Currently, there is interest in the development of molecular-scale devices for use in quantum information processing (QIP). With this application in mind, physical studies on antiferromagnetically coupled molecular wheels [Cr7MF3(Etglu)(O2CtBu)15(phpy)], where M is a divalent metal cation (M = Mn2+, Zn2+, Ni2+) have been pursued. The heterometallic wheels contain an octagon of metal centres, which are bridged by fluoride ions, pivalate groups and a chiral N-ethyl-D-glutamine molecule which is penta-deprotonated and bound to the metal sites through all available O-donors. They are deep purple in colour and they have been named purple-Cr7M. There is antiferromagnetic coupling between adjacent metal centres, J » -8 cm-1, resulting in a non-zero net spin ground state. The spin-Hamiltonian parameters of this family have been determined.At the heterometal site of purple-Cr7M wheels there is a terminal ligand which can be substituted for a variety of N-donor organic ligands. A series of bidentate N-donor linkers has been used to link Cr7Ni wheels (each wheel Seff = 1/2) to create prototype two-qubit systems. Multi-frequency EPR spectroscopy and SQUID magnetometry has been used to extract the spin-Hamiltonian parameters of this family. It has been shown that the single wheels can be linked together electronically as well as chemically. It has been found that for the unsaturated linkers, there is a weaker interaction between Cr7Ni wheels when longer linkers are used. The strength of interaction is smaller for the saturated linkers than for the unsaturated linkers.The formation of 'green'-Cr7M wheels is different, being templated around a cation. Two new types of wheels have been studied: [tBuCONHC6H12NH2C6H12NHCOtBu][Cr7M2+F8(O2CtBu)16] and [Cs?Cr7MF8(O2CtBu)16]·0.5MeCN (where, M = Mn2+, Zn2+, Ni2+), where the former is templated around a long dialkylammonium group and the latter around a caesium cation. The effect of the templating cation on spectroscopic properties has been determined.Physical studies on a family of antiferromagnetically-coupled homometallic clusters have been pursued. They consist of cyclic arrays of homometallic Cr3+ ions in either a octametallic wheel or hexametallic horseshoes. The horseshoes have the general formula: [CrxFx+5L2x-2]n3- (where L = carboxylate). Cr3+ centres are bridged by pivalate groups and fluorides, while Cr3+ centres at the ends of the chain have terminal fluorides completing their coordination sphere. These terminal fluoride groups are labile enough to be substituted, e.g. [EtNH2][Cr6F7(O2CtBu)10(acac)2] is the product of a substitution reaction with acetylacetone.

In the framework of diploma thesis, radical products of the oxidation of selected para­methyl phenols and secondary amines were investigated. In the case of para-methyl phenols using EPR spectroscopy the reaction mechanism was proved, where the abstraction of hydrogen from para-methyl group is involved. In this way benzyl radicals are formed, which can be identified by spin-trapping technique using nitroso compounds. The adducts formed undergo the consecutive rearrangement, which leads to the formation of the corresponding nitrones. In addition to phenols, the radical products of the oxidation of some secondary amines using peroxy radicals and peroxy acids were studied. In both cases new types of nitroxyle radicals were generated and the EPR parameters were determined by spectral simulation.

The determination of anthocyanins in red grapes and wines has been of increasing interest in the last years, as they play an important role in colour quality of red wines revealing also many human health beneficial effects. They contribute e.g. to the reduction of coronary heart disease, but exhibit also antimutagenic, anti-inflammatory, anti-carcinogenic and antioxidant properties. In this doctoral thesis, the complex study of grape skin alcoholic and water extracts, prepared from Alibernet and St. Laurent wine grape varieties is presented. Extracts were prepared from three different amounts (0,5; 1,0 and 1,5 g) of lyophilized grape skin powder using the Pressurized fluid extraction (PFE) and the Pressurized Hot Water Extraction (PHWE) at different temperatures ranging from 40 up to 120°C and pressure of 15 MPa. Methanol, ethanol and water were used as a solvents. Total phenolic compound content (TPC) of individual extracts was determined using the Folin-Ciocalteau assay and their tristimulus color values (CIE Lab) were estimated, using the UV-VIS spectrophotometer. The identification and quantification of anthocyanins by high-performance liquid chromatography with diode array detection (HPLC-DAD) was performed. In addition, pH values of all extracts were also measured using the combinated glass electrode. Antioxidant activity of extracts was tested by EPR spectroscopy in Fenton system (H2O2/Fe2+) generating reactive radicals (•OH, O2-•, •CH3) followed by spin trapping technique, using 5,5-dimethylpyrroline-N-oxide (DMPO) as spin trap. In addition, radical scavenging activity of extracts was assessed applying 2,2-diphenyl-1-picrylhydrazyl (•DPPH) free radical and 2,2'-azino-bis-(3-ethylbenzthiazoline-6-sulfonic acid) cation radical (ABTS•+) assays. All the experimental data were processed with principal component analysis (PCA) and canonical discriminant analysis (CDA) to specify the optimum extraction conditions for extract preparation from the perspective of the potential further application of the extracts as food supplements or food colour enhancers. The results indicated that grape skins of both varieties are a promising source of anthocyanins with prospective application in food industry.

Transfer RNA (tRNA) molecules play a significant role in the translation of the genetic code into protein sequences. The interaction of the messenger RNA (mRNA) codons with the anticodon of the tRNA at the ribosomal subunit results in the discrimination of cognate versus near-cognate and non-cognate codons. In order to do so post-transcriptional modifications at tRNAs have to occur. Among the ~80 identified modifications, the largest diversity can be found in the anticodon wobble position 34 or immediately 3’ adjacent to the anticodon triplet at position 37. The role of these modifications relies in an efficient protein synthesis, the improvement of reading frame maintenance, and codon recognition on the ribosome. Nevertheless, a wide spectrum of human diseases is caused by a growing number of mutations in mitochondrial tRNA genes, whose phenotypic peculiarities are characterized by a broad spectrum of clinical manifestations, including blindness, deafness, dementia, movement disorders, weakness, cardiac failure, diabetes, renal dysfunction, and liver diseases. To analyze the corresponding mechanism of such diseases, model organisms, e.g. E. coli and S. cerrevisiae are frequently used because of the high degree of evolutionary conservation of the modification pathway. In E. coli, the evolutionary conserved protein MnmE together with its interaction partner GidA are involved in the modification of U34 in the wobble position, which has a pivotal role in the structure and function of tRNAs, including structural stabilization, aminoacylation, and codon recognition at the decoding site of small rRNAs. In this work, the G protein MnmE will be investigated by Electron Paramagnetic Resonance (EPR) Spectroscopy in combination with site-directed spin labeling, together with its interaction partner GidA, during different steps of the G domain cycle, to achieve detailed structural and functional insights of those enzymes and to understand their respective biological functions. Thereby, it could be proven directly for the first time that the G domains contact each other in the presence of the triphosphate (GppNHP) or transition state analogue (GDP·AlFx), while they exhibit an open conformation in the Apo- and GDP-bound state. Moreover, it was possible to demonstrate that only the presence of GDP, AlFx and K+ is capable of stabilizing the closed state, and that this effect is specific, since the effect is absent with Na+ and Cs+ and is smaller with similar size cations such as Rb+ and NH4+. This dependency, however, is partly abolished in the presence of GidA, where the results indicate that interaction with this protein on a site remote from the G domains stabilizes the activated GTPase competent MnmE dimer. Moreover, binding of GidA induces conformational changes in MnmE which in turn mediate dimerization of the G domains even in the absence of potassium ions, indicating that GidA binding significantly alters the relative localization of the G domains and thus acts as a co-stimulator of the GTPase reaction.

No description available.

No description available.

This research is focused on the multi-stress aging phenomena and lifetime estimation of 15 kV EPR cable. In order to gain the suitable parameters for the lifetime estimation, the aging study on the EPR cable samples as well as on the cable layers’ dielectrics samples was carried out at the High Voltage Laboratory of Mississippi State University. During the multi-stress aging study of 15 kV EPR cable samples, the EPR cable samples underwent electrical stress, thermal stress, and environmental effects. The aging time for the EPR cables varied from 650 hrs to 1300 hrs. An empirical aging model describing the cables’ lifetime was derived from the partial discharge measurements results. The aging study on the EPR cable layers’ dielectrics was achieved as well. The EPR insulation material samples were aged by combined electrical and thermal stress, while the material samples of inner semi-conducting layer, outer semi-conducting layer, and outer low-density polyethylene (LDPE) jacket were aged by thermal stress. The measurement data was used for the newly proposed lifetime estimation method. A new lifetime estimation method was introduced for the EPR cables. The method assumed that the failures of cables results from the expansion of voids/cavities initiated from the defects in the EPR insulation layer. The proposed lifetime estimation method applied the finite element method (FEM) to solve the electric field distribution inside the EPR cable with the existence of voids/cavities. The parameters were derived from the aging study on the EPR insulation material samples. Assuming the voids/cavities would expand in the direction of the maximum electric field stress, the lifetime of the EPR cables was then estimated through the iteration. The introduced method helped to establish a relationship between the aging study of insulation material samples and the aging of EPR cable samples, which was long missing in the past studies. It also provided a new way to assess the reliability of the EPR cable.

In the present work we investigated various charge transfer processes, as they appear in the versatile world of organic semiconductors by probing the spin states of the corresponding charge carrier species via electron paramagnetic resonance (EPR) spectroscopy. All studied material systems are carbon-based compounds, either belonging to the group of polymers, fullerenes, or single-wall carbon nanotubes (SWNTs). In the first instance, we addressed the change of the open circuit voltage (Voc) with the fullerene blend stoichiometry in fullerene-based solar cells for organic photovoltaics (OPV). The voltage depends strongly on the energy separation between the lowest unoccupied molecular orbital (LUMO) of the donor and the highest occupied molecular orbital (HOMO) of the acceptor. By exploiting the Gaussian distribution of the charge carriers in a two-level system, and thus also their spins in the EPR experiment, it could be shown that the LUMOs get closer by a few to a few hundred meV when going from pure fullerene materials to a fullerene mixture. The reason for this strong energetic effect is likely the formation of a fullerene alloy. Further, we investigated the chemical doping mechanism of SWNTs with a (6,5)-chirality and their behaviour under optical excitation. In order to determine the unintentional (pre)-doping of SWNTs, EPR spectra of the raw material as well as after different purification steps were recorded. This facilitated the determination of nanotube defects and atmospheric p-doping as the causes of the measured EPR signals. In order to deliberately transfer additional charge carriers to the nanotubes, we added the redox-active substance AuCl3 where we determined an associated doping-yield of (1.5±0.2)%. In addition, a statistical occupation model was developed which can be used to simulate the distribution of EPR active, i.e. unpaired and localised charge carriers on the nanotubes. Finally, we investigated the charge transfer behaviour of (6,5)-SWNTs together with the polymer P3HT and the fullerene PC60BM after optical excitation. / Die vorliegende Arbeit untersuchte mit Hilfe der Elektron Paramagnetischen Resonanz Spektroskopie (EPR) die Ladungsträgerspins bei Ladungstransfer-Prozessen in organischen Halbleitern. Insbesondere wurden hier verschiedene Kohlenstoffverbindungen betrachtet, welche zur Gruppe der Polymere, Fullerene, oder Kohlenstoff-Nanoröhren gehören. Zu Beginn gingen wir auf die Veränderung der Leerlaufspannung in Fulleren Solarzellen für organische photovoltaic (OPV) ein, welche mit der Fulleren Stöchiometry variiert. Die Leerlaufspannung ist entscheidend für das Ladungsstransfer-Verhalten nach erfolgreicher optischer Anregung. Sie hängt stark vom Energieabstand des niedrigsten unbesetzten Molekülorbitals (engl. LUMO) des Donators zum höchsten besetzten Molekülorbital (engl. HOMO) des Akzeptors ab. Hierbei wurde die Gaußsche Verteilungs-Statistik der Ladungsträger, und damit auch deren Spins, in einem zwei Niveau System im EPR Experiment ausgenutzt. Es konnte gezeigt werden, dass sich deren Abstand um wenige bis hin zu wenigen Hundert meV annähert wenn man vom reinen Fulleren Material zu einem Fulleren Gemisch übergeht. Die Ursache für diesen starken energetischen Effekt ist wahrscheinlich die Bildung einer Fulleren-Legierung. Des weiteren betrachteten wir speziell einwandige Kohlenstoff-Nanoröhren der Chiralität (6,5). Untersucht wurde zunächst die chemische Dotierung dieser Systeme und anschließend ihr Verhalten bei optischer Anregung. Um zunächst die ungewünschte (vor)-Dotierung der Nanoröhren zu ermitteln, wurden EPR Spektren in unbehandelter Form, als auch nach unterschiedlichen Aufreinigungsschritten aufgenommen. Dies ermöglichte die Bestimmung von Nanorohr-Defekten und atmosphärischer p-Dotierung als Ursache für das gemessene EPR Signal. Um bewusst zusätzliche Ladungsträger auf die Nanoröhren zu übertragen gaben wir die redox-aktive Substanz AuCl3 hinzu, wo wir eine zugehörige Dotiereffizienz von (1,5±0,2)% ermittelten. Darüber hinaus wurde ein statistisches Modell erarbeitet welches die Verteilung von EPR aktiven, d.h. ungepaarten und lokalisierten Ladungsträgern auf den Nanoröhren simulieren kann. Zum Abschluss betrachteten wir das Ladungstransfer-Verhalten von (6,5)-Nanoröhren zusammen mit dem Polymer P3HT und dem Fulleren PC60BM nach optischer Anregung.

La cianotoxina β-N-metil-amino-L-alanina (BMAA) es un aminoácido no proteico producido por cianobacterias, capaz de biomagnificarse en las cadenas tróficas de ecosistemas marinos y terrestres. Dada su capacidad de atravesar la barrera hematoencefálica, su ingesta progresiva se asocia con el desarrollo de ciertas retinopatías, así como también de enfermedades neurodegenerativas, tales como la Esclerosis Lateral Amiotrófica (ELA), la Enfermedad de Parkinson (EP) y la Enfermedad de Alzheimer (EA). Los daños causados por la BMAA son múltiples y originados en mecanismos diversos. Así, la BMAA, en presencia de iones bicarbonato (HCO3 - ), puede formar un compuesto denominado carbamato, cuya estructura química es similar al glutamato (Glut), uno de los neurotransmisores más importantes del sistema nervioso. A su vez, el carbamato se une y activa receptores de Glut, ya sean ionotrópicos (como el receptor de N-metil-D- aspartato) o metabotrópicos. La sobreexcitación de estos receptores ocasionada por la BMAA, promueve mecanismos de excitotoxicidad que conducen a alteraciones neuronales. Por otro lado, la BMAA puede ingresar a las células a través del sistema xc, un sistema de transporte sodio-independiente común para cistina y Glut. Una vez en el interior celular, la toxina puede incorporarse erróneamente en las cadenas polipeptídicas en reemplazo de Serina (Ser). Así, unida a componentes proteicos, puede generar un reservorio endógeno de lenta liberación que expone a las neuronas a una baja pero continua dosis de esta toxina. Entre sus varios efectos subcelulares, la BMAA puede afectar la permeabilidad de las membranas mitocondriales comprometiendo su actividad. Además, puede inducir modificaciones en los niveles de Ca 2+, generar estrés oxidativo, promover fallas en la producción de ATP e inducir estrés en el retículo endoplasmático, lo cual conduce a alteraciones en la síntesis y/o distribución de proteínas. Asociado a esto, se originan alteraciones en el transporte axonal y la fragmentación de estas estructuras. Pese a su trascendencia para la salud, aún son desconocidos los efectos directos que genera la exposición a la BMAA de las neuronas y células gliales de la retina (como las células gliales de Müller –CGM-), o del epitelio pigmentario de la retina (EPR). Además, todavía son mayormente desconocidos aquellos factores o moléculas capaces de modular las vías de señalización involucradas en los efectos deletéreos inducidos por la BMAA. Al respecto, recientemente se ha propuesto que la activación de los receptores X retinoides (RXR) protegerían a las neuronas y modularían la respuesta inflamatoria durante las enfermedades neurodegenerativas del sistema nervioso central, y también en retinopatías. Aún se desconoce si estos receptores ejercen un rol protector contra los daños inducidos por la BMAA. En esta Tesis se estudiaron los mecanismos involucrados en los cambios degenerativos inducidos por la BMAA en células de la retina, así como también en células PC12 diferenciadas a neuronas. Asimismo, se evaluó el valor protector de agonistas de los RXRs frente a los efectos deletéreos inducidos por la BMAA en células de la retina. Para estos estudios, se obtuvieron cultivos puros de neuronas amacrinas y fotorreceptores (FRs), de CGM puros, y cultivos neuro-gliales a partir de retinas de ratas neonatas. Además, se utilizaron cultivos de líneas celulares PC12 y epiteliales ARPE- 19. Todos los cuales fueron tratados con la BMAA para evaluar sus efectos sobre estas células y el posible rol protector de los RXRs. Los resultados obtenidos en este trabajo demostraron que aún bajas concentraciones de la BMAA (de 0,4 μM) alteraron la viabilidad no sólo de las neuronas amacrinas y FRs, sino también de las células PC12 diferenciadas a neuronas, de las CGM e incluso de las células del EPR. La BMAA también, indujo alteraciones en la permeabilidad mitocondrial y en la producción de ROS en las células neuronales, gliales y epiteliales, mientras que en las CGM indujo cambios en la morfología nuclear. Por su parte, en neuronas amacrinas, promovió el crecimiento axonal, aunque generando el colapso de sus conos de crecimiento. Estas alteraciones fueron mediadas por la activación de los receptores NMDA en presencia de iones HCO3 - . Además, en estas células, la BMAA se incorporaría erróneamente en las cadenas polipeptídicas en reemplazo de la Ser, dado que la suplementación del medio de cultivo con este aminoácido previno la toxicidad inducida por la BMAA. En cuanto a la acción protectora de los RXRs, nuestros resultados demostraron que su activación bloqueó los efectos tóxicos que produjo la BMAA sobre las neuronas amacrinas y los FRs, así como también sobre las células del EPR. En resumen, en esta Tesis presentamos evidencias de que la BMAA afecta múltiples estructuras subcelulares en las células que conforman la retina, así como también a células PC12 diferenciadas. Estos resultados sugieren que los daños inducidos por la BMAA representan un potencial riesgo para la salud, y podrían contribuir al desarrollo de retinopatías, así como de varias enfermedades neurodegenerativas. Además, este trabajo indicaría que la activación de los RXRs puede presentar un papel protector al ejercer un rol relevante en la supervivencia de las neuronas amacrinas y FRs, así como también de las células del EPR. En su conjunto, estos hallazgos aportan nuevos conocimientos en relación a los mecanismos deletéreos inducidos por la BMAA y podrían ser de utilidad para el desarrollo de futuras estrategias terapéuticas. / The cyanotoxin β–N-methylamino-L-alanine (BMAA) is a non-proteinogenic amino acid produced by cyanobacteria. It is biomagnified along the food chains in both, marine and terrestrial ecosystems. Due to its ability to cross the brain blood barrier, its ingestion may contribute to the onset of retinopathies, as well as neurodegenerative diseases, like Amyotrophic Lateral Sclerosis, Parkinson (PD) and Alzheimer disease (AD). Damages induced by BMAA are multiple and originated by different mechanisms. In the presence of bicarbonate ions (HCO3 - ), BMAA can produce carbamate, whose chemical structure is similar to that of glutamate (Glut), one of the most important neurotransmitters in the nervous system. In turn, carbamate can bind and activate both ionotropic (like N-Methyl-D-aspartate -NMDA-) and metabotropic Glut receptors. Overactivation of these receptors by BMAA promotes excitotoxicity, which leads to nuclear alterations. On the other hand, BMAA crosses the cell membranes by using the cystine/glutamate antiporter (xc- system), a sodium-independent amino acid transporter. Once inside the cells, the toxin can mistakenly replace the amino acid Serine (Ser) in polypeptide chains, thus generating an endogenous reservoir of BMAA, whose slow- release exposes neurons to a low, but continuous amount of this toxin. Among its various subcellular effects, BMAA can alter mitochondrial membrane permeability compromising mitochondrial activity. Besides, it can alter Ca2+ levels, generate oxidative stress, promote failures in the ATP production and induce endoplasmic reticulum (RE) stress, leading to alterations in the protein synthesis and/or distribution. In this context, BMAA promotes alterations in axonal transport along with fragmentation of these structures. Despite its importance to health, the direct effects of BMAA exposure on retinal neurons and glial cells (such as Müller glial cells –CGM-), or retinal pigment epithelium (RPE) cells, are virtually unknown and the factors or molecules, which could modulate the signaling pathways involved in the deleterious effects induced by BMAA have not been established. In this regard, it has recently been proposed that the activation of Retinoid X Receptors (RXR) can protect neurons and modulate the inflammatory responses during neurodegenerative diseases of the central nervous system, including retinopathies. However, the possible protective roles of RXRs in BMAA-induced damages are still unknown. In this Thesis, we have studied the mechanisms involved in the degenerative changes induced by BMAA into retinal cells, and in neuron-like, differentiated rat pheochromocytoma cells (PC12 cells), as well. We also evaluated the protection of RXR agonists against the deleterious effects of BMAA in retinal cells. For these purposes, we obtained pure neuronal cultures of amacrine neurons and photoreceptors (PHRs); pure MGC cultures, and mixed neuro-glial cultures from newborn rats. In addition, we used PC12 cells and ARPE-19 epithelial cell lines. We treated them with BMAA to evaluate its effects on these cells and the possible protective roles of RXRs. Our results showed that low concentrations of BMAA (0.4 μM) altered, not only the viability of amacrine neurons and PHRs, but also that of neuronally differentiated PC12 cells, MGC and even that of the RPE cells. Also, the cyanotoxin induced alterations in mitochondrial membrane permeability and in ROS production, while in MGC, BMAA induced changes in the nuclear morphology. On the other hand, in amacrine neurons, this toxin promoted axonal growth, although simultaneously generating the collapse of their growth cones. We established that all these alterations were induced by activation of NMDA receptors in the presence of HCO3 - ions. Besides, in all these cell types, BMAA appeared to incorporate into polypeptide chains replacing Ser, since supplementation of the culture media with this amino acid prevented toxicity damages. Regarding the protective roles of RXRs, our results showed that their activation blocked the toxic effects induced by BMAA in amacrine neurons, PHRs, and RPE cells. In summary, in this Thesis we present evidences that BMAA affected multiple subcellular structures in retina cells and in PC12 cells differentiated into neurons. These results suggest that the damaging effects induced by BMAA represent a potential health risk, which could contribute to the development of retinopathies, along with other neurodegenerative diseases. In addition, this work would indicate that RXR activation can promote survival of amacrine PHRs and RPE cells. Taken together, these findings provide new knowledge regarding the deleterious mechanisms induced by BMAA, which could be useful for the development of future effective therapies.

No description available.