Role Of Protons Research Articles

Heterometallic Transition Metal Oxides Containing Lewis Acids as Molecular Catalysts for the Reduction of Carbon Dioxide to Carbon Monoxide with Bimodal Activity.

Electrocatalytic CO2 reduction (e-CO2RR) to CO is replete with challenges including the need to carry out e-CO2RR at low overpotentials. Previously, a tricopper-substituted polyoxometalate was shown to reduce CO2 to CO with a very high faradaic efficiency albeit at -2.5 V versus Fc/Fc+. It is now demonstrated that introducing a nonredox metal Lewis acid, preferably GaIII, as a binding site for CO2 in the first coordination sphere of the polyoxometalate, forming heterometallic polyoxometalates, e.g., [SiCuIIFeIIIGaIII(H2O)3W9O37]8-, leads to bimodal activity optimal both at -2.5 and -1.5 V versus Fc/Fc+; reactivity at -1.5 V being at an overpotential of ∼150 mV. These results were observed by cyclic voltammetry and quantitative controlled potential electrolysis where high faradaic efficiency and chemoselectivity were obtained at -2.5 and -1.5 V. A reaction with 13CO2 revealed that CO2 disproportionation did not occur at -1.5 V. EPR spectroscopy showed reduction, first of CuII to CuI and FeIII to FeII and then reduction of a tungsten atom (WVI to WV) in the polyoxometalate framework. IR spectroscopy showed that CO2 binds to [SiCuIIFeIIIGaIII(H2O)3W9O37]8- before reduction. In situ electrochemical attenuated total reflection surface-enhanced infrared absorption spectroscopy (ATR-SEIRAS) with pulsed potential modulated excitation revealed different observable intermediate species at -2.5 and -1.5 V. DFT calculations explained the CV, the formation of possible activated CO2 species at both -2.5 and -1.5 V through series of electron transfer, proton-coupled electron transfer, protonation and CO2 binding steps, the active site for reduction, and the role of protons in facilitating the reactions.

Role of Protons in and around Strongly Coupled Nitroxide Biradicals for Cross-Effect Dynamic Nuclear Polarization.

In magic angle spinning dynamic nuclear polarization (DNP), biradicals such as bis-nitroxides are used to hyperpolarize protons under microwave irradiation through the cross-effect mechanism. This mechanism relies on electron-electron spin interactions (dipolar coupling and exchange interaction) and electron-nuclear spin interactions (hyperfine coupling) to hyperpolarize the protons surrounding the biradical. This hyperpolarization is then transferred to the bulk sample via nuclear spin diffusion. However, the involvement of the protons in the biradical in the cross-effect DNP process has been under debate. In this work, we address this question by exploring the hyperpolarization pathways in and around bis-nitroxides. We demonstrate that for biradicals with strong electron-electron interactions, as in the case of the AsymPols, the protons on the biradical may not be necessary to quickly generate hyperpolarization. Instead, such biradicals can efficiently, and directly, polarize the surrounding protons of the solvent. The findings should impact the design of the next generation of biradicals.

The Role of Proton in High Power Density Vanadium Redox Flow Batteries.

To design high-performance vanadium redox flow batteries (VRFBs), the influence of proton on electrocatalysts cannot be neglected considering the abundance of proton in a highly acidic electrolyte. Herein, the impact of proton on metal oxide-based electrocatalysts in VRFBs is investigated, and a proton-incorporating strategy is introduced for high power density VRFBs, in addition to unraveling the catalytic mechanism. This study discloses that the metal oxide-based electrocatalyst (WO3) undergoes in situ surface reconstruction by forming H0.5WO3 after incorporating proton. Experimental and theoretical results precisely disclose the catalytic active sites. The battery with H0.5WO3 designed by a proton-incorporating strategy achieves an attractive power density of 1.12 W cm-2 and sustains more than 900 cycles without an obvious decay, verifying the outstanding electrochemical performance of H0.5WO3. This work not only sheds light on the influence of proton on electrocatalysts for rational design of advanced VRFBs catalysts but also provides guidelines for the fundamental understanding of the reaction mechanism, which is highly important for the application of VRFBs.

On the Role of Protons in the Functioning of ATP Synthase Factor F1 during Adenosine-Triphosphate Synthesis

It is shown that the functioning of ATP synthase factor F1, on which the reaction of adenosine triphosphate (ATP) synthesis takes place, involves hydrogen ions that enter F1 after transmembrane transfer through the FO subunit. The protonation of amino-acid residues of glutamate and aspartate significantly changes the conformation of the enzyme, which is an important factor facilitating conformational transitions in ATP synthase along with mechanical movement of the γ subunit described in publications. The binding of protons to ATP synthase factor F1 during enzyme functioning is also necessary to ensure the ATP synthesis reaction itself, which does not slow down under nonequilibrium conditions of alkalization of the near-membrane zone during the functioning of proton pumps thanks to the use of protons transmembranely transferred through FO. The rotation of ATP synthase in this case can catalyze the exchange of protons between the phases of near-membrane and bulk water. It is concluded that the conformational model of ATP synthase should be supplemented and it should take into account protonation of the F1 factor.

Local Structure and Protons in Non-Stoichiometric Pseudo-Cubic Pollucite Mineral by Multinuclear NMR

The pollucite structure is considered as a candidate ceramic crystalline matrix for the ceramic immobilization and long-term storage of 135Cs and 137Cs fission products, and thus, their structural characteristics have particular importance. However, its local structure has not been fully resolved from reciprocal-space techniques and infrared spectroscopy, and important discrepancies exist in the available literature. Two birefringent and non-stoichiometric pollucite specimens from Tanco pegmatite (Cs0.83Na0.20Al1.13Si2.56O6) and from Mt. Mica pegmatite (Cs0.94Na018Al1.23Si2.78O6), with powder X-ray diffraction patterns fully consistent with the cubic Ia-3d space-group symmetry, and with a very different degree of hydrothermal alteration, were used in this work. High-resolution magic-angle spinning multinuclear magnetic resonance (MAS NMR) spectroscopy, including 29Si, 27Al, 23Na, 133Cs, and 1H spectra at 9.4 T, as well as 1H, 27Al, 27Al{1H} dipolar evolutions and 27Al{29Si} Heteronuclear Multiple Quantum Coherence (HMCQ) spectra at 17.6 T, has been used to investigate the local structure of pollucite and the role of protons. The 29Si spectra suggest a local structure with a disordered Si/Al distribution in only one tetrahedral T site, but with a preference of Si atoms for Q41 (3Si,1Al) and Q42 (2Si,2Al) environments, in comparison with random and Loewenstein distributions, due to charge dispersion effects. However, the 27Al{1H} dipolar evolutions suggest two spectroscopically distinct T sites for Al atoms. The 23Na and 133Cs spectra indicate broad site distributions for these cavity cations. The anisotropic character of the long-range disordered pollucite structure, with a pseudo-cubic symmetry and lack of strict periodicity, can be explained from an incipient displacive transition to lower symmetry. These pollucite specimens are essentially anhydrous minerals despite the 1H and the cross-polarization experiments suggesting that some protons exist in the structure as -OH groups, whereas water molecules were only found in relation to the phyllosilicate impurities from alteration in specimen Tanco and perhaps also as liquid water in fluid inclusions.

Insights into the Role of Protonation in Covalent Triazine Framework-Based Photocatalytic Hydrogen Evolution

Insights into the Role of Protonation in Covalent Triazine Framework-Based Photocatalytic Hydrogen Evolution

Computational studies on the Carboni-Lindsey reaction

The Carboni-Lindsey reaction between tetrazines and strained unsaturated hydrocarbons has in recent years become a useful tool in bioorthogonal chemistry for labelling biomolecules in vitro and in vivo. Here, we report computational studies on the mechanism of the Diels-Alder/Retro-Diels-Alder reaction between a commercially available tetrazine and norbornene. Isomerism of the reaction with a norbornene-amide relevant for applications in bioconjugation chemistry, as well as the role of protonation in the mechanism, is assessed.

Addressing the Elusive Polaronic Nature of Multiple Redox States in a π‐Conjugated Ladder‐Type Polymer

AbstractPoly(benzimidazole–benzophenanthroline) (BBL) is a ladder‐type conjugated polymer showing remarkable charge transport properties. Upon doping it displays various conductive regimes, leading to two insulator‐to‐conductor transitions. Such transitions are never fully characterized, limiting understanding of its charged states. Open issues are: i) the electron/hole polaron relaxations, ii) the structure–function relationships of multiple redox states and their connection with the conductive regimes, and iii) the role of protonation. Such knowledge‐gaps are tackled via a comprehensive computational investigation of multiple redox species. Polarons show polyradicaloid character, as revealed by combining broken‐symmetry density functional theory, fragment orbital density, and multireference analysis. Electron/hole polaron relaxations occur on the polymer chain, the former localizing on the benzophenanthroline moieties, the latter on the benzimidazole units. Modeling of multiple charged species, up to one electron per repeat unit (1 eru), reveals a complex scenario of quasidegenerate states each featuring different spin multiplicity. Four redox states are responsible for the BBL insulator‐to‐conductor transitions. The two high conductive states refer to the electron polaron (0.25 eru) and the redox species with 0.75 eru. The insulating regimes refer to the bipolaron (0.50 eru) and the redox state with 1 eru. Protonation is modeled, revealing polaron‐like features in the spectroscopic properties.

Electronic and Optical Properties of Protonated Triazine Derivatives

The peculiar electronic and optical properties of covalent organic frameworks (COFs) are largely determined by protonation, a ubiquitous phenomenon in the solution environment in which they are synthesized. The resulting effects are non-trivial and appear to be crucial for the intriguing functionalities of these materials. In the quantum-mechanical framework of time-dependent density-functional theory, we investigate from first principles the impact of protonation of triazine and amino groups in molecular building blocks of COFs in water solution. In all considered cases, we find that proton uptake leads to a gap reduction and to a reorganization of the electronic structure, driven by the presence of the proton and by the electrostatic attraction between the positively charged protonated species and the negative counterion in its vicinity. Structural distortions induced by protonation are found to play only a minor role. The interplay between band-gap renormalization and exciton binding strength determines the energy of the absorption onsets: when the former prevails on the latter, a red-shift is observed. Furthermore, the spatial and energetic rearrangement of the molecular orbitals upon protonation induces a splitting of the lowest-energy peaks and a decrease of their oscillator strength in comparison with the pristine counterparts. Our results offer quantitative and microscopic insight into the role of protonation on the electronic and optical properties of triazine derivatives as building blocks of COFs. As such, they contribute to rationalize the relationships between structure, property, and functionality of these materials.

The Role of Protons and Hydrides in the Catalytic Hydrogenolysis of Guaiacol at the Ruthenium Nanoparticle–Water Interface

The mechanistic roles of free hydronium ions, surface hydrides, and interfacial protons during guaiacol hydrodeoxygenation (HDO) on ruthenium nanoparticles have been established. As guaiacol adsorb...

Probing Enzymatic Structure and Function in the Dihydroxylating Sesquiterpene Synthase ZmEDS.

Terpene synthases (TPSs) play a vital role in forming the complex hydrocarbon backbones that underlie terpenoid diversity. Notably, some TPSs can add water prior to terminating the catalyzed reaction, leading to hydroxyl groups, which are critical for biological activity. A particularly intriguing example of this is the maize (Zea mays) sesquiterpene TPS whose major product is eudesmanediol, ZmEDS. This production of dual hydroxyl groups is presumably enabled by protonation of the singly hydroxylated transient stable intermediate hedycaryol. To probe the enzymatic structure-function relationships underlying this unusual reaction, protein modeling and docking were used to direct mutagenesis of ZmEDS. Previously, an F303A mutant was shown to produce only hedycaryol, suggesting a role in protonation. Here this is shown to be dependent on the steric bulk positioning of hedycaryol, including a supporting role played by the nearby F299, rather than π-cation interaction. Among the additional residues investigated here, G411 at the conserved kink in helix G is of particular interest, as substitution of this leads to predominant production of the distinct (-)-valerianol, while substitution for the aliphatic I279 and V306 can lead to significant production of the alternative eudesmane-type diols 2,3-epi-cryptomeridiol and 3-epi-cryptomeridol, respectively. Altogether, nine residues that are important for this unusual reaction were investigated here, with the results not only emphasizing the importance of reactant positioning suggested by the stereospecificity observed among the various product types but also highlighting the potential role of the Mg2+-diphosphate complex as the general acid for the protonation-initiated (bi)cyclization of hedycaryol.

Role of Protonation and Isomerism in the Supramolecular Architectures of Heteroaryl-2-imidazole Compounds: Crystal Packing Patterns and Energetics

The influence of isomerism and protonation on crystal packing patterns of three isomeric heteroaryl-2-imidazoles and one related compound was investigated by using single-crystal X-ray diffraction ...

Comparison of kinetic and enzymatic properties of intracellular phosphoserine aminotransferases from alkaliphilic and neutralophilic bacteria

AbstractIntracellular pyridoxal 5´-phosphate (PLP) -dependent recombinant phosphoserine aminotransferases (PSATs; EC 2.6.1.52) from two alkaliphilicBacillusstrains were overproduced inEscherichia coli, purified to homogeneity and their enzymological characteristics were compared to PSAT from neutralophilicE. coli. Some of the enzymatic characteristics of the PSATs from the alkaliphiles were unique, showing high and sharp pH optimal of the activity related to putative internal pH inside the microbes. The specific activities of all of the studied enzymes were similar (42-44 U/mg) as measured at the pH optima of the enzymes. The spectrophotometric acid-base titration of the PLP chromophore of the enzymes from the alkaliphiles showed that the pH optimum of the activity appeared at the pH wherein the active sites were half-protonated. Detachment of PLP from holoenzymes did not take place even at pH up to 11. The kinetics of the activity loss at acid and alkaline pHs were similar in all three enzymes and followed similar kinetics. The available 3-D structural data is discussed as well as the role of protons at the active site of aminotransferases.

Role of Protons on Activity and Selectivity of Fe-N-C Electroctatalysts for Oxygen Reduction Reaction

As one of the most promising candidates to replace Pt-based electrocatalysts for oxygen reduction reaction (ORR) in proton exchange membrane fuel cell (PEMFC), iron-nitrogen-carbon (Fe-N-C) materials have attracted tremendous research interests in recent years. Different from platinum-group-metal (PGM) catalysts, Fe-N-C have several categories of active sites: Fe-Nx has 4-electron pathway; pyrrolic-N and hydrogenated pyridinic-N can partially reduce oxygen into peroxide; pyridinic-N can reduce peroxide into water. Due to this complicated environment and numerous synthesis approaches from various precursors, the electrocatalysis mechanism of Fe-N-C still needs intensive investigation. Here we report the proton independence/dependence of rate-determining step (RDS) for a specific Fe-N-C catalyst via kinetic isotope effect (KIE), in addition to the activity inhibition of proton-involved N active sites via two molecular inhibitors. It is found that Fe-N-C can behave proton independent RDS like conventional Pt electrocatalyst in kinetic-controlled region, but it undergoes proton-coupled electron transfer in mass transport-controlled region, probably attributed to N active sites. It is also discovered that two molecules, tris(hydroxymethyl)aminomethane and etidronic acid, can inhibit and transform two categories of N active sites respectively: tris can effectively poison Fe-Nx active sites and pyridinic-N, resulting in obvious increase of peroxide yielding and drop of half-wave potential; etridonic acid can readily inhibit pyrrolic-N as well as hydrogenated pyridinic-N but no damage to Fe-Nx active sites, leading to decrease of peroxide yielding and no change of half-wave potentials. These results elucidate that Fe-N-C catalysts could, in principle, compete with Pt-based ORR catalysts through careful optimization of structure and morphology. Figure 1

Influence of protons on reduction degree and defect formation in electrochemically reduced graphene oxide

The electrochemical reduction of GO was investigated in aqueous, at both acid and basic pH, and organic media, to identify the possible role of protons (H+) in the reduction mechanism of this material. The obtained rGO films were characterized by FTIR, electrochemical methods, Raman and XPS spectroscopy. Data showed that the reduction was more efficient in acid and basic media due to the presence of protons and the capacity of water that works as a proton donor, resulting in C/O ratios of 3.8 and 7.8, respectively. Mostly hydroxyl, epoxide and carbonyl moieties were removed. In a proton-free organic electrolyte, a C/O ratio of 1.8 was obtained for most of the samples; nevertheless, the graphitic carbon sp2 domains were restored to a large extent in the absence of H+. The characterization of the material showed that the presence of protons, during the electrochemical reduction, caused hydrogenation reactions, which targeted the graphitic domains in rGO and resulted in the loss of sp2 hybridization. The presence of such defects modified the electrochemical properties of the rGO films, where, despite of exhibiting higher C/O ratio, the films reduced in aqueous electrolytes displayed lower electron transfer (e.g. ferrocyanide redox-probe) than those reduced in organic electrolyte.

The Role of Protons and Formation Cu(NH3)2+ During Ammonia-Assisted Solid-State Ion Exchange of Copper(I) Oxide into Zeolites

Solid-state ion exchange in mixtures of copper oxides and zeolites can occur at temperatures as low as 200–250 °C in the presence of ammonia (NH3-SSIE), thus providing a low-temperature method to activate zeolites for selective catalytic reduction nitrogen oxide (NH3-SCR). The ammonia induced solid-state ion exchange process is studied in more detail, by monitoring the development of NH3-SCR activity with duration of NH3-SSIE, formation of a mobile linear [Cu(NH3)2]+ complex with X-ray spectroscopy, and the transfer of Cu to the zeolite by XRD, and evaporation of copper(I)-oxide by TGA. We find that the linear [Cu(NH3)2]+ complex is formed in mixtures of copper-oxide and *BEA and CHA zeolites upon exposure to ammonia. Increasing the temperature for NH3-SSIE to well above 300 °C leads to a less efficient Cu-transfer. This indicates that the [Cu(NH3)2]+ complex is crucial for the NH3-SSIE process. The non-monotonous development of NOx conversion and N2O yield with duration of NH3-SSIE is probably due to an initial enrichment of Cu in the outer shell of the zeolite crystals. We also show that the presence of H+ or NH4+-ions in the zeolite are necessary for the NH3-SSIE, and that the transfer of Cu from the Cu-oxides to the zeolites most likely occurs via a surface diffusion process.

Role of protonation and functional groups in Pd(II) recovery and reuse characteristics of commercial anion exchange resin-synthetic electroless plating solution systems

Deliberating upon the role of solution chemistry complexity, protonation, speciation and pertinent functional groups on the adsorption-desorption characteristics, this work targets the efficacy of Lewatit TP-214 and Amberlyst A21 commercial anion exchange resins for Pd(II) recovery and reuse from synthetic electroless plating (ELP) solutions. For both resins, batch adsorption experiments involved optimality of pH, contact time, adsorbent dosage followed with concentration effect and kinetic studies. Equilibrium, kinetic, and thermodynamic models were considered for their fitness with the obtained Pd(II) batch adsorption characteristics. Among both resins, Lewatit TP-214 resin provided better performance in terms of adsorption and desorption characteristics along with optimal combinations of pH (8), equilibrium time (300 min), and adsorbent dosage (2 g L−1). Overall, both resins provided significantly lower Pd(II) absorption capacity and removal% for ELP systems in comparison with aqueous solutions. This is due to the inhibitory role of Ethylenediaminetetraacetic acid (EDTA) and ammonia during Pd(II) binding with active sites on the resins. The obtained desorption characteristics and associated Pd (II) adsorption and desorption mechanisms are promising to further commercial application of the said resins.

Mixed guanine, adenine base quartets: possible roles of protons and metal ions in their stabilization

Structural variations of the well-known guanine quartet (G4) motif in nucleic acid structures, namely substitution of two guanine bases (G) by two adenine (A) nucleobases in mutual trans positions, are discussed and studied by density functional theory (DFT) methods. This work was initiated by three findings, namely (1) that GA mismatches are compatible with complementary pairing patterns in duplex-DNA structures and can, in principle, be extended to quartet structures, (2) that GA pairs can come in several variations, including with a N1 protonated adeninium moiety (AH), and (3) that cross-linking of the major donor sites of purine nucleobases (N1 and N7) by transition metal ions of linear coordination geometries produces planar purine quartets, as demonstrated by some of us in the past. Here, possible structures of mixed AGAG quartets both in the presence of protons and alkali metal ions are discussed, and in particular, the existence of a putative four-purine, two-metal motif.

Role of Protons in Electrochemical Ammonia Synthesis Using Solid-State Electrolytes

Electrochemical methods of synthesizing ammonia from nitrogen gas have the potential to replace the energy-intensive Haber-Bosch process. In doing so, they offer a CO2-free route to the production of the ever-promising energy carrier. In this study, an effort was made to reveal the relationship between proton involvement in the rate-limiting step of the ammonia synthesis reaction and the overall ammonia synthesis rate, particularly for electrolytic cells using solid-state electrolytes, as no such rule based on the measured parameters of the materials has ever been reported. An empirical atomistic expression was derived to explain the observed correlation between the proton conductivity of the solid-state electrolyte and the ammonia formation rate, by considering the proton excorporation and migration enthalpies. This relationship was determined by examining experimental results from the literature that had been obtained using diverse proton-conducting electrolytes. An almost linear energy relationship was demonstrated for state-of-the-art heterogeneous electrocatalysis.

Ice-Melting Dynamics: The Role of Protons and Interfacial Geometry.

The surface of ice plays a significant role in melting. To better understand the role of the surface, we studied the melting of ice using infrared imaging and pH-sensitive dyes. Ice was allowed to melt in baths of water of varying depths. When the ice melted in a high level of room-temperature water, equal to the height of the ice, the conventional melting pattern appeared. When the ice melted in a chamber with a lower water level, the melting pattern was unexpected. Seconds after the ice was placed in the water, localized regions of low-temperature water appeared around the perimeter of the ice. These regions grew radially outward and seemed to originate as streams coming from inside the ice. Those streams contained high concentrations of protons, as indicated by the color change of a pH-sensitive dye initially placed in the water surrounding the ice. This observation, together with the temperature distribution and ice-shape changes during melting implied that the streams may be propelled by protons from inside the ice. In contrast to conventional melting, which progresses from the outer surface inward, the stream-melting pattern implies a melting process originating inside the ice.