New Insights into the Analysis of the Electrode Kinetics of Flavin Adenine Dinucleotide Redox Center of Glucose Oxidase Immobilized on Carbon Electrodes

Abstract

New insights into electrochemical kinetics of the flavin adenine dinucleotide (FAD) redox center of glucose-oxidase (GlcOx) immobilized on reduced graphene oxide (rGO), single- and multiwalled carbon nanotubes (SW and MWCNT), and combinations of rGO and CNTs have been gained by application of Fourier transformed AC voltammetry (FTACV) and simulations based on a range of models. A satisfactory level of agreement between experiment and theory, and hence establishment of the best model to describe the redox chemistry of FAD, was achieved with the aid of automated e-science tools. Although still not perfect, use of Marcus theory with a very low reorganization energy (≤0.3 eV) best mimics the experimental FTACV data, which suggests that the process is gated as also deduced from analysis of FTACV data obtained at different frequencies. Failure of the simplest models to fully describe the electrode kinetics of the redox center of GlcOx, including those based on the widely employed Laviron theory is demonstrated, as is substantial kinetic heterogeneity of FAD species. Use of a SWCNT support amplifies the kinetic heterogeneity, while a combination of rGO and MWCNT provides a more favorable environment for fast communication between FAD and the electrode.

New Evidence for a Quasi-Simultaneous Proton-Coupled Two-Electron Transfer and Direct Wiring for Glucose Oxidase Captured by the Carbon Nanotube–Polymer Matrix

“…Moreover, we founded our introductory mechanistic analysis on the data-fitting algorithm which is based on the Marcus theory, hence taking into the account the curved character (parabolic shape) of the resultant Gibbs energy wells determining the ET event. Although our further careful analysis indicated that the actual shape of these energy wells is even more complicated (seemingly containing a large higher-order contribution), we rejected Laviron's approach 66 which is based on the Butler−Volmer theory using solely linear approximations for energy wells; see the Supporting Information and ref 23 (pp 97−98, 250) and ref 67. Furthermore, the extended mechanistic analysis based on the generalized charge-transfer theory for the proton-coupled electron transfer, 32−34 furnished by the first-time temperatureallied extension of protein film voltammetry, essentially supports recent deduction by Bond et al 20 in favor of a quasi-simultaneous two-electron transfer mechanism (Note, both, the "true" simultaneous and quasi-simultaneous twoelectron transfer mechanisms, according to basic physicochemical notions, should display very similar CV patterns shown in Figure 1; the quasi-simultaneous mechanism has been deduced based on the well-recognizable condition, viz., the existence of an intrinsic thermodynamic asymmetry between two electrons to be transferred, vide supra; see also ref 20). In addition, the "anomalies" disclosed through the simulation procedure suggest deviations from the "classical" Marcus' model that implied the medium's linear response.…”

“…Application of advanced multilevel Fourier transform ac voltammetry and respective simulations, based on a range of models, allowed only for a general conclusion in favor of quasi-simultaneous two-electron transfer and the extra specific mechanistic complexity, seemingly due to the involvement of a diffusional motion of reactive protons and the accompanying conformational transformation of the FAD cofactor itself, suggested the latter as resulting in the so-called "gated" mechanism for the ET process. 20 Further efforts with the involvement of the Marcus model yielded a rather low value for the reorganization energy (≤0.3 eV, vide infra) and a broad spectrum of floating values for the effective rate constant, spanning over 5−3000 s −1 . 20 Obviously, further in-depth experimental studies and theorybased analyses were required for a better comprehension of the GOx and FAD multifaceted redox mechanisms using additional experimental approaches such as, e.g., temperature-assisted kinetic cyclic voltammetry (CV) studies, allowing independent determination of the additional important physical parameter the enthalpy of activationthus providing a complementary way for the verification of the reorganization energy; hence, our work offers a new prospect for a thorough testing of the ET pattern (vide infra).…”

“…20 Further efforts with the involvement of the Marcus model yielded a rather low value for the reorganization energy (≤0.3 eV, vide infra) and a broad spectrum of floating values for the effective rate constant, spanning over 5−3000 s −1 . 20 Obviously, further in-depth experimental studies and theorybased analyses were required for a better comprehension of the GOx and FAD multifaceted redox mechanisms using additional experimental approaches such as, e.g., temperature-assisted kinetic cyclic voltammetry (CV) studies, allowing independent determination of the additional important physical parameter the enthalpy of activationthus providing a complementary way for the verification of the reorganization energy; hence, our work offers a new prospect for a thorough testing of the ET pattern (vide infra). In the present paper, we report on fundamental studies of electron transfer of GOx that is immobilized into a single-walled carbon nanotube−poly-(ethylenimine) (SWCNT−PEI) matrix.…”

Electrochemical evidence that pyranopterin redox chemistry controls the catalysis of YedY, a mononuclear Mo enzyme

“…Fourier-transformed alternating-current voltammetry (FTacV) is a technique that offers the ability to measure catalytic chemical redox reactions and reversible electron transfer processes in a single experiment (11,12). In the FTacV measurement, a large-amplitude sine wave of frequency f is superimposed on a linear voltage-time sweep (11,(13)(14)(15)) and the resulting current-time response is measured and then Fourier transformed (FT) into the frequency domain to give a power spectrum of harmonic contributions at frequencies f, 2f, 3f, etc. Band selection of the individual harmonics followed by inverse FT resolves the data back into the time domain.…”

Flavin-sensitized electrode system for oxygen evolution using photo-electrocatalysis

“…The obtained value of E°´=−0.48 V (E°´=(E pc +E pa )/2, where E pc is the cathodic and E pa is the anodic peak potential, vs. Ag/AgCl) is very close to the value of -0.44 V reported for the two-electron reduction of flavins in solution. 30 To learn whether electron transfer is governed by diffusioncontrolled processes or an adsorption process, voltammetric scans were performed at different scan rates ranging from 20mV/s to 100 mV/s. A log-log plot of anodic current vs. the scan rate revealed a linear relationship with a slope of 0.95, which is very close to the value of 1 expected for electron transfer by electrode-adsorbed molecules ( Figure 1b) and distinct from the slope of 0.5 expected for diffusion-controlled processes.…”