13 research outputs found

    Photochemical Generation of Strong One-Electron Reductants via Light-Induced Electron Transfer with Reversible Donors Followed by Cross Reaction with Sacrificial Donors

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    This work illustrates a modified approach for employing photoinduced electron transfer reactions coupled to secondary irreversible electron transfer processes for the generation of strongly reducing equivalents in solution. Through irradiation of [Ru­(LL)<sub>3</sub>]<sup>2+</sup> (LL= diimine ligands) with tritolylamine (TTA) as quencher and various alkyl amines as sacrificial electron donors, yields in excess of 50% can be achieved for generation of reductants with E<sup>0</sup>(2+/1+) values between −1.0 and −1.2 V vs NHE. The key to the system is the fact that the TTA cation radical, formed in high yield in reaction with the photoexcited [Ru­(LL)<sub>3</sub>]<sup>2+</sup> complex, reacts irreversibly with various sacrificial electron donating amines <i>that are kinetically unable to directly react with the photoexcited complex</i>. The electron transfer between the TTA<sup>+</sup> and the sacrificial amine is an energetically uphill process. Kinetic analysis of these parallel competing reactions, consisting of bimolecular and pseudo first-order reactions, allows determination of electron transfer rate constants for the cross electron transfer reaction between the sacrificial donor and the TTA<sup>+</sup>. A variety of amines were examined as potential sacrificial electron donors, and it was found that tertiary 1,2-diamines are most efficient among these amines for trapping the intermediate TTA<sup>+</sup>. This electron-donating combination is capable of supplying a persistent reducing flux of electrons to catalysts used for hydrogen production

    Generation of Long-Lived Redox Equivalents in Self-Assembled Bilayer Structures on Metal Oxide Electrodes

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    We report on the synthesis and photophysical properties of a photocathode consisting of a molecular bilayer structure self-assembled on p-type NiO nanostructured films. The resulting photocathode and its nanostructured indium–tin oxide analog absorb visible light and convert it into injected holes with injection yields of ∼30%, measured at the first observation time by nanosecond transient absorption spectroscopy, and long-lived reducing equivalents that last for several milliseconds without applied bias. An initial quantum yield of 15% was achieved for photogeneration of the reduced dye on the p-NiO electrode. Nanosecond transient absorption experiments and detailed analyses of the underlying electron transfer steps demonstrate that the overall efficiency of the cell is limited by hole injection and charge recombination processes. Compared with the highly doped indium–tin oxide photocathode, the NiO photocathode shows superior photoconversion efficiencies for generating reducing equivalents and longer lifetimes of surface-bound redox-separated states due to an inhibition toward charge recombination with the external assembly

    Cooperative Cold Denaturation: The Case of the C‑Terminal Domain of Ribosomal Protein L9

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    Cold denaturation is a general property of globular proteins, but it is difficult to directly characterize because the transition temperature of protein cold denaturation, <i>T</i><sub>c</sub>, is often below the freezing point of water. As a result, studies of protein cold denaturation are often facilitated by addition of denaturants, using destabilizing pHs or extremes of pressure, or reverse micelle encapsulation, and there are few studies of cold-induced unfolding under near native conditions. The thermal and denaturant-induced unfolding of single-domain proteins is usually cooperative, but the cooperativity of cold denaturation is controversial. The issue is of both fundamental and practical importance because cold unfolding may reveal information about otherwise inaccessible partially unfolded states and because many therapeutic proteins need to be stabilized against cold unfolding. It is thus desirable to obtain more information about the process under nonperturbing conditions. The ability to access cold denaturation in native buffer is also very useful for characterizing protein thermodynamics, especially when other methods are not applicable. In this work, we study a point mutant of the C-terminal domain of ribosomal protein L9 (CTL9), which has a <i>T</i><sub>c</sub> above 0 °C. The mutant was designed to allow the study of cold denaturation under near native conditions. The cold denaturation process of I98A CTL9 was characterized by nuclear magnetic resonance, circular dichroism, and Fourier transform infrared spectroscopy. The results are consistent with apparently cooperative, two-state cold unfolding. Small-angle X-ray scattering studies show that the unfolded state expands as the temperature is lowered

    Mechanistic Details for Cobalt Catalyzed Photochemical Hydrogen Production in Aqueous Solution: Efficiencies of the Photochemical and Non-Photochemical Steps

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    A detailed examination of each step of the reaction sequence in the photochemical sacrificial hydrogen generation system consisting of [Ru­(bpy)<sub>3</sub>]<sup>2+</sup>/ascorbate/[Co­(DPA-bpy)­OH<sub>2</sub>]<sup>3+</sup> was conducted. By clearly defining quenching, charge separation, and back electron transfer in the [Ru­(bpy)<sub>3</sub>]<sup>2+</sup>/ascorbate system, the details necessary for evaluation of the efficiency of water reduction with various catalysts are provided. In the particular Co­(III) catalyst investigated, it is clear that the light induced catalytic process is considerably less efficient than the electrocatalytic process. A potential source of catalyst inefficiency in this system is reduction of the products formed in oxidation of the sacrificial electron donor. The data provided for excited state quenching and charge separation in this particular aqueous system are meant to be used by others for thorough investigation of the quantum efficiencies of other aqueous homogeneous and nanoheterogeneous catalysts for water reduction

    Bioremediation Potential of Cr(VI) by <i>Lysinibacillus cavernae</i> CR-2 Isolated from Chromite-Polluted Soil: A Promising Approach for Cr(VI) Detoxification

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    The present study focuses on an efficient Cr(VI)-reducing bacterial strain (CR-2) isolated from an abandoned chromate plant in Qinghai Province, China. CR-2 was confirmed as Lysinibacillus cavernae using 16S rRNA gene sequencing. CR-2 could survive at 500 mg L−1 Cr(VI) and effectively reduce Cr(VI) at concentrations of −1, a pH of 5–9, a temperature of 20–40 °C, and a salinity of 5–15 g L−1. According to the Box–Behnken experimental design, the maximum Cr(VI) removal efficiency by L. cavernae CR-2 was 76.21% under optimum conditions, which comprised a pH of 6.68, a temperature of 28.90 °C, and a salinity of 9.85 g L−1. With regard to Cr(VI) reduction mediated by L. cavernae CR-2, enhancement in efficiency was observed in the presence of Cu2+ and Ca2+, while significant inhibition in the reduction capacity occurred upon exposure to Mg2+, Ba2+, Ni2+, Pb2+, or Cd2+. Moreover, L. cavernae CR-2 tends to use glucose as an electron donor for the reduction of Cr(VI). Results of cell fraction separation and degeneration indicated that the Cr(VI) removal was primarily due to the reduction of Cr(VI) via chromium reductase in the cytoplasm. In addition, bioanalysis of L. cavernae CR-2 by SEM-EDS and TEM-EDS suggested that Cr was distributed both on the surface and in the cell cytoplasm. FT-IR analyses established that multiple functional groups (hydroxyl, carbonyl, amide, amino, and aldehyde groups) participated in the Cr(VI) biosorption on the cell surface. XPS and HPLC also showed that the Cr(III) end-products could be present as Cr(III) hydroxides or as organic–Cr(III) complexes. This study yields insights into the Cr(VI) bioreduction mechanism of L. cavernae CR-2.</p

    Quantitative Proteomic Analysis Identifies Targets and Pathways of a 2‑Aminobenzamide HDAC Inhibitor in Friedreich’s Ataxia Patient iPSC-Derived Neural Stem Cells

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    Members of the 2-aminobenzamide class of histone deacetylase (HDAC) inhibitors show promise as therapeutics for the neurodegenerative diseases Friedreich’s ataxia (FRDA) and Huntington’s disease (HD). While it is clear that HDAC3 is one of the important targets of the 2-aminobenzamide HDAC inhibitors, inhibition of other class I HDACs (HDACs 1 and 2) may also be involved in the beneficial effects of these compounds in FRDA and HD, and other HDAC interacting proteins may be impacted by the compound. To this end, we synthesized activity-based profiling probe (ABPP) versions of one of our HDAC inhibitors (compound 106), and in the present study we used a quantitative proteomic method coupled with multidimensional protein identification technology (MudPIT) to identify the proteins captured by the ABPP 106 probe. Nuclear proteins were extracted from FRDA patient iPSC-derived neural stem cells, and then were reacted with control and ABPP 106 probe. After reaction, the bound proteins were digested on the beads, and the peptides were modified using stable isotope-labeled formaldehyde to form dimethyl amine. The selectively bound proteins determined by mass spectrometry were subjected to functional and pathway analysis. Our findings suggest that the targets of compound 106 are involved not only in transcriptional regulation but also in posttranscriptional processing of mRNA

    Molecular Self-Assembly in Conductive Covalent Networks for Selective Nitrate Electroreduction to Ammonia

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    Electrochemical nitrate (NO3–) reduction in aqueous media provides a useful approach for ammonia (NH3) synthesis. While efforts are focused on developing catalysts, the local microenvironment surrounding the catalyst centers is of great importance for controlling electrocatalytic performance. Here, we demonstrate that a self-assembled molecular iron catalyst integrated in a free-standing conductive hydrogel is capable of selective production of NH3 from NO3– at efficiencies approaching unity. With the electrocatalytic hydrogel, the NH3 selectivity is consistently high under a range of negative biases, which results from the hydrophobicity increase of the polycarbazole-based electrode substrate. In mildly acidic media, proton reduction is suppressed by electro-dewetting of the hydrogel electrode, enhancing the selectivity of NO3– reduction. The electrocatalytic hydrogel is capable of continuous production of NH3 for at least 100 h with NH3 selectivity of ∼89 to 98% at high current densities. Our results highlight the role of constructing an internal hydrophobic surface for electrocatalysts in controlling selectivity in aqueous media

    Brain Proteome Changes Induced by Olfactory Learning in <i>Drosophila</i>

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    For more than 30 years, the study of learning and memory in Drosophila melanogaster (fruit fly) has used an olfactory learning paradigm and has resulted in the discovery of many genes involved in memory formation. By varying learning programs, the creation of different memory types can be achieved, from short-term memory formation to long-term. Previous studies in the fruit fly used gene mutation methods to identify genes involved in memory formation. Presumably, memory creation involves a combination of genes, pathways, and neural circuits. To examine memory formation at the protein level, a quantitative proteomic analysis was performed using olfactory learning and <sup>15</sup>N-labeled fruit flies. Differences were observed in protein expression and relevant pathways between different learning programs. Our data showed major protein expression changes occurred between short-term memory (STM) and long-lasting memory, and only minor changes were found between long-term memory (LTM) and anesthesia-resistant memory (ARM)

    Synthesis and Photophysical Properties of a Covalently Linked Porphyrin Chromophore–Ru(II) Water Oxidation Catalyst Assembly on SnO<sub>2</sub> Electrodes

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    We describe here the preparation and surface photophysical properties of a covalently linked, chromophore-catalyst assembly between a phenyl phosphonate-derivatized pentafluorophenyl-substituted porphyrin and the water oxidation catalyst, [Ru<sup>II</sup>(terpyridine)­(2-benzimidazolylpyridine)­(OH<sub>2</sub>)]<sup>2+</sup>, in a derivatized assembly of porph-Ru<sup>II</sup>–OH<sub>2</sub><sup>2+</sup>. The results of nanosecond transient absorption measurements on nanoparticle SnO<sub>2</sub> electrodes in aqueous acetate buffer at pH 4.7 are consistent with rapid electron injection into SnO<sub>2</sub> with transfer of oxidative equivalents to the assembly. Electron transfer from the singlet excited state of the porphyrin to the conduction band of the electrode, SnO<sub>2</sub>(e<sup>–</sup>)|-porph<sup>+</sup>-Ru<sup>II</sup>–OH<sub>2</sub><sup>2+</sup>, is favored as the porphyrin singlet excited state lies 0.44 eV above the SnO<sub>2</sub> conduction band edge. Electron injection is rapid (⟨τ<sub>inj</sub>⟩ < 10<sup>–8</sup> s), and occurs with high efficiency. Based on measured redox potentials, following excitation and injection, intra-assembly oxidation of the catalyst, -porph<sup>+</sup>-Ru<sup>II</sup>–OH<sub>2</sub><sup>2+</sup> → -porph-Ru<sup>III</sup>–OH<sup>2+</sup> + H<sup>+</sup>, is favored in the transient equilibrium state by 0.62 eV at pH 4.7. However, immediately after the flash, a distribution exists at the surface between isomers with SnO<sub>2</sub>(e<sup>–</sup>)|-porph<sup>+</sup>-Ru<sup>II</sup>–OH<sub>2</sub><sup>2+</sup> undergoing back electron transfer to the surface with an average lifetime of ⟨τ<sub>1</sub>⟩ ∼ 10<sup>–7</sup> s and a slower component for back electron transfer from SnO<sub>2</sub>(e<sup>–</sup>)|-porph-Ru<sup>III</sup>–OH<sup>2+</sup> with ⟨τ<sub>2</sub>⟩ ∼ 4 × 10<sup>–5</sup> s

    Photocathode Chromophore–Catalyst Assembly via Layer-By-Layer Deposition of a Low Band-Gap Isoindigo Conjugated Polyelectrolyte

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    Low band-gap conjugated polyelectrolytes (CPEs) can serve as efficient chromophores for use on photoelectrodes for dye-sensitized photoelectrochemical cells. Herein is reported a novel CPE based on poly­(isoindigo-<i>co</i>-thiophene) with pendant sodium butylsulfonate groups (PiIT) and its use in construction of layer-by-layer (LbL) chromophore–catalyst assemblies with a Pt-based H<sup>+</sup> reduction catalyst (PAA-Pt) for water reduction. A novel Stille polymerization/postpolymerization ion-exchange strategy was used to convert an organic-soluble CPE to the water-soluble poly­(isoindigo-<i>co</i>-thiophene). The anionic PiIT polyelectrolyte- and polyacrylate-stabilized Pt-nanoparticles (PAA-Pt) were codeposited with cationic poly­(diallyldimethylammonium) chloride (PDDA) onto inverse opal (IO), nanostructured indium tin oxide film (nITO) (IO nITO) atop fluorine doped tin oxide (FTO), by using LbL self-assembly. To evaluate the performance of novel conjugated PiIT//PAA-Pt chromphore–catalyst assemblies, interassembly hole transfer was investigated by photocurrent density measurements on FTO//IO nITO electrodes. Enhanced cathodic photocurrent is observed for the polychromophore–catalyst assemblies, compared to electrodes modified with only PiIT, pointing toward photoinduced hole transfer from the excited PilT to the IO nITO. Prolonged photoelectrolysis experiments reveal H<sub>2</sub> production with a Faradaic yield of approximately 45%. This work provides new routes to carry out visible-light-driven water reduction using photocathode assemblies based on low band-gap CPEs
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