Proton Transfer Pathways Research Articles

Effective Proton Conduction in Quasi-Solid Zinc-Manganese Batteries via Constructing Highly Connected Transfer Pathways.

Elusive ion behaviors in aqueous electrolyte remain a challenge to break through the practicality of aqueous zinc-manganese batteries (AZMBs), a promising candidate for safe grid-scale energy storage systems. The proposed electrolyte strategies for this issue most ignore the prominent role of proton conduction, which greatly affects the operation stability of AZMBs. Here we report a water-poor quasi-solid electrolyte with efficient proton transfer pathways based on the large-space interlayer of montmorillonite and strong-hydration Pr3+ additive in AZMBs. Proton conduction is deeply understood in this quasi-solid electrolyte. Pr3+ additive not only dominates the proton conduction kinetics, but also regulates the reversible manganese interfacial deposition. As a result, the Cu@Zn||α-MnO2 cell could achieve a high specific capacity of 433 mAh g-1 at 0.4 mA cm-2 and an excellent stability up to 800 cycles with a capacity retention of 92.2% at 0.8 mA cm-2 in such water-poor quasi-solid electrolyte for the first time. Ah-scale pouch cell with mass loading of 15.19 mg cm-2 sustains 100 cycles after initial activation, which is much better than its counterparts. Our work provides a new path for the development of zinc metal batteries with good sustainability and practicality.

Effective Proton Conduction in Quasi‐Solid Zinc‐Manganese Batteries via Constructing Highly Connected Transfer Pathways

Elusive ion behaviors in aqueous electrolyte remain a challenge to break through the practicality of aqueous zinc‐manganese batteries (AZMBs), a promising candidate for safe grid‐scale energy storage systems. The proposed electrolyte strategies for this issue most ignore the prominent role of proton conduction, which greatly affects the operation stability of AZMBs. Here we report a water‐poor quasi‐solid electrolyte with efficient proton transfer pathways based on the large‐space interlayer of montmorillonite and strong‐hydration Pr3+ additive in AZMBs. Proton conduction is deeply understood in this quasi‐solid electrolyte. Pr3+ additive not only dominates the proton conduction kinetics, but also regulates the reversible manganese interfacial deposition. As a result, the Cu@Zn||α‐MnO2 cell could achieve a high specific capacity of 433 mAh g–1 at 0.4 mA cm–2 and an excellent stability up to 800 cycles with a capacity retention of 92.2% at 0.8 mA cm–2 in such water‐poor quasi‐solid electrolyte for the first time. Ah‐scale pouch cell with mass loading of 15.19 mg cm–2 sustains 100 cycles after initial activation, which is much better than its counterparts. Our work provides a new path for the development of zinc metal batteries with good sustainability and practicality.

Balancing Brightness and Photobasicity: Modulating Excited-State Proton Transfer Pathways in Push-Pull Fluorophores for Biological Two-Photon Imaging.

Push-pull fluorophores with donor-π-acceptor architectures are attractive scaffolds for the design of probes and labels for two-photon microscopy. Such fluorophores undergo a significant charge-delocalization in the excited state, which is essential for achieving a large two-photon absorption cross-section and brightness. The polarized excited state may, however, also facilitate excited-state proton transfer (ESPT) pathways that can interfere with the probe response. Herein, we employed steady-state and time-resolved spectroscopic studies to elucidate whether ESPT is responsible for the pH-dependent emission response of the Zn(II)-selective fluorescent probe chromis-1. Composed of a push-pull architecture with a pyridine ring as the acceptor, the chromis-1 fluorophore core acts as a photobase that promotes ESPT upon acidification. Although the pKa of the pyridine acceptor increases more than six orders of magnitude upon excitation, the photobasicity is not sufficient to deprotonate solvent water molecules under neutral conditions. Rather, the pH-dependent emission response is caused by the pendant bis-isonicotinic acid chelating group which upon protonation facilitates an excited-state intramolecular proton transfer to the pyridine acceptor. A simple permutation of the core pyridine nitrogen from the para- to the ortho-position relative to the thiazole substituent was sufficient to reduce the excited-state basicity by two orders of magnitude without compromising the two-photon excited brightness. These results highlight the importance of choosing the appropriate fluorophore core and chelating moiety for minimizing pH-dependent responses in the design of fluorescent probes for biological imaging.

Role of H2O2 as Oxidant and H2O as Co-catalyst over Anatase-TiO2 (101) for Conversion of Methane to Methanol.

Computations based on density functional theory are carried out to examine the mechanism of photocatalytic oxidation of methane to methanol with H2O2 as oxidant and water as co-catalyst over anatase TiO2 (101) surface. The reaction proceeds with hydrogen abstraction from methane followed by the formation of surface methoxy species, which is reduced to methanol. We compare the reaction energetics for C-H dissociation in the presence and absence of surface defect, but find no discernible impact of O-vacancy on methane oxidation. In comparison, hydroxyl produced as a result of H2O2 or H2O photo-decomposition dramatically reduces the barrier for CH3-H bond cleavage. The reaction proceeds further by the reduction of surface methoxy group and is the rate-limiting step for methanol formation. Additionally, we find that methyl can also react with water to form methanol with a considerably lower barrier, suggesting an active involvement of water for methanol formation. We also study the role of water as a co-catalyst and observe significant reduction in barriers, facilitated by alternate pathway for proton transfer. The reaction pathways presented provide valuable in- sights into the mechanism of methane oxidation in the presence of H2O2 as oxidant and demonstrate the rate-enhancing role of water for these steps.

Regulating Local Electron Distribution of Cu Electrocatalyst via Boron Doping for Boosting Rapid Absorption and Conversion of Nitrate to Ammonia

AbstractThe electrochemical reduction of nitrate to ammonia (NO3RR) is an effective route to ammonia synthesis with the characteristics of low energy input. However, the complex multi‐electron/proton transfer pathways associated with this reaction may trigger the accumulation of competitive by‐products. Herein, boron (B)‐doped Cu electrode (denoted as B–Cu2O/Cu/CP) as “all‐in‐one” catalyst is prepared by one‐step electrodeposition strategy. Caused by the B doping, the charge redistribution and local coordination environment of Cu2O/Cu species are modulated, resulting in the exposure of active sites on the Cu2O/Cu/CP catalyst. In‐situ Fourier transform infrared spectroscopy and theoretical investigations demonstrate that both Cu2O and Cu sites modulated by B can effectively enhance the adsorption of NO3− and facilitate the conversion of intermediate by‐products, thus promoting the direct reduction of NO3− to NH3. Consequently, a remarkable Faradaic efficiency of 92.74% can be obtained on B–Cu2O/Cu/CP catalyst with minimal accumulation of by‐products. It is expected that this work, based on the heterogeneous B doping, will open a maneuverable and versatile way for the design of effective catalysts.

Intermolecular Proton Transfer Enabled Reactive CO2 Capture by the Malononitrile Anion.

Task-specific ionic liquids (ILs) employing carbanions represent a new class of ILs for carbon capture. The deprotonated malononitrile carbanion, [CH(CN)2]-, has shown close to equimolar capacity for reactive CO2 capture. Although the formation of the [C(CN)2COOH]- carboxylic acid was found to be the final product, how the hydrogen atom on the [CH(CN)2]- carbanion transfers to the carboxylate group as a proton has not been fully understood. In this work, we employ density functional theory calculations with an implicit solvation model to investigate the proton transfer mechanisms in forming carboxylic acid from the reaction of the [CH(CN)2]- carbanion with CO2. We find that the intramolecular proton-transfer pathway in [CH(CN)2COO]- to form [C(CN)2COOH]- is unlikely due to the high energy barrier of 152 kJ/mol. Instead, the intermolecular proton transfer pathway between two [CH(CN)2COO]- anions is more feasible to form two molecules of [C(CN)2COOH]-, with a significantly lower activation energy of 50 kJ/mol. Moreover, the [C(CN)2COOH]- dimer is further stabilized by the intermolecular hydrogen bonds of the two -COOH groups in the Z-configuration of the π-conjugated planar geometry. This insight of reactive CO2 capture enabled by intermolecular proton transfer will be useful in designing novel carbanions and ILs for carbon capture and conversion.

An affordable convertible: Engineering proton transfer pathways in microbial rhodopsins

An affordable convertible: Engineering proton transfer pathways in microbial rhodopsins

Modulating the Leverage Relationship in Nitrogen Fixation Through Hydrogen‐Bond‐Regulated Proton Transfer

In the electrochemical nitrogen reduction reaction (NRR), a leverage relationship exists between NH3‐producing activity and selectivity because of the competing hydrogen evolution reaction (HER), which means that high activity with strong protons adsorption causes low product selectivity. Herein, we design a novel metal‐organic hydrogen bonding framework (MOHBF) material to modulate this leverage relationship by a hydrogen‐bond‐regulated proton transfer pathway. The MOHBF material was composited with reduced graphene oxide (rGO) to form a Ni‐N2O2 molecular catalyst (Ni‐N2O2/rGO). The unique structure of O atoms in Ni‐O‐C and N‐O‐H could form hydrogen bonds with H2O molecules to interfere with protons being directly adsorbed onto Ni active sites, thus regulating the proton transfer mechanism and slowing the HER kinetics, thereby modulating the leverage relationship. Moreover, this catalyst has abundant Ni‐single‐atom sites enriched with Ni‐N/O coordination, conducive to the adsorption and activation of N2. The Ni‐N2O2/rGO exhibits simultaneously enhanced activity and selectivity of NH3 production with a maximum NH3 yield rate of 209.7 μg h−1 mgcat.−1 and a Faradaic efficiency of 45.7%, outperforming other reported single‐atom NRR catalysts.

A Competition among Three Ultrafast Photoinduced Events in Thiotropolone: Internal Conversion vs Intersystem Crossing vs ESIPT.

Gas phase excited-state quantum wavepacket dynamics simulations of the thiotropolone demonstrate the ultrafast triplet formation upon photoexcitation to the dipole-allowed S2 state. The dominant relaxation pathway of the S2-T4 intersystem crossing, facilitated by the strong spin-orbit coupling and narrow energy gap, competes with the S2 to S1/S3 internal conversion. The wavepacket populated in T4 via the former pathway decays to lower triplet states. Computed potential energy profiles suggest proton transfer via S2, which might compete with internal conversion and intersystem crossing. The nonadiabatic population transfer from the S2 to S1/S3 states enables proton transfer via the latter states, resulting in multiple proton transfer pathways. Experimental investigations are necessary to shed light on the complex ultrafast photodynamics of thiotropolone.

CdS Quantum Dot Gels as a Direct Hydrogen Atom Transfer Photocatalyst for C-H Activation.

Here, we report CdS quantum dot (QD) gels, a three-dimensional network of interconnected CdS QDs, as a new type of direct hydrogen atom transfer (d-HAT) photocatalyst for C-H activation. We discovered that the photoexcited CdS QD gel could generate various neutral radicals, including α-amido, heterocyclic, acyl, and benzylic radicals, from their corresponding stable molecular substrates, including amides, thio/ethers, aldehydes, and benzylic compounds. Its C-H activation ability imparts a broad substrate and reaction scope. The mechanistic study reveals that this reactivity is intrinsic to CdS materials, and the neutral radical generation did not proceed via the conventional sequential electron transfer and proton transfer pathway. Instead, the C-H bonds are activated by the photoexcited CdS QD gel via a d-HAT mechanism. This d-HAT mechanism is supported by the linear correlation between the logarithm of the C-H bond activation rate constant and the C-H bond dissociation energy (BDE) with a Brønsted slope α=0.5. Our findings expand the currently limited direct hydrogen atom transfer photocatalysis toolbox and provide new possibilities for photocatalytic C-H activation.

CdS Quantum Dot Gels as a Direct Hydrogen Atom Transfer Photocatalyst for C−H Activation

AbstractHere, we report CdS quantum dot (QD) gels, a three‐dimensional network of interconnected CdS QDs, as a new type of direct hydrogen atom transfer (d‐HAT) photocatalyst for C−H activation. We discovered that the photoexcited CdS QD gel could generate various neutral radicals, including α‐amido, heterocyclic, acyl, and benzylic radicals, from their corresponding stable molecular substrates, including amides, thio/ethers, aldehydes, and benzylic compounds. Its C−H activation ability imparts a broad substrate and reaction scope. The mechanistic study reveals that this reactivity is intrinsic to CdS materials, and the neutral radical generation did not proceed via the conventional sequential electron transfer and proton transfer pathway. Instead, the C−H bonds are activated by the photoexcited CdS QD gel via a d‐HAT mechanism. This d‐HAT mechanism is supported by the linear correlation between the logarithm of the C−H bond activation rate constant and the C−H bond dissociation energy (BDE) with a Brønsted slope α=0.5. Our findings expand the currently limited direct hydrogen atom transfer photocatalysis toolbox and provide new possibilities for photocatalytic C−H activation.

Elucidation of the stereochemical mechanism of cystathionine γ-lyase reveals how substrate specificity constrains catalysis.

Pyridoxal phosphate (PLP)-dependent enzymes play essential roles in metabolism and have found applications for organic synthesis and as enzyme therapeutics. The vinylglycine ketimine (VGK) subfamily hosts a growing set of enzymes that play diverse roles in primary and secondary metabolism. However, the molecular determinates of substrate specificity and the complex acid-base chemistry that enables VGK catalysis remain enigmatic. We use a recently discovered amino acid γ-lyase as a model system to probe catalysis in this enzyme family. We discovered that two stereochemically distinct proton transfer pathways occur. Combined kinetic and spectroscopic analysis revealed that progression through the catalytic cycle is correlated with the presence of an H-bond donor after Cγ of an amino acid substrate, suggesting substrate binding is kinetically coupled to a conformational change. High-resolution X-ray crystallography shows that cystathionine-γ-lyases generate an s-trans intermediate and that this geometry is likely conserved throughout the VGK family. An H-bond acceptor in the active site templates substrate binding but does so by pre-organizing substrates away from catalytically productive orientations. Mutagenesis eliminates this pre-organization, such that there is a relaxation of the substrate specificity, but an increase in k cat for diverse substrates. We exploit this information to perform preparative scale α,β,β-tri-deuteration of polar amino acids. Together, these data untangle a complex mode of substrate specificity and provide a foundation for the future study and applications of VGK enzymes.

Searching for proton transfer channels in respiratory complex I

We have explored a strategy to identify potential proton transfer channels using computational analysis of a protein structure based on Voronoi partitioning and applied it for the analysis of proton transfer pathways in redox-driven proton-pumping respiratory complex I. The analysis results in a network of connected voids/channels, which represent the dual structure of the protein; we then hydrated the identified channels using our water placement program Dowser++. Many theoretical water molecules found in the channels perfectly match the observed experimental water molecules in the structure; some other predicted water molecules have not been resolved in the experiments. The channels are of varying cross sections. Some channels are big enough to accommodate water molecules that are suitable to conduct protons; others are too narrow to hold water but require only minor conformational changes to accommodate proton transfer. We provide a preliminary analysis of the proton conductivity of the network channels, classifying the proton transfer channels as open, closed, and partially open, and discuss possible conformational changes that can modulate, i.e., open and close, the channels.

Proton transfer in cytochrome bd-I from E. coli involves Asp-105 in CydB

Cytochrome bds are bacterial terminal oxidases expressed under low oxygen conditions, and they are important for the survival of many pathogens and hence potential drug targets. The largest subunit CydA contains the three redox-active cofactors heme b558, heme b595 and the active site heme d. One suggested proton transfer pathway is found at the interface between the CydA and the other major subunit CydB. Here we have studied the O2 reduction mechanism in E. coli cyt. bd-I using the flow-flash technique and focused on the mechanism, kinetics and pathway for proton transfer.Our results show that the peroxy (P) to ferryl (F) transition, coupled to the oxidation of the low-spin heme b558 is pH dependent, with a maximum rate constant (~104 s−1) that is slowed down at higher pH. We assign this behavior to rate-limitation by internal proton transfer from a titratable residue with pKa ~ 9.7. Proton uptake from solution occurs with the same P➔F rate constant. Site-directed mutagenesis shows significant effects on catalytic turnover in the CydB variants Asp58B➔Asn and Asp105B➔Asn variants consistent with them playing a role in proton transfer. Furthermore, in the Asp105B➔Asn variant, the reactions up to P formation occur essentially as in the wildtype bd-I, but the P➔F transition is specifically inhibited, supporting a direct and specific role for Asp105B in the functional proton transfer pathway in bd-I. We further discuss the possible identity of the high pKa proton donor, and the conservation pattern of the Asp-105B in the cyt. bd superfamily.

Hydrogen Evolving Mono‐Nuclear Manganese Complexes with Carefully Positioned Pyridine Base Ligands

AbstractEnormous efforts have been made by the scientific community in the development of bio‐mimetic/bio‐inspired hydrogen‐evolving complexes as catalysts for an alternative hydrogen energy carrier and a renewable energy sources. In this regard, new bio‐inspired mono‐nuclear Mn(I) carbonyl model complexes fac‐[(Mn(CO)3(κ2‐SN2C7H5)(κ1‐PPh2Py))] 1 and fac‐[(Mn(CO)3(κ2‐S2NC7H4)(κ1‐PPh2Py))] 2 with 2‐mercaptobenzimidazole (N,N) and 2‐mercaptobenzothiazole (S,N) ligands and a N‐based phosphine ligand (PPh2Py) have been synthesized, spectroscopically characterized and evaluated as electro‐catalysts for hydrogen generation. In contrast to related mono‐nuclear (N,N/S,N)−Mn−PPh3 complexes,, the newly introduced pyridine nitrogen atom of the PPh2Py ligand placed in an axial position in the second coordination sphere of the Mn(I) complexes, provides a first site for protonation and enables a fast intra‐molecular proton transfer to the central metal atom with a weaker acid. Theoretical DFT calculations enable a detailed picture of catalytic pathways, assign redox transitions and structural changes. Both the molecular complexes were tested as electro‐catalysts for hydrogen generation in CH3CN. They effectively catalyzed the electrochemical proton reduction with the weaker acetic acid as the proton source and displayed a turnover frequency (TOF/s−1; O.P., η/V) of 610; 1.02 and 615; 1.12, respectively. The computer‐aided design of dedicated proton transfer pathways from second coordination sphere ligands is thus able to allow a hydrogen evolution reactivity with a weaker acid (acetic acid).

Tandem Proton Transfer in Carboxylated Supramolecular Polymer for Highly Efficient Overall Photosynthesis of Hydrogen Peroxide

Proton supply is as critical as O2 activation for artificial photosynthesis of H2O2 via two‐electron oxygen reduction reaction (2e‐ ORR). However, proton release via water dissociation is frequently hindered because of the sluggish water oxidation reaction (WOR), extremely limiting the efficiency of photocatalytic H2O2 production. To tackle this challenge, carboxyl‐enriched supramolecular polymer (perylene tetracarboxylic acid ‐ PTCA) is elaborately prepared by molecular self‐assembly for overall photosynthesis of H2O2. Interestingly, the interconversion between carboxyl as Brønsted acid and its conjugated base realizes rapid proton circulation. Through this efficient tandem proton transfer process, the spatial effect of photocatalytic reduction and oxidation reaction is greatly enhanced with reduced reaction barrier. This significantly facilitates 2e‐ photocatalytic ORR to synthesize H2O2 and in the meanwhile promotes 4e‐ photocatalytic WOR to evolve O2. Consequently, the as‐developed PTCA exhibits a remarkable H2O2 yield of 185.6 μM h‐1 in pure water and air atmosphere under visible light illumination. More impressively, an appreciable H2O2 yield of 78.6 μM h‐1 can be well maintained in an anaerobic system owing to in‐situ O2 generation by 4e‐ photocatalytic WOR. Our study presents a novel concept for artificial photosynthesis of H2O2 via constructing efficient proton transfer pathway to enable rapid proton circulation.

Tandem Proton Transfer in Carboxylated Supramolecular Polymer for Highly Efficient Overall Photosynthesis of Hydrogen Peroxide.

Proton supply is as critical as O2 activation for artificial photosynthesis of hydrogen peroxide (H2O2) via two-electron oxygen reduction reaction (2e- ORR). However, proton release via water dissociation is frequently hindered because of the sluggish water oxidation reaction (WOR), extremely limiting the efficiency of photocatalytic H2O2 production. To tackle this challenge, carboxyl-enriched supramolecular polymer (perylene tetracarboxylic acid-PTCA) is elaborately prepared by molecular self-assembly for overall photosynthesis of H2O2. Interestingly, the interconversion between carboxyl as Brønsted acid and its conjugated base realizes rapid proton circulation. Through this efficient tandem proton transfer process, the spatial effect of photocatalytic reduction and oxidation reaction is greatly enhanced with reduced reaction barrier. This significantly facilitates 2e- photocatalytic ORR to synthesize H2O2 and in the meanwhile promotes 4e- photocatalytic WOR to evolve O2. Consequently, the as-developed PTCA exhibits a remarkable H2O2 yield of 185.6 μM h-1 in pure water and air atmosphere under visible light illumination. More impressively, an appreciable H2O2 yield of 78.6 μM h-1 can be well maintained in an anaerobic system owing to in situ O2 generation by 4e- photocatalytic WOR. Our study presents a novel concept for artificial photosynthesis of H2O2 via constructing efficient proton transfer pathway to enable rapid proton circulation.

Enantio‐ and Z‐Selective δ‐Hydroarylation of Aryl‐Substituted 1,3‐Dienes via Rh‐Catalyzed Conjugate Addition

AbstractMetal‐catalyzed enantioselective conjugate arylations of electron‐poor alkenes are highly selective processes for C(sp2)−C(sp3) bond formation. δ‐Selective hydroarylations of electron‐poor 1,3‐dienes are less well developed and reactions that deliver high enantioselectivity while giving single alkene isomer products are elusive. Here we report the Rh‐catalyzed δ‐arylation of aryl‐substituted 1,3‐dienes that gives nearly exclusive Z‐1,4‐addition products (generally with >95 : 5 positional and geometrical selectivity). This remote functionalization provides access to chiral diarylated alkenes from readily available precursors poised for further functionalization, including in the synthesis of bioactive molecules. Mechanistic studies suggest that protonolysis of a Rh‐allyl intermediate generated by diene insertion into a Rh‐aryl is the turnover limiting step and occurs by an inner‐sphere proton transfer pathway.

Enantio- and Z-Selective δ-Hydroarylation of Aryl-Substituted 1,3-Dienes via Rh-Catalyzed Conjugate Addition.

Metal-catalyzed enantioselective conjugate arylations of electron-poor alkenes are highly selective processes for C(sp2)-C(sp3) bond formation. δ-Selective hydroarylations of electron-poor 1,3-dienes are less well developed and reactions that deliver high enantioselectivity while giving single alkene isomer products are elusive. Here we report the Rh-catalyzed δ-arylation of aryl-substituted 1,3-dienes that gives nearly exclusive Z-1,4-addition products (generally with >95 : 5 positional and geometrical selectivity). This remote functionalization provides access to chiral diarylated alkenes from readily available precursors poised for further functionalization, including in the synthesis of bioactive molecules. Mechanistic studies suggest that protonolysis of a Rh-allyl intermediate generated by diene insertion into a Rh-aryl is the turnover limiting step and occurs by an inner-sphere proton transfer pathway.

Computational Studies of Nucleophilic Substitution at Nitrogen Center: Reactions of NH2Cl with HO-, CH3O- and C2H5O.

The atomic-level mechanisms of the nucleophilic substitution reactions at the nitrogen center (SN2@N) were investigated for the reactions of chloramine (NH2Cl) with the alkoxide ions (RO-, where R = H, CH3, and C2H5) using DFT and MP2 methods. The computed potential energy profiles for the SN2@N pathways involving the back-side attack of the nucleophiles show the typical double-well potential with submerged barriers similar to the SN2@N reactions at the carbon center (SN2@C). However, the pre-reaction and post-reaction complexes are, respectively, the N-H…O and N-H…Cl hydrogen-bonded intermediates, which are different from those generally seen in SN2@C reactions. The SN2@N pathways involving front-side attack of the nucleophiles have high-energy barriers. The potential energy surfaces (PESs) along the proton-transfer pathways were flat. In addition to the proton-transfer and SN2 pathways, we also observed a new path for the methoxide and ethoxide nucleophiles where a hydride transfer from the nucleophile to chloramine resulted in the products Cl- + R'CHO + NH3, (R' = H, CH3), and was the most exoergic. A comparison of the energetics obtained used different DFT and MP2 methods with that of the benchmark coupled-cluster methods reveals that CAM-B3LYP best describes the PESs.