Proton-coupled Electron Transfer Mechanism Research Articles

Histidine-Gated Proton-Coupled Electron Transfer to the CuA Site of Nitrous Oxide Reductase.

Copper-containing nitrous oxide reductase (N2OR) is the only known enzyme to catalyze the conversion of the environmentally critical greenhouse gas nitrous oxide (N2O) to dinitrogen (N2) as the final step of bacterial denitrification. Other than its unique tetranuclear active site CuZ, the binuclear electron entry point CuA is also utilized in other enzymes, including cytochrome c oxidase. In the CuA site of Pseudomonas stutzeri N2OR, a histidine ligand was found to undergo a conformational flip upon binding of the substrate N2O between the two copper centers. Here we report on the systematic mutagenesis and spectroscopic and structural characterization of this histidine and surrounding H-bonding residues, based on an established functional expression system for PsN2OR in E. coli. A single hydrogen bond from Ser550 is sufficient to stabilize an unbound conformation of His583, as shown in a Asp576Ala variant, while the additional removal of the hydrogen bond in a Asp576Ala/Ser550Ala double variant compelled His583 to stay in a bound conformation as a ligand to CuA. Systematic mutagenesis of His583 to Ala, Asp, Asn, Glu, Gln, Lys, Phe, Tyr, and Trp showed that although both the CuZ and CuA sites were present in all the variants, only the ones with a protonable side chain, i.e., His, Asp, and Glu, were able to mediate electron transfer at physiological pH. This observation is in line with a proton-coupled electron transfer mechanism at the CuA site of N2OR.

Functional Water Wires Catalyze Long-Range Proton Pumping in the Mammalian Respiratory Complex I.

The respiratory complex I is a gigantic (1 MDa) redox-driven proton pump that reduces the ubiquinone pool and generates proton motive force to power ATP synthesis in mitochondria. Despite resolved molecular structures and biochemical characterization of the enzyme from multiple organisms, its long-range (∼300 Å) proton-coupled electron transfer (PCET) mechanism remains unsolved. We employ here microsecond molecular dynamics simulations to probe the dynamics of the mammalian complex I in combination with hybrid quantum/classical (QM/MM) free energy calculations to explore how proton pumping reactions are triggered within its 200 Å wide membrane domain. Our simulations predict extensive hydration dynamics of the antiporter-like subunits in complex I that enable lateral proton transfer reactions on a microsecond time scale. We further show how the coupling between conserved ion pairs and charged residues modulate the proton transfer dynamics, and how transmembrane helices and gating residues control the hydration process. Our findings suggest that the mammalian complex I pumps protons by tightly linked conformational and electrostatic coupling principles.

Antioxidant and Anticancer Properties of Functionalized Ferrocene with Hydroxycinnamate Derivatives-An Integrated Experimental and Theoretical Study.

Two ferrocenyl derivatives, Fc-CA and Fc-FA, were synthesized by a condensation reaction between the amino ferrocene and hydroxycinnamic acids, that is, caffeic acid (CA) and ferulic acid (FA). The structures and purity of all compounds were characterized by 1H- and 13C NMR spectroscopies, Mass spectrometry (MS), and elemental analysis. The antioxidant properties of Fc-CA and Fc-FA and of its ligand were studied for free radical scavenging activity toward DPPH•, superoxide anion (O2•-), NO•, and ABTS•+ by UV-vis and electron spin resonance spectroscopies. The cytotoxicity of Fc-CA and Fc-FA against MCF-7 and MDA-MB-231 breast cancer cells and MRC-5 human lung fibroblasts cell was higher than that of cisplatin. The geometry and electronic structures of all compounds were then simulated using density functional theory at M05-2X/6-311+G(d,p) level of theory. Thermodynamics of the free radical quenching reactions by common mechanisms reveal the higher antioxidant properties of the Fc-CA and Fc-FA in comparison to their ligands. An in-depth study of the free radical scavenging activity against HOO• and HO• radicals was performed for two of the most favorable and competitive mechanisms, the hydrogen transfer (either hydrogen atom transfer or proton-coupled electron transfer mechanisms) and the radical adduct formation. The in silico studies indicated that ferrocenyl derivatives exhibited prominent binding affinity to protein models in comparison to CA and FA. Their dock scores were notable at ligand binding sites of ERα, Erβ, and JAK2 proteins. Dock pose analysis also shed light into the possible mechanism of action for the studied compounds.

A Multiple Chirality Switching Device for Spatial Light Modulators.

A new and simple strategy towards electric-field-driven multiple chirality switching device has been designed and fabricated by combining a newly synthesized base-responsive chiroptical polymer switch (R-FLMA) and p-benzoquinone (p-BQ) via proton-coupled electron transfer (PCET) mechanism. Clear and stable triple chirality states (silence, positive, negative) of this device in visible band can be regulated reversibly (>1000 cycles) by adjusting voltage programs. Furthermore, such chiral switching phenomena are also accompanied by apparent changes of color and fluorescence. More importantly, the potential application of this device for a spatial light modulator has also been demonstrated.

Mechanistic Insight into Concerted Proton-Electron Transfer of a Ru(IV)-Oxo Complex: A Possible Oxidative Asynchronicity.

We have thoroughly investigated the oxidation of benzyl alcohol (BA) derivatives by a RuIV(O) complex (RuIV(O)) in the absence or presence of Brønsted acids in order to elucidate the proton-coupled electron-transfer (PCET) mechanisms in C-H oxidation on the basis of a kinetic analysis. Oxidation of BA derivatives by RuIV(O) without acids proceeded through concerted proton-electron transfer (CPET) with a large kinetic isotope effect (KIE). In contrast, the oxidation of 3,4,5-trimethoxy-BA ((MeO)3-BA) by RuIV(O) was accelerated by the addition of acids, in which the KIE value reached 1.1 with TFA (550 mM), indicating an alteration of the PCET mechanism from CPET to stepwise electron transfer (ET) followed by proton transfer (PT). Although the oxidized products of BA derivatives were confirmed to be the corresponding benzaldehydes in the range of acid concentrations (0-550 mM), a one-electron-reduction potential of RuIV(O) was positively shifted with increases in the concentrations of acids. The elevated reduction potential of RuIV(O) strongly influenced the PCET mechanisms in the oxidation of (MeO)3-BA, changing the mechanism from CPET to ET/PT, as evidenced by the driving-force dependence of logarithms of reaction rate constants in light of the Marcus theory of ET. In addition, dependence of activation parameters on acid concentrations suggested that an oxidative asynchronous CPET, which is not an admixture of the CPET and ET/PT mechanisms, is probably operative in the boundary region (0 mM < [TFA] < 50 mM) involving a one-proton-interacted RuIV(O)···H+ as a dominant reactive species.

Nonequilibrium Dynamics of Proton-Coupled Electron Transfer in Proton Wires: Concerted but Asynchronous Mechanisms.

The coupling between electrons and protons and the long-range transport of protons play important roles throughout biology. Biomimetic systems derived from benzimidazole-phenol (BIP) constructs have been designed to undergo proton-coupled electron transfer (PCET) upon electrochemical or photochemical oxidation. Moreover, these systems can transport protons along hydrogen-bonded networks or proton wires through multiproton PCET. Herein, the nonequilibrium dynamics of both single and double proton transfer in BIP molecules initiated by oxidation are investigated with first-principles molecular dynamics simulations. Although these processes are concerted in that no thermodynamically stable intermediate is observed, the simulations predict that they are predominantly asynchronous on the ultrafast time scale. For both systems, the first proton transfer typically occurs ∼100 fs after electron transfer. For the double proton transfer system, typically the second proton transfer occurs hundreds of femtoseconds after the initial proton transfer. A machine learning algorithm was used to identify the key molecular vibrational modes essential for proton transfer: a slow, in-plane bending mode that dominates the overall inner-sphere reorganization, the proton donor–acceptor motion that leads to vibrational coherence, and the faster donor–hydrogen stretching mode. The asynchronous double proton transfer mechanism can be understood in terms of a significant mode corresponding to the two anticorrelated proton donor–acceptor motions, typically decreasing only one donor–acceptor distance at a time. Although these PCET processes appear concerted on the time scale of typical electrochemical experiments, attaching these BIP constructs to photosensitizers may enable the detection of the asynchronicity of the electron and multiple proton transfers with ultrafast two-dimensional spectroscopy. Understanding the fundamental PCET mechanisms at this level will guide the design of PCET systems for catalysis and energy conversion processes.

An Intramolecular Hydrogen-Shift in a Peroxy Radical at Cryogenic Temperatures: The Reaction of 2-Hydroxyphenyl Radical with O2.

Peroxy radical hydrogen-shifts are pivotal elementary reaction steps in the oxidation of small hydrocarbons in autoignition and the lower atmosphere. Although these reactions are typically associated with a substantial barrier, we demonstrate that the [1,5]H-shift in the peroxy species derived from the 2-hydroxyphenyl radical 1 is so facile that it even proceeds rapidly in an argon matrix at 35 K through a proton-coupled electron transfer mechanism. Hydrogen-bound complexes of o-benzoquinone are identified as the main reaction products by infrared spectroscopy although their formation through O-O bond scission is hampered by a barrier of 11.9 kcal mol-1 at the ROCCSD(T)/cc-pVTZ/UB3LYP/6-311G(d,p) level of theory.

Proton coupled electron transfer mechanism for the design and construction of crystalline hybrid photochromic halometallates based on nonphotoactive polypyridine-derivative moieties

Photochromic materials have application in various areas such as anti-counterfeiting, display, switches, etc. Most of reported photochromic materials are supported by photoactive moieties such as diarylethenes, pyridinium, azo compounds. Compared with the broadly investigated photochromic materials driven by photoactive units, the generation of new photochromic materials based on nonphotoactive units is of paramount significance from the perspective of basic research and practical application. Based on the proton coupled electron transfer (PCET) mechanism, the introduction of conjugated triangular tripyridine-derivative unit TPT (2,4,6-tri(4-pyridyl)-1,3,5-triazine) and linear dipyridine-derivative unit BPYB (1,4-di(pyridine-4-yl)benzene) into the zincochloride system generates two hybrid halometallates with photochromic performance, [H2BPYB][ZnCl4]·H2O (1) and [H3TPT]2[Zn3Cl12]·4H2O (2). Both 1 and 2 feature isolated inorganic [ZnCl4]2− tetrahedra (as electron donors and H-bond acceptors) and protonated organic polypyridine derivatives units [H2BPYB]2+ and [H3TPT]3+ (as electron acceptors and H-bond donors). Experimental characterization, together with the structure-property relationship analysis, indicates that the resulting photocoloration and related photoresponsive properties of 1 and 2 is ascribable to the PCET mechanism.

Theoretical Study of Shallow Distance Dependence of Proton-Coupled Electron Transfer in Oligoproline Peptides.

Long-range electron transfer is coupled to proton transfer in a wide range of chemically and biologically important processes. Recently the proton-coupled electron transfer (PCET) rate constants for a series of biomimetic oligoproline peptides linking Ru(bpy)32+ to tyrosine were shown to exhibit a substantially shallower dependence on the number of proline spacers compared to the analogous electron transfer (ET) systems. The experiments implicated a concerted PCET mechanism involving intramolecular electron transfer from tyrosine to Ru(bpy)33+ and proton transfer from tyrosine to a hydrogen phosphate dianion. Herein these PCET systems, as well as the analogous ET systems, are studied with microsecond molecular dynamics, and the ET and PCET rate constants are calculated with the corresponding nonadiabatic theories. The molecular dynamics simulations illustrate that smaller ET donor-acceptor distances are sampled by the PCET systems than by the analogous ET systems. The shallower dependence of the PCET rate constant on the ET donor-acceptor distance is explained in terms of an additional positive, distance-dependent electrostatic term in the PCET driving force, which attenuates the rate constant at smaller distances. This electrostatic term depends on the change in the electrostatic interaction between the charges on each end of the bridge and can be modified by altering these charges. On the basis of these insights, this theory predicted a less shallow distance dependence of the PCET rate constant when imidazole rather than hydrogen phosphate serves as the proton acceptor, even though their pKa values are similar. This theoretical prediction was subsequently validated experimentally, illustrating that long-range electron transfer processes can be tuned by modifying the nature of the proton acceptor in concerted PCET processes. This level of control has broad implications for the design of more effective charge-transfer systems.

An electron density based analysis to establish the electronic adiabaticity of proton coupled electron transfer reactions.

Electrons and protons are the main actors in play in proton coupled electron transfer (PCET) reactions, which are fundamental in many biological (i.e., photosynthesis and enzymatic reactions) and electrochemical processes. The mechanism, energetics and kinetics of PCET reactions are strongly controlled by the coupling between the transferred electrons and protons. Concerted PCET reactions are classified according to the electronical adiabaticity degree of the process. To discriminate among different mechanisms, we propose a new analysis based on the use of electron density based indexes. We choose, as test case, the 3-Methylphenoxyl/phenol system in two different conformations to show how the proposed analysis is a suitable tool to discriminate between the different degree of adiabaticity of PCET processes. The very low computational cost of this procedure is extremely promising to analyze and provide evidences of PCET mechanisms ruling the reactivity of many biological and catalytic systems.

Proton-Coupled Electron Transfer from Tyrosine in the Interior of a de novo Protein: Mechanisms and Primary Proton Acceptor.

Proton-coupled electron transfer (PCET) from tyrosine produces a neutral tyrosyl radical (Y•) that is vital to many catalytic redox reactions. To better understand how the protein environment influences the PCET properties of tyrosine, we have studied the radical formation behavior of Y32 in the α3Y model protein. The previously solved α3Y solution NMR structure shows that Y32 is sequestered ∼7.7 ± 0.3 Å below the protein surface without any primary proton acceptors nearby. Here we present transient absorption kinetic data and molecular dynamics (MD) simulations to resolve the PCET mechanism associated with Y32 oxidation. Y32• was generated in a bimolecular reaction with [Ru(bpy)3]3+ formed by flash photolysis. At pH > 8, the rate constant of Y32• formation (kPCET) increases by one order of magnitude per pH unit, corresponding to a proton-first mechanism via tyrosinate (PTET). At lower pH < 7.5, the pH dependence is weak and shows a previously measured KIE ≈ 2.5, which best fits a concerted mechanism. kPCET is independent of phosphate buffer concentration at pH 6.5. This provides clear evidence that phosphate buffer is not the primary proton acceptor. MD simulations show that one to two water molecules can enter the hydrophobic cavity of α3Y and hydrogen bond to Y32, as well as the possibility of hydrogen-bonding interactions between Y32 and E13, through structural fluctuations that reorient surrounding side chains. Our results illustrate how protein conformational motions can influence the redox reactivity of a tyrosine residue and how PCET mechanisms can be tuned by changing the pH even when the PCET occurs within the interior of a protein.

Catalytic Hydroetherification of Unactivated Alkenes Enabled by Proton-Coupled Electron Transfer.

We report a catalytic, light-driven method for the intramolecular hydroetherification of unactivated alkenols to furnish cyclic ether products. These reactions occur under visible-light irradiation in the presence of an IrIII -based photoredox catalyst, a Brønsted base catalyst, and a hydrogen-atom transfer (HAT) co-catalyst. Reactive alkoxy radicals are proposed as key intermediates, generated by direct homolytic activation of alcohol O-H bonds through a proton-coupled electron-transfer mechanism. This method exhibits a broad substrate scope and high functional-group tolerance, and it accommodates a diverse range of alkene substitution patterns. Results demonstrating the extension of this catalytic system to carboetherification reactions are also presented.

Proton-coupled electron transfer as the mechanism of reaction between triplet state of kynurenic acid and tryptophan

Kynurenic acid (KNA−) is an endogenous UV-A chromophore of the human eye lens able to sensitize photodamages to eye lens proteins. In the present work, we studied in details the mechanism of reaction between KNA− triplet state (TKNA−) and tryptophan (Trp), the most effective quencher of TKNA− among aromatic and sulfur-containing amino acids, by nanosecond laser flash photolysis. Previous studies suggested electron transfer as the reaction mechanism without its direct spectroscopic confirmation. Time-resolved kinetics and spectral data of this work clearly evidence the formation of neutral tryptophanyl radicals immediately after the TKNA− quenching by Trp that indicates either hydrogen transfer or proton-coupled electron transfer (PCET) as the reaction mechanism. Low kinetic isotope effect for the quenching rate constant, kH2O/kD2O = 1.3, speaks in favor of PCET mechanism of the quenching reaction, which includes the diffusion-controlled electron transfer as the primary step and the proton transfer in the radical cage as the final step.

Proton-coupled electron transfer mechanisms of the copper centres of nitrous oxide reductase from Marinobacter hydrocarbonoclasticus – An electrochemical study

Reduction of N2O to N2 is catalysed by nitrous oxide reductase in the last step of the denitrification pathway. This multicopper enzyme has an electron transferring centre, CuA, and a tetranuclear copper-sulfide catalytic centre, “CuZ”, which exists as CuZ*(4Cu1S) or CuZ(4Cu2S). The redox behaviour of these metal centres in Marinobacter hydrocarbonoclasticus nitrous oxide reductase was investigated by potentiometry and for the first time by direct electrochemistry. The reduction potential of CuA and CuZ(4Cu2S) was estimated by potentiometry to be +275 ± 5 mV and +65 ± 5 mV vs SHE, respectively, at pH 7.6. A proton-coupled electron transfer mechanism governs CuZ(4Cu2S) reduction potential, due to the protonation/deprotonation of Lys397 with a pKox of 6.0 ± 0.1 and a pKred of 9.2 ± 0.1. The reduction potential of CuA, in enzyme samples with CuZ*(4Cu1S), is controlled by protonation of the coordinating histidine residues in a two-proton coupled electron transfer process. In the cyclic voltammograms, two redox pairs were identified corresponding to CuA and CuZ(4Cu2S), with no additional signals being detected that could be attributed to CuZ*(4Cu1S). However, an enhanced cathodic signal for the activated enzyme was observed under turnover conditions, which is explained by the binding of nitrous oxide to CuZ0(4Cu1S), an intermediate species in the catalytic cycle.

Electron transfer and spin dynamics of the radical-pair in the cryptochrome from Chlamydomonas reinhardtii by computational analysis

In an effort to elucidate the origin of avian magnetoreception, it was postulated that a radical-pair formed in a cryptochrome upon light activation provided the basis for the mechanism that enables an inclination compass sensitive to the geomagnetic field. Photoreduction in this case involves formation of a flavin adenine dinucleotide (FAD)-tryptophan (TRP) radical-pair, following electron transfer within a conserved TRP triad in the cryptochrome. Recently, an animal-like cryptochrome from Chlamydomonas reinhardtii (CraCRY) was analyzed, demonstrating the role of a fourth aromatic residue, which serves as a terminal electron donor in the photoreduction pathway, resulting in the creation of a more distal radical-pair and exhibiting fast electron transfer. In this work, we investigated the electron transfer in CraCRY with a combination of free energy molecular dynamics (MD) simulations, frozen density functional theory, and QM/MM MD simulations, supporting the suggestion of a proton coupled electron transfer mechanism. Spin dynamics simulations discerned details on the dependence of the singlet yield on the direction of the external magnetic field for the [FAD•- TYRH•+] and [FAD•- TYR•] radical-pairs in CraCRY, in comparison with the previously modeled [FAD•- TRPH•+] radical-pair.

Deprotonation promoted high oxygen evolution activity of plasma functionalized carbon cloth

This paper reports a simple and facile method to prepare surface functionalized commercial carbon cloth via a plasma treatment and its application in electrocatalytic oxygen evolution reaction (OER). It is observed that abundant oxygen containing functional groups formed on the surface of the treated carbon cloth. The functionalized carbon cloth (FCC) exhibits a remarkable of OER activity. Specifically, it can drive a current density of 10 mA cm−2 at a fairly low overpotential of 360 mV and a correspondent small Tafel slope of 66.7 mV dec−1. More unexpectedly, it was found that the OER activity of FCC follows the non-concerted proton-coupled electron transfer mechanism. A possible explanation was proposed that the surface carboxyl groups improve the proton-transfer kinetics, accelerating the deprotonation from OOH to OO* during the OER process.

Evidence for a Proton-Coupled Electron Transfer Mechanism in a Biomimetic System for Monoamine Oxidase B Catalysis.

Mechanistic studies with 5-ethyl-3-methyllumiflavinium (Fl+ ) perchlorate, a biomimetic model for flavoenzyme monoamine oxidase B (MAO-B) catalysis, and the tertiary, allyl amine 1-methyl-4-(1-methyl-1 H-pyrrol-2-yl)-1,2,3,6-tetrahydropyridine (MMTP) reveal that proton-coupled electron transfer (PCET) may be an important pathway for MAO catalysis. The first step involves a single-electron transfer (SET) leading to the free radicals Fl. and MMTP. , the latter produced by deprotonation of the initially formed and highly acidic MMTP.+ . Molecular oxygen (O2 ) is found to play a hitherto unrecognized role in the early steps of the oxidation. MMTP and several structurally similar tertiary amines are the only tertiary amines oxidized by MAO, and their structural/electronic properties provide the key to understanding this behavior. A general hypothesis about the role of SET in MAO catalysis, and the recognition that PCET occurs with appropriately substituted substrates is presented.

Visible Light-Induced Borylation of C-O, C-N, and C-X Bonds.

Boronic acids are centrally important functional motifs and synthetic precursors. Visible light-induced borylation may provide access to structurally diverse boronates, but a broadly efficient photocatalytic borylation method that can effect borylation of a wide range of substrates, including strong C-O bonds, remains elusive. Herein, we report a general, metal-free visible light-induced photocatalytic borylation platform that enables borylation of electron-rich derivatives of phenols and anilines, chloroarenes, as well as other haloarenes. The reaction exhibits excellent functional group tolerance, as demonstrated by the borylation of a range of structurally complex substrates. Remarkably, the reaction is catalyzed by phenothiazine, a simple organic photocatalyst with MW < 200 that mediates the previously unachievable visible light-induced single electron reduction of phenol derivatives with reduction potentials as negative as approximately - 3 V versus SCE by a proton-coupled electron transfer mechanism. Mechanistic studies point to the crucial role of the photocatalyst-base interaction.

QM/MM MD simulations reveal an asynchronous PCET mechanism for nitrite reduction by copper nitrite reductase

Nitrite reductases are enzymes that aid in the denitrification process by catalyzing the reduction of nitrite to nitric oxide gas. Since this reaction is the first committed step that involves gas formation, it is regarded to be a vital step for denitrification. However, the mechanism of copper-containing nitrite reductase is still under debate due to the discrepancy between the theoretical and experimental data, especially in terms of the roles of secondary shell residues Asp98 and His255 and the electron transfer mechanism between the two copper sites. Herein, we revisited the nitrite reduction mechanism of A. faecalis copper nitrite reductase using QM(B3LYP)/MM-based metadynamics. It is found that the intramolecular electron transfer from T1-Cu to T2-Cu occurs via an asynchronous proton-coupled electron transfer (PCET) mechanism, with electron transfer (ET) preceding proton transfer (PT). In particular, we found that the ET process is driven by the conformation conversion of Asp98 from the gatekeeper to the proximal one, which is much more energy-demanding than the PCET itself. These results highlight that the inclusion of an electron donor is vital to investigate electron-transfer related processes such as PCET.

Electrochemical proton-coupled electron transfer of an anthracene-based azo dye

Herein, we report the thermodynamics, kinetics, and mechanism for electrochemical proton-coupled electron transfer involving the anthracene-based azo dye azo-OMe. The peak reduction potential of azo-OMe with organic acids spanning the pKa range of 2.6–23.51 shows a dependence upon the pKa of the acid when the acid pKa falls between 8 and 20 (in acetonitrile). A potential-pKa diagram is constructed and used to estimate the pKa of the azo-OMe species. Heterogeneous electron-transfer rate constants are obtained using rotating disk electrode voltammetry in combination with Koutecký–Levich and Tafel analysis. Electrochemical analysis shows that the reactions are diffusion limited and are kinetically controlled by the electron-transfer step. Kinetic isotope studies indicate a concerted proton, electron transfer event occurs in the pKa-dependent range when using trifluoroacetic acid.