Proton Transport Mechanism Research Articles

Effect of Annealing on Proton Conductivity of Aquivion-Like Proton-Exchange Membrane

Proton-conducting membranes were fabricated from a new short-side chain ionomer Inion (Russian analogue of Aquivion) by solution casting method. A series of temperature treatment experiments was conducted to show that annealing of Inion membranes at the temperature range from 160 °C to 170 °C leads to a significant increase of specific proton conductivity to values even higher than those of commercial membrane Nafion NR212. An explanation of this fact can be given by considering the membranes’ proton transport mechanism and water behavior models in nanopores. Matching the proton conductivity mechanism of the membranes, which is realized in nanostructured channels with the diameter of about several nanometers according to the Grotthuss proton hopping mechanism, and the model of water and ice states in nanopores leads to the comprehensive understanding for the further optimization of the membranes to achieve high transport characteristic. For example, it can be improved by increasing the number of side-chain branches of the polymer.

The crystal structures of a chloride-pumping microbial rhodopsin and its proton-pumping mutant illuminate proton transfer determinants

Microbial rhodopsins are versatile and ubiquitous retinal-binding proteins that function as light-driven ion pumps, light-gated ion channels, and photosensors, with potential utility as optogenetic tools for altering membrane potential in target cells. Insights from crystal structures have been central for understanding proton, sodium, and chloride transport mechanisms of microbial rhodopsins. Two of three known groups of anion pumps, the archaeal halorhodopsins (HRs) and bacterial chloride-pumping rhodopsins, have been structurally characterized. Here we report the structure of a representative of a recently discovered third group consisting of cyanobacterial chloride and sulfate ion-pumping rhodopsins, the Mastigocladopsis repens rhodopsin (MastR). Chloride-pumping MastR contains in its ion transport pathway a unique Thr-Ser-Asp (TSD) motif, which is involved in the binding of a chloride ion. The structure reveals that the chloride-binding mode is more similar to HRs than chloride-pumping rhodopsins, but the overall structure most closely resembles bacteriorhodopsin (BR), an archaeal proton pump. The MastR structure shows a trimer arrangement reminiscent of BR-like proton pumps and shows features at the extracellular side more similar to BR than the other chloride pumps. We further solved the structure of the MastR-T74D mutant, which contains a single amino acid replacement in the TSD motif. We provide insights into why this point mutation can convert the MastR chloride pump into a proton pump but cannot in HRs. Our study points at the importance of precise coordination and exact location of the water molecule in the active center of proton pumps, which serves as a bridge for the key proton transfer.

Insight from perfectly selective and ultrafast proton transport through anhydrous asymmetrical graphene oxide membranes under Grotthuss mechanism

Protons transport profoundly affects diverse fields from proton-exchange membrane fuel cells to storing liquid hydrogen. Recent advances have extended proton-exclusive transport to the two-dimensional channels that use hydrous mechanisms for fast proton transport, where the main challenge is the limited selectivity. However, the physical and chemical properties of 2D nanosheets like GO have the potential to implement full selective and ultrafast proton transport. Here, we uncover the physical potential of anhydrous proton transfer mechanism inside two-dimensional space between graphene nanosheets to exploit the exceptional full proton-selective ability and ultrafast conveyance speed of the Grotthuss mechanism. Reactive molecular dynamics simulations illustrate that the interlayer space between two graphene oxide nanosheets, carpeted with hydroxyl functional groups as additional hopping stages to enable the Grotthuss mechanism, can convey protons without water. Further, we dissect three essential factors that provide a deeper insight into ultrafast proton transport: (i) transitional phase to full anhydrous transport, (ii) outlet size for containing undesired species, and (iii) elastic behavior of the membranes under external strain. Our results show that changes in surface geometry can dramatically increase the diffusion rate in the presence of a small electric field by ~70% compared to hydrous transport. These findings can be used not only to guide the efforts in manufacturing a new generation of sustainable nanochannels but also to advance the pioneering technologies revolving around hydrogen.

Insights into the Proton Transport Mechanism in TiO2 Simple Oxides by In Situ Raman Spectroscopy.

Understanding the mechanisms of proton conduction at the interface of materials enables the development of a new generation of protonic ceramic conductors at low temperatures (<150 °C) through water absorption and proton transport on the surface and grain boundaries. Conductivity measurements under Ar-3% H2O and Ar-3% D2O revealed a σ(H2O)/σ(D2O) ratio of approximately 2, indicating a hopping-based mechanism for proton conduction at the interface. In situ Raman spectroscopy was performed on water-saturated, porous, and nanostructured TiO2 membranes to directly observe the isotope exchange reactions over the temperature range of 25 to 175 °C. The behavior of the isotope exchange reactions suggested a Grotthuss-type proton transport and faster isotope exchange reactions at 175 °C than that at 25 °C with a corresponding activation energy of 9 kJ mol-1. The quantitative and mechanistic kinetic description of the isotope exchange process via in situ Raman spectroscopy represents a significant advance toward understanding proton transport mechanisms and aids in the development of high-performance proton conductors with rapid surface exchange coefficients of importance to contemporary energy conversion and storage material development. In addition, new material systems are proposed, which combine interface and bulk effects at low temperatures (<150 °C), resulting in enhanced proton transport through interfacial engineering at the nanoscale.

Multiscale Simulation Reveals Passive Proton Transport Through SERCA on the Microsecond Timescale

The sarcoplasmic reticulum Ca2+-ATPase (SERCA) transports two Ca2+ ions from the cytoplasm to the reticulum lumen at the expense of ATP hydrolysis. In addition to transporting Ca2+, SERCA facilitates bidirectional proton transport across the sarcoplasmic reticulum to maintain the charge balance of the transport sites and to balance the charge deficit generated by the exchange of Ca2+. Previous studies have shown the existence of a transient water-filled pore in SERCA that connects the Ca2+ binding sites with the lumen, but the capacity of this pathway to sustain passive proton transport has remained unknown. In this study, we used the multiscale reactive molecular dynamics method and free energy sampling to quantify the free energy profile and timescale of the proton transport across this pathway while also explicitly accounting for the dynamically coupled hydration changes of the pore. We find that proton transport from the central binding site to the lumen has a microsecond timescale, revealing a novel passive cytoplasm-to-lumen proton flow beside the well-known inverse proton countertransport occurring in active Ca2+ transport. We propose that this proton transport mechanism is operational and serves as a functional conduit for passive proton transport across the sarcoplasmic reticulum.

RETRACTED: Proton transport enabled by a field-induced metallic state in a semiconductor heterostructure.

Tuning a semiconductor to function as a fast proton conductor is an emerging strategy in the rapidly developing field of proton ceramic fuel cells (PCFCs). The key challenge for PCFC researchers is to formulate the proton-conducting electrolyte with conductivity above 0.1 siemens per centimeter at low temperatures (300 to 600°C). Here we present a methodology to design an enhanced proton conductor by means of a Na x CoO2/CeO2 semiconductor heterostructure, in which a field-induced metallic state at the interface accelerates proton transport. We developed a PCFC with an ionic conductivity of 0.30 siemens per centimeter and a power output of 1 watt per square centimeter at 520°C. Through our semiconductor heterostructure approach, our results provide insight into the proton transport mechanism, which may also improve ionic transport in other energy applications.

Elucidating the Proton Transport Pathways in Liquid Imidazole with First-Principles Molecular Dynamics.

Imidazole is a promising anhydrous proton conductor with a high conductivity comparable to that of water at a similar temperature relative to its melting point. Previous theoretical studies of the mechanism of proton transport in imidazole have relied either on empirical models or on ab initio trajectories that have been too short to draw significant conclusions. Here, we present the results of multiple time-step ab initio molecular dynamics simulations of an excess proton in liquid imidazole reaching 1 ns in total simulation time. We find that the proton transport is dominated by structural diffusion, with the diffusion constant of the proton defect being ∼8 times higher than that of self-diffusion of the imidazole molecules. By using correlation function analysis, we decompose the mechanism for proton transport into a series of first-order processes and show that the proton transport mechanism occurs over three distinct time and length scales. Although the mechanism at intermediate times is dominated by hopping along pseudo-one-dimensional chains, at longer times the overall rate of diffusion is limited by the re-formation of these chains. These results provide a more complete picture of the traditional idealized Grotthuss structural diffusion mechanism.

Proton transfer in the D-channel of cytochrome c oxidase modeled by a transition network approach

BackgroundDetermination of proton uptake pathways in Cytochrome c Oxidase is difficult due to the complexity of the system. The transition networks approach allows sampling of proton transfer pathways without predefined reaction coordinate. MethodsComputation of the proton transfer pathways in a model of the D-channel of cytochrome c oxidase has been performed by a transition network approach that combines discrete, optimisation based and molecular dynamics based sampling. ResultsThe optimal pathway involves an opening of the so-called asparagine gate, hydration of the asparagine region, the formation of a hydrogen-bonded chain, and finally concerted proton hole transport along this chain. The optimal pathway finds the protonation of residue H26 close to the channel entrance favourable for lowering the transition energies of subsequent steps, in particular, opening of the Asn gate and formation of a hydrogen-bonded chain. Residue Y33 plays an important role in shuttling the transferred proton hole. ConclusionsThe optimal pathway found by the transition network approach shows the same important characteristics as pathways determined earlier by other methods. The computed barrier and reaction energies are also in good agreement with previous studies. The transition network approach provides an alternative to explore pathways in complex systems. General significanceThe correct function of the enzyme as oxidase and proton pump depends on the interplay of several redox and proton transport steps. Understanding the proton transport mechanism is therefore key to understanding the protein's function. The complex nature of long- distances proton transfer through a protein requires a non-trivial simulation strategy.

M2 amphipathic helices facilitate pH-dependent conformational transition in influenza A virus

The matrix-2 (M2) protein from influenza A virus is a tetrameric, integral transmembrane (TM) protein that plays a vital role in viral replication by proton flux into the virus. The His37 tetrad is a pH sensor in the center of the M2 TM helix that activates the channel in response to the low endosomal pH. M2 consists of different regions that are believed to be involved in membrane targeting, packaging, nucleocapsid binding, and proton transport. Although M2 has been the target of many experimental and theoretical studies that have led to significant insights into its structure and function under differing conditions, the main mechanism of proton transport, its conformational dynamics, and the role of the amphipathic helices (AHs) on proton conductance remain elusive. To this end, we have applied explicit solvent constant pH molecular dynamics using the multisite λ-dynamics approach (CpHMDMSλD) to investigate the buried ionizable residues comprehensively and to elucidate their effect on the conformational transition. Our model recapitulates the pH-dependent conformational transition of M2 from closed to open state when the AH domain is included in the M2 construct, revealing the role of the amphipathic helices on this transition and shedding light on the proton-transport mechanism. This work demonstrates the importance of including the amphipathic helices in future experimental and theoretical studies of ion channels. Finally, our work shows that explicit solvent CpHMDMSλD provides a realistic pH-dependent model for membrane proteins.

Proton Transport in Mitochondrial UCP2 is Regulated by a Matrix-Oriented Salt-Bridge Network

Uncoupling proteins (UCPs) are ion carrier proteins causing regulated proton leak across the mitochondrial inner membrane, thus reducing the rate of ATP synthesis. Despite this important impact, the mechanism of UCP-mediated proton transport has not been fully understood. It has been speculated that the mitochondrial carrier family members contain two interconnected salt-bridge networks facilitating the substrate transport. One of these networks is close to the matrix side (matrix network) and the other is close to the intermembrane space. This study focuses on the role of matrix network on the regulation of proton transport in UCP2. UCP2 and eleven mutants (in matrix network) are expressed recombinantly in the E. coli inner membrane. Six mutants disrupt a single salt-bridge link by maintaining either a single negative (K to Q mutation) or positive (D to N mutation) charge on one end of the salt-bridge (K38Q, K141Q, K239Q, D35N, D138N and D236N). Two mutants (K38Q/K239Q and D138N/D236N) disrupt two salt-bridges, while the other two, lacking three salt-bridges, disrupt the total matrix network (K38Q/K141Q/K239Q and D35N/D138N/D236N). In the last mutant, the matrix network is inversed by switching the salt-bridge forming couples (D35K/K38D/D236K/K239D/D138K/K141D). Results show that the matrix network has a regulatory role, and perturbation of the network increases the rate of proton transport. It seems that individual salt-bridges within the network are cooperatively and equally responsible for regulation of transport. Various alterations of the constituting elements of matrix salt-bridge network do not have a considerable effect on inhibitor-protein interactions. Conformational analysis of the protein (CD and fluorescence) and MD simulations provided molecular insights into the role of UCP2 in proton transport. This study characterizes the physicochemical role of the matrix-oriented salt-bridge network on proton transport in UCP2 and its inhibition by purine nucleotides.

Vectorial Proton Transport Mechanism of RxR, a Phylogenetically Distinct and Thermally Stable Microbial Rhodopsin

Rubrobacter xylanophilus rhodopsin (RxR) is a phylogenetically distinct and thermally stable seven-transmembrane protein that functions as a light-driven proton (H+) pump with the chromophore retinal. To characterize its vectorial proton transport mechanism, mutational and theoretical investigations were performed for carboxylates in the transmembrane region of RxR and the sequential proton transport steps were revealed as follows: (i) a proton of the retinylidene Schiff base (Lys209) is transferred to the counterion Asp74 upon formation of the blue-shifted M-intermediate in collaboration with Asp205, and simultaneously, a respective proton is released from the proton releasing group (Glu187/Glu197) to the extracellular side, (ii) a proton of Asp85 is transferred to the Schiff base during M-decay, (iii) a proton is taken up from the intracellular side to Asp85 during decay of the red-shifted O-intermediate. This ion transport mechanism of RxR provides valuable information to understand other ion transporters since carboxylates are generally essential for their functions.

A Predictive Thermodynamic-Based Model for Proton Conductivity of Proton Exchange Membranes Based on Poly(Benzimidazole)/Poly(Acrylic Acid) Blend

To predict the novel poly(benzimidazole):poly(acrylic acid) (PBI:PAA) blend membranes proton conductivity, an ionic conductivity equation combined with thermodynamic model is proposed where different proton transport mechanisms including Grotthuss, vehicle and surface hopping mechanisms are considered in calculations. Based on the PAA titration behavior, by increasing the number-average molecular weight (), apparent acidity of PAA decreases. Hence, to calculate the concentration of protons involved in different mechanisms, a new modification in the predictive model is suggested by considering acidity and its relation with Effect of temperature, PAA molar ratio and on the membranes proton conductivity is investigated theoretically and compared with experimental data. The conductivity of membranes is increased by increasing the and molar ratio of PAA where the highest proton conductivity is attributed to the membrane with PAA of = 105 g · mol−1 and molar ratio of PBI:PAA = 1:4. The experimental proton conductivity and predicted results show a good agreement in comparison to the previous models based on Nernst–Einstein equation, indicating that acidic behavior, which is usually omitted in theoretical models, has an important effect on the total proton conductivity of PBI:Polyacid membranes, especially at higher temperatures.

Influence of the acid-base stoichiometry and residual water on the transport mechanism in a highly-Brønsted-acidic proton-conducting ionic liquid.

In this study, Brønsted-acidic proton conducting ionic liquids are considered as potential new electrolytes for polymer membrane fuel cells with operating temperatures above 100 °C. N-Methyltaurine and trifluoromethanesulfonic acid (TfOH) were mixed at various stoichiometric ratios in order to investigate the influence of an acid or base excess. The proton conductivity and self-diffusion of the “neat” and with 6 wt% water samples were investigated by following electrochemical and NMR methods. The composition change in the complete species and the relative proton transport mechanism based on the NMR results are discussed in detail. During fuel cell operation, the presence of significant amounts of residual water is unavoidable. In PEFC electrolytes, the predominating proton transfer process depends on the cooperative mechanism, when PILs are fixed on the polymer matrix within the membrane. Due to the comparable acidity of the cation [2-Sema]+ and the hydroxonium cation, with excess N-methyltaurine or H2O in the compositions, fast proton exchange reactions between the protonated [2-Sema]+ cation, N-methyltaurine and H2O can be envisaged. Thus, an increasing ratio of cooperative proton transport could be observed. Therefore, for polymer membrane fuel cells operating at elevated temperatures, the highly acidic PILs with excess bases are promising candidates for future use as electrolytes.

Transport and Morphology of a Proton Exchange Membrane Based on a Doubly Functionalized Perfluorosulfonic Imide Side Chain Perflourinated Polymer

There is a critical need for higher performing proton exchange membranes for electrochemical energy conversion devices that would enable higher temperature and drier operating conditions to be utilized. A novel approach is to utilize multiacid side chains in a perfluorinated polymer, maintaining the mechanical properties of the material, while dramatically increasing the ion-exchange capacity; however, as we show in this paper, the more complex side chain gives rise to unexpected physical phenomena in the material. We have thoroughly investigated a doubly functionalized perfluorosulfonic imide acid side chain perfluorinated polymer (PFIA), the simplest of many possible multiacid side chains currently being developed. The material is compared to its simpler perfluorosulfonic acid (PFSA) analogue via a battery of characterization and modeling investigations. The doubly functional side chain profoundly influences the properties of the PFIA polymer as it gives rise to both inter- and intraside chain interactions. These affect the nature of thermal decomposition of the material but, more importantly, force the backbone of the polymer into an unusually highly ordered more crystalline configuration. Under water saturated conditions, the PFIA has the same proton conductivity as the PFSA material, indicating that the additional proton does not contribute to the ionic conductivity, but the PFIA shows higher proton conductivity at lower RH conditions owing to dynamic changes in its local molecular environment. A transition is observed between 30 and 60 °C, indicating an order/disorder transition that is not present in the PFSA analogue. The mechanism of proton transport in the PFIA is due to more delocalized protons and more flexible side chains with better-dispersed, smaller water clusters forming the hydrophilic domains than in the PFSA analogue.

Pan-genome diversification and recombination in Cronobacter sakazakii, an opportunistic pathogen in neonates, and insights to its xerotolerant lifestyle

BackgroundCronobacter sakazakii is an emerging opportunistic bacterial pathogen known to cause neonatal and pediatric infections, including meningitis, necrotizing enterocolitis, and bacteremia. Multiple disease outbreaks of C. sakazakii have been documented in the past few decades, yet little is known of its genomic diversity, adaptation, and evolution. Here, we analyzed the pan-genome characteristics and phylogenetic relationships of 237 genomes of C. sakazakii and 48 genomes of related Cronobacter species isolated from diverse sources.ResultsThe C. sakazakii pan-genome contains 17,158 orthologous gene clusters, and approximately 19.5% of these constitute the core genome. Phylogenetic analyses reveal the presence of at least ten deep branching monophyletic lineages indicative of ancestral diversification. We detected enrichment of functions involved in proton transport and rotational mechanism in accessory genes exclusively found in human-derived strains. In environment-exclusive accessory genes, we detected enrichment for those involved in tryptophan biosynthesis and indole metabolism. However, we did not find significantly enriched gene functions for those genes exclusively found in food strains. The most frequently detected virulence genes are those that encode proteins associated with chemotaxis, enterobactin synthesis, ferrienterobactin transporter, type VI secretion system, galactose metabolism, and mannose metabolism. The genes fos which encodes resistance against fosfomycin, a broad-spectrum cell wall synthesis inhibitor, and mdf(A) which encodes a multidrug efflux transporter were found in nearly all genomes. We found that a total of 2991 genes in the pan-genome have had a history of recombination. Many of the most frequently recombined genes are associated with nutrient acquisition, metabolism and toxin production.ConclusionsOverall, our results indicate that the presence of a large accessory gene pool, ability to switch between ecological niches, a diverse suite of antibiotic resistance, virulence and niche-specific genes, and frequent recombination partly explain the remarkable adaptability of C. sakazakii within and outside the human host. These findings provide critical insights that can help define the development of effective disease surveillance and control strategies for Cronobacter-related diseases.

Bulk-like and Interfacial Water Dynamics in Nafion Fuel Cell Membranes Investigated with Ultrafast Nonlinear IR Spectroscopy.

The water confined in the hydrophilic domains of Nafion fuel cell membranes is central to its primary function of ion transport. Water dynamics are intimately linked to proton transfer and are sensitive to the structural features and length scales of confinement. Here, ultrafast polarization-selective pump-probe and two-dimensional infrared vibrational echo (2D IR) experiments were performed on fully hydrated Nafion membranes with sodium counterions to explicate the water dynamics. Like aerosol-OT reverse micelles (AOT RMs), the water dynamics in Nafion are attributed to bulk-like core water in the central region of the hydrophilic domains and much slower interfacial water. Population and orientational dynamics of water in Nafion are slowed by polymer confinement. Comparison of the observed dynamics to those of AOT RMs helps identify local interactions between water and sulfonate anions at the interface and among water molecules in the core. This comparison also demonstrates that the well-known spherical cluster morphology of Nafion is not appropriate. Spectral diffusion of the interfacial water, which arises from structural dynamics, was obtained from the 2D IR experiments taking the core water to have dynamics similar to bulk water. Like the orientational dynamics, spectral diffusion was found to be much slower at the interface compared to bulk water. Together, the dynamics indicate slow reorganization of weakly hydrogen-bonded water molecules at the interface of Nafion. These results provide insights into proton transport mechanisms in fuel cell membranes, and more generally, water dynamics near the interface of confining systems.

Novel proton exchange membranes based on sulfonated-phosphonated poly (p-phenylene-co-aryl ether ketone) terpolymers with microblock structures for passive direct methanol fuel cells

The development of fluorine-free proton exchange membranes (PEMs) to improve alcohol resistance and proton/methanol selectivity is a significant research direction for direct methanol fuel cells (DMFCs). In this work, a range of sulfonated-phosphonated poly (p-phenylene-co-aryl ether ketone) terpolymers with microblock structures are prepared by a Ni (0)-catalyzed coupling reaction at high yield. The experimental results reveal that the formation of P–O–P anhydride bridges by self-condensation in a hydrophilic domain can facilitate the connectivity of the proton conducting channels, through a mixed Grotthuss and Vehicle proton transport mechanism, and improve the alcohol resistance of terpolymers at the same time. The terpolymers have high conductivity (>0.01 S cm-1), which is superior to that of neat sulfonated or neat phosphonated aromatic polymers, and high dimensional stability at 80 °C and 40% relative humidity. A passive DMFC with a terpolymer membrane provides a high power density of 34.4 mW cm−2 at 25 °C, which is higher than that for l-SPP-co-PAEK 1.80 (27.1 mW cm-2) and Nafion®115 (29.3 mW cm-2). By controlling the ratio of phosphonic acid or sulfonic acid to regulate ion aggregation behavior and further construct connected (or cluster-free) proton channels, a new insight into the design PEMs with high proton/methanol selectivity is provided.

Effect of Surface-Sulfonated Silica on the Properties of Pyridine-Containing Polybenzimidazoles

Hybrid membranes based on pyridine-containing polybenzimidazole (PBI–Py) and silica containing sulfonic groups on the surface have been synthesized by two methods, namely, by casting a polymer solution with ready-prepared oxide particles and with a precursor for their synthesis. The effect of the synthesis method on the morphology, conductivity under various conditions, and gas permeability of the hybrid membranes has been studied. It has been shown that the membrane modification with silica leads to an increase in conductivity at a low silica content (2–5 wt %) and an increase in gas permeability. The highest proton conductivity (58 mS/cm at 160°C) has been found for the sample synthesized by casting a PBI–Py solution with a precursor that contains 2 wt % of silica modified with sulfonic groups. Possible mechanisms of proton transport in the synthesized systems have been discussed.

(Invited) Insights into Proton Transport in Superprotonic Solid Acids

Superprotonic solid acid electrolytes, materials with chemical and physical properties intermediate between conventional acids (e.g., H3PO4) and conventional salts (e.g., Cs3PO4) have emerged as attractive candidates for fuel cell and other electrochemical applications. Key characteristics of these materials are tetrahedral oxyanion groups linked by hydrogen bonds, and a polymorphic structural transition to a disordered state at moderate temperatures. Rapid oxyanion reorientation and dynamic disorder of the hydrogen bond network facilitate high proton conductivity in the high temperature phase. Materials exhibiting a superprotonic transition include CsHSO4, Cs3H(SeO4)2, CsH2PO4, and Cs2(HSO4)(H2PO4). Here we review the present state of understanding of proton transport mechanisms and the factors governing the transition behavior as gathered from macroscale measurements of conductivity and thermal properties and from atomistic level studies using nuclear magnetic resonance spectroscopy. We focus in particular on CsH2PO4, the technologically most significant superprotonic solid acid, and its chemical modifications produced by substitution on either the alkali metal or proton site (deuterated materials). Dramatic changes in phase behavior and proton conductivity can be induced by only minor changes in chemistry, suggesting routes for tuning behavior to achieve desired outcomes.

Ultrafast Proton Transport between a Hydroxy Acid and a Nitrogen Base along Solvent Bridges Governed by the Hydroxide/Methoxide Transfer Mechanism.

Aqueous proton transport plays a key role in acid–base neutralization and energy transport through biological membranes and hydrogen fuel cells. Extensive experimental and theoretical studies have resulted in a highly detailed elucidation of one of the underlying microscopic mechanisms for aqueous excess proton transport, known as the von Grotthuss mechanism, involving different hydrated proton configurations with associated high fluxional structural dynamics. Hydroxide transport, with approximately 2-fold-lower bulk diffusion rates compared to those of excess protons, has received much less attention. We present femtosecond UV/IR pump–probe experiments and ab initio molecular dynamics simulations of different proton transport pathways of bifunctional photoacid 7-hydroxyquinoline (7HQ) in water/methanol mixtures. For 7HQ solvent-dependent photoacidity, free-energy–reactivity correlation behavior and quantum mechanics/molecular mechanics (QM/MM) trajectories point to a dominant OH–/CH3O– transport pathway for all water/methanol mixing ratios investigated. Our joint ultrafast infrared spectroscopic and ab initio molecular dynamics study provides conclusive evidence for the hydrolysis/methanolysis acid–base neutralization pathway, as formulated by Manfred Eigen half a century ago. Our findings on the distinctly different acid–base reactivities for aromatic hydroxyl and aromatic nitrogen functionalities suggest the usefulness of further exploration of these free-energy–reactivity correlations as a function of solvent polarity. Ultimately the determination of solvent-dependent acidities will contribute to a better understanding of proton-transport mechanisms at weakly polar surfaces and near polar or ionic regions in transmembrane proton pump proteins or hydrogen fuel cell materials.