Sensitized Electron Transfer Research Articles

A lysosome-targeted fluorescent probe for fluorescence imaging of hypochlorous acid in living cells and in vivo

Lysosomes, as crucial acidic organelles in cells, play a significant role in cellular functions. The levels and distribution of hypochlorous acid (HOCl) within lysosomes can profoundly impact their biological functionality. Hence, real-time monitoring of the concentration of HOCl in lysosomes holds paramount importance for further understanding various physiological and pathological processes associated with lysosomes. In this study, we developed a bodipy-based fluorescent probe derived from pyridine and phenyl selenide for the specific detection of HOCl in aqueous solutions. Leveraging the probe's sensitive photoinduced electron transfer effect from phenyl selenide to the fluorophore, the probe exhibited satisfactory high sensitivity (with a limit of detection of 5.2 nM and a response time of 15 s) to hypochlorous acid. Further biological experiments confirmed that the introduction of the pyridine moiety enabled the probe molecule to selectively target lysosomes. Moreover, the probe successfully facilitated real-time monitoring of HOCl in cell models stimulated by N-acetylcysteine (NAC) and lipopolysaccharide (LPS), as well as in a normal zebrafish model. This provides a universal method for dynamically sensing HOCl in lysosomes.

Nickel/Photoredox-Catalyzed Thioesterification of Carboxylic Acids Enabled by Sensitized Electron Transfer

Nickel/Photoredox-Catalyzed Thioesterification of Carboxylic Acids Enabled by Sensitized Electron Transfer

Radical thioesterification via nickel-catalysed sensitized electron transfer

Multi-catalytic reaction modes have attracted widespread attention in synthetic chemistry. The merger of nickel catalysis with photoredox catalysis has offered a powerful platform for synthesis of molecules with attractive properties. Nonetheless, the conceptual development of nickel-catalysed, sensitized electron transfer is of pivotal relevance, but is still greatly limited. Here we describe the development of a radical cross-thioesterification process by nickel-catalysed sensitized electron transfer. The strategy can produce diverse methyl thioesters, which are not only found in natural products, materials and pharmaceuticals but also are widespread precursors in synthetic chemistry and biological processes. This catalytic mode features high chemoselectivity, good functional group tolerance and excellent scalability. Perhaps more important was the finding that various drugs and amino acids were successfully functionalized in this system. Experimental studies, nanosecond transient spectroscopic analysis, and density functional theory calculations reveal that the merger of photocatalytic electron transfer, energy transfer and nickel catalysis plays an essential role in this radical thioesterification reaction.

Reconstruction of Thiospinel to Active Sites and Spin Channels for Water Oxidation.

Water electrolysis is a promising technique for carbon neutral hydrogen production. A great challenge remains at developing robust and low-cost anode catalysts. Many pre-catalysts are found to undergo surface reconstruction to give high intrinsic activity in the oxygen evolution reaction (OER). The reconstructed oxyhydroxides on the surface are active species and most of them outperform directly synthesized oxyhydroxides. The reason for the high intrinsic activity remains to be explored. Here, a study is reported to showcase the unique reconstruction behaviors of a pre-catalyst, thiospinel CoFe2 S4 , and its reconstruction chemistry for a high OER activity. The reconstruction of CoFe2 S4 gives a mixture with both Fe-S component and active oxyhydroxide (Co(Fe)Ox Hy ) because Co is more inclined to reconstruct as oxyhydroxide, while the Fe is more stable in Fe-S component in a major form of Fe3 S4 . The interface spin channel is demonstrated in the reconstructed CoFe2 S4 , which optimizes the energetics of OER steps on Co(Fe)Ox Hy species and facilitates the spin sensitive electron transfer to reduce the kinetic barrier of O-O coupling. The advantage is also demonstrated in a membrane electrode assembly (MEA) electrolyzer. This work introduces the feasibility of engineering the reconstruction chemistry of the precatalyst for high performance and durable MEA electrolyzers.

Mechanism for the photodegradation of 9,10-dibutoxyanthracene in the presence of air.

The photoreactivity of anthracene has been previously verified for a range of its derivatives. 9,10-Dibutoxyanthracene is commonly used as an electron transfer sensitizer for photopolymerization because of its favorable optical properties. This study experimentally demonstrated that 9,10-dibutoxyanthracene produces an endoperoxide species upon reaction with the oxygen present in air. A secondary decomposition product formed during the photodecomposition of the endoperoxide species was also isolated and identified. The proposed reaction pathway is supported by singlet oxygen scavenger studies and calculations of the singlet–triplet transition energies. Our findings suggest that 9,10-dibutoxyanthracene can be used as a photo-induced oxygen scavenger.

Electron transfer via cytochrome b6f complex displays sensitivity to antimycin A upon STT7 kinase activation.

The cytochrome b6f complex (b6f) has been initially considered as the ferredoxin-plastoquinone reductase (FQR) during cyclic electron flow (CEF) with photosystem I that is inhibited by antimycin A (AA). The binding of AA to the b6f Qi-site is aggravated by heme-ci, which challenged the FQR function of b6f during CEF. Alternative models suggest that PROTON GRADIENT REGULATION5 (PGR5) is involved in a b6f-independent, AA-sensitive FQR. Here, we show in Chlamydomonas reinhardtii that the b6f is conditionally inhibited by AA in vivo and that the inhibition did not require PGR5. Instead, activation of the STT7 kinase upon anaerobic treatment induced the AA sensitivity of b6f which was absent from stt7-1. However, a lock in State 2 due to persisting phosphorylation in the phosphatase double mutant pph1;pbcp did not increase AA sensitivity of electron transfer. The latter required a redox poise, supporting the view that state transitions and CEF are not coercively coupled. This suggests that the b6f-interacting kinase is required for structure-function modulation of the Qi-site under CEF favoring conditions. We propose that PGR5 and STT7 independently sustain AA-sensitive FQR activity of the b6f. Accordingly, PGR5-mediated electron injection into an STT7-modulated Qi-site drives a Mitchellian Q cycle in CEF conditions.

Proton-Coupled Electron Transfer in the Aqueous Nanochannels of Lyotropic Liquid Crystals: Interplay of H-Bonding and Polarity Effects.

A molecular-level description of the aqueous nanochannels in lyotropic liquid crystals (LLCs) is crucial for their widespread utilization in diverse fields. Herein, the polarity and hydrogen bonding effects of LLC water molecules have been simultaneously explored using a single probe, 4'-N,N-dimethylamino-3-hydroxyflavone (DMA3HF), by the unique multiparametric sensitivity of the excited state proton-coupled electron transfer (PCET) phenomenon. The decreased ESIPT efficiency and the significantly retarded ESIPT dynamics (>20 times) of DMA3HF in the LLC phases suggests the dominant influence of strong hydrogen-bonded solute-solvent complexes that leads to a high activation barrier for ESIPT in the mesophases. The effects of hydrogen bonding on ESIPT have been elucidated by enhanced sampling techniques based on classical MD simulations of DMA3HF in explicit water. ESIPT via an extended hydrogen-bonded water wire is associated with a significantly high ESIPT activation barrier, substantiating the experimentally observed slow ESIPT dynamics inside the LLCs.

Remote electron and energy transfer sensitized photoisomerization of encapsulated stilbenes.

Excited state chemistry and physics of molecules, in addition to their inherent electronic and steric features, depend on their immediate microenvironments. This study explores the influence of an organic capsule, slightly larger than the reactant molecule itself, on the excited state chemistry of the encapsulated molecule. Results presented here show that the confined molecule, in fact, is not isolated and can be manipulated from outside even without direct physical interaction. Examples where communication between a confined molecule and a free molecule present outside is brought about through electronic and energy transfer processes are presented. Geometric isomerization of octa acid encapsulated stilbenes induced by energy and electron transfer by cationic sensitizers that attach themselves to the anionic capsule is examined. The fact that isomerization occurs when the sensitizer present outside is excited illustrates that the reactant and sensitizer are communicating across the molecular wall of the capsule. Ability to remotely activate a confined molecule opens up new opportunities to bring about reactions of confined radical ions and triplet excited molecules generated via long distance energy and electron transfer processes.

Birch‐Type Photoreduction of Arenes and Heteroarenes by Sensitized Electron Transfer

AbstractThe direct reduction of arenes and heteroarenes by visible‐light irradiation remains challenging, as the energy of a single photon is not sufficient for breaking aromatic stabilization. Shown herein is that the energy accumulation of two visible‐light photons allows the dearomatization of arenes and heteroarenes. Mechanistic investigations confirm that the combination of energy‐transfer and electron‐transfer processes generates an arene radical anion, which is subsequently trapped by hydrogen‐atom transfer and finally protonated to form the dearomatized product. The photoreduction converts planar aromatic feedstock compounds into molecular skeletons that are of use in organic synthesis.

Birch-Type Photoreduction of Arenes and Heteroarenes by Sensitized Electron Transfer.

The direct reduction of arenes and heteroarenes by visible‐light irradiation remains challenging, as the energy of a single photon is not sufficient for breaking aromatic stabilization. Shown herein is that the energy accumulation of two visible‐light photons allows the dearomatization of arenes and heteroarenes. Mechanistic investigations confirm that the combination of energy‐transfer and electron‐transfer processes generates an arene radical anion, which is subsequently trapped by hydrogen‐atom transfer and finally protonated to form the dearomatized product. The photoreduction converts planar aromatic feedstock compounds into molecular skeletons that are of use in organic synthesis.

Co-substrate strategy for improved power production and chlorophenol degradation in a microbial fuel cell

A co-substrate strategy has the potential to contribute toward minimizing the poisoning of refractories on the anode respiring bacteria (ARB), which is a necessity for microbial fuel cell (MFC) to recover energy from toxic wastewater. However, little is known about the underlying mechanisms. This study employed 4-chlorophenol (4-CP) as target refractory pollutant and acetate as co-substrate. The co-substrate could stimulate 4-CP sensitive extracellular electron transfer (EET) enzymes’ activity under 4-CP stress, such as succinate dehydrogenase (complex II) and cytochromes c OmcA and OmcB. For the microbial community, ARB were still abundant during 4-CP toxification; Azospirillum and Dechloromonas were enriched to conduct aromatic ring breakage and dechlorination. Thus, a 4.3-fold increased power generation was achieved, and the 4-CP and chemical oxygen demand (COD) removals increased by ∼42% and ∼53%, respectively. An easy-to-degrade substrate could improve power re-coverage from refractory wastewater by enhancing EET enzyme activity and optimizing the microbial community.

Correction: pH sensitivity of interfacial electron transfer at a supported graphene monolayer.

Correction for 'pH sensitivity of interfacial electron transfer at a supported graphene monolayer' by Michel Wehrhold et al., Nanoscale, 2019, DOI: 10.1039/c9nr05049c.

PH sensitivity of interfacial electron transfer at a supported graphene monolayer.

Electrochemical devices based on a single graphene monolayer are often realized on a solid support such as silicon oxide, glassy carbon or a metal film. Here, we show that, with graphene on insulating substrates, the kinetics of the electron transfer at graphene with various redox active molecules is dictated by solution pH for electrode reactions that are not proton dependent. We attribute the origin of this unusual phenomenon mainly to electrostatic effects between dissolved/dissociated redox species and the interfacial charge due to trace amounts of ionizable groups at the supported graphene-liquid interface. Cationic redox species show higher electron transfer rates at basic pH, while anionic species undergo faster electron transfer at acidic pH. Although this behavior is observed on graphene on three different insulating substrates, the strength of this effect appears to differ depending on the surface charge density of the underlying substrate. This finding has important implications for the design of electrochemical sensors and electrocatalysts based on graphene monolayers.

In search of stable visible light absorbing photocatalysts: gold nanoclusters $$^{\S }$$ §

Photochemical reactions are beginning to play an important role in the synthesis of complex organic molecules. The impetus to employ light initiated photoreactions as a synthetic tool derives from the fact that these reactions require no reagents except light. The abundance of sunlight has prompted the chemists to search for the visible light absorbing sensitizers (catalysts) to initiate the desired reactions. Our goal in this context is to develop stable and readily available catalysts that would function under sunlight. In this manuscript, we present the results of our experiments with gold nanoclusters (AuNCs) as a visible light absorbing catalysts. AuNCs absorb and emit in the visible region, soluble in water and transfers electron to suitable acceptors. Employing a series of acceptors we found that excited AuNC can transfer one electron to any acceptor whose reduction potential is above −1.1 eV. In the excited state, AuNC does not accept electrons. Also, it did not serve as an energy transfer sensitizer even with molecular oxygen. We are optimistic that AuNP and AuNC could be developed into a stable and water-soluble visible light absorbing photocatalysts to perform useful photoreactions. SYNOPSIS In the context of visible light photocatalysis gold nanoclusters upon light, absorption was found to transfer electron to acceptors whose reduction potential is above −1.1 eV. AuNC has limited value as electron transfer sensitizers in photochemical reactions.

Electron-Phonon Coupling in Cyanobacterial Photosystem I.

One of the fundamental problems in biophysics is whether the protein medium at room temperature can be properly treated as a fluid dielectric or whether its dynamics is determined by a highly ordered molecular structure resembling the properties of crystalline and amorphous solids. Here, we measured the recombination between reduced A1 and the oxidized chlorophyll special pair P700 over a wide temperature range using preparations of photosystem I from the cyanobacterium Synechococcus sp. PCC 7002 depleted of the iron-sulfur clusters. We found that the dielectric properties of the protein matrix in early electron transfer reactions of photosystem I resemble the behavior of solids that require an implicit treatment of electron-phonon coupling even at ambient temperatures. The quantum effects of electron-phonon coupling in proteins could account for a variety of phenomena, such as the weak sensitivity of electron transfer in pigment-protein complexes to changing environmental conditions including temperature, driving force, polarity, and chemical composition.

Chemical reactivity of Ga-based liquid metals with redox active species and its influence on electrochemical processes

Room temperature liquid metals based on gallium demonstrate interesting physical and chemical properties when placed in electrochemical environments. In this work we explore the applicability of hanging liquid metal droplets as electrode materials for simple surface insensitive and surface sensitive electron transfer reactions, namely the electrochemical reduction of [Ru(NH3)6]3+ and [Fe(CN)6]3− ions in solution. Significantly we found that for both redox species the electron transfer process was impeded at galinstan (68.5% Ga, 22.5% In and 10% Sn). This is related to the chemical reactivity of the Ga component which is oxidised when exposed to aqueous solutions of [Ru(NH3)6]3+ and [Fe(CN)6]3− to generate the reduced form of the redox mediator. For the former this results in the production of Ru-red and Ru-brown cationic trimers in solution as well as gallium oxide on the surface of the liquid metal, and in the latter case the formation of solid Prussian Blue.

Visible-light-induced activation of periodate that mimics dye-sensitization of TiO2: Simultaneous decolorization of dyes and production of oxidizing radicals

Inspired by the mechanism behind self-sensitized destruction of dyes on semiconductor photocatalysts, we herein present the first instance of visible-light-induced activation of periodate (IO4−) into reactive iodine radicals via sensitized electron transfer from an organic dye, Rhodamine B (RhB). The IO4− reduction not only leads to oxidative decolorization of RhB but also formation of reactive intermediates that degrade organic compounds. Electron transfer from the excited dye to IO4− was confirmed by detecting RhB radical cation (RhB+) and measuring its lifetime. The efficiency of organic compound degradation was found to significantly vary depending on the target substrate, i.e., phenol, bisphenol A, and 4-chlorophenol were rapidly decomposed, whereas benzoic acid, carbamazepine, 4-nitrophenol, and sulfamethoxazole exhibited moderate decomposition rate. Lines of evidence in addition to the substrate specificity, such as insignificant hydroxylation, non-stoichiometric dechlorination, and marginal quenching effects of organic/inorganic compounds (e.g., methanol, natural organic matters, and chloride ion), points toward the involvement of iodate radical (IO3). The dye-sensitized IO4− activation process was also found to be highly effective in inactivation of MS2 bacteriophage.

A highly selective and sensitive photoinduced electron transfer (PET) based HOCl fluorescent probe in water and its endogenous imaging in living cells.

A probe based on the phenothiazine-acridine orange conjugate (Ptz-AO) has been designed and synthesized for the sensitive and selective detection of HOCl. Ptz-AO has excellent properties, including pH-independence of fluorescence, high resistance to photobleaching, and response in real time. The value of Ptz-AO was confirmed by exogenous, endogenous and the real-time imaging of HOCl in vitro using a fluorescence microscope.

Viologen embedded polyaromatic hydrocarbons (VPAH2+): synthesis, computational, photophysical, and electrochemical characterizations of 3,8-diazaphenanthrenyl viologens

Several new dicationic viologen sensitizers are synthesized and characterized both photo- and electro-chemically. Benzo-annulation of methyl viologen increases its fluorescence lifetime by a factor of 6.6. Similar lifetime increases were observed for methylated derivatives, that have the added advantage of enhanced solubilities. These electron transfer sensitizers can oxidize organic substrates with oxidation potentials of approximately 2.4eV and lower.

Regiochemistry and substituent effects on pyrylogen and thiopyrylogen stability and electronic character

A physical organic analysis of a collection of 57 data sets generated by B3LYP/6-311+G(2d,p) geometry optimizations and TD-DFT calculations is presented. The analysis engenders recommendations for syntheses of new pyrylogen electron-transfer sensitizers that are potentially optimized for low energy absorbance and high extinction coefficients. It is argued that bathochromically shifted intense absorbance is especially useful to avoid competitive absorbance by electron-transfer substrates and for use at the low concentrations likely achievable in experimentally attractive nonpolar solvents.