Electrochemical Studies on the Biopigment Eumelanin

Pour répondre à la demande énergétique mondiale croissante, nous devons fabriquer des dispositifs de stockage d'énergie pour de multiples besoins futurs. La durabilité motive la recherche de matériaux / produits chimiques abondants, non toxiques et bon marché pour des dispositifs de stockage d'énergie à faible consommation d'énergie et dont l’utilisation n’a pas d’impact négatif sur l'environnement. Les matières organiques actives redox extraites de sources naturelles (bio-sourcées) sont intrigantes pour de tels dispositifs de stockage d'énergie. L'eumélanine qui est un bio-pigment brun-noir à base de quinone est un candidat prometteur pour les électrodes organiques conçus à partir de matériaux provenant des bio-sources. L'eumélanine présente des propriétés fonctionnelles intéressantes, notamment l'absorption optique à large bande, l'activité redox (propriétés de transfert d'électrons) et des propriétés antioxydantes (par chélation des ions métalliques et piégeage des radicaux). Parmi ces propriétés fonctionnelles, le transfert d'électrons est essentiel pour le stockage d'énergie ainsi que pour exploiter les propriétés anti-oxydantes du biopigment. L'eumélanine naturelle est composée de deux éléments de base, le 5,6-dihydroxyindole (DHI) et l'acide 5,6-dihydroxyindole-2-carboxylique (DHICA). Cependant, son hétérogénéité chimique est reconnue comme le principal défi pour l'étude de ses propriétés redox. Dans cette thèse de doctorat, nous avons fabriqué des eumélanines contrôlées chimiquement à partir leurs éléments constitutifs, à savoir DHI-mélanine, DHICA-mélanine et DHI-DHICA-mélanine dans des rapports contrôlés. L'objectif principal de ce travail est de faire la lumière sur les propriétés redox de l'eumélanine à l'aide de méthodes électrochimiques. Dans l'article 1 et l'article 3, nous avons caractérisé l’eumélanine par les techniques de voltampérométrie et une microscopie électronique à balayage (SEM) dans le but de comprendre et de contrôler davantage ses propriétés électrochimiques basées sur l'effet des ions métalliques, les valeurs de pH des électrolytes, etc. À l'article 2, nous avons étudié l'effet de l'irradiation lumineuse sur les propriétés de stockage d'énergie de l'eumélanine, dans le but d'améliorer lesdites propriétés par la lumière solaire qui est considérée comme source d’énergie durable. En effet, une augmentation de la capacité / capacité d'environ 50% de l'eumélanine est observée sous irradiation lumineuse.----------Abstract To meet the growing global energy demand, we need to fabricate energy storage devices for multiple future needs. Sustainability motivates the search for abundant, non-toxic, low-cost materials/chemicals for low-embedded energy and eco-friendly energy storage devices. Redox active organic materials extracted from natural sources (bio-sourced) are intriguing for such energy storage devices. The quinone-based brown-black biopigment eumelanin is a promising candidate for bio-sourced organic electrode materials. Eumelanin features interesting functional properties including broadband optical absorption, redox activity (electron transfer properties), and antioxidant properties (through metal ion chelation and radical scavenging). Among these functional properties, electron transfer is essential for energy storage as well as to exploit the antioxidant properties of the biopigment. Natural eumelanin is composed of two building blocks, 5,6-dihydroxyindole (DHI) and 5,6-dihydroxyindole-2-carboxylic acid (DHICA). Natural eumelanin features chemical heterogeneity, which is recognized as the main challenge for studying its redox properties. In this work, we fabricated chemically controlled eumelanins from the building blocks, i.e. DHI-melanin, DHICA-melanin, and DHI-DHICA-melanin in controlled ratios. The main goal of this work is to shed light on the redox properties of eumelanin using electrochemical methods. In Article 1 and Article 3, we conducted cyclic voltammetry and scanning electron microscopy (SEM) on eumelanin, aiming at understanding and further controlling its electrochemical properties based on the effect of metal ions, pH values of the electrolytes, etc. In Article 2, we studied the effect of light irradiation on the energy storage properties of eumelanin, aiming at enhancing its energy storage properties by the sustainable source solar light. Indeed, ca 50% enhanced capacity/capacitance of eumelanin is observed under light irradiation. In Article 3, the antioxidant/prooxidant dual properties of eumelanin are studied by exposing the samples to reactive oxygen species (ROS) and transition metal ions. Cyclic voltammetry, X-ray photoelectron spectroscopy (XPS) and SEM are used to characterize such effects. Article 3 aims at understanding the dual properties of eumelanin and proposing possible methods to suppress the prooxidant properties of eumelanin, which is reported to be the main cause of neurondegeneration-related diseases

Low-temperature formation of polycyclic aromatic hydrocarbons in Titan’s atmosphere

The detection of benzene in Titan’s atmosphere led to the emergence of polycyclic aromatic hydrocarbons (PAHs) as potential nucleation agents triggering the growth of Titan’s orange-brownish haze layers. However, the fundamental mechanisms leading to the formation of PAHs in Titan’s low-temperature atmosphere have remained elusive. We provide persuasive evidence through laboratory experiments and computations that prototype PAHs like anthracene and phenanthrene (C14H10) are synthesized via barrierless reactions involving naphthyl radicals (C10H7•) with vinylacetylene (CH2=CH–C≡CH) in low-temperature environments. These elementary reactions are rapid, have no entrance barriers, and synthesize anthracene and phenanthrene via van der Waals complexes and submerged barriers. This facile route to anthracene and phenanthrene—potential building blocks to complex PAHs and aerosols in Titan—signifies a critical shift in the perception that PAHs can only be formed under high-temperature conditions, providing a detailed understanding of the chemistry of Titan’s atmosphere by untangling elementary reactions on the most fundamental level

Multilevel Nitrogen Additions Alter Chemical Composition and Turnover of the Labile Fraction Soil Organic Matter via Effects on Vegetation and Microorganisms

Global nitrogen (N) deposition greatly impacts soil carbon sequestration. A 2- yr multiple N addition (0, 10, 20, 40, 80, and 160 kg N·ha- 1·yr- 1) experiment was conducted in alpine grassland to illustrate the mechanisms underlying the observed soil organic matter (SOM) dynamics on the Qinghai- Tibet Plateau (QTP). Labile fraction SOM (LF- SOM) fingerprints were characterized by pyrolysis- gas chromatography/tandem- mass spectrometry, and microbial functional genes (GeoChip 4.6) were analyzed in conjunction with LF- SOM fingerprints to decipher the responses of LF- SOM transformation to N additions. The significant correlations between LF- SOM and microbial biomass, between organic compounds in LF- SOM and compound degradation- related genes, and between LF- SOM and net ecosystem exchange implied LF- SOM were the main fraction utilized by microorganisms and the most sensitive fraction to N additions. The LF- SOM increased at the lowest N addition levels (10 and 20 kg N·ha- 1·yr- 1) and decreased at higher N addition levels (40 to 160 kg N·ha- 1·yr- 1), but the decrease of LF- SOM was weakened at 160 kg N·ha- 1·yr- 1 addition. The nonlinear response of LF- SOM to N additions was due to the mass balance between plant inputs and microbial degradation. Plant- derived compounds in LF- SOM were more sensitive to N addition than microbial- derived and aromatic compounds. It is predicted that when the N deposition rate increased by 10 kg N·ha- 1·yr- 1 on the QTP, carbon sequestration in the labile fraction may increase by nearly 170% compared with that under the current N deposition rate. These findings provide insight into future N deposition impacts on LF- SOM preservation on the QTP.Key PointsThe LF- SOM quantity increased at the lowest N additions (N10 and N20) and decreased from N40 to N160, but the decrease was weakened at the highest N addition (N160)Plant- derived compounds in LF- SOM were more sensitive to N addition than microbial- derived and aromatic compoundsThe organic compounds in LF- SOM were significantly correlated with compound degradation- related genesPeer Reviewedhttps://deepblue.lib.umich.edu/bitstream/2027.42/154963/1/jgrg21637_am.pdfhttps://deepblue.lib.umich.edu/bitstream/2027.42/154963/2/jgrg21637.pd

Isolation and antibacterial activity of anabaena phycocyanin

The isolation and antibacterial activity of anabaena phycocyanin were investigated. The result indicates that three kinds of protein ingredients: PC-A, PC-B and PC-C were obtained using high performance liquid chromatography. The estimated molecular masses of PC-A and PC-B were 14 to 18 kD. PC-B and PC-C had certain antibacterial activity on Bibrio parahemolyticus, Bacillus mucilaginosus and Sarcina lutea. In addition, PC-C had certain antibacterial activity on Vibrio harveyi. PC-A did not possess antibacterial activity in the study.Keywords: Anabaena, phycocyanin, liquid chromatogram, antibacterialAfrican Journal of Biotechnology Vol. 12(15), pp. 1869-187

Vorticity, phase stiffness and the cuprate phase diagram

We review results obtained from vortex-Nernst experiments in cuprates. Evidence for a loss of phase coherence at the Meissner transition $T_{c0}$ is derived from vortex-like excitations that persist to high temperature. Below $T_{c0}$, the Nersnt signal provides a determination of the upper critical field $H_{c2}$ vs. doping $x$. Implications for the cuprate phase diagram are discussed.Comment: 6 pages, 8 figures, Plenary talk of the 7th International Conference on Materials and Mechanisms of Superconductivity and High Temperature Superconductors. To appear in Physica C, the proceeding of M2S-HTSC-VI