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
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 [RuÂ(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
[RuÂ(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
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
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
A detailed examination of each step
of the reaction sequence in
the photochemical sacrificial hydrogen generation system consisting
of [RuÂ(bpy)<sub>3</sub>]<sup>2+</sup>/ascorbate/[CoÂ(DPA-bpy)ÂOH<sub>2</sub>]<sup>3+</sup> was conducted. By clearly defining quenching,
charge separation, and back electron transfer in the [RuÂ(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 CoÂ(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
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
Molecular Self-Assembly in Conductive Covalent Networks for Selective Nitrate Electroreduction to Ammonia
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
Quantitative Proteomic Analysis Identifies Targets and Pathways of a 2‑Aminobenzamide HDAC Inhibitor in Friedreich’s Ataxia Patient iPSC-Derived Neural Stem Cells
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
Brain Proteome Changes Induced by Olfactory Learning in <i>Drosophila</i>
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
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)Â(2-benzimidazolylpyridine)Â(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
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 polyÂ(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 polyÂ(isoindigo-<i>co</i>-thiophene). The
anionic PiIT polyelectrolyte- and polyacrylate-stabilized Pt-nanoparticles
(PAA-Pt) were codeposited with cationic polyÂ(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