ET Kinetics Research Articles

Theory for Heterogeneous Electron Transfer Kinetics on Nanocorrugated Atomic Stepped Metal Electrodes

We develop a novel mean-field theoretical approach for the outer-sphere heterogeneous electron transfer (OS-HET) rate constant at an atomically stepped nanocorrugated metal surface. Theory accounts for the contributions from the nature of the metal, local curvature of the nanocorrugated atomic step, the density of atomic steps and kinks, dipolar solvents, and the frontier molecular orbital of electroactive molecules. Our theoretical approach develops a novel model for the free energy of activation obtained using the alignment of the Fermi energy level in the metal with one of the frontier molecular orbitals of the electroactive species. The activation free energy is obtained as a product of a modified work function (WF) and the fractional electronic charge of activation for the alignment of levels. Metal surface local curvatures at atomic steps are modeled as a hyperbolic tangent functional with a random nanocorrugated step edge. The influence of step density and edge fluctuations (or kinks) on the activation free energy is included through the surface average mean curvature and the mean-square gradient dependent modified WF. The theoretical results highlight the step density-dependent nonmonotonic variation in WF, fractional electronic charge, and activation free energy. The increase in step density shifts the Fermi energy level toward LUMO and away from HOMO energy levels. This causes the reduced (for the LUMO) and enhanced (for the HOMO) activation free energy for ET kinetics compared to the basal plane. The dichotomy in the OS-HET kinetics at the atomically stepped metal results in anomalous enhancement and suppression for the LUMO and HOMO, respectively. The strong influence of kinks along the step edge on the HET rate constant is predicted up to 6 decades (compared to the basal surface of Pt metal). Finally, theory shows agreement with the reported experimental data of WF and exchange current density for single crystal Pt electrodes.

Adiabatic versus non-adiabatic electron transfer at 2D electrode materials

2D electrode materials are often deployed on conductive supports for electrochemistry and there is a great need to understand fundamental electrochemical processes in this electrode configuration. Here, an integrated experimental-theoretical approach is used to resolve the key electronic interactions in outer-sphere electron transfer (OS-ET), a cornerstone elementary electrochemical reaction, at graphene as-grown on a copper electrode. Using scanning electrochemical cell microscopy, and co-located structural microscopy, the classical hexaamineruthenium (III/II) couple shows the ET kinetics trend: monolayer > bilayer > multilayer graphene. This trend is rationalized quantitatively through the development of rate theory, using the Schmickler-Newns-Anderson model Hamiltonian for ET, with the explicit incorporation of electrostatic interactions in the double layer, and parameterized using constant potential density functional theory calculations. The ET mechanism is predominantly adiabatic; the addition of subsequent graphene layers increases the contact potential, producing an increase in the effective barrier to ET at the electrode/electrolyte interface.

Electrocatalytic Cross-Coupling Reactions Assisted By Redox-Active Mediators

The seminar will detail electrocatalytic C–C and C–N bond-forming methodologies enabled by redox-active mediators and shuttles. Here, we will demonstrate a symbiotic relationship between energy storage and metal-catalyzed organic synthesis. Two research initiatives will be discussed that highlight the use of a mediator to (i) protect a coupling catalyst from overreduction through a degenerate shuttling of electrons and (ii) assist in oxidation of complexes with slow ET kinetics under electrochemical conditions.The concept of protecting catalyst from degradation using shuttles will be discussed in the context of cross-electrophile coupling (XEC). Couplings of alkyl and aryl halides promoted by electrochemistry represents an attractive alternative to conventional methods that require stoichiometric quantities of high-energy reductants. Most importantly, electroreduction can readily exceed the reducing potentials of chemical reductants to activate catalysts with improved reactivities and selectivities that are otherwise incompatible with conventional reductants. This work details the mechanistically-driven development of an electrochemical methodology for XEC that utilizes redox-active shuttles developed by the energy-storage community to protect reactive coupling catalysts from overreduction. The resulting electrocatalytic system is practical, scalable, and broadly applicable to the reductive coupling of a wide range of aryl, heteroaryl, or vinyl bromides with primary or secondary alkyl bromides. The impact of overcharge protection as a strategy for electrosynthetic methodologies is underscored by the dramatic differences in yields from coupling reactions with added redox shuttles (generally >80%) and those without (generally <20%). In addition to excellent yields for a wide range of substrates, reactions protected from overreduction can be performed at high currents and on multigram scalesThe second half of the talk will demonstrate the use of mediators to protect ligandless complexes that are susceptible to electroplating on a cathode and to facilitate oxidation of the electrochemically-inactive catalysts to the active form. This strategy is applied to an electrochemically driven, scalable Chan-Lam-Evans reaction that utilizes simple copper salts to promote C-N bond formation under an inert atmosphere. This work represents a rare example of air-free CLE coupling. This oxidative reaction is performed in an undivided cell with a simple Cu salt that would otherwise rapidly plate out of solution. Mediator-assisted CLE coupling provides access to a broad range of amine products that are otherwise challenging to form even in air. The methodology seeks to eliminate the need for O2 on scale and provide an attractive alternative for process groups in industry.

Lanthanide (III) ions as multichannel acceptors for bimolecular photoinduced electron transfer reactions with coumarin dyes

Understanding the kinetics and energetics of photoinduced electron transfer (PET) reactions is of considerable research interest in photochemical sciences. In this study, interactions between trivalent lanthanide ions (Ln(III)) as electron acceptors and coumarin dyes as electron donors have been investigated in aqueous media using ground state absorption, steady-state (SS) emission, time-resolved (TR) fluorescence and laser flash photolysis (LFP) techniques. Among different Ln(III) ions, only Eu(III), Yb(III) and Sm(III) are found to display substantial interaction with the excited singlet (S1) state of the coumarin dyes, which correspond well with the favorable free energy changes (negative ΔG0) associated with the PET reactions in these coumarin-Ln(III) systems. For other Ln(III) ions, as ΔG0 is not favorable, they do not show any interaction with the excited coumarin dyes. Among the interacting coumarin-Ln(III) systems, bimolecular quenching rate constant (kq) increases consistently with the increasing exergonicity (−ΔG0), when each of the Ln(III) ions are considered independently, except for Eu(III), for which the kq values have already reached the diffusion controlled limit. However, considering all the Ln(III) ions together, the kq values do not show any common correlation with the changing ΔG0 values. The kq values are in general much lower for Yb(III) quencher as compared to Sm(III), even though the exergonicity values for coumarin-Yb(III) systems are more favorable than the coumarin-Sm(III) systems. Correlating the observed results based on the diffusion mediated bimolecular reaction model, we infer that depending on the ΔG0 values, some Ln(III) quenchers possibly also engage their upper electronic energy states as well to assist multichannel ET with the excited coumarin donors, and thereby modulating the kq values very substantially for different coumarin-Ln(III) systems. Apparently, the involvement of multichannel ET causes the total reorganization energy (λ) for ET reactions to differ largely for different Ln(III) ions, which would otherwise be very similar for all the coumarin-Ln(III) pairs. Observed results suggest that the intramolecular reorganization energy (λi) associated with lower electronic states of the hydrated Ln(III) ions is much larger than that associated with higher electronic states of these ions. While the involvement of multichannel ET is itself an intriguing inference, modulation in the ET kinetics and total λ values for different Ln(III) quenchers through the differential participation of the multichannel PET is also expected to find potential uses in the controlled utilization of such PET reactions in many practical areas.

Twisting of diarylnitroxides: An efficient tool for redox tuning

The electrochemical behavior of a new type of stable diarylnitroxide radicals with bulky substituent in the ortho-position of one of the phenyl rings is described. The presence of the substituent results in broken conjugation of one of the phenyl rings with the NO moiety (the other ring stays conjugated) giving rise to so called “twisted nitroxides”. New diphenylnitroxides with t-Bu and/or CF3 substituents were synthesized and are the first examples of diarylnitroxides which are extremely stable in solution and form both stable oxidized and reduced forms (oxoammonium cations and aminoxyl anions). These compounds exhibit fast ET kinetics and reversible electrochemical oxidation and reduction. Varying the torsion angle θ shifts Eox and Ered values in the opposite directions thus increasing the electrochemical potential gap, contrary to the traditional effects of substituents that commonly shift both potential values in the same direction. The electronic effect of the substituents in the ortho-position as well as the total electronic effect of both rings on the electrochemical potential values was estimated using the linear fit for the relationship between E1/2Ox+E1/2Red vs. the sum of Hammett constants (σp + σo·cosθ). The ρ value (the slope in the linear fit) is 0.501 indicating significant sensitivity of the potential values to the electronic effects and wide possibilities for tuning. The results obtained shed light on the efficient principles of molecular design for stable electroactive nitroxide-based materials.

Effect of the ligand framework of cobalt clathrochelates on hydrogen evolution electrocatalysis: electrochemical, spectroscopic and Density Functional Theory analyses

Water electrolysis in acidic media requires electrocatalysts that are chemically stable at low pH, such as platinum group metals (PGM). Due to the scarcity and cost of these PGMs, there is a need to find alternatives. Molecular complexes of 3d metals such as cobalt, nickel and iron are potential candidates. Cage metal complexes such as their tris-dioximate clathrochelates have already demonstrated an interesting electrochemical activity, in particular with regard to the hydrogen evolution reaction (HER). In this work, we report the characterization of three different cobalt clathrochelates and discuss their individual activity and performance with regard to the HER. Our objective is to unravel the effect of different peripheric substituents in the caging ligands and to discuss their role in the observed electrocatalytic activities. Theoretical (DFT) and spectroscopic (EPR and XPS) techniques were employed to assess the electronic structure of the different catalysts, which was subsequently coupled to the results obtained by cyclic voltammetry. The electrochemical kinetics of the systems was determined by Nicholson’s method. In all cases, the standard rate constant presented values circa 10−3cm/s, characteristic of quasi-reversible electron transfers. Thus, the differences in electrocatalytic activities were not due to dissimilarities in the ET kinetics among the molecules. The electroactivity towards the HER was determined by additions of sulfuric acid to the electrochemical cell. The clathrochelate with most strong electron-withdrawing groups presented the lowest overpotential for the HER.

Kinetics of Quantized Charging of Au144 Nanoclusters

AbstractElectron transfer (ET) in monolayer‐protected gold clusters (Au MPCs) is crucial for revealing their fundamental energetic and kinetic properties and potential electrochemical and optical applications. In this Communication, we measured the kinetics of quantized charging processes of the Au144 MPCs. A detailed mathematical model was developed, including ET kinetics and diffusion in MPCs. The charging processes of controlled core states were treated as multistep ET reactions, and the corresponding rate constants were determined by steady‐state voltammetry and scanning electrochemical microscopy (SECM).

Single-Nanoparticle Resolved Biomimetic Long-Range Electron Transfer and Electrocatalysis of Mixed-Valence Nanoparticles

Long-range electron transfer (LRET) is a core elementary step in a wealth of processes central to chemistry and biology, including photosynthesis, respiration, and catalysis. In nature, biological catalysis is performed by enzymes. However, enzymes are structurally fragile and have limited stability in vitro. Development of robust biomimetic nanostructures is therefore highly desirable. Here, with Prussian blue nanoparticles (PBNPs) as an example we have demonstrated the preparation of highly stable and water-soluble mixed-valence nanoparticles under mild conditions. We have mapped their enzyme-mimicking catalytic properties and controlled LRET to single-nanoparticle resolution. PBNPs show high substrate binding affinity and tunable electrocatalytic efficiency toward hydrogen peroxide reduction, resembling the patterns for similar size redox metalloenzymes. We have further disclosed a correlation between electrocatalytic efficiency and distance-dependent interfacial ET kinetics. Given their high stability...

Diffusion Assisted Bimolecular Electron Injection to CdS Quantum Dots: Existence of Different Regimes in Time Dependent Sink Term of Collins–Kimball Model

Excited state lifetime and steady state fluorescence of a series of CdS quantum dots (QDs) with different sizes in toluene were quenched by electron donor molecule N-methyl aniline (NMA). Static quenching Collins–Kimball (SQCK) diffusion model enabled convincing fittings to the steady-state and time-resolved data using nearly a same set of parameters, only after considering the presence of inherent quencher sites statistically distributed over the quantum dot surface. Electron injection rate shows strong chemical driving force dependency. QD with largest dimension (∼5.4 nm) used in this study exhibits a slightly higher chemical driving force (−ΔG0 = 0.80 eV) of electron transfer as compared to that (−ΔG0 = 0.79 eV) obtained for the smallest size QD (∼3.8 nm). However, such a small change in driving force causes nearly ∼3 times acceleration of the ET rate coefficient (k0 = 8.30 × 109 M–1 s–1) within the larger size QD as compared to that (k0 = 2.74 × 109 M–1 s–1) observed in smaller size QD. The time evolution of the sink term obtained from the Collins–Kimball fitting of ET kinetics shows different regimes of the kinetics (static and nonstationary).

Electrocatalytic Activity and Electroanalytical Performance of Nanodiamond-Derived Carbon Nano-Onions

Carbon nano-onions (CNOs) are an emerging, novel class of carbon nanomaterials. A CNO is a 0-D carbon analogue of a 1-D carbon nanotube (CNT) and a 2-D graphene. It is composed of sp2-bonded, concentric carbon shells surrounding a hollow core. The curved morphology of a CNO loosens the conjugation along the graphene sheet and results in the change in electronic structure. The unique structure as well as surface chemical functional groups of CNOs will significantly influence physicochemical, electronic, and electrochemical properties.In this talk, the physicochemical and electrochemical properties of CNOs will be presented. First, morphology and microstructure of CNOs were characterized by scanning and transmission electron microscopies (SEM, TEM), X-ray diffraction (XRD), and Raman spectroscopy. Chemical composition and chemical functional groups were probed by X-ray photoelectron spectroscopy (XPS). Raman spectra of CNOs showed a large value of ID/IG ratio, indicating that carbon concentric shells of CNOs include defects, i.e., exposed edge planes. Second, heteroatomic doping of CNOs and its electrocatalytic activity toward oxygen reduction reaction will be presented, in comparison with other carbon nanomaterials. Last, redox activities and electron transfer kinetics of CNOs were studied. CNOs showed remarkable electrochemical activities for the detection of biologically important molecules (such as high sensitivity, high selectivity and fast ET kinetics). Electrochemical measurements were carried out with standard redox couples such as Ru(NH3)6 3+/2+, Fe2+/3+, IrCl6 2 - /3 - and Fe(CN)6 3-/4- as well as redox-active biomolecules such as dopamine, epinephrine, and norepinephrine. The results showed a stable and fast electrochemical response of CNOs for all of the redox analytes and often surpassed the performance of other carbon electrodes like carbon nanotubes, graphite nanoflakes, and glassy carbon.

Electrocatalytic actvity of modified gold electrodes based on self-assembled monolayers of 4-mercaptopyridine and 4-aminothiophenol on Au(111) surfaces chemically functionalized with substituted and unsubstituted iron phthalocyanines

Abstract Systems containing self-assembled monolayers (SAMs) of thiols on metallic and nonmetallic surfaces are attractive as they provide structurally well-defined surfaces with a controllable chemical functionality, in addition to their stability and stiffness. These special features have inspired several studies addressed to electron transport ability through the SAMs and to the influence of the chemical functionality of thiols. We studied the electrocatalytic activity for the oxidation of l -cysteine of gold electrodes modified with self-assembled monolayers of 4-mercaptopyridine (4MPy) and 4-aminothiophenol (4ATP) on Au(111) surfaces chemically functionalized with substituted and unsubstituted Fe(II)-phthalocyanines (FePc). Attention was focused on the study of the effect of the thiol-end-FePc on the electron transfer rate from l -cysteine to the gold–SAM–FePc assembly, as well as on the effect of Pc-ring substituents on the ET kinetics. It is found that the effect of substituents on the Pc-ring (using Hammett parameters) on the Fe(III)/(II) redox potential is weak when FePc molecules are confined directly on Au and with the 4MPy SAMs/FePc, but their more pronounced when FePc molecules are located on the outermost position of the SAM assembly with a 4ATP molecules.

Distance-independent charge recombination kinetics in cytochrome c-cytochrome c peroxidase complexes: compensating changes in the electronic coupling and reorganization energies.

Charge recombination rate constants vary no more than 3-fold for interprotein ET in the Zn-substituted wild type (WT) cytochrome c peroxidase (CcP):cytochrome c (Cc) complex and in complexes with four mutants of the Cc protein (i.e., F82S, F82W, F82Y, and F82I), despite large differences in the ET distance. Theoretical analysis indicates that charge recombination for all complexes involves a combination of tunneling and hopping via Trp191. For three of the five structures (WT and F82S(W)), the protein favors hopping more than that in the other two structures that have longer heme → ZnP distances (F82Y(I)). Experimentally observed biexponential ET kinetics is explained by the complex locking in alternative coupling pathways, where the acceptor hole state is either primarily localized on ZnP (slow phase) or on Trp191 (fast phase). The large conformational differences between the CcP:Cc interface for the F82Y(I) mutants compared to that the WT and F82S(W) complexes are predicted to change the reorganization energies for the CcP:Cc ET reactions because of changes in solvent exposure and interprotein ET distances. Since the recombination reaction is likely to occur in the inverted Marcus regime, an increased reorganization energy compensates the decreased role for hopping recombination (and the longer transfer distance) in the F82Y(I) mutants. Taken together, coupling pathway and reorganization energy effects for the five protein complexes explain the observed insensitivity of recombination kinetics to donor-acceptor distance and docking pose and also reveals how hopping through aromatic residues can accelerate long-range ET.

Study on Electron Transfer Across Nucleobases Terminated Surface Assembly

3-Mercaptopropionic acid(MPA),Mg2+ and nucleotide monophosphate(NTMP)(including guanosine,adenosine,cytidine,uridine-5′-monophosphate,and abbreviations were GMP,AMP,CMP,UMP respectively) were adsorbed onto Au substrates stepwise to form nucleobases terminated surface.Electron transfer(ET) process across the NTMP/Mg2+/MPA/Au multilayer assembly was studied and the ET kinetics were determined using electrochemical methods including cyclic voltammetry(CV),chronoamperometry(CA) and electrochemical impedance spectroscopy(EIS).The experimental results obtained from electrochemistry showed that the rate of ET kinetics decreased in the order: GMP/Mg2+/MPA/AuAMP/Mg2+/MPA/AuUMP/Mg2+/MPA/AuCMP/Mg2+/MPA/Au.And the calculated energy gaps(ΔEgap=ELUMO-EHOMO) of their corresponding Mg-NTMP complexes motif increased in the order: ΔEMg-GMP(0.89 eV)ΔEMg-AMP(1.40 eV)ΔEMg-UMP(1.57 eV)ΔEMg-CMP(1.64 eV),which suggested that complexation of Mg with the NTMP,and their corresponding ΔEgap may play important role in their ET process across the nucleobases terminated multilayer assembly.

Enzymatically modified peptide surfaces: towards general electrochemical sensor platform for protein kinase catalyzed phosphorylations

We hereby present an electrochemical approach for monitoring the three protein kinases sarcoma-related kinase (Src), extracellular signal-regulated kinase 1 (Erk1), and cyclin A-dependent kinase 2 (CDK2/cyclin A). The electrochemical sensor is based on the ability of kinases to transfer a redox-labeled phosphoryl group to surface-bound peptides that are highly specific substrates for the particular protein kinase (EGIYDVP, EPLTPSG, and HHASPRK, respectively). The detection method relies on the use of 5'-γ-ferrocenoyl-ATP (Fc-ATP) as a co-substrate for peptide phosphorylation. The peptides themselves are attached to a Au substrate, which acts as the working electrode. In this process a Fc-phosphoryl group is transferred to the peptide and the presence of the redox active Fc group is detected electrochemically. All peptide films were fully characterized by cyclic voltammetry (CV), square wave voltammetry (SWV), and electrochemical impedance spectroscopy (EIS). Particular attention was given to the electron transfer rates, k(ET), in peptide films after Fc-phosphorylation which were found to be on the order of seconds. The slow ET kinetics is presumably a result of the negative charge on the phosphoryl group. Time-of-flight secondary ion mass spectrometry (TOF-SIMS) and X-ray photoelectron spectroscopy (XPS) experiments based on the peptide modified Au surfaces reveal significant ferrocene and phosphate group content introduced using the kinase-catalyzed phosphorylation reaction.

Fundamental Electrochemical Properties of Carbon Nanotube Electrodes

We report on the fundamental electrochemical properties of inherently hydrophobic Multiwalled Carbon Nanotubes MWNT electrodes and acid treated MWNTs. The electrochemical response of the electrodes was investigated by cyclic voltammetry and ac impedance spectroscopy using the redox probe of (Fe(CN)6 3−/Fe(CN)6 4−, which has well‐known kinetic behaviour on carbon surfaces. Comparisons were made between the electrochemical behaviour observed at MWNT electrodes and electrochemically polished glassy carbon, GCE. Confirmation of carbon oxygen functionalities on the surface of acid treated nanotubes was accomplished by X‐ray photoelectron spectroscopy. Slow electron transfer kinetics with a peak separation larger than 200 eV, and an electron transfer resistance of approximately 370 Ω were observed on pristine MWNTs. However acid treatment was found to significantly improve the electron transfer kinetics of Fe(CN)6 −3/−4 redox couple, approaching almost reversible ET kinetics. Acid treatment modifies the charge carrier density at the surface by introducing surface states and serves to facilitate electron transfer. The better wetting properties of functionalised carbon nanotubes and the higher local density of states as compared to untreated MWNT could be responsible for favouring faster ET kinetics.

Fundamental Electrochemical Properties of Carbon Nanotube Electrodes

We report on the fundamental electrochemical properties of inherently hydrophobic Multi‐walled Carbon Nanotubes (MWNT) electrodes and acid‐treated MWNTs. The electrochemical response of the electrodes was investigated by cyclic voltammetry and AC impedance spectroscopy using the redox probe of (Fe(CN)6 3−/Fe(CN)6 4−, which has well‐known kinetic behaviour on carbon surfaces. Comparisons were made between the electrochemical behaviour observed at MWNT electrodes and electrochemically polished glassy carbon, GCE. Confirmation of carbon oxygen functionalities on the surface of acid‐treated nanotubes was accomplished by x‐ray photoelectron spectroscopy. Slow electron transfer kinetics with a peak separation larger than 200 eV, and an electron transfer resistance of approximately 370 Ω were observed on pristine MWNTs. However, acid treatment was found to significantly improve the electron transfer kinetics of Fe(CN)6 −3/−4 redox couple, approaching almost reversible ET kinetics. Acid treatment modifies the charge carrier density at the surface by introducing surface states and serves to facilitate electron transfer. The better wetting properties of functionalised carbon nanotubes and the higher local density of states as compared to untreated MWNT could be responsible for favouring faster ET kinetics.

Kinetics of the Reduction of Dialkyl Peroxides. New Insights into the Dynamics of Dissociative Electron Transfer1

The concerted dissociative reduction of di-tert-butyl peroxide (DTBP), dicumyl peroxide (DCP), and di-n-butyl peroxide (DNBP) is evaluated by both heterogeneous and homogeneous electron transfer using electrochemical methods. Electrochemical and thermochemical determination of the O−O bond energies and the standard potentials of the alkoxyl radicals allow the standard potentials for dissociative reduction of the three peroxides in N,N-dimethylformamide and acetonitrile to be evaluated. These values allowed the kinetics of homogeneous ET reduction of DTBP and DCP by a variety of radical anion donors to be evaluated as a function of overall driving force. Comparison of the heterogeneous ET kinetics of DTBP and DNBP as a function of driving force for ET allowed the distance dependence on the reduction kinetics of the former to be estimated. Results indicate that the kinetics of ET to DTBP is some 0.8 order of magnitude slower in reactivity than DNBP because of a steric effect imposed by the bulky tert-butyl ...

Transient and time-resolved resonance Raman investigation of photoinitiated electron transfer in ruthenated cytochromes c.

Ruthenation of exterior amino acid residues of heme proteins provides an effective means by which biological ET reactions can be studied within the context of highly complex protein environments. Resonance Raman spectroscopy can probe both ET kinetics and structural dynamics at the molecular level. Here we present the first comprehensive use of time-resolved and transient resonance Raman spectroscopies to examine photoinduced ET in cytochromes. Two ruthenated cytochromes c, Ru(lys72)-cyt.c and Ru(cyt102)cyt.c, were studied with TRRS using 10 ns laser pulses and with TRRRS on a 10 ns to 10 ms time scale. It was found that resonance Raman protocols can effectively trace ET kinetics and associated heme--protein structural dynamics. Care must be exercised, however, when drawing comparisons to measurements made by other methods (i.e., transient absorbance). The TRRS studies directly probe the heme and its local environment and reveal that the heme dynamics accompanying ET are very rapid relative to phenomenological ET kinetics. The heme and its local environment evolve to their equilibrium (ferrous) structure in less than 10 ns subsequent to ET, with no evidence for the existence of metastable heme pocket geometries analogous to those observed in the dynamic response of hemoglobins and oxidases. Finally, species-specific differences are observed in the photoinduced ET kinetics and heme structural dynamics. However, these differences are confined to nanosecond or faster time scales.