses of 8-demethoxyrunanine and cepharatines A, C, and D were accomplished in 9-11 steps from commercially available starting materials.

More recent efforts have focused on applying the sulfinimine methodology to the synthesis of a more structurally complex propellane alkaloid, acutumine. Extensive studies have determined that a properly functionalized dihydroindolone undergoes a photochemical [2+2] cycloaddition followed by a lactone fragmentation/Dieckmann cyclization to establish the carbocyclic framework of the natural product. The preparation of more appropriately oxidized propellane intermediates is currently under investigation, and is anticipated to facilitate our synthetic endeavors toward acutumine.

Discounting the value of natural resources in costbenefit analysis: a case study for policy making

4 p.

A unified strategy to ent-kauranoid natural products : total syntheses of (–)-maoecrystal Z, (–)-trichorabdal A, and (–)-longikaurin E

The diterpenoid constituents of the Isodon plants have attracted reasearchers interested in both their chemical structures and biological properties for more than a half-century. In recent years, the isolations of new members displaying previously unprecedented ring systems and highly selective biological properties have piqued interest from the synthetic community in this class of natural products.

Reported herein is the first total synthesis of such a recently isolated diterpenoid, (–)-maoecrystal Z. The principal transformations implemented in this synthesis include two highly diastereoselective radical cyclization reactions: a Sm^(II)-mediated reductive cascade cyclization, which forms two rings and establishes four new stereocenters in a single step, and a Ti^(III)-mediated reductive epoxide-acrylate coupling that yields a functionalized spirolactone product, which forms a core bicycle of maoecrystal Z.

The preparation of two additional ent-kauranoid natural products, (–)-trichorabdal A and (–)-longikaurin E, is also described from a derivative of this key spirolactone. These syntheses are additionally enabled by the palladium-mediated oxidative cyclization reaction of a silyl ketene acetal precursor that is used to install the bridgehead all-carbon quaternary stereocenter and bicyclo[3.2.1]octane present in each natural product. These studies have established a synthetic relationship among three architecturally distinct ent-kaurane diterpenoids and have forged a path for the preparation of interesting unnatural ent-kauranoid structural analogs for more thorough biological study.

The development of an asymmetric palladium-catalyzed conjugate addition and its application toward the total syntheses of taiwaniaquinoid natural products

The asymmetric synthesis of quaternary stereocenters remains a challenging problem in organic synthesis. Past work from the Stoltz laboratory has resulted in methodology to install quaternary stereocenters α- or γ- to carbonyl compounds. Thus, the asymmetric synthesis of β-quaternary stereocenters was a desirable objective, and was accomplished by engineering the palladium-catalyzed addition of arylmetal organometallic reagents to α,β-unsaturated conjugate acceptors.

Herein, we described the rational design of a palladium-catalyzed conjugate addition reactions utilizing a catalyst derived from palladium(II) trifluoroacetate and pyridinooxazole ligands. This reaction is highly tolerant of protic solvents and oxygen atmosphere, making it a practical and operationally simple reaction. The mild conditions facilitate a remarkably high functional group tolerance, including carbonyls, halogens, and fluorinated functional groups. Furthermore, the reaction catalyzed conjugate additions with high enantioselectivity with conjugate acceptors of 5-, 6-, and 7-membered ring sizes. Extension of the methodology toward the asymmetric synthesis of flavanone products is presented, as well.

A computational and experimental investigation into the reaction mechanism provided a stereochemical model for enantioinduction, whereby the α-methylene protons adjacent the enone carbonyl clashes with the tert-butyl groups of the chiral ligand. Additionally, it was found that the addition of water and ammonium hexafluorophosphate significantly increases the reaction rate without sacrificing enantioselectivity. The synergistic effects of these additives allowed for the reaction to proceed at a lower temperature, and thus facilitated expansion of the substrate scope to sensitive functional groups such as protic amides and aryl bromides. Investigations into a scale-up synthesis of the chiral ligand (S)-tert-butylPyOx are also presented. This three-step synthetic route allowed for synthesis of the target compound of greater than 10 g scale.

Finally, the application of the newly developed conjugate addition reaction toward the synthesis of the taiwaniaquinoid class of terpenoid natural products is discussed. The conjugate addition reaction formed the key benzylic quaternary stereocenter in high enantioselectivity, joining together the majority of the carbons in the taiwaniaquinoid scaffold. Efforts toward the synthesis of the B-ring are presented.

The Equivalency Principle for Discounting the Value of Natural Assets: An Application to an Investment Project in the Basque Coast

25 p.