Valence-tautomeric RbMnFe Prussian blue analogues:composition and time stability investigation

Three different stoichiometric forms of RbMn[Fe(CN) ]y·zHO [x = 0.96, y = 0.98, z = 0.75 (1); x = 0.94, y = 0.88, z = 2.17 (2); x = 0.61, y = 0.86, z = 2.71 (3)] Prussian blue analogues were synthesized and investigated by magnetic, calorimetric, Raman spectroscopic, X-ray diffraction, and Fe Mössbauer spectroscopic methods. Compounds 1 and 2 show a hysteresis loop between the high-temperature (HT) Fe(S = 1/2)-CN-Mn(S = 5/2) and the low-temperature (LT) Fe(S = 0)-CN-Mn(S = 2) forms of 61 and 135 K width centered at 273 and 215 K, respectively, whereas the third compound remains in the HT phase down to 5 K. The splitting of the quadrupolar doublets in the Fe Mössbauer spectra reveal the electron-transfer-active centers. Refinement of the X-ray powder diffraction profiles shows that electron-transfer-active materials have the majority of the Rb ions on only one of the two possible interstitial sites, whereas nonelectron-transfer-active materials have the Rb ions equally distributed. Moreover, the stability of the compounds with time and following heat treatment is also discussed. © Wiley-VCH Verlag GmbH & Co. KGaA, 2009.

Effects of oxidative stress and neuroprotection in apoptosis in neuronal cell models

The PC12 and SH-SY5Y cell models have been proposed as potentially realistic models to investigate neuronal cell toxicity. The effects of oxidative stress (OS) caused by both H2O2 and Aβ on both cell models were assessed by several methods. Cell toxicity was quantitated by measuring cell viability using the 3-(4,5-dimethylthiazol-2-yl)-2,5-diphenyltetrazolium (MTT) viability assay, an indicator of the integrity of the electron transfer chain (ETC), and cell morphology by fluorescence and video microscopy, both of which showed OS to cause decreased viability and changes in morphology. Levels of intracellular peroxide production, and changes in glutathione and carbonyl levels were also assessed, which showed OS to cause increases in intracellular peroxide production, glutathione and carbonyl levels. Differentiated SH-SY5y cells were also employed and observed to exhibit the greatest sensitivity to toxicity. The neurotrophic factor, nerve growth factor (NGF) was shown to cause protection against OS. Cells pre-treated with NGF showed higher viability after OS, generally less apoptotic morphology, recorded less apoptotic nucleiods, generally lower levels of intracellular peroxides and changes in gene expression. The neutrophic factor, brain derived growth factor (BDNF) and ascorbic acid (AA) were also investigated. BDNF showed no specific neuroprotection, however the preliminary data does warrant further investigation. AA showed a 'janus face' showing either anti-oxidant action and neuroprotection or pro-oxidant action depending on the situation. Results showed that the toxic effects of compounds such as Aβ and H2O2 are cell type dependent, and that OS alters glutathione metabolism in neuronal cells. Following toxic insult, glutathione levels are depleted to low levels. It is herein suggested that this lowering triggers an adaptive response causing alterations in glutathione metabolism as assessed by evaluation of glutathione mRNA biosynthetic enzyme expression and the subsequent increase in glutathione peroxidase (GPX) levels.

Three-dimensional nitrogen-doped graphene supported molybdenum disulfide nanoparticles as an advanced catalyst for hydrogen evolution reaction

An efficient three-dimensional (3D) hybrid material of nitrogen-doped graphene sheets (N-RGO) supporting molybdenum disulfide (MoS2) nanoparticles with high-performance electrocatalytic activity for hydrogen evolution reaction (HER) is fabricated by using a facile hydrothermal route. Comprehensive microscopic and spectroscopic characterizations confirm the resulting hybrid material possesses a 3D crumpled few-layered graphene network structure decorated with MoS2 nanoparticles. Electrochemical characterization analysis reveals that the resulting hybrid material exhibits efficient electrocatalytic activity toward HER under acidic conditions with a low onset potential of 112 mV and a small Tafel slope of 44 mV per decade. The enhanced mechanism of electrocatalytic activity has been investigated in detail by controlling the elemental composition, electrical conductance and surface morphology of the 3D hybrid as well as Density Functional Theory (DFT) calculations. This demonstrates that the abundance of exposed active sulfur edge sites in the MoS2 and nitrogen active functional moieties in N-RGO are synergistically responsible for the catalytic activity, whilst the distinguished and coherent interface in MoS 2 /N-RGO facilitates the electron transfer during electrocatalysis. Our study gives insights into the physical/chemical mechanism of enhanced HER performance in MoS2/N-RGO hybrids and illustrates how to design and construct a 3D hybrid to maximize the catalytic efficiency.

Impact of co-adsorbed oxygen on crotonaldehyde adsorption over gold nanoclusters:a computational study

Crotonaldehyde (2-butenal) adsorption over gold sub-nanometer particles, and the influence of co-adsorbed oxygen, has been systematically investigated by computational methods. Using density functional theory, the adsorption energetics of crotonaldehyde on bare and oxidised gold clusters (Au , d = 0.8 nm) were determined as a function of oxygen coverage and coordination geometry. At low oxygen coverage, sites are available for which crotonaldehyde adsorption is enhanced relative to bare Au clusters by 10 kJ mol. At higher oxygen coverage, crotonaldehyde is forced to adsorb in close proximity to oxygen weakening adsorption by up to 60 kJ mol relative to bare Au. Bonding geometries, density of states plots and Bader analysis, are used to elucidate crotonaldehyde bonding to gold nanoparticles in terms of partial electron transfer from Au to crotonaldehyde, and note that donation to gold from crotonaldehyde also becomes significant following metal oxidation. At high oxygen coverage we find that all molecular adsorption sites have a neighbouring, destabilising, oxygen adatom so that despite enhanced donation, crotonaldehyde adsorption is always weakened by steric interactions. For a larger cluster (Au, d = 1.1 nm) crotonaldehyde adsorption is destabilized in this way even at a low oxygen coverage. These findings provide a quantitative framework to underpin the experimentally observed influence of oxygen on the selective oxidation of crotyl alcohol to crotonaldehyde over gold and gold-palladium alloys. © 2014 the Partner Organisations.

Ag alloyed Pd single-atom catalysts for efficient selective hydrogenation of acetylene to ethylene in excess ethylene

Semihydrogenation of acetylene in an ethylene-rich stream is an industrially important process. Conventional supported monometallic Pd catalysts offer high acetylene conversion, but they suffer from very low selectivity to ethylene due to overhydrogenation and the formation of carbonaceous deposits. Herein, a series of Ag alloyed Pd single-atom catalysts, possessing only ppm levels of Pd, supported on silica gel were prepared by a simple incipient wetness coimpregnation method and applied to the selective hydrogenation of acetylene in an ethylene-rich stream under conditions close to the front-end employed by industry. High acetylene conversion and simultaneous selectivity to ethylene was attained over a wide temperature window, surpassing an analogous Au alloyed Pd single-atom system we previously reported. Restructuring of AgPd nanoparticles and electron transfer from Ag to Pd were evidenced by in situ FTIR and in situ XPS as a function of increasing reduction temperature. Microcalorimetry and XANES measurements support both geometric and electronic synergetic effects between the alloyed Pd and Ag. Kinetic studies provide valuable insight into the nature of the active sites within these AgPd/SiO2 catalysts, and hence, they provide evidence for the key factors underpinning the excellent performance of these bimetallic catalysts toward the selective hydrogenation of acetylene under ethylene-rich conditions while minimizing precious metal usage.

Multimode Analysis of Nanoscale Biomolecular Interactions

Biomolecular interactions, including protein-protein, protein-DNA, and protein-ligand interactions, are of special importance in all biological systems. These interactions may occer during the loading of biomolecules to interfaces, the translocation of biomolecules through transmembrane protein pores, and the movement of biomolecules in a crowded intracellular environment. The molecular interaction of a protein with its binding partners is crucial in fundamental biological processes such as electron transfer, intracellular signal transmission and regulation, neuroprotective mechanisms, and regulation of DNA topology. In this dissertation, a customized surface plasmon resonance (SPR) has been optimized and new theoretical and label free experimental methods with related analytical calculations have been developed for the analysis of biomolecular interactions. Human neuroglobin (hNgb) and cytochrome c from equine heart (Cyt c) proteins have been used to optimize the customized SPR instrument. The obtained Kd value (~13 µM), from SPR results, for Cyt c-hNgb molecular interactions is in general agreement with a previously published result. The SPR results also confirmed no significant impact of the internal disulfide bridge between Cys 46 and Cys 55 on hNgb binding to Cyt c. Using SPR, E. coli topoisomerase I enzyme turnover during plasmid DNA relaxation was found to be enhanced in the presence of Mg2+. In addition, a new theoretical approach of analyzing biphasic SPR data has been introduced based on analytical solutions of the biphasic rate equations. In order to develop a new label free method to quantitatively study protein-protein interactions, quartz nanopipettes were chemically modified. The derived Kd (~20 µM) value for the Cyt c-hNgb complex formations matched very well with SPR measurements (Kd ~16 µM). The finite element numerical simulation results were similar to the nanopipette experimental results. These results demonstrate that nanopipettes can potentially be used as a new class of a label-free analytical method to quantitatively characterize protein-protein interactions in attoliter sensing volumes, based on a charge sensing mechanism. Moreover, the molecule-based selective nature of hydrophobic and nanometer sized carbon nanotube (CNT) pores was observed. This result might be helpful to understand the selective nature of cellular transport through transmembrane protein pores.

Nano-grids of Yeast Cytochrome C

One innovative thought in biomolecular electronics is the exploitation of electron transfer proteins. Using nature's self assembly techniques, proteins can build highly organized edifices with retained functional activity, and they can serve as platforms for biosensors. In this research work, Yeast Cytochrome C (YCC) is immobilized with a help of a linker molecule, 3-Mercaptopropyltrimethoxysilane (3-MPTS) on a hydroxylated surface of a silicon substrate. Atomic Force Microscopy (AFM) is used for characterization. AFM data shows immobilization of one YCC molecule in between eight grids that are formed by the linker molecules. 3-MPTS monolayers are organized in grids that are 1.2 nm apart. Immobilization of 3-MPTS was optimized using a concentration of 5 mM in a completely dehydrated state for 30 minutes. The functionally active grids of YCC can now be incorporated with Cytochrome C oxidase on a Platinum electrode surface for transfer of electrons in development of biosensors, such as nitrate sensor, that are small in size, cheaper, and easier to manufacture than the top-down approach of fabrication of molecular biodevices

AFM and SPR studies of protein adsorption at solid/liquid interface

Membrane-like structure formed by surfactant molecules of didodecyldimethylammonium bromide (DDAB) on both HOPG and gold electrodes were studied with AFM and SPR techniques. The study shows that the thickness of the adsorbed layer of DDAB is strongly dependent on the concentration of the vesicle solution. We have also investigated the adsorption of redox protein, Cytochrome c, on graphite electrode with in situ tapping mode AFM. The protein adsorbs spontaneously onto the electrode covered with an adsorbed phosphate layer and forms a uniform monolayer. The adsorbed protein exhibits a reversible electron transfer at 0.17 V (Ag/AgCI) once the electrode potential has been increased to 0.75V. Using surface plasmon resonance spectroscopy we have measured subtle conformational change in protein, Cyt c, due to electron transfer of a single electron on MPA-coated gold electrode. The electron transfer induced change in the resonant angle is about 0.006 deg., which corresponds to ~ 0.2 A decreases in the thickness. This is consistent with that reduced state is more compact than the oxidized state.

Photocatalytic activities of tin(IV) oxide surface-modified titanium(IV) dioxide show a strong sensitivity to the TiO 2 crystal form

Surface modification of rutile TiO2 with extremely small SnO2 clusters gives rise to a great increase in its UV light activity for degradation of model organic water pollutants, while the effect is much smaller for anatase TiO2. This crystal form sensitivity is rationalized in terms of the difference in the electronic modification of TiO2 through the interfacial Sn−O−Ti bonds. The increase in the density of states near the conduction band minimum of rutile by hybridization with the SnO2 cluster levels intensifies the light absorption, but this is not seen with modified anatase. The electronic transition from the valence band to the conduction band causes the bulk-to-surface interfacial electron transfer to enhance charge separation. Further, electrons relaxed to the conduction minimum are smoothly transferred to O2 due to the action of the SnO2 species as an electron transfer promoter.

Loading effect in copper(II) oxide cluster-surface-modified titanium(IV) oxide on visible- and UV-light activities

Cu(acac)2 is chemisorbed on TiO2 particles [P-25 (anatase/rutile = 4/1 w/w), Degussa] via coordination by surface Ti–OH groups without elimination of the acac ligand. Post-heating of the Cu(acac)2-adsorbed TiO2 at 773 K yields molecular scale copper(II) oxide clusters on the surface (CuO/TiO2). The copper loading amount (Γ/Cu ions nm–2) is controlled in a wide range by the Cu(acac)2 concentration and the chemisorption–calcination cycle number. Valence band (VB) X-ray photoelectron and photoluminescence spectroscopy indicated that the VB maximum of TiO2 rises up with increasing Γ, while vacant midgap levels are generated. The surface modification gives rise to visible-light activity and concomitant significant increase in UV-light activity for the degradation of 2-naphthol and p-cresol. Prolonging irradiation time leads to the decomposition to CO2, which increases in proportion to irradiation time. The photocatalytic activity strongly depends on the loading, Γ, with an optimum value of Γ for the photocatalytic activity. Electrochemical measurements suggest that the surface CuO clusters promote the reduction of adsorbed O2. First principles density functional theory simulations clearly show that, at Γ < 1, unoccupied Cu 3d levels are generated in the midgap region, and at Γ > 1, the VB maximum rises and the unoccupied Cu 3d levels move to the conduction band minimum of TiO2. These results suggest that visible-light excitation of CuO/TiO2 causes the bulk-to-surface interfacial electron transfer at low coverage and the surface-to-bulk interfacial electron transfer at high coverage. We conclude that the surface CuO clusters enhance the separation of photogenerated charge carriers by the interfacial electron transfer and the subsequent reduction of adsorbed O2 to achieve the compatibility of high levels of visible and UV-light activities.

Design, Synthesis, and Photophysical Investigation of Porphyrin-containing Polymer-wrapped Single-walled Carbon Nanotubes

Significant advances in understanding the fundamental photophysical behavior of single-walled carbon nanotubes (SWNTs) have been made possible by the development of ionic, conjugated aryleneethynylene polymers that helically wrap SWNTs with well-defined morphology. My contribution to this work was the design and synthesis of porphyrin-containing polymers and the photophysical investigation of the corresponding polymer-wrapped SWNTs. For these new constructs, the polymer acts as more than just a solubilization scaffold; such assemblies can provide benchmark data for evaluating spectroscopic signatures of energy and charge transfer events and lay the groundwork for further, rational development of polymers with precisely tuned redox properties and electronic coupling with the underlying SWNT. The first design to incorporate a zinc porphyrin into the polymer backbone, PNES-PZn, suffered from severe aggregation in solution and was redesigned to produce the porphyrin-containing polymer S-PBN-PZn. This polymer was utilized to helically wrap chirality-enriched (6,5) SWNTs, which resulted in significant quenching of the porphyrin-based fluorescence. Time-resolved spectroscopy revealed a simultaneous rise and decay of the porphyrin radical cation and SWNT electron polaron spectroscopic signatures indicative of photoinduced electron transfer. A new polymer, S-PBN(b)-Ph2PZn3, was then synthesized which incorporated a meso-ethyne linked zinc porphyrin trimer. By changing the absorption profile and electrochemical redox potentials of the polymer, the photophysical behavior of the corresponding polymer-wrapped (6,5)-SWNTs was dramatically changed, and the polymer-wrapped SWNTs no longer showed evidence for photoinduced electron transfer.

Experimental and Theoretical Models to Probe Mechanisms of Biological Charge Flow

Nature is challenged to move charge efficiently over many length scales. From sub-nm to μm distances, electron-transfer proteins orchestrate energy conversion, storage, and release both inside and outside the cell. Uncovering the detailed mechanisms of biological electron-transfer reactions, which are often coupled to bond-breaking and bond-making events, is essential to designing durable, artificial energy conversion systems that mimic the specificity and efficiency of their natural counterparts. Here, we use theoretical modeling of long-distance charge hopping (Chapter 3), synthetic donor-bridge-acceptor molecules (Chapters 4, 5, and 6), and de novo protein design (Chapters 5 and 6) to investigate general principles that govern light-driven and electrochemically driven electron-transfer reactions in biology. We show that fast, μm-distance charge hopping along bacterial nanowires requires closely packed charge carriers with low reorganization energies (Chapter 3); singlet excited-state electronic polarization of supermolecular electron donors can attenuate intersystem crossing yields to lower-energy, oppositely polarized, donor triplet states (Chapter 4); the effective static dielectric constant of a small (~100 residue) de novo designed 4-helical protein bundle can change upon phototriggering an electron transfer event in the protein interior, providing a means to slow the charge-recombination reaction (Chapter 5); and a tightly-packed de novo designed 4-helix protein bundle can drastically alter charge-transfer driving forces of photo-induced amino acid radical formation in the bundle interior, effectively turning off a light-driven oxidation reaction that occurs in organic solvent (Chapter 6). This work leverages unique insights gleaned from proteins designed from scratch that bind synthetic donor-bridge-acceptor molecules that can also be studied in organic solvents, opening new avenues of exploration into the factors critical for protein control of charge flow in biology.

Hydroxyamidinates et polymères de coordination : suspicions et valences mixtes

Dans un contexte où l’énergie représente un enjeu majeur pour les pays et organisations à économies émergentes et développées, la recherche de nouvelles sources renouvelables et la démocratisation des vecteurs énergétiques permettant l’approvisionnement mondial de façon durable constitue un devoir pour la communauté scientifique internationale. D’ailleurs, il serait essentiel que les nombreuses disciplines de la chimie concertent leurs efforts. Plus particulièrement, la croissance de la recherche en chimie de coordination orientée vers la photosynthèse artificielle ainsi que le développement de matériaux fonctionnels démontre l’importance indéniable de ce champ de recherche. Ce travail présente dans un premier temps l’étude des différentes voies de synthèse d’hydroxyamidines, un ligand chélatant aux propriétés de coordination prometteuses ne recevant que très peu d’attention de la part de la communauté scientifique. Dans un deuxième temps, nous présenterons le développement d’une stratégie d’assemblage de leurs complexes supramoléculaires impliquant des métaux de transition abondants et peu dispendieux de la première rangée. Dans un troisième temps, il sera question de l’investigation de leurs propriétés photophysiques et électrochimiques à des fins d’applications au sein de matériaux fonctionnels. Pour ce faire, les différentes voies de synthèse des hydroxyamidines et de leurs amidines correspondantes qui ont précédemment été étudiées par les membres du groupe seront tout d’abord perfectionnées, puis investiguées afin de déterminer leur versatilité. Ensuite, les propriétés de complexation des amox résultantes comportant des motifs sélectionnés seront déterminées pour enfin étudier les propriétés photophysiques et électrochimiques d’une série de complexes de métaux de transition de la première rangée. En somme, plusieurs designs qu’offrent les amox et bis-amox sont étudiés et les propriétés des architectures résultantes de leur auto-assemblage sont déterminées.

Interactive effects of ocean acidification and nitrogen limitation on the diatom Phaeodactylum tricornutum

Climate change is expected to bring about alterations in the marine physical and chemical environment that will induce changes in the concentration of dissolved CO2 and in nutrient availability. These in turn are expected to affect the physiological performance of phytoplankton. In order to learn how phytoplankton respond to the predicted scenario of increased CO2 and decreased nitrogen in the surface mixed layer, we investigated the diatom Phaeodactylum tricornutum as a model organism. The cells were cultured in both low CO2 (390 µatm) and high CO2 (1000 µatm) conditions at limiting (10 µmol/L) or enriched (110 µmol/L) nitrate concentrations. Our study shows that nitrogen limitation resulted in significant decreases in cell size, pigmentation, growth rate and effective quantum yield of Phaeodactylum tricornutum, but these parameters were not affected by enhanced dissolved CO2 and lowered pH. However, increased CO2 concentration induced higher rETRmax and higher dark respiration rates and decreased the CO2 or dissolved inorganic carbon (DIC) affinity for electron transfer (shown by higher values for K1/2 DIC or K1/2 CO2). Furthermore, the elemental stoichiometry (carbon to nitrogen ratio) was raised under high CO2 conditions in both nitrogen limited and nitrogen replete conditions, with the ratio in the high CO2 and low nitrate grown cells being higher by 45% compared to that in the low CO2 and nitrate replete grown ones. Our results suggest that while nitrogen limitation had a greater effect than ocean acidification, the combined effects of both factors could act synergistically to affect marine diatoms and related biogeochemical cycles in future oceans.

Tegument Glycoproteins and Cathepsins of Newly Excysted Juvenile Fasciola hepatica Carry Mannosidic and Paucimannosidic N-glycans

Recently, the prevalence of Fasciola hepatica in some areas has increased considerably and the availability of a vaccine to protect livestock from infection would represent a major advance in tools available for controlling this disease. To date, most vaccine-target discovery research on this parasite has concentrated on proteomic and transcriptomic approaches whereas little work has been carried out on glycosylation. As the F. hepatica tegument (Teg) may contain glycans potentially relevant to vaccine development and the Newly Excysted Juvenile (NEJ) is the first lifecycle stage in contact with the definitive host, our work has focused on assessing the glycosylation of the NEJTeg and identifying the NEJTeg glycoprotein repertoire. After in vitro excystation, NEJ were fixed and NEJTeg was extracted. Matrix-assisted laser desorption ionisation-time of flight-mass spectrometry (MALDI-TOF-MS) analysis of released N-glycans revealed that oligomannose and core-fucosylated truncated N-glycans were the most dominant glycan types. By lectin binding studies these glycans were identified mainly on the NEJ surface, together with the oral and ventral suckers. NEJTeg glycoproteins were affinity purified after targeted biotinylation of the glycans and identified using liquid chromatography and tandem mass spectrometry (LC-MS/MS). From the total set of proteins previously identified in NEJTeg, eighteen were also detected in the glycosylated fraction, including the F. hepatica Cathepsin B3 (FhCB3) and two of the Cathepsin L3 (FhCL3) proteins, among others. To confirm glycosylation of cathepsins, analysis at the glycopeptide level by LC-ESI-ion-trap-MS/MS with collision-induced dissociation (CID) and electron-transfer dissociation (ETD) was carried out. We established that cathepsin B1 (FhCB1) on position N80, and FhCL3 (BN1106_s10139B000014, scaffold10139) on position N153, carry unusual paucimannosidic Man2GlcNAc2 glycans. To our knowledge, this is the first description of F. hepatica NEJ glycosylation and the first report of N-glycosylation of F. hepatica cathepsins. The significance of these findings for immunological studies and vaccine development is discussed.