This study discusses the problem of methanol crossover of a direct methanol fuel cell. By increasing the difficulty in the movement of methanol molecules, and blocking the larger holes in the membrane methanol crossover is reduced. The experimental results were further validated through the MEA performance, and its long term stability, and the carbon dioxide reduction in the cathode. The experimental results show that the most effective way is to soak the membrane in the Nafion solution with ultrasonic vibration applied. Silica, instead of Nafion solution, is able to reduce the methanol crossover the most. However, the MEA thus produced performs poorly because the silica tends to roughen the membrane surface and interfere with the cohesion between the membrane and the electrode. Further studies are needed on this subject.

Les crossing-overs (CO) sont issus d’échange réciproque de matériel génétique entre les chromosomes homologues. Les COs produisent de la diversité génétique et sont essentiels chez la plupart des eucaryotes, pour la distribution équilibrée des chromosomes lors de la méiose. Malgré leur importance, et un large excès de précurseurs moléculaires, le nombre de CO est très limité dans la grande majorité des espèces (Typiquement 1 à 4 par paire de chromosomes). Cela suggère que les COs sont étroitement régulés, mais les mécanismes sous-jacents sont mal connus. Pour identifier les gènes qui limitent la formation des CO, l’équipe a mené un crible génétique chez Arabidopsis thaliana. Ces travaux ont mené à l’identification de plusieurs facteurs anti-CO, définissant trois voies : (i) L’hélicase FANCM et ses co-facteurs ; (ii) L’AAA-ATPase FIDGETIN-LIKE-1 (FIGL1) ; (iii) Le complexe RECQ4-Topoisomerase 3α-RMI1.Le premier objectif de ma thèse est d’explorer les relations entre ces trois voies en s’attachant aux questions suivantes ; (1) Jusqu’où peut-on augmenter la recombinaison en combinant les mutations dans FANCM, FIGL1 et RECQ4 ? Nous avons montré que la plus forte augmentation de recombinaison était obtenue dans recq4 figl1, atteignant 7,5 fois la fréquence du sauvage en moyenne sur le génome. (2) Quel est la distribution de ces extra-COs ? L’augmentation de recombinaison n’est pas homogène le long du génome : Les fréquences de CO augmente fortement des centromères vers les télomères, avec les plus hautes fréquences observées dans les régions distales. (3) La modification des fréquences de recombinaison est-elle identique lors des méioses mâles et femelle ? Chez le sauvage, la fréquence de recombinaison est plus élevée lors de la méiose mâle que femelle. Au contraire, la recombinaison femelle devient plus élevée que la recombinaison mâle chez les mutants recq4 et recq4 figl1. Ceci suggère que des contraintes qui s’appliquent sur la formation des CO lors de la méiose femelle sont relâchées chez ces mutants. En poursuivant le crible génétique, un nouveau mutant hyper-recombinant a été identifié. Le second objectif de ma thèse fut d’identifier et de caractériser fonctionnellement le gène correspondant. Une cartographie génétique et des études d’interactions protéine-protéine, ont mené à l’identification d’un facteur qui interagit directement avec FIGL1 et semble former un complexe conservé depuis les plantes jusqu’au mammifères. Nous avons baptisé cette protéine FLIP (Fidgetin-like-1 interacting protein). Les fréquences de recombinaisons sont augmentées dans flip-1, confirmant que FLIP1 limite la formation des COs. Des études d’épistasie ont montré que FLIP et FIGL1 agissent dans la même voie. De plus les protéines FIGL1/FLIP d’Arabidopsis ou humaine, interagissent avec RAD51 et DMC1, les deux protéines qui catalyse une étape clef de la recombinaison, l’invasion d’un ADN homologue. Finalement, dans flip comme dans figl1, la dynamique de DMC1 est modifiée. Nous proposons donc un modèle dans lequel le complexe FLIP-FIGL1 régule négativement l’activité de RAD51/DMC1 pour limiter la formation des COs. L’étude du complexe conservé FLIP-FIGL1 a mis en évidence un nouveau mode de régulation de la recombinaison, qui agit vraisemblablement à l’étape clé de l’échange de brin homologue. De plus, l’augmentation des CO sans précédent obtenues chez recq4 figl1 peut être d’un grand intérêt pour l’amélioration des plantes en permettant de diversité de nouvelles combinaisons alléliques. / Meiotic crossovers (CO) are formed by reciprocal exchange of genetic material between the homologous chromosomes. CO generate genetic diversity and are essential for the proper segregation of chromosomes during meiosis in most eukaryotes. Despite their significance and a large excess of CO precursors, CO number is very low in vast majority of species (typically one to three per chromosome pair). This indicates that COs are tightly regulated but the underlying mechanisms of this limit remain elusive. In order to identify genes that limit COs, a genetic screen was performed in Arabidopsis thaliana. This led to the identification and characterization of several anti-CO factors belonging to three different pathways: (i) The FANCM helicase and its cofactors (ii) The AAA-ATPase FIDGETIN-LIKE-1 (FIGL1) (iii) The RECQ4 -Topoisomerase 3α-RMI1 complex. The first objective was to understand the functional relationship between these three pathways and to address following questions: (1) how far can we increase recombination when combining mutations in FANCM, FIGL1 and RECQ4? We show that the highest increase in recombination was obtained in figl1 recq4, reaching to 7.5 fold the wild type level, on average genome wide. (2) How is the distribution of recombination events genome wide in mutants? The increased CO frequency in the mutants was not uniform throughout the genome. CO frequency rises from the centromere to telomeres, with distal intervals having highest COs (3) is the recombination frequency increase same in both male and female? In Arabidopsis wild type, male has higher recombination than female meiosis. In contrast, in recq4 and recq4 figl1, female recombination was higher than male. This suggests that certain constraints that apply to CO formation in wild type females are relieved in the mutant. By continuing the same genetic screen, a novel anti-CO mutant was identified. The second objective was to identify and functionally characterize the corresponding gene. Genetic mapping and protein interaction studies led to the identification of a factor that directly interacts with FIGL1 and appears to form a conserved complex both in Arabidopsis and humans. Hence, the factor was named FLIP (Fidgetin-like-1 interacting protein). Recombination frequency is increased in flip, confirming that FLIP limit COs. Epistasis studies showed that FLIP and FIGL1 act in same pathway. Further, FIGL1/FLIP proteins of Arabidopsis and humans directly interact with the recombinases RAD51 and DMC1 which catalyze a crucial step of homologous recombination, the inter homolog strand invasion. In addition flip like figl1 modifies dynamics of DMC1. We thus propose a model wherein the FLIP-FIGL1 complex negatively regulates RAD51/DMC1 to limit CO formation. Studying the conserved FIGL1-FLIP complex led to the identification of a novel mode of regulation of recombination, that likely acts at the key step of homologous strand invasion. Further the unprecedented level of CO increase in recq4figl1 in hybrids could be of great interest for crop improvement, allowing the production of novel allele combinations.

No description available.

The major focus of this thesis involves a new approach to spin-crossover (SCO) in iron(II) dinuclear complexes. In order to acquire SCO properties, a series of novel ligands (L1M, L2M, L3M, L4M), together with the corresponding iron(II) and/or nickel(II) complexes were synthesized and characterized. Beyond the successful synthesis and characterization of these ligands, some interesting aspects of their synthesis are discussed. The reported ligands are methyl-derivatives from ligands previously studied in the group. The methyl group in the position 6 of the pyridine group is introduced to induce steric hindrance in the complexes and decrease the ligand field strength. As a result, the influence of the methyl group may induce SCO in low spin (LS) complexes, or tune the SCO properties (such as transition hysteresis or transition temperature) in complexes with SCO. Ligands were divided into three different types according to their spacers: a ligand with an aromatic spacer (L1M), ligands with aliphatic spacer (L2M, L3M) and a ligand with no spacer (L4M). The difference between their structures helped us to better understand the coordination chemistry of the corresponding complexes. The nickel(II) complexes formed provided useful background information on the coordination chemistry of iron(II), such as the crystal field stabilization energy (10Dq) value. An approximation of 10Dq value of iron(II) was calculated based on the 10Dq value of analogous nickel(II) complexes using an empirical rule, to provide a prediction of SCO property. / Thesis (Master, Chemistry) -- Queen's University, 2013-09-20 10:34:02.756

In this thesis, a long-term operation direct methanol fuel cell (DMFC) stack with no performance decay is developed. In a passive DMFC, its performance will decay after a short period, if the weak methanol solution is supplied. Although the strong methanol solution may slightly increase the operating time, it tends to crossover leading to cathode Pt particle poisoned, and it is still unable to maintain long-term operation stably. In our passive DMFC, there are two chambers in the anode. They are a pure methanol chamber and a 2~3M methanol solution chamber. By adjusting the diffusion gate outlet of the pure methanol chamber to control the diffusion rate of pure methanol, the appropriate pure methanol deliver to maintain methanol concentration in the anode reaction chamber stably. If the anode reaction chamber is initially filled with 0.8 c.c., 3M methanol solution and without additional fuel supply, experimental results show that the performance of the DMFC begin decay after about 30 minutes. If the appropriate amount of methanol is supplied, the cell can maintain a long-term operation with no performance decay. The methanol concentration between 1.5 and 3M only has a little impact on the current output. Experimental results also show that the initial methanol concentration 2M is able to maintain a cell operation more stable than it in 3M. However, the initial methanol concentration and appropriate methanol supply is also important to maintain a DMFC for a long-term operation stably.

Molecules with functional properties, especially ones that display spin crossover behaviour, are becoming increasingly well researched due to their potential applications as the active components in molecular memory, optical displays or sensors.1,2 To this end, a series of Fe(II) coordination compounds utilising imidazolylimine and pyridylimine based ligands have been synthesised and their spin crossover behaviour analysed through a variety of techniques including variable temperature X-ray crystallography, magnetic susceptibility, surface reflectivity and UV-Visible spectroscopy. Light-Induced Excited Spin-State Trapping (LIESST) effects have also been analysed via photomagnetic measurements. Particular attention was paid to the supramolecular interactions present within the solid-state and the effect that these interactions may have on spin crossover behaviour. Subtle changes to the complexes through counter-anion exchange, solvent effects and ligand design were explored and careful structural analysis completed. This study was separated into two distinct categories. The first describes the synthesis, structural characterisation and spin crossover behaviour of Fe(II) dinuclear triple helicates. Four dinuclear triple helicates have been synthesised, three of which display spin crossover behaviour. Weak LIESST effects were observed in two of these examples. Variation in the ligand field strength between the complexes was achieved through modification of the coordinating “head group” and differences between the flexibility of the ligand backbone were also explored. The latter part of this study describes the synthesis, structural characterisation and spin crossover behaviour of novel Fe(II) mononuclear complexes. A series of related ligands which differ in the p-substituent have been synthesised and their coordination chemistry with Fe(II) metal ions examined. Subtle changes to the counter-anion and crystallised solvent molecules were also explored during this study. Five mononuclear Fe(II) complexes were shown to undergo spin crossover and the structural changes associated with the spin-state conversion have been analysed via variable temperature X-ray crystallography and compared with magnetic susceptibility measurements. Two complexes were also analysed for LIESST behaviour and these complexes showed full photo-excitation into the high-spin state at 10 K.

No description available.

Youth involved with the United States juvenile legal system (JLS) experience multiple institutional transitions — movements into, between, and out of state agencies — throughout their lifecourse. For youth and young adults dually-involved with both the child welfare system (CWS) and the JLS, two primary forms of institutional transitions include:1) crossover from the CWS to the JLS; and 2) reentry back into the community following prolonged system involvement. While all system-impacted youth experience adverse outcomes and navigate significant challenges during these transitions, a growing body of evidence demonstrates that dually-involved Black, Indigenous, and Youth of Color (BIYOC) fare even worse than their white system-involved peers. Research on dual involvement has focused on the demographic factors, events, and contexts associated with the crossover phenomenon, while research on reentry — particularly for dually-involved youth — has focused on the barriers and facilitators of positive reentry experiences, predominantly conceptualizing recidivism as the indicator by which to measure successful reentry. Less attention has been paid to the subjective experiences of dually-involved BIYOC, including how they conceptualize, navigate, and make sense of these transitions. Drawing from the Phenomenological Variant of Ecological Systems Theory and phenomenological approaches, this three-paper mixed methods dissertation contributes to scholarship on dually-involved youth by focusing on the subjective experiences of youth and young adults of color during crossover and reentry. Paper one is a qualitative study based on in depth semi-structured interviews (N=31) with dually-involved Black and Latino youth and young adults in Massachusetts. This study uses Reflexive Thematic Analyses to investigate the factors youth and young adults identify as contributing to their transitions from the CWS to the JLS and their subjective experiences of the CWS, JLS, and crossover. Paper two uses a subset of these interviews with Black and Latina girls and young women (N=10) and draws on Interpretative Phenomenological Analysis to examine their experiences with the CWS, JLS, and during reentry, tracing their life trajectories and their goals, hopes, and expectations beyond incarceration. Paper three uses linked child welfare and juvenile legal administrative data (N=1226) to further understand dually-involved youth and young adults’ reentry experiences, conceptualizing participation in reentry programming as a strengths-based metric for successful reentry. One central finding is that the CWS and JLS are highly porous, with nearly two thirds of youth committed to the Massachusetts JLS between 2015-2019 having experienced an open case with the CWS, and 43.6% having been removed from their homes. Youth and young adults’ narratives suggest that factors including CWS rules and regulations, subpar living conditions within congregate care facilities, and surveillance from multiple systems facilitate crossover into the JLS. Findings also suggest that when juxtaposed to CWS placements, dually-involved Black and Latino youth and young adults view the JLS context more favorably, describing the existence of a reactive but robust social safety net within JLS placements. This safety net is not only characterized by social services and support, but several forms of care including what I refer to as care-as-structure and care-as-relationships. Relationships between youth and JLS caseworkers are especially salient for girls and young women, who order their system trajectories through the lens of their relational constellations. The implications are manyfold and include the need for further research examining information-sharing practices between youth-facing agencies, the youth categorization mechanisms that facilitate crossover, and the nature of care within carceral contexts. Policy and structural implications include implementing youth programming focused on relationships rather than rules, redistributing social services to communities upstream of agency involvement, and facilitating kinship and chosen family networks within communities. / 2025-05-15T00:00:00Z

Meiosis is a specialized division of germ cells in sexually reproducing organisms, which is a fundamental process with key implications for evolution and biodiversity. In two consecutive rounds of cell division, meiosis I and meiosis II, a normal, diploid set of chromosome is halved. From diploid mother cells haploid gametes are generated to create genetic individual cells. This genetic uniqueness is obtained during prophase of meiosis I by essential meiotic processes in meiotic recombination, as double strand break (DSB) formation and repair, formation of crossovers (CO) and holiday junctions (HJs). Checkpoint mechanisms ensure a smooth progress of these events. Despite extensive research key mechanisms are still not understood. Based on an analysis of Massive Parallel Sequencing (MPS) data I could identify 2 genes, Mcmdc2 and Prr19, with high implication in meiotic recombination. In the absence of Mcmdc2 both sexes are infertile and meiocytes arrest at a stage equivalent to mid-­‐pachytene in wt. Investigations of the synaptonemal complex (SC) formation revealed severe defects suggesting a role for MCMDC2 in homology search. Moreover, MCMDC2 does not seem to be essential for DSB repair, as DSB markers of early and mid recombination nodules, like DMC1 and RPA, are decreased in oocytes. Nevertheless, late recombination nodules, which are positive for MutL homolog 1 (MLH1), do not form in both sexes. The absence of the asynapsis surveillance checkpoint mechanism in Hormad2 deficient ovaries with Mcmdc2 mutant background allowed survival of oocytes. This points into the direction that Mcmdc2 knock­out oocytes get eliminated after prophase I due to failed homologous synapsis. Interestingly, MCMDC2 contains a conserved helicase domain, like the MCM protein family members MCM8 and MCM9. I therefore hyphothesize that Mcmdc2 promotes homolgy search.

La recombinaison méiotique est au cœur de l'hérédité Mendélienne, de l'évolution et de l'amélioration des plantes, car elle assure, grâce aux crossovers, une transmission fidèle des chromosomes et le brassage de l’information génétique au fil des générations. Deux voies de formation des crossovers coexistent chez les plantes. La voie principale (voie I) dépend de la protéine MSH4 (et de quelques autres). La voie secondaire ne produit que quelques crossovers (dits de voie II) au cours de la méiose d’une plante de type sauvage ; ils sont indépendants de MSH4 et leur nombre est limité par des protéines telles que FANCM. Si ces deux voies ont été bien décrites chez des espèces diploïdes, ce n’est pas le cas chez des plantes allopolyploïdes, pourtant très fréquentes parmi les plantes cultivées. Il s'agit là d'une lacune importante, car la présence de plusieurs jeux de chromosomes apparentés conduit à augmenter le nombre de partenaires susceptibles de former des crossovers et le nombre de copies de tous les gènes méiotiques, rendant la recombinaison méiotique plus complexe. Cette thèse vise à explorer l'interaction entre les voies de formation des crossovers et la polyploïdie en utilisant des mutants de colza (Brassica napus; AACC) et d’un de ces parents diploïdes (B. rapa; AA) pour deux gènes de la recombinaison méiotique.J'ai tout d'abord testé dans quelle mesure la formation de crossovers entre chromosomes homologues et entre homéologues (chromosomes A et C) est tributaire des voies I et II en évaluant l’effet d’une diminution du nombre de copies fonctionnelles de MSH4 sur le nombre de crossovers. J'ai montré que ce dernier n'est altéré que lorsque les deux copies MSH4 sont inactivées, toute autre combinaison de mutations conduisant au même nombre de crossovers inter-homologues que chez le sauvage. J'ai également montré que la proportion de crossovers de voie II chez des mutants msh4 de colza est bien supérieure à celles observées chez d’autres plantes mutantes pour msh4. Cette observation reste vraie chez des mutants msh4 de B. rapa, suggérant que la proportion accrue de crossovers de voie II n’est pas spécifique au colza, mais probablement une caractéristique des Brassicaceae. Chez des plantes allohaploïdes (AC) de colza, chez lesquelles les crossovers ne peuvent se former qu’entre homéologues, les copies MSH4 ne se compensent plus complétement ; le nombre de crossovers de voie I fluctue au contraire proportionnellement au dosage de MSH4, devenant presque nul lorsque toutes les copies sont inactivées. Mes résultats illustrent deux nouvelles propriétés spécifiques des crossovers entre homéologues: une plus grande sensibilité vis-à-vis du dosage MSH4 pour les crossovers de voie I et une plus faible efficacité des crossovers de voie II.Dans un second temps, j'ai caractérisé cytologiquement des mutants fancm de colza pour vérifier que l'augmentation des crossovers de voie II ne nuit pas à au bon déroulement de sa méiose. Cette question est restée en suspens, les mutants fancm de colza n’étant pas complètement nuls. Cet écueil m'a incité à développer une approche de TILLING par séquençage pour identifier de nouveaux mutants de recombinaison chez le colza. J'ai alors combiné les mutations fancm et msh4 chez B. rapa pour vérifier si les premières suffisent à corriger les défauts méiotiques induits par les secondes. J'ai montré que, conformément à ce qui avait été observé chez Arabidopsis thaliana, la mutation fancm augmente le nombre de crossovers à un point tel qu’elle restaure la formation de bivalents dans un mutant msh4. La fonction de FANCM est donc conservée chez B. rapa.Mes résultats ont fait progresser la compréhension des voies de formation des crossovers lors d’une méiose allopolyploïde. Ils indiquent que la transmission des chromosomes chez ces espèces implique principalement des crossovers de voie I, et qu’elle pourrait être assurée en limitant l’efficacité de cette voie (e.g. en diminuant le nombre de copies de gènes). / Meiotic recombination ensures, through the formation of crossovers (COs), both faithful chromosome transmission and allelic shuffling over generations; it is at the heart of Mendelian heredity, evolution and plant breeding. Two crossover pathways co-exist in plants. The main pathway (class I) is dependent on MSH4 (and additional proteins). The secondary pathway produces only a few MSH4-independent (class II) crossovers during wild-type meiosis that are limited in number by anti-crossover proteins such as FANCM. These pathways have been extensively described in diploid species, disregarding one of the most pervasive features of crop genomes: polyploidy. This is a major gap in our understanding because the presence of more than two related sets of chromosomes leads both to extra partners for crossover formation and additional copies for all meiotic genes, which make meiotic recombination more intricate. This thesis aims at exploring the interplay between meiotic recombination pathways and polyploidy using mutants for two recombination genes in allotetraploid Brassica napus (AACC; 2n=38) and its diploid progenitor, B. rapa (AA; 2n=20). I have first tested the extent to which class I and class II pathways contribute to inter-homolog and inter-homoeolog (between A and C chromosomes) crossover formation by analyzing how crossovers are affected as the number of functional MSH4 copies decreases. I showed that inter-homolog crossover formation is impaired only when the two MSH4 copies are lost, any other combination of msh4 mutations resulting in wild-type crossover numbers. I also observed that, when class I crossovers are completely abolished in B. napus, the highest frequency of class II crossover ever reported among plant msh4 mutants is observed. I reproduced this result using B. rapa msh4 mutants, thereby demonstrating that increased class II crossover frequencies is not specific to B.napus, but could instead be a general feature of the Brassicaceae. In B. napus allohaploids (AC), where crossovers are forced to occur between homeologs, MSH4 copies no longer complement each other perfectly; counter to the situation in euploids, the number of MSH4-dependent crossovers formed between homoeologs fluctuates with MSH4 dosage in these plants, and approximate zero when all MSH4 copies are depleted. Altogether, my results illustrate two novel specific properties of inter-homeolog crossovers: a greater sensitivity to MSH4 dosage for class I pathway and a lower efficiency for class II.Next, I characterized cytologically B. napus fancm mutants to confirm that boosting class II crossovers would not be detrimental to B. napus meiosis. However, a prudential interpretation of these results is demanded since the B. napus fancm alleles retained residual anti-crossover activity. This has prompted me to set up a TILLING-by-sequencing procedure in order to produce new recombination mutants in B. napus. I also combined the B. rapa fancm and msh4 mutations to test whether the former is sufficient to fix the meiotic defects resulting from the latter. I showed that, similarly to what had been observed in Arabidopsis thaliana, fancm mutation boost COs to such a point that it restores bivalent formation in B. rapa msh4 background. My results therefore confirmed that the function of FANCM is conserved in B. rapa. Overall, the findings and achievements of this thesis make a step forward dissection of CO pathways during allopolyploid meiosis. They indicate that meiotic adaptation to allopolyploidy mainly involve the class I crossover pathway and could be achieved by limiting its efficiency (e.g. by decreasing gene copy number).