Ion-pair Dissociation Research Articles

Counterion Blockade in a Heterogeneously Charged Single-File Water Channel.

The Possion-Nernst-Planck theories fail to describe the ionic transport in Angstrom channels, where conduction deviates from Ohm's law, which is attributed to the dehydration/self-energy barrier and dissociation of Bjerrum ion pairs in previous work. Here, we find that the cations can be strongly bound to the surface charge, which blocks the ionic transport in a single-file water channel, causing nonlinear current-voltage curves. The presence of free ions significantly increases the probability of bound ions being released, resulting in an ionic current. We find that ionic conduction gradually becomes Ohmic as the surface charge density increases, but the conduction amplitude decreases due to the increased friction from the bound ions. We rationalize the ionic transport using 1D Kramers' escape theory framework, which describes nonlinear ionic current and the impact of surface charge density on the I-V curves. Our results show that the strong Coulomb interaction between the counterion and surface charge may cause ionic blockade in Angstrom channels.

Impact of interfacial curvature on molecular properties of aqueous interfaces.

The curvature of soft interfaces plays a crucial role in determining their mechanical and thermodynamic properties, both at macroscopic and microscopic scales. In the case of air/water interfaces, particular attention has recently focused on water microdroplets, due to their distinctive chemical reactivity. However, the specific impact of curvature on the molecular properties of interfacial water and interfacial reactivity has so far remained elusive. Here, we use molecular dynamics simulations to determine the effect of curvature on a broad range of structural, dynamical, and thermodynamical properties of the interface. For a droplet, a flat interface, and a cavity, we successively examine the structure of the hydrogen-bond network and its relation to vibrational spectroscopy, the dynamics of water translation, rotation, and hydrogen-bond exchanges, and the thermodynamics of ion solvation and ion-pair dissociation. Our simulations show that curvature predominantly impacts the hydrogen-bond structure through the fraction of dangling OH groups and the dynamics of interfacial water molecules. In contrast, curvature has a limited effect on solvation and ion-pair dissociation thermodynamics. For water microdroplets, this suggests that the curvature alone cannot fully account for the distinctive reactivity measured in these systems, which are of great importance for catalysis and atmospheric chemistry.

Ceria Quantum Dot Filler-Modified Polymer Electrolytes for Three-Dimensional-Printed Sodium Solid-State Batteries.

Solid polymer electrolytes have been considered as promising candidates for solid-state batteries (SSBs), owing to their excellent interfacial compatibility and high mechanical toughness; however, they suffer from intrinsic low ionic conductivity (lower than 10-6 S/cm) and large thickness (usually surpassed over 100 μm or even 500 μm), which has a negative influence on the interface resistance and ionic migration. In this work, ceria quantum dot (CQD)-modified composite polymer electrolyte (CPE) membranes with a thickness of 20 μm were successfully manufactured via 3D printing technology. The CQD fillers can reduce the crystallinity of the polymer, and the oxygen vacancies on CQDs can facilitate the dissociation of ion pairs in the NaTFSI salt to release more free Na+, improving the ionic conductivity. Meanwhile, tailoring the thickness of the CPE-CQDs membrane via 3D printing can further promote the migration and transport of Na+. Furthermore, the printed NNM//CPE-CQDs//Na SSB exhibited outstanding rate capability and cycling stability. The combination of CQD modification and thickness tailoring through 3D printing paves a new avenue for achieving high performance solid electrolyte membranes for practical application in Na SSBs.

Electric Field Effect of the Plasma-Initiated Polymerization of Methyl Methacrylate: A Negatively Charged Long-Lived Radical.

Plasma-initiated polymerization (PIP) is generally attributed to a radical process due to its inhibiting property. However, its unique polymerization behaviors like long-lived radical and solvent effect do not comply well with the traditional radical mechanism. Herein, the PIP of methyl methacrylate (MMA) was conducted in a high-voltage DC electric field to investigate the charged nature of its radicals. Consequently, the polymerization presented a preferential distribution of polymers at the anode but not the cathode, revealing the negatively charged nature of the growing radicals. An acceleration phenomenon, accompanied by the growth in molecular weights and the reduction in molecular weight distributions (Ð), was observed at the voltages above 16 kV, suggesting the dissociation of ion pairs of growing radicals. The PIP yielded PMMA with analogous chemical and steric structures to those of PMMA from traditional radical initiation, whether in the presence or absence of the external electric field. This work offers new insights into the PIP of vinyl monomers, wherein a one-electron transfer reaction is inferred to be involved in the monomer activation.

ION-PAIR CONVERSION THERMODYNAMICS IN ALCOHOL SOLUTIONS OF HYDROGEN HALIDES

The calculation of thermodynamic characteristics of dissociation of contact and solvent-separated ion pairs into ions, conversion of contact ion pairs into solvent-separated ion pairs of ionogens HCl, HBr and HI in n-alcohols from methyl alcohol to n-octyl alcohol by the method described earlier for the systems HCl – n-alcohol in the same solvents at the same temperatures was carried out. The following regularities were established in this work: a) positive values of the change in the Gibbs energy of dissociation (ΔdisG°) of contact and solvent-separated ion pairs increase in the case of increasing temperature and the number of carbon atoms in the n-alcohol molecule and decreasing radius of the halide ion, and their sign and magnitude are determined by the entropic component (–TΔdisS°). In this case, the values of ΔdisG° of contact ion pairs exceed the same values for ion pairs separated by solvent; b) the values of the Gibbs energy change (ΔconvG°) for the studied ionogens HCl, HBr, and HI are also positive, except for the values of ΔconvG° of ionogens in methanol and HBr solutions in ethanol. In these cases, the ΔdisG° values for solvent-separated ion pairs exceed the same values for contact ion pairs, and the ΔconvG° values are negative. With increasing temperature and radius of the halide ion, ΔconvG° become more negative, and vice versa with increasing hydrocarbon radical; c) the concentration of contact ion pairs increases in the methanol – n-octanol series for all ionogens, decreases slightly with increasing temperature and radius of the anion, and varies from ~30% (methanol) to ~95% (n-octanol). In methanol, solvent-separated ion pairs predominate; in alcohols from n-propyl to n-octyl, contact ion pairs predominate, i.e., deconversion of ion pairs occurs.

Multipolar Conjugated Polymer Framework Derived Ionic Sieves via Electronic Modulation for Long-Life All-Solid-State Li Batteries.

Here, we build a tunable multipolar conjugated polymer framework platform via pore wall chemistry to probe the role of electronic structure engineering in improving the Li+ conduction by theoretical studies. Guided by theoretical prediction, we develop a new cyano-vinylene-linked multipolar polymer framework namely CNF-COF, which can act as efficient ion sieves to modify solid polymer electrolytes to simultaneously tune Li+ migration and stable Li anodes for long-lifespan all-solid-state (ASS) Li metal batteries at high rate. The dual-decoration of cyano and fluorine groups in CNF-COF favorably regulates electronic structure via multipolar donor-acceptor electronic effects to afford proper energy band structure and abundant electron-rich sites for enhanced oxidative stability, facilitated ion-pair dissociation and suppressed anion movements. Thus, the CNF-COF incorporation into poly (ethylene oxide) (PEO) electrolytes not only renders fast selective Li+ transport but also facilitates the Li dendrite suppression. Specifically, the constructed PEO composite electrolyte with an ultra-low CNF-COF content of only 0.5 wt % is endowed with a wide electrochemical window, a high ionic conductivity of 0.634 mS cm-1 at 60 °C and a large Li+ transference number of 0.81-remarkably outperforming CNF-COF-free counterparts (0.183 mS cm-1 and 0.22). As such, the Li symmetric cell delivers stable Li plating/stripping over 1400 h at 0.1 mA cm-2. Impressively, by coupling with LiFePO4 (LFP) cathodes, the assembled ASS Li battery under 60 °C allows for stable cycling over 2000 cycles at 1 C and over 1000 cycles even at 2 C with a large capacity retention of ~75 %, surpassing most reported ASS Li batteries using PEO-based electrolytes.

Absolute partial and total ionization cross sections of carbon monoxide with electron collision from 350 eV to 8000 eV

The absolute partial and total cross sections for electron impact ionization of carbon monoxide are reported for electron energies from 350 eV to 8000 eV. The product ions (CO+, C+, O+, CO2+, C2+, and O2+) are measured by employing an ion imaging mass spectrometer and two ion-pair dissociation channels (C+ + O+ and C2+ + O+) are identified. The absolute cross sections for producing individual ions and their total, as well as for the ion-pair dissociation channels are obtained by normalizing the data of CO+ to that of Ar+ from CO–Ar mixture target with a fixed 1:1 ratio. The overall errors are evaluated by considering various kinds of uncertainties. A comprehensive comparison is made with the available data, which shows a good agreement with each other over the energy ranges that are overlapped. This work presents new cross-section data with electron energies above 1000 eV.

Probing Acid-Induced Compaction of Denatured Proteins by High-Pressure Electrospray Mass Spectrometry.

Further increase in the acidity in the most denaturing acidic solution is known to induce compaction of the fully unfolded protein into a compact molten globule. The phenomenon of "acid-induced folding of proteins" takes place at pH ∼1 in strong acid aqueous solutions with high electrical conductivity and surface tension, a condition that is difficult to handle using conventional electrospray ionization methods for mass spectrometry. Here, high-pressure electrospray ionization (HP-ESI) is used to produce well-resolved mass spectra for proteins in strong acids with pH as low as 1. The compaction of protein conformation is indicated by a large shift in the charge state from high charges to native-like low charges. The addition of salt to the protein in the most denaturing condition also reproduces the compaction effect, thereby supporting the role of anions in this phenomenon. Similar compaction of proteins is also observed in organic solvent/acid mixtures. The charge state of the compacted protein depends on the type of anions that formed ion pairs with a positive charge on the protein. The dissociation of ion pairs during the ionization process forms neutral acids that can be observed by HP-ESI using a soft ion introduction configuration.

A Water-Free Ionic Covalent Organic Polymer with High Electrorheological Activity and Temperature Stability

Ionic polymers are important electrorheological (ER) materials, whose ER effect originates from ion motion-induced interfacial polarization. However, the first-generation ionic polymer ER material based on traditional polyelectrolyte needs adsorbed water to make ion movement to activate the ER effect. This results in poor temperature and cycling stability. Although the second-generation ionic polymer ER material based on polyethylene oxide-salt complexes and the currently developed poly(ionic liquid) ER material do not require adsorption of water to activate the ER effect due to decreased ion-pair dissociation energy, low glass transition temperature still limits their temperature and cycling stability. Herein, we report an ionic polymer ER material based on ionic covalent organic polymer (iCOP) synthesized via the Menshutkin reaction and ion exchange, which exhibits not only a highly anhydrous ER effect due to lots of hydrophobic fluoric counterion-induced strong interfacial polarization but also excellent temperature stability and cycling stability due to a unique covalent organic framework structure. In particular, its leaking current density and power consumption are far lower than those of the ER system of traditional polyelectrolytes, polyethylene oxide-salt complexes, and poly(ionic liquid)s. These make iCOP possess large potential as a platform to develop next-generation ionic polymer ER material with high performance.

Ion Channel-Restructured Zwitterionic Covalent Organic Framework Solid Electrolyte for All-Solid-State Lithium Metal Batteries.

Organic solid electrolytes offer an effective route for safe and high energy-density all-solid-state Li metal batteries. However, it remains a challenge to devise a new strategy to promote the dissociation of strong ion pairs and the transport of ionic components in organic solid electrolytes. Herein, a zwitterionic covalent organic framework (Zwitt-COF) with well-defined chemical and pore structures is prepared as a solid electrolyte capable of accelerating the dissociation and transport of Li ions. The Zwitt-COF solid electrolyte exhibits a high room-temperature ionic conductivity of 1.65 × 10-4 S cm-1 with a wide electrochemical stability window. Besides, the Zwitt-COF solid electrolyte displays stable Li plating/stripping behavior via effective inhibition of the formation of Li dendrites and dead Li, leading to superior long-term cycle performance with retention of 99% discharge capacity and 98% Coulombic efficiency in an all-solid-state Li metal battery. Theoretical simulations reveal that the incorporation of zwitterionic groups into COF can facilitate the dissociation of strong ion pairs and reconstruct the AA-stacking configuration by dissociative adsorption of Li+ ions on Zwitt-COF producing linear hexagonal ion channels in the Zwitt-COF solid electrolyte. This strategy based on Zwitt-COF can provide an alternative way to construct various solid-state Li batteries. This article is protected by copyright. All rights reserved.

ION PAIR CONVERSION THERMODYNAMICS IN HYDROGEN BROMIDE ALCOHOL SOLUTIONS

The thermodynamic quantities of dissociation of contact and solvent-separated ion pairs into ions, conversion of contact ion pairs into solvent-separated ion pairs of HBr ionogen in n-alcohols from methyl to n-octyl have been calculated by the procedure we set forth earlier for the HCl – n-alcohol systems in the same solvents at 278.15–328.15 K. The following regularities were established in this work: a) positive values of ΔdisGº of contact and solvent-separated ion pairs increase with increasing temperature, the number of carbon atoms in the n-alcohol molecule, and decreasing radius of halide ion, and their sign and magnitude are determined by the entropic component (–TΔdis Sº). In this case, the values of ΔdisGº of contact ion pairs exceed the same values for solvent-separated ion pairs; b) ΔconvGº values for HCl and HBr are also positive, except for ΔconvGº values in methanol at 278.15–328.15 K and HBr solutions at the same temperatures in ethanol. For these cases, by contrast, ΔdisGº(RIP) > ΔdisGº(CIP) and ΔconvGº are negative. As the temperature and radius of the halide ion increase, ΔconvGº become more negative, and vice versa as the hydrocarbon radical increases; c) the concentration of contact ion pairs increases in the methanol-n-octanol series, decreases slightly with increasing temperature and anion radius, and changes within ~30 % (methanol) to 95 % (n-octanol) at 278.15 K. In methanol, solvent-separated ion pairs predominate; in ethanol, the concentration of both types of ion pairs is approximately the same; in other n-octanols, contact ion pairs predominate.

The impact of ionomeric active binder on Li-ion battery charge–discharge and rate capability, Part I: Electrolyte

Ionically functionalized polymers as active binders in porous electrodes of Li-ion batteries have been studied recently to improve the rate capability of thick electrodes. Li+ ion depletion and the accompanying rapid drop of electric potential act as limiting factors for thick cathodes in Li-ion batteries. We show that the incorporation of lithiated additives containing covalently bound anions, including both ionomers and nanoparticle salts, can enhance the nominal voltage and utilization of Li-ion cathodes. A mathematical model is proposed to describe the influence of the lithiated ionic additives in liquid electrolyte and is implemented. The simulation results elaborate the effect of varying ion-pair dissociability for the ionic additives. A steady state ion concentration gradient is achieved for low current density, but operation at high current density results in Li+ depletion in the electrolyte phase. The use of dissociable ionic additives can overcome this limitation and reduce the potential polarization across the electrolyte. Higher concentration of ionomer ligands can improve performance for poorly dissociable ionic additives. The model validation shows that the potential drop can be lessened when ion-pair dissociation is considered while ion mobility remains unchanged. The modeling technique can be extended to a whole battery cell.

Breakdown of dipole Born approximation and the role of Rydberg’s predissociation for the electron-induced ion-pair dissociation to oxygen in the presence of background gases

We study the electron-induced ion-pair dissociation to gas-phase oxygen molecules using a state-of-the-art velocity-map ion-imaging technique. The analysis is entirely based on the conical time-gated wedge-shaped velocity slice images of O-/O2 nascent anionic fragments, and the resulting observations are in favor of Van Brunt et al.'s report [R. J. Van Brunt and L. J. Kieffer, J. Chem. Phys. 60, 3057 (1974)]. A new image reconstruction method, Jacobian over parallel slicing, is introduced to overcome the drawback of ion exaggeration in determining the kinetic energy distribution from the time-gated parallel slicing technique, which offers an alternative approach to the wedge slicing method. Most importantly, the role of the quintet-heavy Rydberg state has been drawn out to the complex ion-pair formalism. The extracted kinetic energy and angular distributions from the wedge slice images reveal a high momentum transfer during the ion-pair dissociation process, which could be the finest rationale to observe the breakdown of dipole Born approximation driven by multipole moment associated with the incident electron beam. Three distinct dissociative momentum bands have been precisely identified for O- dissociation. However, radiationless Rydberg's predissociation continuum (≥15%) has become an inherent character of electron-induced ion-pair dissociation, which could be dealt with using the beyond Born-Oppenheimer treatment. The incoherent sum of Σ and Π symmetric-associated ion-pair final states has been precisely identified by modeling the angular distribution of O-/O2 for each of the kinetic energy bands. A negligibly small amount of forward-backward asymmetry is observed in the angular distribution of O-/O2, which might be explained by the dissociative state-specific quantum coherence mechanism as reported [Krishnakumar et al., Nat. Phys. 14, 149 (2018); Kumar et al., arXiv:2206.15024 (2022)] by Prabhudesai et al.

Molecular and biochemical characterization of a recombinant glycosyl hydrolase family 3 β-glucosidase overexpressed in Escherichia. coli; bioprospecting metagenomes for cellulolytic processing function

Enzyme activity on a synthetic substrate (p-nitrophenyl-β-D-glucopyranoside (pNPG)) ranked by specificity (kcat/Km), placed Bgl-3 (a recombinant P. carotovorum subsp. carotovorum-ß-glucosidase expressed in Escherichia coli) in a group of ß-glucosidases, whereas ranking according to turnover number (kcat) placed Bgl-3 at the top of the group, implying its potential for high activity in biomass processing. The role of Lys211, His212, Arg172, and Asp114, in substrate recognition and stabilization of a glucose unit in catalysis is proposed here due to interactions with the substrate C1-C6 hydroxyls through hydrogen bonding, as well as the role of two methionines, Met255, Met322, in hydrophobic stabilization. However, enzyme inactivation due to ion pair dissociation, at pH range >8 and <5, of the enzyme nucleophile and the acid-base, can influence enzyme-intermediate complex formation, suggesting the latter as rate limiting in Bgl3 catalysis. Asp290 and Glu517 in the substrate binding clefts, are the nucleophile and the catalytic acid/base suggested, respectively. The close proximity of His212 and His522 stabilizes the enzyme glycosyl-intermediate through hydrogen bonding.

Additive Engineering Enables Ionic-Liquid Electrolyte-Based Supercapacitors To Deliver Simultaneously High Energy and Power Density

Ionic liquid (IL) electrolytes with a high potential window are promising candidates to high-energy-density supercapacitors; however, they commonly suffer from serious kinetic barriers that lead to poor power density. In this work, we propose an additive engineering method to promote rapid dynamics of IL-based supercapacitors. Additive engineering is based on adding cetyltrimethylammonium bromide-grafted Ti3C2 MXene (Ti3C2-CTAB) into 1-ethyl-3-methylimidazolium tetrafluoroborate (EMIMBF4), a typical IL electrolyte for supercapacitors. Remarkably, IL electrolytes show a considerable increase by 38% in ionic conductivity and great reduction in solid–liquid surface energy from 18.03 to 12.37 mN m–1. We prove that electrostatic force and hydrogen bonds generated from the interaction between Ti3C2-CTAB and EMIMBF4 facilitate considerable dissociation of electrolyte ion pairs and ion-transfer capability. Consequently, additive engineering-designed IL-based supercapacitors deliver simultaneously a high energy density of 28.3 Wh kg–1 and power density of 18.3 kW kg–1. The increased high-power characteristics are supported by a faster ion diffusion coefficient (1.50 × 10–12 vs 4.04 × 10–13 cm2 s–1) and shorter relaxation time (3.83 vs 6.81 s). In addition, additive engineering guarantees a stable cycling life of 83.6% capacitance retention after 9000 cycles at the depth potential window from 0 to 3.0 V.

Li+ affinity ultra-thin solid polymer electrolyte for advanced all-solid-state lithium-ion battery

The chasing for all-solid-state lithium-ion batteries (ASSLIBs) is based on the need for safer and higher energy density batteries. In this regard, solid polymer electrolytes (SPEs) are well-renowned for their processability and electrochemical stability, yet slimmer and more flexible SPE with higher ionic conductivity is still desired. Herein, an ultrathin (35 µm), rigidity-enhancing co-blending SPEs design using electrospinning was proposed, blending bio-polyamide with a N-substituted pyrrolidone ring (IBD) with polyethylene oxide/lithium bis(trifluoromethanesulfonyl)imide (PEO/LiTFSI). IBD is confirmed to possess the dominant Li+ affinity. Then IBD with flexible chain segments of PEO, triggers a concerted ion-transport mechanism synergistically, in which IBD is responsible for the processes of enhanced ion-pair dissociation resulting in dynamic association between the mobile cations and the long-chain molecules that constitute the SPE. Meanwhile, it could broaden the ion transport channel in some extent. The ionic conductivity of the SPE is up to 4.26 × 10−4 S cm−1 at 50 ℃. Furthermore, the high strength modulus, low crystallinity, and ultra-thin characteristic of IBD-PEO/LiTFSI power-assisted the ASSLIBs of LiFePO4//Li to achieve extraordinary cycle performance (80.5 % capacity retention after 580 cycles at C/2 rate) and the average coulomb efficiency exceeds 99.5 % at 50 ℃. Moreover, it has the ability to withstand folding and bending conditions.

Ionic Conduction in Polymer-Based Solid Electrolytes.

Good safety, high interfacial compatibility, low cost, and facile processability make polymer-based solid electrolytes promising materials for next-generation batteries. Key issues related to polymer-based solid electrolytes, such as synthesis methods, ionic conductivity, and battery architecture, are investigated in past decades. However, mechanistic understanding of the ionic conduction is still lacking, which impedes the design and optimization of polymer-based solid electrolytes. In this review, the ionic conduction mechanisms and optimization strategies of polymer-based solid electrolytes, including solvent-free polymer electrolytes, composite polymer electrolytes, and quasi-solid/gel polymer electrolytes, are summarized and evaluated. Challenges and strategies for enhancing the ionic conductivity are elaborated, while the ion-pair dissociation, ion mobility, polymer relaxation, and interactions at polymer/filler interfaces are highlighted. This comprehensive review is especially pertinent for the targeted enhancement of the Li-ion conductivity of polymer-based solid electrolytes.

Photoionization and Photofragmentation Dynamics of I2 in Intense Laser Fields: A Velocity-Map Imaging Study.

The photoionization and photofragmentation dynamics of I2 in intense femtosecond near-infrared laser fields were studied using velocity-map imaging of cations, electrons, and anions. A series of photofragmentation pathways originating from different cationic electronic states were observed following single ionization, leading to I+ fragments with distinct kinetic energies, which could not be resolved in previous studies. Photoelectron spectra indicate that these high-lying dissociative states are primarily produced through nonresonant ionization from several molecular orbitals (MO) of the neutral. The photoelectron spectra also show clear signatures of resonant ionization pathways (Freeman resonances) to low-lying bound ionic states via Rydberg states of the neutral moiety. To investigate the role of these Rydberg states further, we imaged anionic products (I-) formed through ion-pair dissociations of neutral molecules excited to these Rydberg states by the intense femtosecond laser pulse. Collectively, these results shed significant new light on the complex dynamics of I2 molecules in intense laser fields and on the important role of neutral Rydberg states in a full description of strong-field phenomena in molecules.

Investigation on Tethered Anion Effects in Solid Polymer Electrolytes for Li-Ion Batteries

Solid polymer electrolytes (SPEs) have been proposed to reduce concentration polarizations within batteries by immobilizing anions to the polymer matrix, which can help stabilize the electrodeposition process and extend Li-ion battery lifetime. Unfortunately, single-ion conducting SPEs continue to suffer from poor conductivity due to coupling with the slow segmental mobility of polymer chains and poor ion pair dissociation. Our group systematically investigated how different tethered anions affect the Li-ion conduction and polymer segmental mobility. Copolymers were synthesized using a common neutral monomer, poly(ethylene glycol) methyl ether methacrylate (PEGMA), with a series of different anionic monomers: 4-styrene sulfonate, 4-styrene phosphonate, 4-styrene TFSI, and methacrylate TFSI. These monomers presented a series of anions of varying strength with the same styrene structure, sulfonate vs. phosphonate vs. TFSI, as well as exploring structural monomer changes for the same TFSI anion, styrene vs. methacrylate, that allows for systematic changing of polymer electrolyte properties. Each copolymer series was further expanded by changing the composition of neutral-to-anionic monomers, ranging from 10-67 mol% anionic. The increased ion exchange capacity of the phosphonate groups led to a high degree of ion aggregation that exhibited hard mechanical properties and very low Li-ion conductivities. Increasing the electron delocalization of the ions from sulfonate to TFSI greatly improved the conductivities and lowered activation energies of ion transport, with methacrylate being slightly better than styrene. Implications of these results will be discussed for next-generation polymer electrolyte design.

Electric fields control water-gated proton transfer in cytochrome c oxidase

Aerobic life is powered by membrane-bound enzymes that catalyze the transfer of electrons to oxygen and protons across a biological membrane. Cytochrome c oxidase (CcO) functions as a terminal electron acceptor in mitochondrial and bacterial respiratory chains, driving cellular respiration and transducing the free energy from O2 reduction into proton pumping. Here we show that CcO creates orientated electric fields around a nonpolar cavity next to the active site, establishing a molecular switch that directs the protons along distinct pathways. By combining large-scale quantum chemical density functional theory (DFT) calculations with hybrid quantum mechanics/molecular mechanics (QM/MM) simulations and atomistic molecular dynamics (MD) explorations, we find that reduction of the electron donor, heme a, leads to dissociation of an arginine (Arg438)-heme a3 D-propionate ion-pair. This ion-pair dissociation creates a strong electric field of up to 1 V Å-1 along a water-mediated proton array leading to a transient proton loading site (PLS) near the active site. Protonation of the PLS triggers the reduction of the active site, which in turn aligns the electric field vectors along a second, "chemical," proton pathway. We find a linear energy relationship of the proton transfer barrier with the electric field strength that explains the effectivity of the gating process. Our mechanism shows distinct similarities to principles also found in other energy-converting enzymes, suggesting that orientated electric fields generally control enzyme catalysis.