Modelización de procesos de soldadura (GMAW y GTAW) de la aleación de magnesio AZ31B. Estudio y análisis del comportamiento de este tipo de uniones soldadas frente a la corrosión en medios salinos

El magnesio y sus aleaciones representan un interesante campo de investigación dentro de la ingeniería de materiales debido a los retos que plantean tanto su conformabilidad como durabilidad. Las características físicas y mecánicas del magnesio y sus aleaciones los convierten en materiales de gran interés desde el punto de vista industrial al tratarse de uno de los materiales más abundantes y ligeros en un mundo en el que día a día los recursos y materias primas son más escasos por lo que el acceso a materiales abundantes y ligeros que permitan economizar el uso de energía cobrará mayor importancia en el futuro. En la coyuntura actual es por tanto necesario revisar procesos y procedimientos, investigando y tratando de ampliar desde este punto de vista si es posible mejorar los procedimientos de fabricación de los materiales disponibles actualmente o el desarrollo de nuevos, mejores que los anteriores, que permitan ayudar a la sostenibilidad del planeta. El magnesio, pese a ser un material muy abundante y relativamente barato, presenta una serie de inconvenientes que limitan de manera muy seria su aplicación industrial, su alta reactividad en presencia de oxígeno y su mal comportamiento frente a la corrosión así como limitaciones en su conformabilidad han estado limitando su uso y aplicaciones, los investigaciones dentro del campo de la metalurgia física de este material y el desarrollo de nuevas aleaciones han permitido su empleo en múltiples aplicaciones dentro de la industria aeroespacial, militar, automovilística, electrónica, deportiva y médica. La motivación para esta tesis doctoral ha sido tratar de aportar más luz sobre el comportamiento de una de las aleaciones comerciales base magnesio más empleadas, la AZ31B, tratando de modelizar como le afectan los procesos de soldadura y estudiando desde un punto de vista experimental como se ve modificada su microestructura, su comportamiento mecánico y su resistencia frente a la corrosión. Aunque en un principio se pensó en el empleo de métodos electroquímicos para el estudio de la corrosión de estos materiales, rápidamente se decidió prescindir de su uso dada la dificultad observada tanto durante los trabajos de investigación de esta Tesis como los encontrados por otros investigadores. Mediante microdurezas se han caracterizado mecánicamente las soldaduras de aleación de magnesio tipo AZ31 en función de diferentes materiales de aporte, observándose que el empleo de las aleaciones con mayor contenido de aluminio y zinc no contribuye a una mejora significativa de las propiedades mecánicas. Se han podido establecer correlaciones entre los modelos de simulación desarrollados y las microestructuras resultantes de los procesos reales de soldadura que permiten definir a priori que estructuras se van a obtener. De igual forma ha sido posible completar un estudio micrográfico y químico completo de las diferentes fases y microconstituyentes originados durante los procesos de soldadura, gracias a estos resultados se ha propuesto como hipótesis una explicación que justifica el comportamiento frente a la corrosión de estas aleaciones una vez soldadas. Los ensayos de corrosión realizados han permitido determinar correlaciones matemáticas que indican las velocidades de corrosión esperables de este tipo de aleaciones. Desde el punto de vista del diseño, los resultados obtenidos en este trabajo permitirán a otros investigadores y diseñadores tomar decisiones a la hora de decidir qué materiales de aporte emplear junto con las implicaciones que conllevan desde el punto de vista metalúrgico, mecánico o corrosivo las diferentes alternativas. Por último indicar que gracias al trabajo desarrollado se han definido modelos matemáticos para predecir el comportamiento frente a la corrosión de estas aleaciones, se han determinado las posibles causas y mecanismos por las que se gobierna la corrosión en la soldadura de chapas de aleación AZ31B y los motivos por los que se debe considerar el empleo de un material de aporte u otro. Los modelos de simulación desarrollados también han ayudado a comprender mejor la microestructura resultante de los procesos de soldadura y se han determinado que fases y microconstituyentes están presentes en las soldaduras de estas aleaciones. ABSTRACT Magnesium and its alloys represent and interesting research field in the material science due to the challenges of their fabrication and durability. The physical and mechanical properties of magnesium and its alloys make them a very interesting materials from and industrial point of view being one of the most abundant and lightest materials in a world in which day by day the lacking of resources and raw materials is more important, the use of light materials which allow to save energy will become more important in a near future. So that it is necessary to review processes and procedures, investigating and trying to improve current fabrication procedures and developing new ones, better than the former ones, in order to help with the sustainability of the planet. Although magnesium is a very common and relatively cheap material, it shows some inconveniences which limit in a major way their industrial application; its high reactivity in presence of oxygen, its poor corrosion resistance and some manufacturing problems had been limiting their use and applications, metallurgical investigations about this material and the development of new alloys have allowed its use in multiple applications in the aerospacial, military, automobile, electronics, sports and medical industry. The motivation for this thesis has been trying to clarify the behavior of one most used commercial base magnesium alloys, the AZ31, trying to modeling how its affected by thermal cycles of the welding process and studying from an experimental point of view how its microstructure is modified and how these modifications affect its mechanical behavior and corrosion resistance. Although at the beginning of this works it was though about the using of electrochemical techniques to evaluate the corrosion of these materials, rapidly it was decided not to use them because of the difficulty observed by during this research and by other investigators. The results obtained in this thesis have allowed to characterize mechanically AZ31 magnesium welding alloys considering different filler metals, according to this study using filler metals with a high content of aluminum and zinc does not represent an important improve It has been possible to establish correlations between simulation models and the resultant microstructures of the real melting processes originated during welding processes which allow to predict the structures which will be obtained after the welding. In addition to that it is possible to complete a complete micrographic and chemical analysis of the different phases and microconstituents created during welding, due to these results and hypothesis to explain the corrosion behavior of these welded alloys. Corrosion tests carried out have allowed defining mathematical correlations to predict corrosion rates of this kind of alloys. From a designing point of view, the results obtained in this work will let other investigators and designers to make decisions taking into account which implications have the different options from a metallurgical, mechanic and corrosive point of view. Finally we would like to indicate that thanks to this work it has been possible to define mathematical models to predict the corrosion behavior, the causes and the mechanism of this corrosion in the AZ31 welding sheets have been also determined and the reasons for using of one filler metal or another, the developed simulation models have also help to get a better understanding of the result microstructure determining the phases and the microconstituents present in the welding of this alloys.

Improvement of root architecture under abiotic stress through control of auxin homeostasis in Arabidopsis and Brassica crops

Auxin plays an important role in many aspects of plant development including stress responses. Here we briefly summarize how auxin is involved in salt stress, drought (i.e. mostly osmotic stress), waterlogging and nutrient deficiency in Brassica plants. In addition, some mechanisms to control auxin levels and signaling in relation to root formation (under stress) will be reviewed. Molecular studies are mainly described for the model plant Arabidopsis thaliana, but we also like to demonstrate how this knowledge can be transferred to agriculturally important Brassica species, such as Brassica rapa, Brassica napus and Brassica campestris. Moreover, beneficial fungi could play a role in the adaptation response of Brassica roots to abiotic stresses. Therefore, the possible influence of Piriformospora indica will also be covered since the growth promoting response of plants colonized by P. indica is also linked to plant hormones, among them auxin.

Polymerase recognition of synthetic oligodeoxyribonucleotides incorporating degenerate pyrimidine and purine bases

A universal base that is capable of substituting for any of the four natural bases in DNA would be of great utility in both mutagenesis and recombinant DNA experiments. This paper describes the properties of oligonucleotides incorporating two degenerate bases, the pyrimidine base 6H,8H-3,4-dihydropyrimido[4,5-c][1,2]oxazin-7-one and the purine base N6-methoxy-2,6-diaminopurine, designated P and K, respectively. An equimolar mixture of the analogues P and K (called M) acts, in primers, as a universal base. The thermal stability of oligonucleotide duplexes were only slightly reduced when natural bases were replaced by P or K. Templates containing the modified bases were copied by Taq polymerase; P behaved as thymine in 60% of copying events and as cytosine in 40%, whereas K behaved as if it were guanine (13%) or adenine (87%). The dUTPase gene of Caenorhabditis elegans, which we have found to contain three nonidentical homologous repeats, was used as a model system to test the use of these bases in primers for DNA synthesis. A pair of oligodeoxyribonucleotides, each 20 residues long and containing an equimolar mixture of P and K at six positions, primed with high specificity both T7 DNA polymerase in sequencing reactions and Taq polymerase in PCRs; no nonspecific amplification was obtained on genomic DNA of C. elegans. Use of P and K can significantly reduce the complexity of degenerate oligonucleotide mixtures, and when used together, P and K can act as a universal base.

Streaming potential measurements in αβγ-rat epithelial Na+ channel in planar lipid bilayers

Streaming potentials across cloned epithelial Na+ channels (ENaC) incorporated into planar lipid bilayers were measured. We found that the establishment of an osmotic pressure gradient (Δπ) across a channel-containing membrane mimicked the activation effects of a hydrostatic pressure differential (ΔP) on αβγ-rENaC, although with a quantitative difference in the magnitude of the driving forces. Moreover, the imposition of a Δπ negates channel activation by ΔP when the Δπ was directed against ΔP. A streaming potential of 2.0 ± 0.7 mV was measured across αβγ-rat ENaC (rENaC)-containing bilayers at 100 mM symmetrical [Na+] in the presence of a 2 Osmol/kg sucrose gradient. Assuming single file movement of ions and water within the conduction pathway, we conclude that between two and three water molecules are translocated together with a single Na+ ion. A minimal effective pore diameter of 3 Å that could accommodate two water molecules even in single file is in contrast with the 2-Å diameter predicted from the selectivity properties of αβγ-rENaC. The fact that activation of αβγ-rENaC by ΔP can be reproduced by the imposition of Δπ suggests that water movement through the channel is also an important determinant of channel activity.

Arabidopsis thaliana PAD4 encodes a lipase-like gene that is important for salicylic acid signaling

The Arabidopsis PAD4 gene previously was found to be required for expression of multiple defense responses including camalexin synthesis and PR-1 gene expression in response to infection by the bacterial pathogen Pseudomonas syringae pv. maculicola. This report describes the isolation of PAD4. The predicted PAD4 protein sequence displays similarity to triacyl glycerol lipases and other esterases. The PAD4 transcript was found to accumulate after P. syringae infection or treatment with salicylic acid (SA). PAD4 transcript levels were very low in infected pad4 mutants. Treatment with SA induced expression of PAD4 mRNA in pad4–1, pad4–3, and pad4–4 plants but not in pad4–2 plants. Induction of PAD4 expression by P. syringae was independent of the regulatory factor NPR1 but induction by SA was NPR1-dependent. Taken together with the previous observation that pad4 mutants have a defect in accumulation of SA upon pathogen infection, these results suggest that PAD4 participates in a positive regulatory loop that increases SA levels, thereby activating SA-dependent defense responses.

The chitinase PfCHT1 from the human malaria parasite Plasmodium falciparum lacks proenzyme and chitin-binding domains and displays unique substrate preferences

Within hours after the ingestion of a blood meal, the mosquito midgut epithelium synthesizes a chitinous sac, the peritrophic matrix. Plasmodium ookinetes traverse the peritrophic matrix while escaping the mosquito midgut. Chitinases (EC 3.2.1.14) are critical for parasite invasion of the midgut: the presence of the chitinase inhibitor, allosamidin, in an infectious blood meal prevents oocyst development. A chitinase gene, PgCHT1, recently has been identified in the avian malaria parasite P. gallinaceum. We used the sequence of PgCHT1 to identify a P. falciparum chitinase gene, PfCHT1, in the P. falciparum genome database. PfCHT1 differs from PgCHT1 in that the P. falciparum gene lacks proenzyme and chitin-binding domains. PfCHT1 was expressed as an active recombinant enzyme in Escherichia coli. PfCHT1 shares with PgCHT1 a substrate preference unique to Plasmodium chitinases: the enzymes cleave tri- and tetramers of GlcNAc from penta- and hexameric oligomers and are unable to cleave smaller native chitin oligosaccharides. The pH activity profile of PfCHT1 and its IC50 (40 nM) to allosamidin are distinct from endochitinase activities secreted by P. gallinaceum ookinetes. Homology modeling predicts that PgCHT1 has a novel pocket in the catalytic active site that PfCHT1 lacks, which may explain the differential sensitivity of PfCHT1 and PgCHT1 to allosamidin. PfCHT1 may be the ortholog of a second, as yet unidentified, chitinase gene of P. gallinaceum. These results may allow us to develop novel strategies of blocking human malaria transmission based on interfering with P. falciparum chitinase.

Cloning and Characterization of peter pan, a Novel Drosophila Gene Required for Larval Growth

We identified a new Drosophila gene, peter pan (ppan), in a screen for larval growth–defective mutants. ppan mutant larvae do not grow and show minimal DNA replication but can survive until well after their heterozygotic siblings have pupariated. We cloned the ppan gene by P-element plasmid rescue. ppan belongs to a highly conserved gene family that includes Saccharomyces cerevisiae SSF1 and SSF2, as well as Schizosaccharomyces pombe, Arabidopsis, Caenorhabditis elegans, mouse, and human homologues. Deletion of both SSF1 and SSF2 in yeast is lethal, and depletion of the gene products causes cell division arrest. Mosaic analysis of ppan mutant clones in Drosophila imaginal disks and ovaries demonstrates that ppan is cell autonomous and required for normal mitotic growth but is not absolutely required for general biosynthesis or DNA replication. Overexpression of the wild-type gene causes cell death and disrupts the normal development of adult structures. The ppan gene family appears to have an essential and evolutionarily conserved role in cell growth.

A fusion pore phenotype in mast cells of the ruby-eye mouse

Using patch-clamp capacitance and amperometric techniques, we have identified an exocytotic phenotype that affects the function of the fusion pore, the molecular structure that connects the lumen of a secretory vesicle with the extracellular environment during exocytosis. Direct observation of individual exocytotic events in mast cells from the ruby-eye mouse (ru/ru) showed a 3-fold increase in the fraction and duration of transient fusion events with respect to wild-type mice. The fraction of the total fusion events that were transient increased from 0.22 ± 0.02 (wild type) to 0.65 ± 0.02 (ru/ru), and the average duration of these events increased from 418 ± 32 ms (wild type) to 1207 ± 89 ms (ru/ru). We also show that this phenotype can reduce and delay an evoked secretory response by causing the fusion of vesicles that have been previously emptied by repeated cycles of transient fusion. The exocytotic phenotype that we describe here may be a cause of diseases like platelet storage pool deficiency and prolonged bleeding times for which the ruby-eye mouse serves as an animal model. Furthermore, the identification of the gene causing the fusion pore phenotype reported here will illuminate the molecular mechanisms regulating exocytotic fusion.

Integration of circadian and phototransduction pathways in the network controlling CAB gene transcription in Arabidopsis

The transcription of CAB genes, encoding the chlorophyll a/b-binding proteins, is rapidly induced in dark-grown Arabidopsis seedlings following a light pulse. The transient induction is followed by several cycles of a circadian rhythm. Seedlings transferred to continuous light are known to exhibit a robust circadian rhythm of CAB expression. The precise waveform of CAB expression in light–dark cycles, however, reflects a regulatory network that integrates information from photoreceptors, from the circadian clock and possibly from a developmental program. We have used the luciferase reporter system to investigate CAB expression with high time resolution. We demonstrate that CAB expression in light-grown plants exhibits a transient induction following light onset, similar to the response in dark-grown seedlings. The circadian rhythm modulates the magnitude and the kinetics of the response to light, such that the CAB promoter is not light responsive during the subjective night. A signaling pathway from the circadian oscillator must therefore antagonize the phototransduction pathways controlling the CAB promoter. We have further demonstrated that the phase of maximal CAB expression is delayed in light–dark cycles with long photoperiods, due to the entrainment of the circadian oscillator. Under short photoperiods, this pattern of entrainment ensures that dawn coincides with a phase of high light responsiveness, whereas under long photoperiods, the light response at dawn is reduced.

Oxaloacetate Transport into Plant Mitochondria1

The properties of oxaloacetate (OA) transport into mitochondria from potato (Solanum tuberosum) tuber and pea (Pisum sativum) leaves were studied by measuring the uptake of 14C-labeled OA into liposomes with incorporated mitochondrial membrane proteins preloaded with various dicarboxylates or citrate. OA was found to be transported in an obligatory counterexchange with malate, 2-oxoglutarate, succinate, citrate, or aspartate. Phtalonate inhibited all of these countertransports. OA-malate countertransport was inhibited by 4,4′-dithiocyanostilbene-2,2′-disulfonate and pyridoxal phosphate, and also by p-chloromercuribenzene sulfonate and mersalyl, indicating that a lysine and a cysteine residue of the translocator protein are involved in the transport. From these and other inhibition studies, we concluded that plant mitochondria contain an OA translocator that differs from all other known mitochondrial translocators. Major functions of this translocator are the export of reducing equivalents from the mitochondria via the malate-OA shuttle and the export of citrate via the citrate-OA shuttle. In the cytosol, citrate can then be converted either into 2-oxoglutarate for use as a carbon skeleton for nitrate assimilation or into acetyl-coenzyme A for use as a precursor for fatty acid elongation or isoprenoid biosynthesis.

Plants and animals share functionally common bacterial virulence factors

By exploiting the ability of Pseudomonas aeruginosa to infect a variety of vertebrate and nonvertebrate hosts, we have developed model systems that use plants and nematodes as adjuncts to mammalian models to help elucidate the molecular basis of P. aeruginosa pathogenesis. Our studies reveal a remarkable degree of conservation in the virulence mechanisms used by P. aeruginosa to infect hosts of divergent evolutionary origins.

The Regulation of Photosynthetic Electron Transport during Nutrient Deprivation in Chlamydomonas reinhardtii1

The light-saturated rate of photosynthetic O2 evolution in Chlamydomonas reinhardtii declined by approximately 75% on a per-cell basis after 4 d of P starvation or 1 d of S starvation. Quantitation of the partial reactions of photosynthetic electron transport demonstrated that the light-saturated rate of photosystem (PS) I activity was unaffected by P or S limitation, whereas light-saturated PSII activity was reduced by more than 50%. This decline in PSII activity correlated with a decline in both the maximal quantum efficiency of PSII and the accumulation of the secondary quinone electron acceptor of PSII nonreducing centers (PSII centers capable of performing a charge separation but unable to reduce the plastoquinone pool). In addition to a decline in the light-saturated rate of O2 evolution, there was reduced efficiency of excitation energy transfer to the reaction centers of PSII (because of dissipation of absorbed light energy as heat and because of a transition to state 2). These findings establish a common suite of alterations in photosynthetic electron transport that results in decreased linear electron flow when C. reinhardtii is limited for either P or S. It was interesting that the decline in the maximum quantum efficiency of PSII and the accumulation of the secondary quinone electron acceptor of PSII nonreducing centers were regulated specifically during S-limited growth by the SacI gene product, which was previously shown to be critical for the acclimation of C. reinhardtii to S limitation (J.P. Davies, F.H. Yildiz, and A.R. Grossman [1996] EMBO J 15: 2150–2159).

Suppression of diet-induced atherosclerosis in low density lipoprotein receptor knockout mice overexpressing lipoprotein lipase.

Lipoprotein lipase (LPL) is a key enzyme in the hydrolysis of triglyceride-rich lipoproteins. Conflicting results have been reported concerning its role in atherogenesis. To determine the effects of the overexpressed LPL on diet-induced atherosclerosis, we have generated low density lipoprotein receptor (LDLR) knockout mice that overexpressed human LPL transgene (LPL/LDLRKO) and compared their plasma lipoproteins and atherosclerosis with those in nonexpressing LDLR-knockout mice (LDLRKO). On a normal chow diet, LPL/LDLRKO mice showed marked suppression of mean plasma triglyceride levels (32 versus 236 mg/dl) and modest decrease in mean cholesterol levels (300 versus 386 mg/dl) as compared with LDLRKO mice. Larger lipoprotein particles of intermediate density lipoprotein (IDL)/LDL were selectively reduced in LPL/LDLRKO mice. On an atherogenic diet, both mice exhibited severe hypercholesterolemia. But, mean plasma cholesterol levels in LPL/ LDLRKO mice were still suppressed as compared with that in LDLRKO mice (1357 versus 2187 mg/dl). Marked reduction in a larger subfraction of IDL/LDL, which conceivably corresponds to remnant lipoproteins, was observed in the LPL/LDLRKO mice. LDLRKO mice developed severe fatty streak lesions in the aortic sinus after feeding with the atherogenic diet for 8 weeks. In contrast, mean lesion area in the LPL/LDLRKO mice was 18-fold smaller than that in LDLRKO mice. We suggest that the altered lipoprotein profile, in particular the reduced level of remnant lipoproteins, is mainly responsible for the protection by LPL against atherosclerosis.

Isolation of virus-neutralizing RNAs from a large pool of random sequences.

RNA and ribonuclease-resistant RNA analogs that bound and neutralized Rous sarcoma virus (RSV) were isolated from a large pool of random sequences by multiple cycles of in vitro selection using infectious viral particles. The selected RNA pool of RSV-binding sequences at a concentration of 0.16 microM completely neutralized the virus. Of 19 sequences cloned from the selected pool, 5 inhibited RSV infection. The selected RNA and RNA analogs were shown to neutralize RSV by interacting with the virus, rather than by adversely affecting the host cells. The selection of the anti-RSV RNA and RNA analogs by intact virions immediately suggests the potential application of this approach to develop RNA and RNA analogs as inhibitors of other viruses such as human immunodeficiency virus.

Dinâmica do fósforo no solo em função da adição de ácidos orgânicos de baixa massa molar

A baixa eficiência da adubação fosfatada em solos altamente intemperizados é devido, entre outros fatores, à adsorção do fósforo (P) à superfície das argilas silicatadas do tipo 1:1 e, principalmente, dos (hidr)óxidos de Fe e de Al. Manejos do solo que induzem a solubilização de formas de P indisponíveis para as plantas têm sido intensamente estudados nos últimos anos. Uma tentativa de aumentar a concentração de P disponível na solução do solo para sua absorção pelas plantas é a mobilização de P por ânions de ácidos orgânicos de baixa massa molar (AOBMM). Ânions derivados de AOBMM exsudados pelas raízes de plantas ou excretados por microrganismos são associados com algumas condições de rizosfera como deficiência de P e fitotoxidez de Al e interagem com o solo de forma a aumentar a biodisponibilidade de P. Dependendo dos atributos do solo, do grau de dissociação, das propriedades e do número de grupos carboxílicos dos ânions orgânicos, o P pode ser mobilizado do solo principalmente devido à dissolução complexométrica de minerais e à adsorção competitiva dos grupos funcionais carboxílicos e fosfato nos sítios de superfície coloidais. A capacidade dos ânions citrato, malato e oxalato em mobilizar P de amostras de um Neossolo Quartzarênico típico (RQ) e de um Latossolo Vermelho ácrico (LVwf) foi avaliada por meio de um estudo de lixiviação de ânions em colunas. Devido a não detecção de P nos efluentes das colunas com LVwf, foi realizado outro estudo em colunas, no qual somente citrato foi lixiviado, mas num volume maior, e as alterações das formas de P nas amostras desse solo induzidas pela lixiviação de citrato foram identificadas por espectroscopia de absorção de raios-X na borda K do fósforo (X-ray absorption near edge structure -XANES - spectroscopy). A capacidade dos ânions de AOBMM em solubilizar P foi mais dependente do teor de P disponível e de outros atributos do solo que do número de grupos funcionais carboxílicos dos ânions orgânicos. Somente o oxalato mobilizou P do RQ, enquanto todos os ânions de AOBMM foram capazes de mobilizar P do LVwf. Quando baixos volumes de solução contendo ânions de AOBMM foram lixiviados no solo, além do aumento do pH, a mobilização de P foi acompanhada pela mobilização de Al no RQ (pH água = 5), e pela mobilização de Ca no LVwf (pH água = 5.6), o que indica solubilização de P pela complexação de Al, Ca, ou Fe, de fosfatos insolúveis, ou pela inibição da precipitação de P com esses metais. Ao lixiviar um volume maior de citrato no LVwf, o P também não foi detectado nos efluentes das colunas, mas houve lixiviação intensa de Al e Fe, bem como mudanças nas proporções de formas de P no solo caracterizadas pelos espectros XANES. Embora tenhamos encontrado indícios da ação dos principais mecanismos de solubilização de P (dissolução complexométrica de minerais e troca de ligantes entre grupos funcionais carboxílicos e P adsorvido ao solo), os ânions de AOBMM mostraram pouco potencial de efetivamente aumentar a biodisponibilidade de P.