Spanning four mechanistic regions of intramolecular proton-coupled electron transfer in a Ru(bpy)3(2+)-tyrosine complex

Proton Domino Reactions at an Imidazole Relay Control the Oxidation of a TyrZ-His190 Artificial Mimic of Photosystem II

A close mimic of P680 and the TyrosineZ-Histidine190 pair in photosystem II (PS II) has been synthesized using a ruthenium chromophore and imidazole-phenol ligands. The intramolecular oxidation of the ligands by the photoproduced Ru(III) species is characterized by a small driving force, very similar to PS II where the complexity of kinetics was attributed to the reversibility of electron transfer steps. Laser flash photolysis revealed biphasic kinetics for ligand oxidation. The fast phase (τ<50 ns) corresponds to partial oxidation of the imidazole-phenol ligand, proton transfer within the hydrogen bond, and formation of a neutral phenoxyl radical. The slow phase (5-9 μs) corresponds to full oxidation of the ligand which is kinetically controlled by deprotonation of the distant 1-nitrogen of the imidazolium. These results show that imidazole with its two protonatable sites plays a special role as a proton relay in a 'proton domino' reaction.

Zinc-indium-sulfide favors efficient C - H bond activation by concerted proton-coupled electron transfer

C - H bond activation is a ubiquitous reaction that remains a major challenge in chemistry. Although semiconductor-based photocatalysis is promising, the C - H bond activation mechanism remains elusive. Herein, we report value-added coupling products from a wide variety of biomass and fossil-derived reagents, formed via C - H bond activation over zinc-indium-sulfides (Zn-In-S). Contrary to the commonly accepted stepwise electron-proton transfer pathway (PE-ET) for semiconductors, our experimental and theoretical studies evidence a concerted proton-coupled electron transfer (CPET) pathway. A pioneering microkinetic study, considering the relevant elementary steps of the surface chemistry, reveals a faster C - H activation with Zn-In-S because of circumventing formation of a charged radical, as it happens in PE-ET where it retards the catalysis due to strong site adsorption. For CPET over Zn-In-S, H abstraction, forming a neutral radical, is rate-limiting, but having lower energy barriers than that of PE-ET. The rate expressions derived from the microkinetics provide guidelines to rationally design semiconductor catalysis, e.g., for C - H activation, that is based on the CPET mechanism.

Switching the proton-coupled electron transfer mechanism for non-canonical tyrosine residues in a de novo protein

The proton-coupled electron transfer (PCET) reactions of tyrosine (Y) are instrumental to many redox reactions in nature. This study investigates how the local environment and the thermodynamic properties of Y influence its PCET characteristics. Herein, 2- and 4-mercaptophenol (MP) are placed in the well-folded α3C protein (forming 2MP-α3C and 4MP-α3C) and oxidized by external light-generated [Ru(L)3]3+ complexes. The resulting neutral radicals are long-lived (>100 s) with distinct optical and EPR spectra. Calculated spin-density distributions are similar to canonical Y˙ and display very little spin on the S-S bridge that ligates the MPs to C32 inside the protein. With 2MP-α3C and 4MP-α3C we probe how proton transfer (PT) affects the PCET rate constants and mechanisms by varying the degree of solvent exposure or the potential to form an internal hydrogen bond. Solution NMR ensemble structures confirmed our intended design by displaying a major difference in the phenol OH solvent accessible surface area (≤∼2% for 2MP and 30-40% for 4MP). Additionally, 2MP-C32 is within hydrogen bonding distance to a nearby glutamate (average O-O distance is 3.2 ± 0.5 Å), which is suggested also by quantum mechanical/molecular mechanical (QM/MM) molecular dynamics simulations. Neither increased exposure of the phenol OH to solvent (buffered water), nor the internal hydrogen bond, was found to significantly affect the PCET rates. However, the lower phenol pKa values associated with the MP-α3C proteins compared to α3Y provided a sufficient change in PT driving force to alter the PCET mechanism. The PCET mechanism for 2MP-α3C and 4MP-α3C with moderately strong oxidants was predominantly step-wise PTET for pH values, but changed to concerted PCET at neutral pH values and below when a stronger oxidant was used, as found previously for α3Y. This shows how the balance of ET and PT driving forces is critical for controlling PCET mechanisms. The presented results improve our general understanding of amino-acid based PCET in enzymes.

Neutral Phenolic Contaminants Are Not Necessarily More Resistant to Permanganate Oxidation Than Their Dissociated Counterparts: Importance of Proton-Coupled Electron Transfer

Despite decades of research on phenols oxidation by permanganate, there are still considerable uncertainties regarding the mechanisms accounting for the unexpected parabolic pH-dependent oxidation rate. Herein, the pH effect on phenols oxidation was reinvestigated experimentally and theoretically by highlighting the previously unappreciated proton transfer. The results revealed that the oxidation of protonated phenols occurred via proton-coupled electron transfer (PCET) pathways, which can switch from ETPT (electron transfer followed by proton transfer) to CEPT (concerted electron-proton transfer) or PTET (proton transfer followed by electron transfer) with an increase in pH. A PCET-based model was thus established, and it could fit the kinetic data of phenols oxidation by permanganate well. In contrast with what was previously thought, both the simulating results and the density functional theory calculation indicated the rate of CEPT reaction of protonated phenols with OH- as the proton acceptor was much higher than that of deprotonated phenols, which could account for the pH-rate profiles for phenols oxidation. Analysis of the quantitative structure-activity relationships among the modeled rate constants, Hammett constants, and pKa values of phenols further supports the idea that the oxidation of protonated phenols is dominated by PCET. This study improves our understanding of permanganate oxidation and suggests a new pattern of reactivity that may be applicable to other systems.

Testing the Limits of Imbalanced CPET Reactivity: Mechanistic Crossover in H-Atom Abstraction by Co(III)-Oxo Complexes

Transition metal-oxo complexes are key intermediates in a variety of oxidative transformations, notably C-H bond activation. The relative rate of C-H bond activation mediated by transition metal-oxo complexes is typically predicated on substrate bond dissociation free energy in cases with a concerted proton-electron transfer (CPET). However, recent work has demonstrated that alternative stepwise thermodynamic contributions such as acidity/basicity or redox potentials of the substrate/metal-oxo may dominate in some cases. In this context, we have found basicity-governed concerted activation of C-H bonds with the terminal CoIII-oxo complex PhB(tBuIm)3CoIIIO. We have been interested in testing the limits of such basicity-dependent reactivity and have synthesized an analogous, more basic complex, PhB(AdIm)3CoIIIO, and studied its reactivity with H-atom donors. This complex displays a higher degree of imbalanced CPET reactivity than PhB(tBuIm)3CoIIIO with C-H substrates, and O-H activation of phenol substrates displays mechanistic crossover to stepwise proton transfer-electron transfer (PTET) reactivity. Analysis of the thermodynamics of proton transfer (PT) and electron transfer (ET) reveals a distinct thermodynamic crossing point between concerted and stepwise reactivity. Furthermore, the relative rates of stepwise and concerted reactivity suggest that maximally imbalanced systems provide the fastest CPET rates up to the point of mechanistic crossover, which results in slower product formation.

Understanding the Key Roles of pH Buffer in Accelerating Lignin Degradation by Lignin Peroxidase

pH buffer plays versatile roles in both biology and chemistry. In this study, we unravel the critical role of pH buffer in accelerating degradation of the lignin substrate in lignin peroxidase (LiP) using QM/MM MD simulations and the nonadiabatic electron transfer (ET) and proton-coupled electron transfer (PCET) theories. As a key enzyme involved in lignin degradation, LiP accomplishes the oxidation of lignin via two consecutive ET reactions and the subsequent C-C cleavage of the lignin cation radical. The first one involves ET from Trp171 to the active species of Compound I, while the second one involves ET from the lignin substrate to the Trp171 radical. Differing from the common view that pH = 3 may enhance the oxidizing power of Cpd I via protonation of the protein environment, our study shows that the intrinsic electric fields have minor effects on the first ET step. Instead, our study shows that the pH buffer of tartaric acid plays key roles during the second ET step. Our study shows that the pH buffer of tartaric acid can form a strong H-bond with Glu250, which can prevent the proton transfer from the Trp171-H^•+ cation radical to Glu250, thereby stabilizing the Trp171-H^•+ cation radical for the lignin oxidation. In addition, the pH buffer of tartaric acid can enhance the oxidizing power of the Trp171-H^•+ cation radical via both the protonation of the proximal Asp264 and the second-sphere H-bond with Glu250. Such synergistic effects of pH buffer facilitate the thermodynamics of the second ET step and reduce the overall barrier of lignin degradation by ∼4.3 kcal/mol, which corresponds to a rate acceleration of 10³-fold that agrees with experiments. These findings not only expand our understanding on pH-dependent redox reactions in both biology and chemistry but also provide valuable insights into tryptophan-mediated biological ET reactions.

Photoelectrochemical water oxidation improved by pyridine N-oxide as a mimic of tyrosine-Z in photosystem II

Artificial photosynthesis provides a way to store solar energy in chemical bonds with water oxidation as a major challenge for creating highly efficient and robust photoanodes that mimic photosystem II. We report here an easily available pyridine N-oxide (PNO) derivative as an efficient electron transfer relay between an organic light absorber and molecular water oxidation catalyst on a nanoparticle TiO2 photoanode. Spectroscopic and kinetic studies revealed that the PNO/PNO+˙ couple closely mimics the redox behavior of the tyrosine/tyrosyl radical pair in PSII in improving light-driven charge separation via multi-step electron transfer. The integrated photoanode exhibited a 1 sun current density of 3 mA cm-2 in the presence of Na2SO3 and a highly stable photocurrent density of >0.5 mA cm-2 at 0.4 V vs. NHE over a period of 1 h for water oxidation at pH 7. The performance shown here is superior to those of previously reported organic dye-based photoanodes in terms of photocurrent and stability.

A Semiconductor-Mediator-Catalyst Artificial Photosynthetic System for Photoelectrochemical Water Oxidation

In fabricating an artificial photosynthesis (AP) electrode for water oxidation, we have devised a semiconductor-mediator-catalyst structure that mimics photosystem II (PSII). It is based on a surface layer of vertically grown nanorods of Fe₂ O₃ on fluorine doped tin oxide (FTO) electrodes with a carbazole mediator base and a Ru(II) carbene complex on a nanolayer of TiO₂ as a water oxidation co-catalyst. The resulting hybrid assembly, FTO|Fe₂ O₃ |-carbazole|TiO₂ |-Ru(carbene), demonstrates an enhanced photoelectrochemical (PEC) water oxidation performance compared to an electrode without the added carbaozle base with an increase in photocurrent density of 2.2-fold at 0.95 V vs. NHE and a negatively shifted onset potential of 500 mV. The enhanced PEC performance is attributable to carbazole mediator accelerated interfacial hole transfer from Fe₂ O₃ to the Ru(II) carbene co-catalyst, with an improved effective surface area for the water oxidation reaction and reduced charge transfer resistance.

Strategies for switching the mechanism of proton-coupled electron transfer reactions illustrated by mechanistic zone diagrams

The mechanism by which proton-coupled electron transfer (PCET) occurs is of fundamental importance and has great consequences for applications, e.g. in catalysis. However, determination and tuning of the PCET mechanism is often non-trivial. Here, we apply mechanistic zone diagrams to illustrate the competition between concerted and stepwise PCET-mechanisms in the oxidation of 4-methoxyphenol by Ru(bpy)₃³⁺-derivatives in the presence of substituted pyridine bases. These diagrams show the dominating mechanism as a function of driving force for electron and proton transfer (ΔG⁰_ET and ΔG⁰_PT) respectively [Tyburski et al., J. Am. Chem. Soc., 2021, 143, 560]. Within this framework, we demonstrate strategies for mechanistic tuning, namely balancing of ΔG⁰_ET and ΔG⁰_PT, steric hindrance of the proton-transfer coordinate, and isotope substitution. Sterically hindered pyridine bases gave larger reorganization energy for concerted PCET, resulting in a shift towards a step-wise electron first-mechanism in the zone diagrams. For cases when sufficiently strong oxidants are used, substitution of protons for deuterons leads to a switch from concerted electron–proton transfer (CEPT) to an electron transfer limited (ETPT_lim) mechanism. We thereby, for the first time, provide direct experimental evidence, that the vibronic coupling strength affects the switching point between CEPT and ETPT_lim, i.e. at what driving force one or the other mechanism starts dominating. Implications for solar fuel catalysis are discussed.

The mechanism by which proton-coupled electron transfer (PCET) occurs is of fundamental importance and has great consequences for applications, e.g. in catalysis.

Water-Assisted Concerted Proton-Electron Transfer at Co(II)-Aquo Sites in Polyoxotungstates With Photogenerated RuIII (bpy)33+ Oxidant

The cobalt substituted polyoxotungstate [Co6 (H2 O)2 (α-B-PW9 O34 )2 (PW6 O26 )]17- (Co6) displays fast electron transfer (ET) kinetics to photogenerated RuIII (bpy)33+ , 4 to 5 orders of magnitude faster than the corresponding ET observed for cobalt oxide nanoparticles. Mechanistic evidence has been acquired indicating that: (i) the one-electron oxidation of Co6 involves Co(II) aquo or Co(II) hydroxo groups (abbreviated as Co6(II)-OH2 and Co6(II)-OH, respectively, whose speciation in aqueous solution is associated to a pKa of 7.6), and generates a Co(III)-OH moiety (Co6(III)-OH), as proven by transient absorption spectroscopy; (ii) at pH>pKa , the Co6(II)-OH→RuIII (bpy)33+ ET occurs via bimolecular kinetics, with a rate constant k close to the diffusion limit and dependent on the ionic strength of the medium, consistent with reaction between charged species; (iii) at pH Collapse

Key Words

• Co-aquo moiety
• cobalt polyoxometalate
• flash photolysis
• hydrogen bonding
• proton coupled electron transfer

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MESH Headings

• Cobalt/chemistry
• Coordination Complexes/chemical synthesis
• Coordination Complexes/chemistry
• Electrons
• Kinetics
• Light
• Organometallic Compounds/chemistry
• Organometallic Compounds/radiation effects
• Oxidants/chemistry
• Oxidants/radiation effects
• Oxidation-Reduction
• Polymers/chemical synthesis
• Polymers/chemistry
• Protons
• Ruthenium/chemistry
• Ruthenium/radiation effects
• Tungsten Compounds/chemical synthesis
• Tungsten Compounds/chemistry
• Water/chemistry

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Grants

• Fondazione Cariparo
• 10BIRD2018-UNIPD Department of Chemical Sciences at University of Padova
• 2017PBXPN4 Department of Chemical Sciences at University of Padova
• FAR 2020 University of Ferrara
• Fondazione Cariparo
• University of Ferrara

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Affiliation(s)

Francesco Rigodanza Department of Chemical SciencesUniversity of Padovavia Marzolo 135131PadovaItaly Consiglio Nazionale delle Ricerche (C.N.R.)Institute on Membrane Technology section of Padovavia Marzolo 135131PadovaItaly
Nadia Marino Department of Chemistry and Chemical TechnologiesUniversity of Calabria87036Arcavacata di Rende (CS)Italy
Alessandro Bonetto Dept. Environmental Sciences, Informatics and StatisticsUniversity Ca' Foscari Venice VegaparkVia delle Industrie 21/830175Marghera, VeniceItaly
Antonio Marcomini Dept. Environmental Sciences, Informatics and StatisticsUniversity Ca' Foscari Venice VegaparkVia delle Industrie 21/830175Marghera, VeniceItaly
Marcella Bonchio Department of Chemical SciencesUniversity of Padovavia Marzolo 135131PadovaItaly Consiglio Nazionale delle Ricerche (C.N.R.)Institute on Membrane Technology section of Padovavia Marzolo 135131PadovaItaly
Mirco Natali Department of Chemical, Pharmaceutical and Agricultural Sciences (DOCPAS)University of Ferrara, and Centro Interuniversitario per la Conversione Chimica dell'Energia Solare (SOLARCHEM) sez. di Ferraravia L. Borsari 4644121FerraraItaly
Andrea Sartorel Department of Chemical SciencesUniversity of Padovavia Marzolo 135131PadovaItaly

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Photoinduced Electron vs. Concerted Proton Electron Transfer Pathways in SnIV (l-Tryptophanato)2 Porphyrin Conjugates

Aromatic amino acids such as l‐tyrosine and l‐tryptophan are deployed in natural systems to mediate electron transfer (ET) reactions. While tyrosine oxidation is always coupled to deprotonation (proton‐coupled electron‐transfer, PCET), both ET‐only and PCET pathways can occur in the case of the tryptophan residue. In the present work, two novel conjugates 1 and 2, based on a Sn^IV tetraphenylporphyrin and Sn^IV octaethylporphyrin, respectively, as the chromophore/electron acceptor and l‐tryptophan as electron/proton donor, have been prepared and thoroughly characterized by a combination of different techniques including single crystal X‐ray analysis. The photophysical investigation of 1 and 2 in CH₂Cl₂ in the presence of pyrrolidine as a base shows that different quenching mechanisms are operating upon visible‐light excitation of the porphyrin component, namely photoinduced electron transfer and concerted proton electron transfer (CPET), depending on the chromophore identity and spin multiplicity of the excited state. The results are compared with those previously described for metal‐mediated analogues featuring Sn^IV porphyrin chromophores and l‐tyrosine as the redox active amino acid and well illustrate the peculiar role of l‐tryptophan with respect to PCET.

Proton-Coupled Electron Transfer Guidelines, Fair and Square

Proton-coupled electron transfer (PCET) reactions are fundamental to energy transformation reactions in natural and artificial systems and are increasingly recognized in areas such as catalysis and synthetic chemistry. The interdependence of proton and electron transfer brings a mechanistic richness of reactivity, including various sequential and concerted mechanisms. Delineating between different PCET mechanisms and understanding why a particular mechanism dominates are crucial for the design and optimization of reactions that use PCET. This Perspective provides practical guidelines for how to discern between sequential and concerted mechanisms based on interpretations of thermodynamic data with temperature-, pressure-, and isotope-dependent kinetics. We present new PCET-zone diagrams that show how a mechanism can switch or even be eliminated by varying the thermodynamic (ΔGPT° and ΔGET°) and coupling strengths for a PCET system. We discuss the appropriateness of asynchronous concerted PCET to rationalize observations in organic reactions, and the distinction between hydrogen atom transfer and other concerted PCET reactions. Contemporary issues and future prospects in PCET research are discussed.

Gated Proton Release during Radical Transfer at the Subunit Interface of Ribonucleotide Reductase

The class Ia ribonucleotide reductase of Escherichia coli requires strict regulation of long-range radical transfer between two subunits, α and β, through a series of redox-active amino acids (Y₁₂₂•[β] ↔ W₄₈?[β] ↔ Y₃₅₆[β] ↔ Y₇₃₁[α] ↔ Y₇₃₀[α] ↔ C₄₃₉[α]). Nowhere is this more precarious than at the subunit interface. Here, we show that the oxidation of Y₃₅₆ is regulated by proton release involving a specific residue, E₅₂[β], which is part of a water channel at the subunit interface for rapid proton transfer to the bulk solvent. An E₅₂Q variant is incapable of Y₃₅₆ oxidation via the native radical transfer pathway or non-native photochemical oxidation, following photosensitization by covalent attachment of a photo-oxidant at position 355[β]. Substitution of Y₃₅₆ for various F_nY analogues in an E₅₂Q-photoβ₂, where the side chain remains deprotonated, recovered photochemical enzymatic turnover. Transient absorption and emission data support the conclusion that Y₃₅₆ oxidation requires E₅₂ for proton management, suggesting its essential role in gating radical transport across the protein-protein interface.

Proton-Coupled Electron Transfer for Photoinduced Generation of Two-Electron Reduced Species of Quinone

Purpose-built molecules that follow the fundamental process of photosynthesis have significance in developing better insight into the natural photosynthesis process. Quinones have a significant role as electron acceptors in natural photosynthesis, and their reduction is assisted through H-bond donation or protonation. The major challenge in such studies is to couple the multielectron and proton-transfer process and to achieve a reasonably stable charge-separated state for the elucidation of the mechanistic pathway. We have tried to address this issue through the design of a donor-acceptor-donor molecular triad (2RuAQ) derived from two equivalent [Ru(bpy)₃]²⁺ derivatives and a bridging anthraquinone moiety (AQ). Photoinduced proton-coupled electron transfer (PCET) for this molecular triad was systematically investigated in the absence and presence of hexafluoroisopropanol and p-toluenesulfonic acid (PTSA) using time-resolved absorption spectroscopy in the ultrafast time domain. Results reveal the generation of a relatively long-lived charge-separated state in this multi-electron transfer reaction, and we could confirm the generation of AQ^2- and Ru^III as the transient intermediates. We could rationalize the mechanistic pathway and the dynamics associated with photoinduced processes and the role of H-bonding in stabilizing charge-separated states. Transient absorption spectroscopic studies reveal that the rates of intramolecular electron transfer and the mechanistic pathways associated with the PCET process are significantly different in different solvent compositions having different polarities. In acetonitrile, a concerted PCET mechanism prevails, whereas the stepwise PCET reaction process is observed in the presence of PTSA. The results of the present study represent a unique model for the mechanistic diversity of PCET reactions.

Shallow Distance Dependence for Proton-Coupled Tyrosine Oxidation in Oligoproline Peptides

We have explored the kinetic effect of increasing electron transfer (ET) distance in a biomimetic, proton-coupled electron-transfer (PCET) system. Biological ET often occurs simultaneously with proton transfer (PT) in order to avoid the high-energy, charged intermediates resulting from the stepwise transfer of protons and electrons. These concerted proton-electron-transfer (CPET) reactions are implicated in numerous biological ET pathways. In many cases, PT is coupled to long-range ET. While many studies have shown that the rate of ET is sensitive to the distance between the electron donor and acceptor, extensions to biological CPET reactions are sparse. The possibility of a unique ET distance dependence for CPET reactions deserves further exploration, as this could have implications for how we understand biological ET. We therefore explored the ET distance dependence for the CPET oxidation of tyrosine in a model system. We prepared a series of metallopeptides with a tyrosine separated from a Ru(bpy)32+ complex by an oligoproline bridge of increasing length. Rate constants for intramolecular tyrosine oxidation were measured using the flash-quench transient absorption technique in aqueous solutions. The rate constants for tyrosine oxidation decreased by 125-fold with three added proline residues between tyrosine and the oxidant. By comparison, related intramolecular ET rate constants in very similar constructs were reported to decrease by 4-5 orders of magnitude over the same number of prolines. The observed shallow distance dependence for tyrosine oxidation is proposed to originate in part from the requirement for stronger oxidants, leading to a smaller hole-transfer effective tunneling barrier height. The shallow distance dependence observed here and extensions to distance-dependent CPET reactions have potential implications for long-range charge transfers.

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 (k_PCET) 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. k_PCET 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.

Elucidating Proton-Coupled Electron Transfer Mechanisms of Metal Hydrides with Free Energy- and Pressure-Dependent Kinetics

Proton-coupled electron transfer (PCET) was studied in a series of tungsten hydride complexes with pendant pyridyl arms ([(PyCH₂Cp)WH(CO)₃], PyCH₂Cp = pyridylmethylcyclopentadienyl), triggered by laser flash-generated Ru^III-tris-bipyridine oxidants, in acetonitrile solution. The free energy dependence of the rate constant and the kinetic isotope effects (KIEs) showed that the PCET mechanism could be switched between concerted and the two stepwise PCET mechanisms (electron-first or proton-first) in a predictable fashion. Straightforward and general guidelines for how the relative rates of the different mechanisms depend on oxidant and base are presented. The rate of the concerted reaction should depend symmetrically on changes in oxidant and base strength, that is on the overall ΔG⁰_PCET, and we argue that an "asynchronous" behavior would not be consistent with a model where the electron and proton tunnel from a common transition state. The observed rate constants and KIEs were examined as a function of hydrostatic pressure (1-2000 bar) and were found to exhibit qualitatively different dependence on pressure for different PCET mechanisms. This is discussed in terms of different volume profiles of the PCET mechanisms as well as enhanced proton tunneling for the concerted mechanism. The results allowed for assignment of the main mechanism operating in the different cases, which is one of the critical questions in PCET research. They also show how the rate of a PCET reaction will be affected very differently by changes of oxidant and base strength, depending on which mechanism dominates. This is of fundamental interest as well as of practical importance for rational design of, for example, catalysts for fuel cells and solar fuel formation, which operate in steps of PCET reactions. The mechanistic richness shown by this system illustrates that the specific mechanism is not intrinsic to a specific synthetic catalyst or enzyme active site but depends on the reaction conditions.

A stable dye-sensitized photoelectrosynthesis cell mediated by a NiO overlayer for water oxidation

In the development of photoelectrochemical cells for water splitting or CO₂ reduction, a major challenge is O₂ evolution at photoelectrodes that, in behavior, mimic photosystem II. At an appropriate semiconductor electrode, a water oxidation catalyst must be integrated with a visible light absorber in a stable half-cell configuration. Here, we describe an electrode consisting of a light absorber, an intermediate electron donor layer, and a water oxidation catalyst for sustained light driven water oxidation catalysis. In assembling the electrode on nanoparticle SnO₂/TiO₂ electrodes, a Ru(II) polypyridyl complex was used as the light absorber, NiO was deposited as an overlayer, and a Ru(II) 2,2'-bipyridine-6,6'-dicarboxylate complex as the water oxidation catalyst. In the final electrode, addition of the NiO overlayer enhanced performance toward water oxidation with the final electrode operating with a 1.1 mA/cm² photocurrent density for 2 h without decomposition under one sun illumination in a pH 4.65 solution. We attribute the enhanced performance to the role of NiO as an electron transfer mediator between the light absorber and the catalyst.

Proton-coupled electron transfer oxidation of O–H bond by the N-radical cation of Wurster's blue salt (TMPDA˙+)

Beyond a single electron oxidant, the N-radical cation is also a good PCET reagent.

Proton-coupled multi-electron transfer and its relevance for artificial photosynthesis and photoredox catalysis

Photoinduced PCET meets catalysis, and the accumulation of multiple redox equivalents is of key importance.

Switching between Stepwise and Concerted Proton-Coupled Electron Transfer Pathways in Tungsten Hydride Activation

Catalytic processes to generate (or oxidize) fuels such as hydrogen are underpinned by multiple proton-coupled electron transfer (PCET) steps that are associated with the formation or activation of metal-hydride bonds. Fully understanding the detailed PCET mechanisms of metal hydride transformations holds promise for the rational design of energy-efficient catalysis. Here we investigate the detailed PCET mechanisms for the activation of the transition metal hydride complex CpW(CO)₂(PMe₃)H (Cp = cyclopentadienyl) using stopped-flow rapid mixing coupled with time-resolved optical spectroscopy. We reveal that all three limiting PCET pathways can be accessed by changing the free energy for elementary proton, electron, and proton-electron transfers through the choice of base and oxidant, with the concerted pathway occurring exclusively as a secondary parallel route. Through detailed kinetics analysis, we define free energy relationships for the kinetics of elementary reaction steps, which provide insight into the factors influencing reaction mechanism. Rate constants for proton transfer processes in the limiting stepwise pathways reveal a large reorganization energy associated with protonation/deprotonation of the metal center (λ = 1.59 eV) and suggest that sluggish proton transfer kinetics hinder access to a concerted route. Rate constants for concerted PCET indicate that the concerted routes are asynchronous. Additionally, through quantification of the relative contributions of parallel stepwise and concerted mechanisms toward net product formation, the influence of various reaction parameters on reactivity are identified. This work underscores the importance of understanding the PCET mechanism for controlling metal hydride reactivity, which could lead to superior catalyst design for fuel production and oxidation.

Deciphering the incognito role of water in a light driven proton coupled electron transfer process

Light induced multisite electron proton transfer in two different phenol (simple and phenol carrying an intramolecularly hydrogen bonded base) pendants on acridinedione dye (ADD) and an NADH analogue was studied by following fluorescence quenching dynamics in an ultrafast timescale. In a simple phenol derivative (ADDOH), photo-excited acridinedione acquires an electron from phenol intramolecularly, coupled with the transfer of a proton to solvent water. But in a phenol carrying hydrogen bonded base (ADDDP), both electron and proton transfer occur completely intramolecularly. The sequence of this electron and proton transfer process was validated by discerning the pH dependency of the reaction kinetics. Since photo-excited ADDs are stronger oxidants, the sequential electron first proton transfer mechanism (ETPT) was observed in ADDOH and hence there is no change in the PCET reaction kinetics kETPT ∼ 6.57 × 109 s-1 in the entire pH range (pH 2-12). But the phenol carrying hydrogen bonded base (ADDDP) unleashes concerted electron proton transfer where the PCET reaction rate decreases upon decreasing the pH below its pKa. Noticeably, the concerted EPT process in ADDDP mimics the donor side of photosystem II and it occurs by two distinct pathways: (i) through direct intramolecular hydrogen bonding between the phenol and amine, kDEPT ∼ 12.5 × 1010 s-1 and (ii) through the bidirectional hydrogen bond extended by the water molecule trapped in between the proton donor and acceptor, which mediates the proton transfer and serves as a proton wire, kWMEPT ∼ 2.85 × 1010 s-1. These results unravel the incognito role played by water in mediating the proton transfer process when the structural elements do not favor direct hydrogen bonding between the proton donor and acceptor in a concerted PCET reaction.

Mimicking the Key Functions of Photosystem II in Artificial Photosynthesis for Photoelectrocatalytic Water Splitting

It has been anticipated that learning from nature photosynthesis is a rational and effective way to develop artificial photosynthesis system, but it is still a great challenge. Here, we assembled a photoelectrocatalytic system by mimicking the functions of photosystem II (PSII) with BiVO₄ semiconductor as a light harvester protected by a layered double hydroxide (NiFeLDH) as a hole storage layer, a partially oxidized graphene (pGO) as biomimetic tyrosine for charge transfer, and molecular Co cubane as oxygen evolution complex. The integrated system exhibited an unprecedentedly low onset potential (0.17 V) and a high photocurrent (4.45 mA cm^-2), with a 2.0% solar to hydrogen efficiency. Spectroscopic studies revealed that this photoelectrocatalytic system exhibited superiority in charge separation and transfer by benefiting from mimicking the key functions of PSII. The success of the biomimetic strategy opened up new ways for the rational design and assembly of artificial photosynthesis systems for efficient solar-to-fuel conversion.

Formation of a long-lived radical pair in a Sn(iv) porphyrin–di(l-tyrosinato) conjugate driven by proton-coupled electron-transfer

A surprisingly long-lived radical pair state is achieved in a tin-porphyrin/l-tyrosine conjugate by exploiting a photochemical PCET quenching mechanism.

Ultrafast Spectroscopic Dynamics of Quinacrine-Riboflavin Binding Protein Interactions

Redox active cofactors play a dynamic role inside protein binding active sites because the amino acids responsible for binding participate in electron transfer (ET) reactions. Here, we use femtosecond transient absorption (FsTA) spectroscopy to examine the ultrafast ET between quinacrine (Qc), an antimalarial drug with potential anticancer activity, and riboflavin binding protein (RfBP) with a known K_d = 264 nM. Steady-state absorption reveals a ∼ 10 nm red-shift in the ground state when QcH₃²⁺ is titrated with RfBP, and a Stern-Volmer analysis shows ∼84% quenching and a blue-shift of the QcH₃²⁺ photoluminescence to form a 1:1 binding ratio of the QcH₃²⁺-RfBP complex. Upon selective photoexcitation of QcH₃²⁺ in the QcH₃²⁺-RfBP complex, we observe charge separation in 7 ps to form ¹[QcH_{3_red}^•+-RfBP^•+], which persists for 138 ps. The FsTA spectra show the spectroscopic identification of QcH_{3_red}^•+, determined from spectroelectrochemical measurements in DMSO. We correlate our results to literature and report lifetimes that are 10-20× slower than the natural riboflavin, Rf-RfBP, complex and are oxygen independent. Driving force (ΔG) calculations, corrected for estimated dielectric constants for protein hydrophobic pockets, and Marcus theory depict a favorable one-electron ET process between QcH₃²⁺ and nearby redox active tyrosine (Tyr) or tryptophan (Trp) residues.

Kinetics of photoinduced electron transfer reactions of ruthenium(II) complexes and phenols, tyrosine, N-acetyl-tyrosine and tryptophan in aqueous solutions measured with modulated fluorescence spectroscopy

Photooxidation kinetics of phenol, 1-naphthol, 2-naphthol, tyrosine (TyrOH) and N-acetyl-tyrosine (AcTyrOH), tryptophan (TrpH) by ruthenium(II) polypyridyl complexes: [Ru(bpy)₃]Cl₂ (1), [Ru(phen)₃]Cl₂ (2), [Ru(bpy)(phen)(bpg)]Cl₂ (3), and [Ru(dpq)₂(bxbg)]Cl₂ (4) where bpy is 2,2'-bipyridine, phen - 1,10-phenanthroline, bpg - bipyridine-glycoluril, dpq - dipyrido[3,2-d:2',3'-f]quinoxaline, and bxbg - bis(o-xylene)bipyridine-glycoluril are investigated. Rate constants have been measured by steady-state luminescence and phase-modulation fluorometry in aqueous solutions at different pH's. The rates for the oxidation of the phenols and phenolic aromatic amino acids spreads over a wide range from 4.2×10⁶ to 6.8×10⁹M^-1s^-1, depending on pH and the nature of solutes. At pH>pK_a of the quenchers, the presence of reactive species (PhO^-) in the alkaline solutions is accounted for the rapid ET rates. In the pH range between 4 and 10 (pH<pK_a), the ETPT mechanism becomes dominate and the rate constants are relatively low. It reveals that the important parameters that influence the quenching reaction rates, others than the driving forces ∆G⁰ are the steric and hydrophobic interactions arising from the structure of the compounds. This is clearly seen in the case of photoreaction between the Ru(phen)₃²⁺ complex and AcTyrOH. Phen ligands and acetyl group cause a steric effect, but strengthen the hydrophobic interactions and thus promote the quenching process. The pH-dependent equation of the observed rate constant for PhOH/AcTyrOH oxidation is expressed as a sum of rates for its protonated, neutral and deprotonated forms.

Marcus-type driving force correlations reveal the mechanism of proton-coupled electron transfer for phenols and [Ru(bpy)3]3+ in water at low pH

We examined PCET between a series of phenol derivatives and photogenerated [Ru(bpy)₃]³⁺ in low pH (≤4) water using the laser flash-quench technique.

Proton-coupled electron transfer (PCET) from tyrosine and other phenol derivatives in water is an important elementary reaction in chemistry and biology. We examined PCET between a series of phenol derivatives and photogenerated [Ru(bpy)₃]³⁺ in low pH (≤4) water using the laser flash-quench technique. From an analysis of the kinetic data using a Marcus-type free energy relationship, we propose that our model system follows a stepwise electron transfer-proton transfer (ETPT) pathway with a pH independent rate constant at low pH in water. This is in contrast to the concerted or proton-first (PTET) mechanisms that often dominate at higher pH and/or with buffers as primary proton acceptors. The stepwise mechanism remains competitive despite a significant change in the pK_a and redox potential of the phenols which leads to a span of rate constants from 1 × 10⁵ to 2 × 10⁹ M^–1 s^–1. These results support our previous studies which revealed separate mechanistic regions for PCET reactions and also assigned phenol oxidation by [Ru(bpy)₃]³⁺ at low pH to a stepwise PCET mechanism.

Proton-coupled electron hopping in Ru-modified P. aeruginosa azurin

We constructed two artificial multiple-step electron transfer (hopping) systems based on Pseudomonas aeruginosa azurin where a tyrosine (YOH) is situated between Ru(2,2'-bipyridine)2(imidazole)(histidine) and the native copper site: RuH107YOH109 and RuH124-YOH122. We investigated the rates of Cu(I) oxidation by flash-quench generated Ru(III) over a range of conditions that probed the role of proton-coupled oxidation/reduction of YOH in the reaction. Rates of Cu(I) oxidation were enhanced over single-step electron transfer by factors between 3 and 80, depending on specific scaffold and buffer conditions.

Proton coupled electron transfer from the excited state of a ruthenium(ii) pyridylimidazole complex

Transfer of one electron and one proton from [Ru(bpy)₂pyimH]²⁺ to monoquat (MQ⁺) upon photoexcitation, corresponding to net transfer of a hydrogen atom.

Direct Interfacial Y731 Oxidation in α2 by a Photoβ2 Subunit of E. coli Class Ia Ribonucleotide Reductase

Proton-coupled electron transfer (PCET) is a fundamental mechanism important in a wide range of biological processes including the universal reaction catalysed by ribonucleotide reductases (RNRs) in making de novo, the building blocks required for DNA replication and repair. These enzymes catalyse the conversion of nucleoside diphosphates (NDPs) to deoxynucleoside diphosphates (dNDPs). In the class Ia RNRs, NDP reduction involves a tyrosyl radical mediated oxidation occurring over 35 Å across the interface of the two required subunits (β2 and α2) involving multiple PCET steps and the conserved tyrosine triad [Y356(β2)-Y731(α2)-Y730(α2)]. We report the synthesis of an active photochemical RNR (photoRNR) complex in which a Re(I)-tricarbonyl phenanthroline ([Re]) photooxidant is attached site-specifically to the Cys in the Y356C-(β2) subunit and an ionizable, 2,3,5-trifluorotyrosine (2,3,5-F3Y) is incorporated in place of Y731 in α2. This intersubunit PCET pathway is investigated by ns laser spectroscopy on [Re356]-β2:2,3,5-F3Y731-α2 in the presence of substrate, CDP, and effector, ATP. This experiment has allowed analysis of the photoinjection of a radical into α2 from β2 in the absence of the interfacial Y356 residue. The system is competent for light-dependent substrate turnover. Time-resolved emission experiments reveal an intimate dependence of the rate of radical injection on the protonation state at position Y731(α2), which in turn highlights the importance of a well-coordinated proton exit channel involving the key residues, Y356 and Y731, at the subunit interface.

Concerted proton-coupled electron transfer from a metal-hydride complex

Metal hydrides are key intermediates in the catalytic reduction of protons and CO2 as well as in the oxidation of H2. In these reactions, electrons and protons are transferred to or from separate acceptors or donors in bidirectional protoncoupled electron transfer (PCET) steps. The mechanistic interpretation of PCET reactions of metal hydrides has focused on the stepwise transfer of electrons and protons. A concerted transfer may, however, occur with a lower reaction barrier and therefore proceed at higher catalytic rates. Here we investigate the feasibility of such a reaction by studying the oxidation–deprotonation reactions of a tungsten hydride complex. The rate dependence on the driving force for both electron transfer and proton transfer—employing different combinations of oxidants and bases—was used to establish experimentally the concerted, bidirectional PCET of a metal-hydride species. Consideration of the findings presented here in future catalyst designs may lead to more-efficient catalysts.

Moving protons and electrons in biomimetic systems

An enormous variety of biological redox reactions are accompanied by changes in proton content at enzyme active sites, in their associated cofactors, in substrates and/or products, and between protein interfaces. Understanding this breadth of reactivity is an ongoing chemical challenge. A great many workers have developed and investigated biomimetic model complexes to build new ways of thinking about the mechanistic underpinnings of such complex biological proton-coupled electron transfer (PCET) reactions. Of particular importance are those model reactions that involve transfer of one proton (H(+)) and one electron (e(-)), which is equivalent to transfer of a hydrogen atom (H(•)). In this Current Topic, we review key concepts in PCET reactivity and describe important advances in biomimetic PCET chemistry, with a special emphasis on research that has enhanced efforts to understand biological PCET reactions.

Photochemical tyrosine oxidation in the structurally well-defined α3Y protein: proton-coupled electron transfer and a long-lived tyrosine radical

Tyrosine oxidation–reduction involves proton-coupled electron transfer (PCET) and a reactive radical state. These properties are effectively controlled in enzymes that use tyrosine as a high-potential, one-electron redox cofactor. The α₃Y model protein contains Y32, which can be reversibly oxidized and reduced in voltammetry measurements. Structural and kinetic properties of α₃Y are presented. A solution NMR structural analysis reveals that Y32 is the most deeply buried residue in α₃Y. Time-resolved spectroscopy using a soluble flash-quench generated [Ru(2,2′-bipyridine)₃]³⁺ oxidant provides high-quality Y32–O• absorption spectra. The rate constant of Y32 oxidation (k_PCET) is pH dependent: 1.4 × 10⁴ M^–1 s^–1 (pH 5.5), 1.8 × 10⁵ M^–1 s^–1 (pH 8.5), 5.4 × 10³ M^–1 s^–1 (pD 5.5), and 4.0 × 10⁴ M^–1 s^–1 (pD 8.5). k^H/k^D of Y32 oxidation is 2.5 ± 0.5 and 4.5 ± 0.9 at pH(D) 5.5 and 8.5, respectively. These pH and isotope characteristics suggest a concerted or stepwise, proton-first Y32 oxidation mechanism. The photochemical yield of Y32–O• is 28–58% versus the concentration of [Ru(2,2′-bipyridine)₃]³⁺. Y32–O• decays slowly, t_1/2 in the range of 2–10 s, at both pH 5.5 and 8.5, via radical–radical dimerization as shown by second-order kinetics and fluorescence data. The high stability of Y32–O• is discussed relative to the structural properties of the Y32 site. Finally, the static α₃Y NMR structure cannot explain (i) how the phenolic proton released upon oxidation is removed or (ii) how two Y32–O• come together to form dityrosine. These observations suggest that the dynamic properties of the protein ensemble may play an essential role in controlling the PCET and radical decay characteristics of α₃Y.