Redox and redox-coupled processes of heme proteins and enzymes at electrochemical interfaces

Electron Tunneling in Biology: When Does it Matter?

Electrons can tunnel between cofactor molecules positioned along biological electron transport chains up to a distance of ≃ 20 Å on the millisecond time scale of enzymatic turnover. This tunneling range determines the design of biological energy chains facilitating the cross-membrane transport of electrons. Tunneling distance and cofactors' redox potentials become the main physical parameters affecting the rate of electron transport. In addition, universal charge-transport properties are assigned to all proteins, making protein identity, flexibility, and dynamics insignificant. This paradigm is challenged by dynamical models of electron transfer, showing that the electron hopping rate is constant within the crossover distance R* ≃ 12 Å, followed with an exponential falloff at longer distances. If this hypothesis is fully confirmed, natural and man-made energy chains for electron transport should be best designed by placing redox cofactors near the crossover distance R*. Protein flexibility and dynamics affect the magnitude of the maximum hopping rate within the crossover distance. Changes in protein flexibility between forward and backward transitions contribute to vectorial charge transport. For biological energy chains, charge transport through proteins is not defined by universal parameters, and protein identity matters.

Cyclic voltammetry, square wave voltammetry or electrochemical impedance spectroscopy? Interrogating electrochemical approaches for the determination of electron transfer rates of immobilized redox proteins

In this article, we cross-examine three well-established electrochemical approaches, namely cyclic voltammetry (CV), cyclic square-wave voltammetry (SWV) and electrochemical impedance spectroscopy (EIS) to dissect the electron transfer (ET) rate of electrostatically immobilized cytochrome c on Ag electrodes. A detailed analysis supported by simulations of redox transition provided three distinct values for the heterogeneous electron transfer (HET) rate constant of cyt c interfaced on COOH-terminated C₁₀-long alkanethiol, i.e., k_HET= 47.8 (±2,91) s^-1 in CV, k_HET= 64.8 (±1,27) s^-1 in SWV, and k_HET= 26.5 s^-1 in EIS. We discuss the obtained discrepancies obtained from electrochemical methods and compare them with the data from spectro-electrochemical experiments. A comprehensive selection list is created from which the most applicable approach can be chosen for studying proteins of interest. CV is most applicable to study the interfaced proteins exhibiting k_HET of ca. 0.5 - 70 s^-1, SWV is suitable for a broader range of k_HET of 5 - 120 s^-1 and EIS for k_HET of 0.5 to 5 s^-1 if alkanethiols are used as immobilization strategy.

Effect of Water Deuteration on Protein Electron Transfer

Traditional theories of long-range protein electron transfer describe the reaction rate in terms of the tunneling distance and the reaction free energy. They do not recognize two physical effects: (i) local wetting of the active site by hydration water and (ii) protein identity affecting the rate through dynamics and flexibility. We find, by molecular dynamics simulations, a significant, ∼25 times, slowing down of the rate of protein electron transfer upon deuteration. H/D substitution changes the rate constant pre-exponential factor in the regime of electron transfer controlled by medium dynamics. Switching from light to heavy water increases the effective medium relaxation time. The effect is caused by both a global change in the flexibility of the protein backbone and locally stronger hydrogen bonds to charged residues.

Theory of Protein Charge Transfer: Electron Transfer between Tryptophan Residue and Active Site of Azurin

One reaction step in the conductivity relay of azurin, electron transfer between the Cu-based active site and the tryptophan residue, is studied theoretically and by classical molecular dynamics simulations. Oxidation of tryptophan results in electrowetting of this residue. This structural change makes the free energy surfaces of electron transfer nonparabolic as described by the Q-model of electron transfer. We analyze the medium dynamical effect on protein electron transfer produced by coupled Stokes-shift dynamics and the dynamics of the donor-acceptor distance modulating electron tunneling. The equilibrium donor-acceptor distance falls in the plateau region of the rate constant, where it is determined by the protein-water dynamics, and the probability of electron tunneling does not affect the rate. The crossover distance found here puts most intraprotein electron-transfer reactions under the umbrella of dynamical control. The crossover between the medium-controlled and tunneling-controlled kinetics is combined with the effect of the protein-water medium on the activation barrier to formulate principles of tunability of protein-based charge-transfer chains. The main principle in optimizing the activation barrier is the departure from the Gaussian-Gibbsian statistics of fluctuations promoting activated transitions. This is achieved either by incomplete (nonergodic) sampling, breaking the link between the Stokes-shift and variance reorganization energies, or through wetting-induced structural changes of the enzyme's active site.

Label-Free Surface-Enhanced Raman Spectroscopic Analysis of Proteins: Advances and Applications

Surface-enhanced Raman spectroscopy (SERS) is powerful for structural characterization of biomolecules under physiological condition. Owing to its high sensitivity and selectivity, SERS is useful for probing intrinsic structural information of proteins and is attracting increasing attention in biophysics, bioanalytical chemistry, and biomedicine. This review starts with a brief introduction of SERS theories and SERS methodology of protein structural characterization. SERS-active materials, related synthetic approaches, and strategies for protein-material assemblies are outlined and discussed, followed by detailed discussion of SERS spectroscopy of proteins with and without cofactors. Recent applications and advances of protein SERS in biomarker detection, cell analysis, and pathogen discrimination are then highlighted, and the spectral reproducibility and limitations are critically discussed. The review ends with a conclusion and a discussion of current challenges and perspectives of promising directions.

How to Turn an Electron Transfer Protein into a Redox Enzyme for Biosensing

Cytochrome c is a small globular protein whose main physiological role is to shuttle electrons within the mitochondrial electron transport chain. This protein has been widely investigated, especially as a paradigmatic system for understanding the fundamental aspects of biological electron transfer and protein folding. Nevertheless, cytochrome c can also be endowed with a non-native catalytic activity and be immobilized on an electrode surface for the development of third generation biosensors. Here, an overview is offered of the most significant examples of such a functional transformation, carried out by either point mutation(s) or controlled unfolding. The latter can be induced chemically or upon protein immobilization on hydrophobic self-assembled monolayers. We critically discuss the potential held by these systems as core constituents of amperometric biosensors, along with the issues that need to be addressed to optimize their applicability and response.

Identifying conformational changes of aptamer binding to theophylline: A combined biolayer interferometry, surface-enhanced Raman spectroscopy, and molecular dynamics study

Theophylline is a potent bronchodilator for the treatment of asthma, bronchitis, and emphysema. Its narrow therapeutic window (20-100 μM) demands that the blood concentration of theophylline be monitored carefully, which can be achieved by aptamer capture. Thus, an understanding of what occurs when aptamers bind to theophylline is critical for identifying a high-affinity and high-specificity aptamer, which improve the sensitivity and selectivity of theophylline detection. Consequently, there is an urgent need to develop a simple, convenient, and nondestructive method to monitor conformational changes during the binding process. Here, we report the determination of the affinity of a selected aptamer and theophylline via biolayer interferometry (BLI) experiments. Additionally, using surface-enhanced Raman spectroscopy (SERS), the conformational changes on theophylline-aptamer binding were identified from differences in the SER spectra. Finally, molecular dynamics (MD) simulations were used to identify the specific conformational changes of the aptamer during the binding process. Such a combined BLI-SERS-MD method provides an in-depth understanding of the theophylline-aptamer binding processes and a comprehensive explanation for conformational changes, which helps to select, design, and modify an aptamer with high affinity and specificity. It can also be used as a scheme for the study of other aptamer-ligand interactions, which can be applied to the detection, sensing, clinical diagnosis, and treatment of diseases.

Probing Structure and Reaction Dynamics of Proteins Using Time-Resolved Resonance Raman Spectroscopy

The mechanistic understanding of protein functions requires insight into the structural and reaction dynamics. To elucidate these processes, a variety of experimental approaches are employed. Among them, time-resolved (TR) resonance Raman (RR) is a particularly versatile tool to probe processes of proteins harboring cofactors with electronic transitions in the visible range, such as retinal or heme proteins. TR RR spectroscopy offers the advantage of simultaneously providing molecular structure and kinetic information. The various TR RR spectroscopic methods can cover a wide dynamic range down to the femtosecond time regime and have been employed in monitoring photoinduced reaction cascades, ligand binding and dissociation, electron transfer, enzymatic reactions, and protein un- and refolding. In this account, we review the achievements of TR RR spectroscopy of nearly 50 years of research in this field, which also illustrates how the role of TR RR spectroscopy in molecular life science has changed from the beginning until now. We outline the various methodological approaches and developments and point out current limitations and potential perspectives.

Strongly Coupled Redox-Linked Conformational Switching at the Active Site of the Non-Heme Iron-Dependent Dioxygenase, TauD

2-Oxoglutarate (2OG)-dependent dioxygenases catalyze C-H activation while performing a wide range of chemical transformations. In contrast to their heme analogues, non-heme iron centers afford greater structural flexibility with important implications for their diverse catalytic mechanisms. We characterize an in situ structural model of the putative transient ferric intermediate of 2OG:taurine dioxygenase (TauD) by using a combination of spectroelectrochemical and semiempirical computational methods, demonstrating that the Fe(III/II) transition involves a substantial, fully reversible, redox-linked conformational change at the active site. This rearrangement alters the apparent redox potential of the active site between -127 mV for reduction of the ferric state and +171 mV for oxidation of the ferrous state of the 2OG-Fe-TauD complex. Structural perturbations exhibit limited sensitivity to mediator concentrations and potential pulse duration. Similar changes were observed in the Fe-TauD and taurine-2OG-Fe-TauD complexes, thus attributing the reorganization to the protein moiety rather than the cosubstrates. Redox-difference infrared spectra indicate a reorganization of the protein backbone in addition to the involvement of carboxylate and histidine ligands. Quantitative modeling of the transient redox response using two alternative reaction schemes across a variety of experimental conditions strongly supports the proposal for intrinsic protein reorganization as the origin of the experimental observations.

Axial ligands tailoring the ORR activity of cobalt porphyrin

In an effort to provide visualization and understanding to the electronic "push effect" of axial ligands on the catalytic activity of cobalt macrocyclic molecules, we design a simple model system involving an [5,10,15,20-tetrakis(4-methoxyphenyl)porphyrin]cobalt(II) (TMMPCo) monolayer axially-coordinated on thiol ligand modified Au electrode and explore the activity of the axial-ligand coordinated TMPPCo toward oxygen reduction reaction (ORR) in acidic medium. Three different ligands, with a decreasing order of coordinating ability as: 4-mercaptopyridine (MPy) > 4-aminothiolphenol (APT) > 4-mercaptobenzonitrile (MBN) are used and a maximum difference in ORR onset potential of 80 mV is observed between the MPy (highest onset potential) and MBN systems (lowest onset potential). The ORR activity of TMPPCo increases with the increase in binding strength of the axial ligand. A detailed mechanism study reveals that ORR on the three ligand coordinated TMPPCo systems shares the same 2-electron mechanism with H₂O₂ as the terminal product. Theoretical calculation into the structure of the ligand coordinated cobalt porphyrins uncovers the variation in atomic charge of the Co(II) center and altered frontier molecular orbital distribution among the three ligand systems. Both properties have great influence on the back-bonding formation between the Co(II) center and O₂ molecules, which has been suggested to be critical toward the O₂ adsorption and subsequent activation process.

Fourier Transform Infrared Spectrovoltammetry and Quantitative Modeling of Analytes in Kinetically Constrained Redox Mixtures

Redox-active analytes that do not support direct electron transfer on the electrode, such as proteins with buried redox centers, pose challenges to characterization of their structural and thermodynamic properties. Investigations of indirect transitions in analytes supported by complex redox mixtures require a careful balance between kinetic limitations and spectral interference from the mediators. Using methylene green and thionine acetate as redox mediators and myoglobin as the analyte, we demonstrate that normal pulse spectrovoltammetry (NPSV) with Fourier transform infrared (FT-IR) detection and subsequent global spectral regression analysis can resolve structural and thermodynamic properties simultaneously with little a priori information. Both the E_1/2 and unbiased redox difference FT-IR spectra of the Fe(II)/Fe(III) redox couple of myoglobin in reduction and oxidation NPSV modes were in good agreement with those reported earlier by independent techniques. The thermodynamic and kinetic limitations of mediators/analyte interactions were investigated using comprehensive semiempirical kinetic simulation models. This modeling effort yielded a flexible computational tool capable of quantitatively predicting the redox response in mediated electrochemical studies and defining its limitations, thus greatly expanding the range and precision of the formal mediator/analyte concentration ratio rule.

Structural Changes in Adsorbed Cytochrome c upon Applied Potential Characterized by Neutron Reflectometry

The structural behavior of an electron-transfer protein, cytochrome c, at the 316L stainless steel electrode/aqueous interface was investigated over a range of applied potentials using neutron reflectometry supported by solution depletion isotherms, X-ray reflectometry, and quartz crystal microbalance measurements. A custom-made electrochemical cell allowed in situ observation of the adsorbed protein across a range of applied potentials; models fitted to the NR data showed a compact inner protein layer at the metal/electrolyte interface and a further thicker but highly diffuse layer that could be removed by rinsing. The overall amount adsorbed was found to be strongly dependent on the applied potential and buffer pH. Subtle but significant changes in the structure of the adsorbed protein layer were seen as the potential was swept between ±0.40 V, reflecting changing attractive/repulsive interactions between the protein's charged side groups and the surface. At greater applied potentials, irreversible changes in the stainless steel film structure were also observed and attributed to deuterium absorption into the metal.

The influence of pH and divalent/monovalent cations on the internal electron transfer (IET), enzymatic activity, and structure of fructose dehydrogenase

We report on the influence of pH and monovalent/divalent cations on the catalytic current response, internal electron transfer (IET), and structure of fructose dehydrogenase (FDH) by using amperometry, spectrophotometry, and circular dichroism (CD). Amperometric measurements were performed on graphite electrodes, onto which FDH was adsorbed and the effect on the response current to fructose was investigated when varying the pH and the concentrations of divalent/monovalent cations in the contacting buffer. In the presence of 10 mM CaCl₂, a current increase of up to ≈ 240% was observed, probably due to an intra-complexation reaction between Ca²⁺ and the aspartate/glutamate residues found at the interface between the dehydrogenase domain and the cytochrome domain of FDH. Contrary to CaCl₂, addition of MgCl₂ did not show any particular influence, whereas addition of monovalent cations (Na⁺ or K⁺) led to a slight linear increase in the maximum response current. To complement the amperometric investigations, spectrophotometric assays were carried out under homogeneous conditions in the presence of a 1-electron non-proton-acceptor, cytochrome c, or a 2-electron-proton acceptor, 2,6-dichloroindophenol (DCIP), respectively. In the case of cytochrome c, it was possible to observe a remarkable increase in the absorbance up to 200% when 10 mM CaCl₂ was added. However, by further increasing the concentration of CaCl₂ up to 50 mM and 100 mM, a decrease in the absorbance with a slight inhibition effect was observed for the highest CaCl₂ concentration. Addition of MgCl₂ or of the monovalent cations shows, surprisingly, no effect on the electron transfer to the electron acceptor. Contrary to the case of cytochrome c, with DCIP none of the cations tested seem to affect the rate of catalysis. In order to correlate the results obtained by amperometric and spectrophotometric measurements, CD experiments have been performed showing a great structural change of FDH when increasing the concentration CaCl₂ up to 50 mM, at which the enzyme molecules start to agglomerate, hindering the substrate access to the active site probably due to a chelation reaction occurring at the enzyme surface with the glutamate/aspartate residues.

Graphical Abstract

Structure, electrocatalysis and dynamics of immobilized cytochrome PccH and its microperoxidase

Geobacter sulfurreducens cells have the ability to exchange electrons with conductive materials, and the periplasmic cytochrome PccH plays an essential role in the direct electrode-to-cell electron transfer in this bacterium. It has atypically low redox potential and unique structural features that differ from those observed in other c-type cytochromes. We report surface enhanced resonance Raman spectroscopic and electrochemical characterization of the immobilized PccH, together with molecular dynamics simulations that allow for the rationalization of experimental observations. Upon attachment to electrodes functionalized with partially or fully hydrophobic self-assembled monolayers, PccH displays a distribution of native and non-native heme spin configurations, similar to those observed in horse heart cytochrome c. The native structural and thermodynamic features of PccH are preserved upon attachment mixed hydrophobic (-CH₃/-NH₂) surfaces, while pure -OH, -NH₂ and -COOH surfaces do not provide suitable platforms for its adsorption, indicating that its still unknown physiological redox partner might be membrane integrated. Neither of the employed immobilization strategies results in electrocatalytically active PccH capable of the reduction of hydrogen peroxide. Pseudoperoxidase activity is observed in immobilized microperoxidase, which is enzymatically produced from PccH and spectroscopically characterized. Further improvement of PccH microperoxidase stability is required for its application in electrochemical biosensing of hydrogen peroxide.

Protein bioelectronics: a review of what we do and do not know

We review the status of protein-based molecular electronics. First, we define and discuss fundamental concepts of electron transfer and transport in and across proteins and proposed mechanisms for these processes. We then describe the immobilization of proteins to solid-state surfaces in both nanoscale and macroscopic approaches, and highlight how different methodologies can alter protein electronic properties. Because immobilizing proteins while retaining biological activity is crucial to the successful development of bioelectronic devices, we discuss this process at length. We briefly discuss computational predictions and their connection to experimental results. We then summarize how the biological activity of immobilized proteins is beneficial for bioelectronic devices, and how conductance measurements can shed light on protein properties. Finally, we consider how the research to date could influence the development of future bioelectronic devices.

Hydrogen evolution by cobalt hangman porphyrins under operating conditions studied by vibrational spectro-electrochemistry

The structure of cobalt hangman complexes adsorbed on electrodes during HER was analysed via surface enhanced Raman spectroscopy.

Insights into Interprotein Electron Transfer of Human Cytochrome c Variants Arranged in Multilayer Architectures by Means of an Artificial Silica Nanoparticle Matrix

The redox behavior of proteins plays a crucial part in the design of bioelectronic systems. We have demonstrated several functional systems exploiting the electron exchange properties of the redox protein cytochrome c (cyt c) in combination with enzymes and photoactive proteins. The operation is based on an effective reaction at modified electrodes but also to a large extent on the capability of self-exchange between cyt c molecules in a surface-fixed state. In this context, different variants of human cyt c have been examined here with respect to an altered heterogeneous electron transfer (ET) rate in a monolayer on electrodes as well as an enhanced self-exchange rate while being incorporated in multilayer architectures. For this purpose, mutants of the wild-type (WT) protein have been prepared to change the chemical nature of the surface contact area near the heme edge. The structural integrity of the variants has been verified by NMR and UV-vis measurements. It is shown that the single-point mutations can significantly influence the heterogeneous ET rate at thiol-modified gold electrodes and that electroactive protein/silica nanoparticle multilayers can be constructed with all forms of human cyt c prepared. The kinetic behavior of electron exchange for the mutant proteins in comparison with that of the WT has been found altered in some multilayer arrangements. Higher self-exchange rates have been found for K79A. The results demonstrate that the position of the introduced change in the charge situation of cyt c has a profound influence on the exchange behavior. In addition, the behavior of the cyt c variants in assembled multilayers is found to be rather similar to the situation of cyt c self-exchange in solution verified by NMR.

Cofactor-specific covalent anchoring of cytochrome b562on a single-walled carbon nanotube by click chemistry

Redox-active cytochromeb₅₆₂with a tethered azide group on the heme propionate side chain is covalently linked to an acetylene moiety introduced on the sidewall of a single-walled carbon nanotube by copper-catalyzed click chemistry.

Gold nanoparticle-mediated electron transfer of cytochrome c on a self-assembled surface

Gold nanoparticles provide short-range electron transfer between cytochrome c and gold electrode.

2nd coordination sphere controlled electron transfer of iron hangman complexes on electrodes probed by surface enhanced vibrational spectroscopy

Surface enhanced vibrational spectroscopy shows the correlation between electron transfer kinetics and protonation degree of Fe Hangman complexes on electrodes.

Iron hangman complexes exhibit improved catalytic properties regarding O₂ and H₂O₂ reduction, which are attributed to the presence of a proton donating group in defined vicinity of the catalytic metal centre. Surface enhanced resonance Raman (SERR) and IR (SEIRA) spectro-electrochemistry has been applied concomitantly for the first time to analyse such iron hangman porphyrin complexes attached to electrodes in aqueous solution. While the SERR spectra yield information about the redox state of the central iron, the SEIRA spectra show protonation and deprotonation events of the 2^nd coordination sphere. To investigate the influence of a proton active hanging group on the heterogeneous electron transfer between the iron porphyrin and the electrode, two hangman complexes with either an acid or ester functional group were compared. Using time resolved SERR spectroscopy the electron transfer rates of both complexes were determined. Complexes with an acid group showed a slow electron transfer rate at neutral pH that increased significantly at pH 4, while complexes with an ester group exhibited a much faster, but pH independent rate. SEIRA measurements were able to determine directly for the first time a pK_a value of 3.4 of a carboxylic hanging group in the immobilized state that shifted to 5.2 in D₂O buffer solution. The kinetic data showed an increase of the heterogeneous electron transfer rate with the protonation degree of the acid groups. From these results, we propose a PCET which is strongly modulated by the protonation state of the acid hanging group via hydrogen bond interactions.

Surface Enhanced Resonance Raman Spectroscopy Reveals Potential Induced Redox and Conformational Changes of Cytochrome c Oxidase on Electrodes

Immobilization of Cytochrome c oxidase (CcO) on electrodes makes voltage-driven reduction of oxygen to water possible. Efficient catalytic turnover in CcO/electrode systems is, however, often observed at large overpotentials that cannot be rationalized by the redox properties of the enzyme itself. To understand the structural basis for this observation, CcO was electrostatically adsorbed on amino-functionalized Ag electrodes, and the redox transitions of heme a and a3 were monitored via surface enhanced resonance Raman spectroscopy (SERRS) as a function of applied potential. Under completely anaerobic conditions, the reduction of heme a3 could be seen at potentials close to those measured in solution indicating an intact catalytic center. However, in the immobilized state, a new non-native heme species was observed that exhibited a redox potential much more negative than measured for the native hemes. Analysis of the high and low frequency SERR spectra indicated that this new species is formed from heme a upon axial loss of one histidine ligand. It is concluded that the formation of the non-native heme a species alters the potential-dependent electron supply to the catalytic reaction and, thus, can have a impact on the applicability of this enzyme in biofuel cells.

Nature of the Surface-Exposed Cytochrome-Electrode Interactions in Electroactive Biofilms of Desulfuromonas acetoxidans

Metal-respiring bacteria are microorganisms capable of oxidizing organic pollutants present in wastewater and transferring the liberated electrons to an electrode. This ability has led to their application as catalysts in bioelectrochemical systems (BESs), a sustainable technology coupling bioremediation to electricity production. Crucial for the functioning of these BESs is a complex protein architecture consisting of several surface-exposed multiheme proteins, called outer membrane cytochromes, wiring the cell metabolism to the electrode. Although the role of these proteins has been increasingly understood, little is known about the protein-electrode interactions and their impact on the performance of BESs. In this study, we used surface-enhanced resonance Raman spectroscopy in combination with electrochemical techniques to unravel the nature of the protein-electrode interaction for the outer membrane cytochrome OmcB from Desulfuromonas acetoxidans (Dace). Comparing the spectroelectrochemical properties of OmcB bound directly to the electrode surface with those of the same protein embedded inside an electroactive biofilm, we have shown that the surface-exposed cytochromes of Dace biofilms are in direct contact with the electrode surface. Even if direct binding causes protein denaturation, the biofilm possesses the ability to minimize the extent of the damage maximizing the amount of cells in direct electrical communication with the electrode.

Towards the understanding of the effect of oxygen on the electrocatalytic activity of microbial biofilms: a spectroelectrochemical study

Metal-respiring bacteria oxidize an organic substrate and transfer the liberated electrons to the electrode. Molecular oxygen interrupts the current flow by cutting the electrical circuit wiring the cell metabolism to the electrode.

Temperature-Driven Changeover in the Electron-Transfer Mechanism of a Thermophilic Plastocyanin

Electron-transfer kinetics of the thermophilic protein Plastocyanin from Phormidium laminosum adsorbed on 1,ω-alkanedithiol self-assembled monolayers (SAMs) deposited on gold have been investigated. The standard electron-transfer rate constant has been determined as a function of electrode-protein distance and solution viscosity over a broad temperature range (0-90 °C). For either thin or thick SAMs, the electron-transfer regime remains invariant with temperature, whereas for the 1,11-undecanethiol SAM of intermediate chain length, a kinetic regime changeover from a gated or friction-controlled mechanism at low temperature (0-30 °C) to a nonadiabatic mechanism above 40 °C is observed. To the best of our knowledge, this is the first time a thermal-induced transition between these two kinetic regimes is reported for a metalloprotein.

Spectroelectrochemical analyses of electroactive microbial biofilms

Understanding the mechanism of ET (electron transfer) through electroactive microbial biofilms is a challenge in the field of fundamental and applied life sciences. To date, electrochemical techniques such as CV (cyclic voltammetry) have been applied successfully to study the ET process in intact microbial biofilms on electrodes, providing important insight into their redox properties. However, CV as such does not provide any structural information about the species involved in the redox process. This shortcoming may limit the understanding of the ET process in microbial biofilms. To overcome this restriction, spectroelectrochemical techniques have been designed consisting of a spectroscopic technique performed in combination with electrochemical methods on the same electrode sample. These analytical approaches allow in vivo measurements of microbial biofilms under physiologically relevant conditions and controlled applied potential. The present review describes these spectroelectrochemical methodologies and critically addresses their impact on the understanding of the ET through biofilms.