Proton Donor Research Articles

Highly Intrinsic Thermal Conductivity of Aramid Nanofiber Films by Manipulating Intermolecular Hydrogen Bonding Interactions

AbstractLightweight, flexible, and thermostable thermally conductive materials are essential for enhancing heat dissipation efficiency in advanced electronics. The development of intrinsic thermally conductive polymers is the key to expanding the space for improving the thermal conductivity of polymer‐based thermal management materials. In order to balance the thermal conductivity and mechanical performance of bulk polymers, thermally conductive aramid nanofiber (ANF) films are assembled by manipulating the proton‐donating ability of solvents. Compared to water as a conventional proton donor, ethanol‐induced multi‐scale structures composed of dense hydrogen bonding interaction, large grain size, and uniform fiber topology endow the resulting ANF films with enhanced intrinsic thermal conductivity up to 5.05 W m−1 K−1 with a 34% increase, salient mechanical performance with the tensile strength of 181.4 MPa, and exceptional thermal stability higher than 500 °C. These outstanding properties of ANF films provide many possibilities for the preparation of polymer‐based thermally conductive materials.

Electrolyte Effects in Electrocatalytic Kinetics†

Comprehensive SummaryTuning electrolyte properties is a widely recognized strategy to enhance activity and selectivity in electrocatalysis, drawing increasing attention in this domain. Despite extensive experimental and theoretical studies, debates persist about how various electrolyte components influence electrocatalytic reactions. We offer a concise review focusing on current discussions, especially the contentious roles of cations. This article further examines how different factors affect the interfacial solvent structure, particularly the hydrogen‐bonding network, and delves into the microscopic kinetics of electron and proton‐coupled electron transfer. We also discuss the overarching influence of solvents from a kinetic modeling perspective, aiming to develop a robust correlation between electrolyte structure and reactivity. Lastly, we summarize ongoing research challenges and suggest potential directions for future studies on electrolyte effects in electrocatalysis. Key ScientistsIn 1956, Marcus theory was developed to describe the mechanism of outer‐sphere electron transfer (OS‐ET). In 1992, Nocera et al. directly measured proton‐coupled electron transfer (PCET) kinetics for the first time, and their subsequent research in 1995 investigated the effects of proton motion on electron transfer (ET) kinetics. In 1999 and 2000, Hammes‐schiffer et al. developed the multistate continuum theory for multiple charge reactions and deduced the rate expressions for nonadiabatic PCET reactions in solution, laying the theoretical foundation for the analysis of PCET kinetics in electrochemical processes. In 2006, Saveant et al. verified the concerted proton and electron transfer (CPET) mechanism in the oxidation of phenols coupled with intramolecular amine‐driven proton transfer (PT). Their subsequent work in 2008 reported the pH‐dependent pathways of electrochemical oxidation of phenols.Electrolyte effects in electrocatalysis have gained emphasis in recent years. In 2009, Markovic's pioneering work proposed non‐covalent interactions between hydrated alkaline cations and adsorbed OH species in oxygen reduction reaction (ORR)/hydrogen oxidation reaction (HOR). In 2011, Markovic et al. significantly enhanced hydrogen evolution reaction (HER) activity in alkaline solution by improving water dissociation, which was assumed to dominate the sluggish HER kinetics in such media. In comparation, Yan et al. applied hydrogen binding energy (HBE) theory in 2015 to explain the pH‐dependent HER/HOR activity. Cations play a significant role in regulating the selectivity and activity of carbon dioxide reduction (CO2RR). In 2016 and 2017, Karen Chan et al. introduced the electric field generated by solvated cations to explain the cation effects on electrochemical CO2RR. Conversely, in 2021, Koper et al. suggested that short‐range electrostatic interactions between partially desolvated metal cations and CO2 stabilized CO2 and promoted CO2RR.Recent researches have combined the exploration of the electrical double layer (EDL) structure with theoretical analysis of PCET kinetics. In 2019, Huang et al. developed a microscopic Hamiltonian model to quantitatively understand the sluggish hydrogen electrocatalysis in alkaline media. In 2021, two meticulous studies from Shao‐Horn's group analyzed the effects of cations on reorganization energy and the impacts of hydrogen bonds between proton donors and acceptors on proton tunneling kinetics, respectively. Electrolyte effects on proton transport process were researched in recent years. In 2022, Hu et al. and Chen et al. proposed that the cation‐induced electric field distribution and pH‐dependent hydrogen bonding network connectivity played essential roles in proton transport, separately.

2.6-Å resolution cryo-EM structure of a class Ia ribonucleotide reductase trapped with mechanism-based inhibitor N3CDP

Ribonucleotide reductases (RNRs) reduce ribonucleotides to deoxyribonucleotides using radical-based chemistry. For class Ia RNRs, the radical species is stored in a separate subunit (β2) from the subunit housing the active site (α2), requiring the formation of a short-lived α2β2 complex and long-range radical transfer (RT). RT occurs via proton-coupled electron transfer (PCET) over a long distance (~32-Å) and involves the formation and decay of multiple amino acid radical species. Here, we use cryogenic electron microscopy and a mechanism-based inhibitor 2'-azido-2'-deoxycytidine-5'-diphosphate (N3CDP) to trap a wild-type α2β2 complex of Escherichia coli class Ia RNR. We find that one α subunit has turned over and that the other is trapped, bound to β in a midturnover state. Instead of N3CDP in the active site, forward RT has resulted in N2 loss, migration of the third nitrogen from the ribose C2' to C3' positions, and attachment of this nitrogen to the sulfur of cysteine-225. In this study, an inhibitor has been visualized as an adduct to an RNR. Additionally, this structure reveals the positions of PCET residues following forward RT, complementing the previous structure that depicted a preturnover PCET pathway and suggesting how PCET is gated at the α-β interface. This N3CDP-trapped structure is also of sufficient resolution (2.6 Å) to visualize water molecules, allowing us to evaluate the proposal that water molecules are proton acceptors and donors as part of the PCET process.

Cleavage of a Peroxide Bond via a Dual Attack by FunctionalMimics of Glutathione Peroxidase.

Nonmetal-containing peroxidase enzymes, including glutathione peroxidase (GPx), and peroxiredoxins, control cellular redox levels by catalyzing the reduction of H2O2. The remarkably higher reactivity of GPx enzyme as compared to the fully dissociated synthetic selenolate/thiolate molecule is probably due to the dual-attack on the peroxide bond (HO1-O2H) by the enzyme; The first one is a nucleophilic attack of the selenolate/thiolate moiety to O1 atom and the second attack at the O2 atom of the peroxide bond by the acidic "parked proton" from Trp or His residue present at the enzyme's active site, leading to the facile cleavage of O-O bond. Herein, we report two synthetic compounds (1 and 2), having a selenolate (Se-) and a proton donor (imidazolium or -COOH group) moieties, which showed excellent GPx-like activity via dual-attack on the peroxide bond. The combined effect of selenolate moiety that donates electrons to the antibondingorbital of O1-O2 bond and the imidazolium or carboxylic acid moiety at the side chain that forms a strong H-bonding with the O2 atom facilitates O-O bond cleavage of H2O2 more efficiently. 1 and 2 exhibit remarkable ability in protecting Cu(I)-complex [TpmCu(CH3CN)]+ (9) against H2O2 by acting as a sacrificial antioxidant, thereby preventing metal-mediated ROS production.

Stereodivergent Hydrohalogenation of SCF3 Alkynes and Cross- Coupling Reactions.

SCF3-substituted internal alkynes undergo regio- and stereoselective hydrohalogenation to provide the corresponding β-halo-α-SCF3-alkenes. The regioselectivity is ensured by the strong polarization of the C≡C triple bond in SCF3-alkynes, while the stereoselectivity is governed by the strength of the proton donor and/or by the solvent polarity. The potential of β-iodo-α-SCF3-alkenes was illustrated in several cross-coupling, protodeiodination, and cyanation reactions.

Formic Acid-Intensified Photoreduction of NOx on Iron Minerals Triggers Daytime HONO Formation through Active Hydrogen.

Nitrous acid (HONO) is crucial in atmospheric chemistry as a precursor to morning peak hydroxyl radicals and significantly affects urban air quality by forming secondary pollutants, yet the mechanisms of its daytime formation is not fully understood. This study investigates the role of formic acid (HCOOH), a prevalent electron and proton donor, in the transformation of nitrogen oxides (NOx) and the formation of HONO on photoactive mineral dust. Exploiting hematite (Fe2O3) as an environmental indicator, we demonstrate that HCOOH significantly promotes the photoreduction of NO2 to HONO, while suppressing nitrate accumulation. This occurs through the formation of a surface ≡Fe-OOCH complex, where sunlight activates the C-H bond to generate and transfer active hydrogen, directly converting NO2 to HONO. Additionally, HCOOH can trigger the photolysis of nitrates as predeposited on Fe2O3, further increasing HONO production. These findings show that HCOOH-mediated photochemical reactions on iron minerals may contribute to elevated atmospheric HONO levels, highlighting a crucial pathway with broad effects on atmospheric chemistry and public health.

Potential-dependent kinetics of oxygen chemisorption as the crucial step of oxygen reduction reaction: GCDFT study

Nitrogen-doped carbon materials (NCMs) are widely regarded as promising alternatives to expensive platinum-based electrocatalysts for the oxygen reduction reaction (ORR). While NCMs exhibit considerable electrochemical activity in alkaline media, their performance in acidic environments remains a significant challenge. However, acidic conditions are commercially desirable for ORR’s catalysis in proton-exchange membrane fuel cells (PEMFCs). The dramatic pH dependence of NCM effectiveness has sparked ongoing debate, with several factors under consideration, including surface protonation, variations in hydrogen binding energy, differences in proton donors, and interface structure. In this work, we present a grand canonical density functional theory (GCDFT) study of the chemisorption step on pristine and nitrogen-doped graphene. Through nudged elastic band (NEB) calculations at various electrode potentials, we propose a potential-dependent (and thus pH-dependent) mechanism of oxygen chemisorption at graphitic nitrogen (Ngr) defects, offering new insights into the pH dependency of the onset potential in NCM catalysts.

Oxide Acidity Modulates Structural Transformations in Hydrogen Titanates during Electrochemical Li-Ion Insertion.

Hydrogen titanates (HTOs) form a diverse group of metastable, layered titanium oxides with an interlayer containing both water molecules and structural protons. We investigated how the chemistry of this interlayer environment influenced electrochemical Li+-insertion in a series of HTOs, H2TiyO2y+1·nH2O (y = 3, 4, and 5). We correlated the electrochemical response with the physical and chemical properties of HTOs using operando X-ray diffraction, in situ differential electrochemical mass spectroscopy, solid-state proton nuclear magnetic resonance, and quasi-elastic neutron scattering. We found that the potential for the first reduction reaction trended with the relative acidity of the structural protons. This mechanism was supported with first-principles density functional theory (DFT) calculations. We propose that the electrochemical reaction involves reduction of the structural protons to yield hydrogen gas and formation of a lithiated hydrogen titanate (H2-xLixTiyO2y+1). The hydrogen gas is confined within the HTO lattice until the titanate structure expands upon subsequent oxidation. Our work has implications for the electrochemical behavior of insertion hosts containing hydrogen and structural water molecules, where hydrogen evolution is expected at potentials below the hydrogen reduction potential and in the absence of electrolyte proton donors. This behavior is an example of electrochemical electron transfer to a nonmetal element in a metal oxide host, in analogy to anion redox.

Photoredox-Catalyzed Cross-Coupling of In Situ Generated Quinoxalinones with Indoles for the Synthesis of Tertiary Alcohols.

A visible light-driven photoredox-catalyzed direct C(sp2)-H functionalization of N-H free indoles with quinoxalinones generated in situ from 2,2-dihydroxy-1H-indene-1,3(2H)-dione and phenylene-1,2-diamines has been reported with the aid of Na2-Eosin Y as the photocatalyst and the Hünig base as the sacrificial electron and proton donor. The reaction provides easy access to a variety of quaternary-centered C-3 selective indole-substituted tertiary alcohols in good yields. Mechanistic studies demonstrated the realization of photoredox-catalyzed in situ quinoxalinone formation and their proton-coupled single electron reduction to the corresponding ketyl radicals followed by cross-coupling with indoles. The potential applications of the synthesized tertiary alcohols in photoacid-catalyzed carbon-carbon and carbon-sulfur bond-forming reactions feature the key findings of the present work.

Molecular basis of plastoquinone reduction in plant cytochrome b6f.

A multi-subunit enzyme, cytochrome b6f (cytb6f), provides the crucial link between photosystems I and II in the photosynthetic membranes of higher plants, transferring electrons between plastoquinone (PQ) and plastocyanin. The atomic structure of cytb6f is known, but its detailed catalytic mechanism remains elusive. Here we present cryogenic electron microscopy structures of spinach cytb6f at 1.9 Å and 2.2 Å resolution, revealing an unexpected orientation of the substrate PQ in the haem ligand niche that forms the PQ reduction site (Qn). PQ, unlike Qn inhibitors, is not in direct contact with the haem. Instead, a water molecule is coordinated by one of the carbonyl groups of PQ and can act as the immediate proton donor for PQ. In addition, we identify water channels that connect Qn with the aqueous exterior of the enzyme, suggesting that the binding of PQ in Qn displaces water through these channels. The structures confirm large movements of the head domain of the iron-sulfur protein (ISP-HD) towards and away from the plastoquinol oxidation site (Qp) and define the unique position of ISP-HD when a Qp inhibitor (2,5-dibromo-3-methyl-6-isopropylbenzoquinone) is bound. This work identifies key conformational states of cytb6f, highlights fundamental differences between substrates and inhibitors and proposes a quinone-water exchange mechanism.

A strong metal-support interaction strategy for enhanced proton-coupled electron transfer and promoted photocatalytic H2O2 production in pure water

Proton-coupled electron transfer (PCET) is a key factor in determining the H2O2 production efficiency from oxygen reduction reaction (ORR) or direct H2/O2 reaction, which usually requires the aid of proton donors including alcohols. Here, we report a strong metal-support interaction (SMSI) strategy to facilitate the PCET process of a typical Pd/TiO2 photocatalyst for photocatalytic ORR into H2O2 production in pure water. Compared to conventional Pd/TiO2 photocatalysts, the Pd/TiO2 photocatalyst with SMSI between Pd and TiO2 (Pd/TiO2-850H) promotes photocatalytic H2O2 production in pure water by 4.21-fold, which can be ascribed to that the SMSI simultaneously promotes the H2O dissociation and the photogenerated charge transfer from TiO2 to Pd catalyst, enabling highly efficient proton transfer to be participated into converting •O2- to H2O2. Moreover, the formation of TiO2-x overlayer caused by SMSI suppresses the H2O2 decomposition and stabilizes Pd NPs, finally achieving an H2O2 production rate of 4.10 mmol g−1 h−1, which surpasses most metal/TiO2 photocatalytic system.

Ionic liquid catalysed synthesis of benzimidazolone from Ortho-Phenylenediamine in the presence of carbon dioxide as a carbonyl source under solvent free condition

In this study, we report synthesis of benzimidazolones from o-phenylene diamine by utilizing carbon dioxide as a carbonyl source under mild reaction conditions in the presence of ionic liquid as a catalyst. Six bifunctional protonic ionic liquids (PILs) were synthesised by reacting superbases 1,5,7-Triazabicyclo[4.4.0]dec-5-ene (TBD) and 1,8-diazabicyclo-[5.4.0]undec-7-ene (DBU) with proton donors such as Trifluoroethanol (TFE), Cyclopropane carboxylic acid (CPCA) and 2,2,2-Trifluoroacetic acid (TFA). Among them, [TBDH+][TFE−] was the most effective, achieving high conversion rates of o-phenylene diamine to 2-benzimidazolone at 1 bar CO2 pressure and 60° C. This PIL could be reused for up to six cycles. Our method offers a green and efficient approach for producing 2-benzimidazolone derivatives, with potential applications in pharmaceuticals and materials science.

Versatile photoluminescence behavior of polycyclic hydroxybenzimidazoles driven by intermolecular hydrogen bonding

Herein, the synthesis of polycyclic hydroxybenzimidazole based on 4-hydroxyphthalimide is presented and two isomeric structures are formed. The isomeric structures are capable of forming intermolecular hydrogen-bonded molecular associates. Hydroxybenzimidazole hydrogen-bonded organic frameworks have been shown to be sensitive to different solvent polarity, particularly in proton donor media, resulting in a blue shift in emission. The role of proton donor media has been evaluated using the binary mixture of acetonitrile/water and protonation by trifluoroacetic acid. The results show that by tuning the environment, the aggregation induced emission has appeared in the blue region and larger aggregates are formed compared to the less polar aprotic solvents. Under acidic conditions, the disruption of the hydrogen-bonded dimers was estimated, resulting in deep blue emission. This provides an opportunity to control the molecular associates and tune the optical behavior.

Mesomorphic thermal stabilities and nonlinear optical properties of fluorine containing liquid crystals

H-bonded liquid crystals (HBLCs) are newly synthesized with PyBF viz., (4-pyridyl)-benzylidene-4′-fluoro aniline as the proton acceptor which is non-mesogenic and alkyloxy benzoic acids viz., nOBAs (n = 3, 12) as the mesogenic proton donors. The H-bond formed is confirmed by FTIR spectroscopy. Polarizing Optical Microscopy confirmed the H-bonded compound’s mesomorphic textures, and the thermal analysis is carried out using a Differential Scanning Calorimeter. The influence of fluorine atom on mesomorphic stability is studied. The nonlinear optical properties of the H-bonded compound is studied using DFT theoretical approach. This work supports the SDG-9 of the United Nations.Graphical abstract

Dual Active Intermediates Induced LiOH Formation via the OH‐Rich Proton Donor in Li−O2 Batteries

Dual Active Intermediates Induced LiOH Formation via the OH‐Rich Proton Donor in Li−O₂ Batteries

The effect of weak proton donors on the steady-state behavior of hydrogen evolution in mildly acidic media

The influence of anionic and cationic electro-inactive species on the steady-state current of the Hydrogen Evolution Reaction (HER) is often not considered. Here, the effects of these widely-used electrolyte species on the steady-state behavior of HER are investigated on a Au RDE at a bulk pH of 4. A current enhancement is observed from the coupling of proton reduction with homogeneous acid-base equilibria, with HSO4− having the largest influence despite the electrolyte condition being far beyond its typical buffering range, while the cations, specifically K+ and Cs+, serve as weak buffers with limited buffer capacity. Further quantitative analysis illustrates that from the steady-state current of HER the thermodynamic parameters of the homogeneous reactions can be evaluated, taking advantage of the linear dependence of the Koutecký-Levich Slope on the concentration of the electro-inactive species. Based on this evaluation, the pKa‘s of HSO4−, K+ and Cs+ were determined to be 2.06, 2.52 and 2.48, respectively. These results confirm the previously postulated proton-donating capability of certain (alkali) cations and show that these effects influence the HER current in mildly acidic media.

Heterologous expression, characterisation and 3D-structural insights of GH18 chitinases derived from sago palm (Metroxylon sagu)

Although plants don't have chitins, they produce chitinases to protect themselves from biotic and abiotic stressors. There are two forms of chitinases found in organisms: glycosyl hydrolase 18 (GH18) and 19 (GH19) families. Plant GH19 chitinases are well known for their role in protecting against pathogens, but the roles of GH18 chitinases have not been fully elucidated. This study aimed to produce and characterise two recombinant GH18 chitinases from Metroxylon sagu. Two GH18 chitinase genes, MsChi1 and MsChi2, were identified, with nucleotide sequences of 1009 and 1308 bp, respectively. The proteins encoded by MsChi1 and MsChi2 genes were single polypeptide chains of 310 and 300 amino acids with predicted molecular masses of 31.21 and 30.15 kDa, respectively. Both cDNAs were cloned and expressed in the GS115 strain of Pichia pastoris. Recombinant MsChi1 and MsChi2 exhibited optimal activity at 60 °C with acidic pH 4.0 and 5.0, respectively. Both recombinant enzymes could hydrolyze synthetic and natural substrates (colloidal chitin). rMsChi1 preferred 4-nitrophenol N,N′-diacetyl-β-D chitobioside, while rMsChi2 preferred 4-nitrophenol N,N′,N″-triacetyl-β-D chitotriose, suggesting they might function as exochitinase and endochitinase, respectively. They also demonstrated antifungal activities against tested fungi. Homology modeling indicated ASP and GLU as essential residues for proton donation and acceptance.

At least five: Benefit origins of potassium and sodium co-doping on carbon nitride for integrating pharmaceuticals degradation and hydrogen peroxide production

Benefit origins of potassium (K+) and sodium (Na+) co-doping on carbon nitride for integrating pharmaceutical degradation and hydrogen peroxide (H2O2) production was investigated. K+ and Na+ co-doped carbon nitride (CN-K/Na) with modified crystallinity and surface structure was synthesized by ionothermal polymerization of urea. The CN-K/Na exhibited an apparent quantum yield of 26.2% in H2O2 photosynthesis at 400nm (isopropanol as proton donor), and it was better at extracting proton from pharmaceutical-laden wastewater to produce H2O2 than pristine carbon nitride. These superior performances are attributed to the benefits directly or indirectly caused by the co-doping: i) Na+ optimizes in-plane charge transfer, ii) K+ builds channel for interplane charge transfer, iii) cyano group as Lewis acid site adsorbs and activates oxygen, iv) amino group as Lewis base site extracts and releases protons, v) increased visible-light absorption. This work offers significant insights into designing polymeric photocatalysts for environmental management and energy conservation.

Transition Metal Complexes with Appended Benzimidazole Groups for Sensing Dihydrogenphosphate.

Four new complexes [Ru(bpy)2(bbib)](PF6)2, [Ru(phen)2(bbib)](PF6)2, [Re(CO)3(bbib)(py)](PF6) and [Ir(ppy)2(bbib)](PF6) [where bbib=4,4'-bis(benzimidazol-2-yl)-2,2'-bipyridine] have been prepared and their photophysical properties determined. Their behaviour has been studied with a variety of anions in acetonitrile, DMSO and 10 % aquated DMSO. Acetate and dihydrogenphosphate demonstrate a redshift in the bbib ligand associated absorptions suggesting that the ligand is strongly interacting with these anions. The 3MLCT emissive state is sensitive to the introduction of small quantities of anion (sub-stoichiometric quantities) and significant quenching is typically observed with acetate, although this is less pronounced in the presence of water. The emissive behaviour with dihydrogenphosphate is variable, showing systematic changes as anion concentration increases with several distinct interactions evident. 1H- and 31P-NMR titrations in a 10 % D2O-DMSO-D6 mixture suggest that with dihydrogenphosphate, the imidazole group is able to act as both a proton acceptor and donor. It appears that all four complexes can form a {[complex]2-H2PO4} "dimer", a one-to-one species (which the X-ray crystallography study suggests is dimeric in the solid-state), and a complex with a combined bis(dihydrogenphosphate) complex anion. The speciation relies on complex equilibria dependent on several factors including the complex charge, the hydrophobicity of the associated ligands, and the solvent.

The Initiation Mechanism of the Isoolefin Oligomerization Reaction in the Presence of Ethylaluminum Dichloride – Protonodonor Complex Catalysts

The mechanism of isoolefins initiation in the presence of ethylaluminum dichloride – proton donor (water, phenol, hydrochloric acid) complex catalysts has been studied by ab initio HF.3.21G. The energetics of these reactions was estimated, the values of its activation energy and thermal effects were obtained. It was found that among the studied catalysts, an increase in the activation energy of the reaction of initiation of oligomerization of isoolefins contributes to an increase in the selectivity of the process.