Energetics Of Electron Transfer Research Articles

Delineating redox cooperativity in water-soluble and membrane multiheme cytochromes through protein design.

Nature has evolved diverse electron transport proteins and multiprotein assemblies essential to the generation and transduction of biological energy. However, substantially modifying or adapting these proteins for user-defined applications or to gain fundamental mechanistic insight can be hindered by their inherent complexity. De novo protein design offers an attractive route to stripping away this confounding complexity, enabling us to probe the fundamental workings of these bioenergetic proteins and systems, while providing robust, modular platforms for constructing completely artificial electron-conducting circuitry. Here, we use a set of de novo designed mono-heme and di-heme soluble and membrane proteins to delineate the contributions of electrostatic micro-environments and dielectric properties of the surrounding protein medium on the inter-heme redox cooperativity that we have previously reported. Experimentally, we find that the two heme sites in both the water-soluble and membrane constructs have broadly equivalent redox potentials in isolation, in agreement with Poisson-Boltzmann Continuum Electrostatics calculations. BioDC, a Python program for the estimation of electron transfer energetics and kinetics within multiheme cytochromes, also predicts equivalent heme sites, and reports that burial within the low dielectric environment of the membrane strengthens heme-heme electrostatic coupling. We conclude that redox cooperativity in our diheme cytochromes is largely driven by heme electrostatic coupling and confirm that this effect is greatly strengthened by burial in the membrane. These results demonstrate that while our de novo proteins present minimalist, new-to-nature constructs, they enable the dissection and microscopic examination of processes fundamental to the function of vital, yet complex, bioenergetic assemblies.

Fusion Position-Dependent Aromatic Transitions of Ligand Backbone Rings for Controlling the Redox Energetics of Photoredox Catalysts.

To reveal, quantify, and rationalize the effect of backbone π-extension on ligand redox activity, we studied the ground- and excited-state reduction potentials of eight ruthenium photoredox catalysts with the formula Ru(ppy)2L (L is the redox-active ligand of the bipyridine family) using density functional theory. Our research underlines the profound importance of the fusion position of backbone aromatic C6 rings on the redox activity of ligands in transition metal photoredox catalysts. Namely, certain fusion positions lead to the dearomatization of C6 rings in ligand-centered electron transfer events, resulting in a thermodynamic penalty equivalent to a half-volt negative shift in the reduction potential. Contrarily, the extent of backbone delocalization shows a minimal impact on redox energetics, which can be explained by the charge concentration at the nitrogen contact atoms in ligand-centered reductions. Grounded in Caulton's conceptual framework, we reaffirm the predictive potency of Lewis structures in ligand-centered redox energetics with qualitative and quantitative data. Our hypothesis regarding the effect of backbone ring dearomatization on redox energetics is further corroborated using magnetic and structure-based aromaticity indicators. Highlighting fusion-dependent dearomatization as a determining factor of ligand-centered electron transfer energetics, our findings hold implications for molecular-level design in advanced electroactive materials and catalysts.

Utilising Fourier Transform Voltammetry to Advance the Enzyme Electrochemistry Toolkit

Electron transfer and redox chemistry drives life and Biology can provide inspiration for the design of sustainable fuel catalysts that achieve highly active and selective small molecule transformations. In particular, the Parkin group are interested in hydrogenases,1 biological H2-producing enzymes, and lytic polysaccharide monooxygenases (LPMOs),2, 3 enzymes that breakdown cellulose. We seek to understand how the rate and energetics of electron transfer controls catalysis in such enzymes;4 to do this we collaborate with the Gavaghan and Bond groups to develop more powerful bioelectrochemical methodologies.5 This talk will describe our most recent efforts to integrate sinusoidal voltammetry into our enzyme-electrochemistry toolkit.6, 7 The technique offers advantages in terms of simulation speed, which in turn enables the powerful application of Bayesian statistical analysis to visualise the uncertainty in the modelling approach. However, we still rely on large amplitude Fourier transform voltammetry and direct current methodologies to visualise the Faradaic current and readily define redox reaction parameter bounds.

Combined Effects of Hemicolligation and Ion Pairing on Reduction Potentials of Biphenyl Radical Cations.

Formal reduction potentials of highly oxidizing and short-lived radical cations of substituted biphenyls generated by pulse radiolysis in 1,2-dichloroethane (DCE) were measured using a redox equilibrium ladder method. The effect of halide ion-radical interactions on reduction potentials of biphenyls was examined by utilizing the ability of DCE to release Cl- in the vicinity of the radical cation. The Hammett correlation of measured potentials across a range of over 700 mV shows saturation at high Hammett sigma values. This effect has been explained by both ion-pairing and hemicolligation interactions between biphenyl radical cations and Cl- and appears to modulate reduction potentials by as much as 400 mV. This finding offers a convenient way to manipulate the energetics of electron transfer involving organic redox species.

The energetics and evolution of oxidoreductases in deep time.

The core metabolic reactions of life drive electrons through a class of redox protein enzymes, the oxidoreductases. The energetics of electron flow is determined by the redox potentials of organic and inorganic cofactors as tuned by the protein environment. Understanding how protein structure affects oxidation-reduction energetics is crucial for studying metabolism, creating bioelectronic systems, and tracing the history of biological energy utilization on Earth. We constructed ProtReDox (https://protein-redox-potential.web.app), a manually curated database of experimentally determined redox potentials. With over 500 measurements, we can begin to identify how proteins modulate oxidation-reduction energetics across the tree of life. By mapping redox potentials onto networks of oxidoreductase fold evolution, we can infer the evolution of electron transfer energetics over deep time. ProtReDox is designed to include user-contributed submissions with the intention of making it a valuable resource for researchers in this field.

Kinetics and Energetics of Electron Transfer to Dimer Radical Cations.

Spectra of the dimer cations naphthalene (Nap2•+) and ethene (Ethene2•+) were measured in liquid dichloromethane (DCM). The spectra peak at very different energies, 1.2 and 3.3 eV. In DCM dimerization stabilizes Nap2•+ by ΔGd°(Nap2•+) = -218 meV relative to the monomer Nap•+ as determined from the dimerization equilibrium constant. Both dimers can transfer a positive charge to hole acceptor molecules, but for both the rate constants rise more gradually with reaction energetics than do many charge transfer reactions previously studied. A striking observation finds that the rate constant for hole transfer from the Nap2•+ dimer to phenanthrene is smaller by two decades than that from biphenyl•+ monomer to Nap, although both reactions have the same -ΔG° = 0.05 eV. A plausible interpretation for these observations is the presence of an energy of reorganization, λ(M2), for the dimer that involves movement apart of the two partners in the dimer. While the dimerization equilibrium cannot be measured for Ethene2•+, the charge transfer data imply that both ΔGd°(Ethene2•+) and λ(Ethene2•+) are considerably larger, perhaps by factors of 2-4 than for Nap2•+.

Band-structure tunability via the modulation of excitons in semiconductor nanostructures: manifestation in photocatalytic fuel generation.

Understanding the energetics of electron transfer at the semiconductor interface is crucial for the development of solar harvesting technologies, including photovoltaics, photocatalysis, and solar fuel systems. However, modern artificial photosynthetic materials are not efficient and limited by their fast charge recombination with high binding energy of excitons. Hence, reducing the exciton binding energy can increase the generation of charge carriers, which improve the photocatalytic activities. Extensive research has been dedicated to improving the exciton dissociation efficiency through rational semiconductor design via heteroatom doping, vacancy engineering, the construction of heterostructures, and donor-π-acceptor (D-π-A) interfaces to extend the charge carrier migration, promoting the dissociation of excitons. Consequently, functionalized photocatalysts have demonstrated remarkable photocatalytic performances for solar fuel production under visible light irradiation. This review provides the fundamental aspects of excitons in semiconductor nanostructures, having a high binding energy and ultrafast exciton formation together with promising photo-redox properties for solar to fuel conversion application. In particular, this review highlights the significant role of the excitonic effect in the photocatalytic activity of newly developed functional materials and the underlying mechanistic insight for tuning the performance of nanostructured semiconductor photocatalysts for water splitting, CO2 reduction, and N2 fixation reactions.

Samarium Diiodide Acting on Acetone-Modeling Single Electron Transfer Energetics in Solution.

Samarium diiodide is a versatile single electron transfer (SET) agent with various applications in organic chemistry. Lewis structures regularly insinuate the existence of a ketyl radical when samarium diiodide binds a carbonyl group. The study presented here investigates this electron transfer by the means of computational chemistry. All electron CASPT2 calculations with the inclusion of scalar relativistic effects predict an endotherm electron transfer from samarium diiodide to acetone. Energies calculated with the PBE0-D3(BJ) functional and a small core pseudopotential are in good agreement with CASPT2. The calculations confirm the experimentally measured increase of the samarium diiodide reduction potential through the addition of hexamethylphosphoramide also known as HMPA.

Assessing Thermal Response of Redox Conduction for Anti-Arrhenius Kinetics in a Microbial Cytochrome Nanowire.

A micrometers-long helical homopolymer of the outer-membrane cytochrome type S (OmcS) from Geobacter sulfurreducens is proposed to transport electrons to extracellular acceptors in an ancient respiratory strategy of biogeochemical and technological significance. OmcS surprisingly exhibits higher conductivity upon cooling (anti-Arrhenius kinetics), an effect previously attributed to H-bond restructuring and heme redox potential shifts. Herein, the temperature sensitivity of redox conductivity is more thoroughly examined with conventional and constant-redox and -pH molecular dynamics and quantum mechanics/molecular mechanics. A 30 K drop in temperature constituted a weak perturbation to electron transfer energetics, changing electronic couplings (⟨Hmn⟩), reaction free energies (ΔGmn), reorganization energies (λmn), and activation energies (Ea) by at most |0.002|, |0.050|, |0.120|, and |0.045| eV, respectively. Changes in ΔGmn reflected -0.07 ± 0.03 V shifts in redox potentials that were caused in roughly equal measure by altered electrostatic interactions with the solvent and protein. Changes in intraprotein H-bonding reproduced the earlier observations. Single-particle diffusion and multiparticle steady-state flux models, parametrized with Marcus theory rates, showed that biologically relevant incoherent hopping cannot qualitatively or quantitatively describe electrical conductivity measured by atomic force microscopy in filamentous OmcS. The discrepancy is attributed to differences between solution-phase simulations and solid-state measurements and the need to model intra- and intermolecular vibrations explicitly.

Mapping the Double Layer Using Proton-Coupled Electron Transfer at Functionalized Carbon Electrodes

Conductive electrodes functionalized with molecularly well-defined species are emerging as effective active materials for a wide range of applications, from electrocatalytic CO2 and O2 reduction, to high performance lithium-sulfur batteries.1–3 For redox-active species that participate in proton-coupled electron transfer (PCET), the extent of conjugation, applied potential, and distance between the redox center and electrode determine the potential and electric field experienced at the site of electron transfer. This electric field in turn determines the pH-dependent energetics of electron transfer (i.e. Pourbaix behavior) and the rate PCET. We discuss how measurements of PCET to redox-active species functionalized to carbon electrodes can yield insight into the potential and electric field profiles within the electrode-electrolyte double layer. Systematically increasing the distance between the redox-active moiety and the electrode results in the moiety experiencing a progressively weaker electric field, as determined by changes to PCET behavior. Based on these results, we propose strategies to quantify the strength of the electric field as a function of this distance. This work can inform strategies for effective pH modulation at electrified interfaces in ways that can enhance electrocatalytic processes and other applications, such as electrochemical CO2 capture based on pH swing.References Ren, G. et al. Porous Core-Shell Fe3C Embedded N-doped Carbon Nanofibers as an Effective Electrocatalysts for Oxygen Reduction Reaction. ACS Applied Materials and Interfaces 8, 4118–4125 (2016).Zhang, S., Fan, Q., Xia, R. & Meyer, T. J. CO2 Reduction: From Homogeneous to Heterogeneous Electrocatalysis. Accounts of Chemical Research 53, 255–264 (2020).Zhao, C.-X. et al. Semi-Immobilized Molecular Electrocatalysts for High-Performance Lithium–Sulfur Batteries. Journal of the American Chemical Society 143, 19865–19872 (2021).

Active-Site Environmental Factors Customize the Photophysics of Photoenzymatic Old Yellow Enzymes.

The development of non-natural photoenzymatic systems has reinvigorated the study of photoinduced electron transfer (ET) within protein active sites, providing new and unique platforms for understanding how biological environments affect photochemical processes. In this work, we use ultrafast spectroscopy to compare the photoinduced electron transfer in known photoenzymes. 12-Oxophytodienoate reductase 1 (OPR1) is compared to Old Yellow Enzyme 1 (OYE1) and morphinone reductase (MR). The latter enzymes are structurally homologous to OPR1. We find that slight differences in the amino acid composition of the active sites of these proteins determine their distinct electron-transfer dynamics. Our work suggests that the inside of a protein active site is a complex/heterogeneous dielectric network where genetically programmed heterogeneity near the site of biological ET can significantly affect the presence and lifetime of various intermediate states. Our work motivates additional tunability of Old Yellow Enzyme active-site reorganization energy and electron-transfer energetics that could be leveraged for photoenzymatic redox approaches.

Designing Nanoparticles for Optimized Pseudocapacitive Energy Storage

Power harvesting from intermittent renewable energy sources requires high charge storage capacity without compromising charging speed. These properties are achievable within hybrid capacitors by enhancing the pseudocapacitive (Faradaic) charge storage relative to the double-layer (non-Faradaic) charge storage, via the overlapping of equally-spaced redox reactions to reach a near ideal voltammetry profile. In this work, we examine how the structure of nanoparticles may be utilized to tune the pseudocapacitive response. This is accomplished by utilizing Gerischer-Hopfield electron transfer theory to capture the effects of redox species solvation through the solvent reorganization energy. Our model translates the total-energy picture by Marcus-Hush theory into single-particle picture for electron transfer in Faradaic charge storage processes, thereby arriving at a general phenomenological description of how pseudocapacitive properties can be engineered at the nanoscale. We also explore the design limitations of such nanoparticle-derived pseudocapacitance through a fundamental consideration of electron transfer energetics and kinetics. In general, our analysis is intended to aid the development enhanced pseudocapacitive properties in hybrid capacitor systems.

Energetics of Electron Transfer and Charge Separation in Moderate to Low Polarity Solvents

The absorption of light in natural photosynthesis occurs in a low polarity region allowing photoelectrons to store absorbed energy in chemical bonds instead of emitting heat via solvent interaction. Consequently, studying electron transfer and charge separation in low polarity solvents may have application in solar energy storage as such a study will make electron transfer in low polarity become more thoroughly understood. Conducted in solvents of various dielectric constants, the study combines strong electron donors and electron acceptors to promote ion formation by thermal equilibria. The extent to which these molecules formed ion pairs and free ions was measured via conductivity and UV-Vis spectroscopy. Gibbs free energy changes were calculated for ion formation and separation using acetonitrile as a standard highly polar solvent. With values of ionization potentials and electron affinities, free energy changes observed in the moderately polar solvent, tetrahydrofuran, were fit to theoretical Born solvation and Coulomb potential models by determining effective ion radii. Model predictions for free energy changes in other solvents disagreed with experimental data because unexpected charge transfer complexes become significant in concentration. Approximations of free energy changes were difficult in low polarity solvents due to low conductivity measurements and charge transfer complexes that appear in UV-Vis absorption spectra. Observed free energy changes were found to be significantly less favorable than redox potentials of the same reactions determined electrochemically not in this study. The difference is attributed to the unintended stabilization of ions caused by salt in solution required for the electrochemical method.

Synthesis, characterization and the photoinduced electron-transfer energetics of CdTe/CdSe type-II core/shell quantum dots

The CdTe/CdSe type-II core/shell quantum dots (QDs) were chemically synthesized in a noncoordinating solvent. The optical properties and structures of the synthesized QDs were characterized by absorption spectroscopy (Abs), photoluminescence (PL) spectroscopy, PL-decay lifetime, X-ray diffraction (XRD), transmission electron microscopy (TEM), and energy dispersive X-ray spectroscopy (EDX). The PL peak of the CdTe/CdSe core/shell QDs could be tuned from 698 nm (1.77 eV) to 780 nm (1.59 eV) by changing the thickness of CdSe shell and fixing the size of CdTe core. The PL quantum yields (QYs) of these QDs were from 14.1% to 31.7% and their full width at half maximum (FWHM) of PL peaks was kept below 0.12 eV during the growth process of CdSe shell on CdTe core. Compared to CdTe cores, the CdTe/CdSe type-II core/shell QDs have much longer PL decay times, up to ~100 ns? Based on Marcus theory, the charge transfer (CT) of CdTe/CdSe 1–5 monolayers (MLs) type-II core/shell QDs has been studied and the effects of quantum confinement on the structure parameters have been revealed. The reorganization energies (RE) of the CdTe/CdSe (1–5 ML) type-II core/shell QDs for the electron transfer (ET) were in the range from 60 meV to 106 meV and was found that ET takes place in the Marcus inverted region (-ΔG0ET > λCT) which is typical for these QDs.

Smooth path through a molecular catalyst

Electrochemistry Molecular electrocatalysts accelerate reactions by shuttling electrons between the electrodes and reactants. This process typically is constrained by the energetics of electron transfer between the electrode and the catalyst. However, Jackson et al. now show that, when linked carefully to the electrode, the catalyst can act as an electron conduit without being reduced ahead of time. They specifically conjugated a rhodium hydrogen-evolution catalyst to a graphite electrode through a pyrazine linker. The ensuing reaction proceeded through concerted proton-coupled electron transfer. J. Am. Chem. Soc. 10.1021/jacs.9b04981 (2019).

Electron transfer mediates vibrational relaxation of CO in collisions with Ag(111).

We report experimental results on the state-to-state vibrational relaxation of CO(v = 17) in collisions with Ag(111) at incidence translational energies between 0.27 eV and 0.57 eV. These together with previous results provide a comprehensive set of data on two molecules (CO and NO)-one open and one closed shell-and two metals (Ag and Au). In all four cases, the incidence vibrational energy has been varied over several eV. We find a unifying relation between the probability of vibrational relaxation and the energetics of electron transfer from the metal to the molecule. This argues strongly that electronic friction based theories are not capable of explaining these data.

Charge-transfer excited state in pyrene-1-carboxylic acids adsorbed on titanium dioxide nanoparticles

The electronic structure of excited photosensitizer adsorbed at the surface of a solid is the key factor in the electron transfer processes that underlie the efficiency of dye-sensitized solar cells and photocatalysts. In this work, Stark effect (electroabsorption) spectroscopy has been used to measure the polarizability and dipole moment changes in electronic transitions of pyrene-1-carboxylic (PCA), -acetic (PAA) and -butyric (PBA) acids in ethanol, both free and adsorbed on colloidal TiO2, in glassy ethanol at low temperature. The lack of appreciable increase of dipole moment in the excited state of free and adsorbed PAA and PBA points that two or more single bonds completely prevent the expansion of π-electrons from the aromatic ring towards the carboxylic group, thus excluding the possibility of direct electron injection into TiO2. In free PCA, the pyrene's forbidden S0→S1 transition has increased intensity, exhibits a long progression in 1400cm−1 Ag mode and is associated with |∆μ| of 2 D. Adsorption of PCA on TiO2 causes a broadening and red shift of the S0→S1 absorption band and an increase in dipole moment change on electronic excitation to |∆μ|=6.5 D. This value increased further to about 15 D when the content of acetic acid in the colloid was changed from 0.2% to 2%, and this effect is ascribed to the surface electric field. The large increase of |∆μ| points that the electric field effect can not only change the energetics of electron transfer from the excited sensitizer into the solid, but can also shift the molecular electronic density, thus directly influencing the electronic coupling factor relevant for electron transfer at the molecule-solid interface.

Binding and Energetics of Electron Transfer between an Artificial Four-Helix Mn-Protein and Reaction Centers from Rhodobacter sphaeroides.

The ability of an artificial four-helix bundle Mn-protein, P1, to bind and transfer an electron to photosynthetic reaction centers from the purple bacterium Rhodobacter sphaeroides was characterized using optical spectroscopy. Upon illumination of reaction centers, an electron is transferred from P, the bacteriochlorophyll dimer, to QA, the primary electron acceptor. The P1 Mn-protein can bind to the reaction center and reduce the oxidized bacteriochlorophyll dimer, P+, with a dissociation constant of 1.2 μM at pH 9.4, comparable to the binding constant of c-type cytochromes. Amino acid substitutions of surface residues on the Mn-protein resulted in increases in the dissociation constant to 8.3 μM. The extent of reduction of P+ by the P1 Mn-protein was dependent on the P/P+ midpoint potential and the pH. Analysis of the free energy difference yielded a midpoint potential of approximately 635 mV at pH 9.4 for the Mn cofactor of the P1 Mn-protein, a value similar to those found for other Mn cofactors in proteins. The linear dependence of -56 mV/pH is consistent with one proton being released upon Mn oxidation, allowing the complex to maintain overall charge neutrality. These outcomes demonstrate the feasibility of designing four-helix bundles and other artificial metalloproteins to bind and transfer electrons to bacterial reaction centers and establish the usefulness of this system as a platform for designing sites to bind novel metal cofactors capable of performing complex oxidation-reduction reactions.

Impact of Lattice Water on Solid-State Electron Transfer in Viologen Pseudopolymorphs: Modulation of Photo- and Piezochromism.

Stimuli-induced solid-state electron transfer (ET) underlies the use of viologen compounds as responsive materials, but unequivocal structure-property correlations for solid-state ET are still lacking. With different pseudopolymorphic solids derived from N,N'-bis(4-carboxylphenyl)viologen ([H2bcpV]2+), here we report a systematic study on photo- and piezochromic properties associated with ET. We show that the higher the water content in the lattice, the less sensitively the compounds respond to light and pressure. It is proposed that the lattice water does not act as an electron donor but serves to change the ET energetics through its unique polarity and hydrogen bonding capability. The impedimental impact of water on solid-state ET of viologen compounds has not yet been recognized and elucidated prior to this work. The study also suggests that pressure is more powerful than light in inducing ET.

Proton-Promoted Electron Transfer in Photocatalysis: Key Step for Photocatalytic Hydrogen Evolution on Metal/Titania Composites

Metal cocatalysts are widely utilized for enhancing photocatalytic conversion. In TiO2-based photocatalysts, a wide range of metals dispersed on TiO2 surfaces were observed to be effective for photocatalytic hydrogen production. To clarify the metal/oxide synergistic effect in photocatalysis and the insensitivity of photoactivity on metal types, here we investigate the mechanism of electron transfer from semiconductor to the cocatalyst by using ab initio molecular dynamics and hybrid density functional theory calculations. By determining the optimal geometry of a Pt13 subnano cluster on anatase TiO2(101) and quantifying the electron transfer energetics, we find that the electron transfer from oxide to the metal cluster is significantly boosted (exothermic by more than 0.3 eV) by the adsorption of proton on the metal cluster, which is otherwise endothermic without the presence of proton. This cooperative effect between oxide, subnano metal cluster, and adsorbed proton is rationalized from electronic struct...