Quantification of Enzymatic Biofilm Removal Using the Sauerbrey Equation: Application to the Case of Pseudomonas protegens

A methodology for the quantitative analysis of enzymatic removal of biofilms (BF) was developed, based on a quartz crystal microbalance (QCM) under stationary conditions. This was applied to the case of Pseudomonas protegens (PP) BFs, through a series of five enzymes, whose removal activity was screened using the presented methodology. The procedure is based on the following: when BFs can be modeled as rigid materials, QCM can be used as a balance under stationary conditions for determining the BFs mass reduction by enzymatic removal. For considering a BF as a rigid model, energy dissipation effects, associated with viscoelastic properties of the BF, must be negligible. Hence, a QCM system with detection of dissipation (referred to as QCM with dissipation) was used for evaluating the energy losses, which, in fact, resulted in negligible energy losses in the case of dehydrated PP BFs, validating the application of the Sauerbrey equation for the change of mass calculations. The stationary methodology reduces operating times and simplifies data analysis in comparison to dynamic approaches based on flow setups, which requires the incorporation of dissipation effects due to the liquid media. By carrying out QCM, glycosidase-type enzymes showed BF removal higher than 80% at enzyme concentration 50 ppm, reaching removal over 90% in the cases of amylase and cellulase/xylanase enzymes. The highest removal percentage produced a reduction from about 15 to 1 μg in the BF mass. Amylase enzyme was tested from below 50 to 1 ppm, reaching around 60% of removal at 1 ppm. The obtained results were supported by other instrumental techniques such as Raman spectroscopy, attenuated total reflection Fourier transform infrared spectroscopy, atomic force microscopy, high performance anion exchange chromatography, thermogravimetric analysis, and differential scanning calorimetry. The removal quantifications obtained with QCM were compared with those obtained by well-established screening techniques (UV-vis spectrophotometry using crystal violet and agar diffusion test). The proposed methodology expands the possibility of using a quartz microbalance to perform enzymatic activity screening.

Operando detection and suppression of spurious singlet oxygen in Li-O2 batteries

The rechargeable lithium air (oxygen) battery (Li-O2) has very high energy density, comparable to that of fossil fuels (∼3600 W h kg-1). However, the parasitic reactions of the O2 reduction products with solvent and electrolyte lead to capacity fading and poor cyclability. During the oxygen reduction reaction (ORR) in aprotic solvents, the superoxide radical anion (O2˙-) is the main one-electron reaction product, which in the presence of Li+ ions undergoes disproportionation to yield Li2O2 and O2, a fraction of which results in singlet oxygen (1O2). The very reactive 1O2 is responsible for the spurious reactions that lead to high charging overpotential and short cycle life due to solvent and electrolyte degradation. Several techniques have been used for the detection and suppression of 1O2 inside a Li-O2 battery under operation and to test the efficiency and electrochemical stability of different physical quenchers of 1O2: azide anions, 1,4-diazabicyclo[2.2.2]octane (DABCO) and triphenylamine (TPA) in different solvents (dimethyl sulfoxide (DMSO), diglyme and tetraglyme). Operando detection of 1O2 inside the battery was accomplished by following dimethylanthracene fluorescence quenching using a bifurcated optical fiber in front-face mode through a quartz window in the battery. Differential oxygen-pressure measurements during charge-discharge cycles vs. charge during battery operation showed that the number of electrons per oxygen molecule was n > 2 in the absence of physical quenchers of 1O2, due to spurious reactions, and n = 2 in the presence of physical quenchers of 1O2, proving the suppression of spurious reactions. Battery cycling at a limited specific capacity of 500 mA h gC-1 for the MWCNT cathode and 250 mA gC-1 current density, in the absence and presence of a physical quencher or a physical quencher plus the redox mediator I3-/I- (with a lithiated Nafion® membrane), showed increasing cyclability according to coulombic efficiency and cell voltage data over 100 cycles. Operando Raman studies with a quartz window at the bottom of the battery allowed detection of Li2O2 and excess I3- redox mediator during discharge and charge, respectively.

Reduction of metmyoglobin by inorganic disulfide species

The mechanism of the metal centered reduction of metmyoglobin (MbFeIII) by inorganic disulfide species has been studied by combined spectroscopic and kinetic analyses, under argon atmosphere. The process is kinetically characterized by biexponential time traces, for variable ratios of excess disulfide to protein, in the pH interval 6.6-8.0. Using UV-vis and resonance Raman spectroscopies, we observed that MbFeIII is converted into a low spin hexacoordinated ferric complex, tentatively assigned as MbFeIII(HSS-)/MbFeIII(SS2-), in an initial fast step. The complex is slowly converted into a pentacoordinated ferrous form, assigned as MbFeII according to the resonance Raman records. The reduction is a pH-dependent process, but independent of the initial disulfide concentration, suggesting the unimolecular decomposition of the intermediate complex following a reductive homolysis. We estimated the rate of the fast formation of the complex at pH 7.4 (kon = 3.7 × 103 M-1 s-1), and a pKa2 = 7.5 for the equilibrium MbFeIII(HSS-)/MbFeIII(SS2-). Also, we estimated the rate for the slow reduction at the same pH (kred = 10-2 s-1). A reaction mechanism compliant with the experimental results is proposed. This mechanistic study provides a differential kinetic signature for the reactions of disulfide compared to sulfide species on metmyoglobin, which may be considered in other hemeprotein systems.

Autocatalytic Mechanism in the Anaerobic Reduction of Metmyoglobin by Sulfide Species

The mechanism of the metal centered reduction of metmyoglobin (MbFeIII) by sulfide species (H2S/HS-) under an argon atmosphere has been studied by a combination of spectroscopic, kinetic, and computational methods. Asymmetric S-shaped time-traces for the formation of MbFeII at varying ratios of excess sulfide were observed at pH 5.3 < pH < 8.0 and 25 °C, suggesting an autocatalytic reaction mechanism. An increased rate at more alkaline pHs points to HS- as relevant reactive species for the reduction. The formation of the sulfanyl radical (HS•) in the slow initial phase was assessed using the spin-trap phenyl N-tert-butyl nitrone. This radical initiates the formation of S-S reactive species as disulfanuidyl/ disulfanudi-idyl radical anions and disulfide (HSSH•-/HSS•2- and HSS-, respectively). The autocatalysis has been ascribed to HSS-, formed after HSSH•-/HSS•2- disproportionation, which behaves as a fast reductant toward the intermediate complex MbFeIII(HS-). We propose a reaction mechanism for the sulfide-mediated reduction of metmyoglobin where only ferric heme iron initiates the oxidation of sulfide species. Beside the chemical interest, this insight into the MbFeIII/sulfide reaction under an argon atmosphere is relevant for the interpretation of biochemical aspects of ectopic myoglobins found on hypoxic tissues toward reactive sulfur species.

Electrochemical Actuation of a DyP Peroxidase: A Facile Method for Drastic Improvement of the Catalytic Performance

Dye decolorizing peroxidases (DyP) have attracted interest for applications such as dye-containing wastewater remediation and biomass processing. So far, efforts to improve operational pH ranges, activities, and stabilities have focused on site-directed mutagenesis and directed evolution strategies. Here, we show that the performance of the DyP from Bacillus subtilis can be drastically boosted without the need for complex molecular biology procedures by simply activating the enzyme electrochemically in the absence of externally added H₂O₂. Under these conditions, the enzyme shows specific activities toward a variety of chemically different substrates that are significantly higher than in its canonical operation. Moreover, it presents much broader pH activity profiles with the maxima shifted toward neutral to alkaline. We also show that the enzyme can be successfully immobilized on biocompatible electrodes. When actuated electrochemically, the enzymatic electrodes have two orders of magnitude higher turnover numbers than with the standard H₂O₂-dependent operation and preserve about 30% of the initial electrocatalytic activity after 5 days of operation-storage cycles.

Direct Electrochemical Generation of Catalytically Competent Oxyferryl Species of Classes I and P Dye Decolorizing Peroxidases

This work introduces a novel way to obtain catalytically competent oxyferryl species for two different dye-decolorizing peroxidases (DyPs) in the absence of H2O2 or any other peroxide by simply applying a reductive electrochemical potential under aerobic conditions. UV-vis and resonance Raman spectroscopies show that this method yields long-lived compounds II and I for the DyPs from Bacillus subtilis (BsDyP; Class I) and Pseudomonas putida (PpDyP; Class P), respectively. Both electrochemically generated high valent intermediates are able to oxidize ABTS at both acidic and alkaline pH. Interestingly, the electrocatalytic efficiencies obtained at pH 7.6 are very similar to the values recorded for regular catalytic ABTS/H2O2 assays at the optimal pH of the enzymes, ca. 3.7. These findings pave the way for the design of DyP-based electrocatalytic reactors operable in an extended pH range without the need of harmful reagents such as H2O2.

Reactivity of inorganic sulfide species towards a pentacoordinated heme model system

The reactivity of inorganic sulfide towards ferric bis(N-acetyl)- microperoxidase 11 in sodium dodecyl sulfate has been explored by means of visible absorption and resonance Raman spectroscopies. The reaction has been previously studied in buffered solutions at neutral pH and in the presence of excess sulfide, revealing the formation of a moderately stable hexacoordinated low spin ferric sulfide complex that yields the ferrous form in the hour's timescale. In the surfactant solution, instead, the ferrous form is rapidly formed. The spectroscopic characterization of the heme structure in the surfactant milieu revealed the stabilization of a major ferric mono-histidyl high spin heme, which may be ascribed to out of plane distortions prompting the detachment of the axially ligated water molecule, thus leading to a differential reactivity. The ferric bis(N-acetyl)- microperoxidase 11 in sodium dodecyl sulfate provides a model for pentacoordinated heme platforms with an imidazole-based ligand.

In Situ Spectroelectrochemical Investigations of Electrode-Confined Electron-Transferring Proteins and Redox Enzymes

This perspective analyzes recent advances in the spectroelectrochemical investigation of redox proteins and enzymes immobilized on biocompatible or biomimetic electrode surfaces. Specifically, the article highlights new insights obtained by surface-enhanced resonance Raman (SERR), surface-enhanced infrared absorption (SEIRA), protein film infrared electrochemistry (PFIRE), polarization modulation infrared reflection-absorption spectroscopy (PMIRRAS), Förster resonance energy transfer (FRET), X-ray absorption spectroscopy (XAS), electron paramagnetic resonance (EPR), and differential electrochemical mass spectrometry (DMES)-based spectroelectrochemical methods on the structure, orientation, dynamics, and reaction mechanisms for a variety of immobilized species. This includes small heme and copper electron shuttling proteins, large respiratory complexes, hydrogenases, multicopper oxidases, alcohol dehydrogenases, endonucleases, NO-reductases, and dye decolorizing peroxidases, among other enzymes. Finally, I discuss the challenges and foreseeable future developments toward a better understanding of the functioning of these complex macromolecules and their exploitation in technological devices.

Liquid-polymer triboelectricity: chemical mechanisms in the contact electrification process

Liquid-polymer contact electrification between sliding water drops and the surface of polytetrafluoroethylene (PTFE) was studied as a function of the pH and ionic strength of the drop as well as ambient relative humidity (RH). The PTFE surface was characterized by using SEM, water-contact-angle measurements, FTIR spectroscopy, XPS, and Raman spectroscopy. The charge acquired by the drops was calculated by detecting the transient voltage induced on a specifically designed capacitive sensor. It is shown that water drops become positively charged at pH > pH_zch (pH_zch being the zero charge point of the polymer) while they become negatively charged for pH < pH_zch. The addition of non-hydrolysable salts (NaCl or CaCl₂) to water decreases the electrical charge induced in the drop. The charge also decreases with increasing RH. These results suggest proton or hydroxyl transfer from the liquid to the hydrophobic polymer surface. A proposed thermodynamic model for the ion transfer process allows explaining the observed effects of RH, pH and ionic strength.

CuA-based chimeric T1 copper sites allow for independent modulation of reorganization energy and reduction potential

Attaining rational modulation of thermodynamic and kinetic redox parameters of metalloproteins is a key milestone towards the (re)design of proteins with new or improved redox functions. Here we report that implantation of ligand loops from natural T1 proteins into the scaffold of a CuA protein leads to a series of distorted T1-like sites that allow for independent modulation of reduction potentials (E°') and electron transfer reorganization energies (λ). On the one hand E°' values could be fine-tuned over 120 mV without affecting λ. On the other, λ values could be modulated by more than a factor of two while affecting E°' only by a few millivolts. These results are in sharp contrast to previous studies that used T1 cupredoxin folds, thus highlighting the importance of the protein scaffold in determining such parameters.

Purple Mixed-Valent Copper A

CuA is a binuclear copper center acting as an electron transfer hub in terminal oxidases such as cytochrome c oxidase and nitrous oxide reductase. Its unique electronic structure is intimately linked to its function and has puzzled the community of biological inorganic chemistry for decades. Here we review the insights provided by different spectroscopic techniques of CuA centers, and the different experimental approaches to tackle its study, that encompass the synthesis of model compounds as well as protein engineering efforts. The contribution of the electronic structure to the thermodynamic and kinetic of electron transfer is extensively discussed. We also describe the proposed mechanism of CuAassembly in different organisms. The recent discovery of a novel CuA site opens new perspectives to this field.

Unexpected electron spin density on the axial methionine ligand in CuA suggests its involvement in electron pathways

The CuA center is a paradigm for the study of long-range biological electron transfer. This metal center is an essential cofactor for terminal oxidases like cytochrome c oxidase, the enzymatic complex responsible for cellular respiration in eukaryotes and in most bacteria. CuA acts as an electron hub by transferring electrons from reduced cytochrome c to the catalytic site of the enzyme where dioxygen reduction takes place. Different electron transfer pathways have been proposed involving a weak axial methionine ligand residue, conserved in all CuA sites. This hypothesis has been challenged by theoretical calculations indicating the lack of electron spin density in this ligand. Here we report an NMR study with selectively labeled methionine in a native CuA. NMR spectroscopy discloses the presence of net electron spin density in the methionine axial ligand in the two alternative ground states of this metal center. Similar spin delocalization observed on two second sphere mutants further supports this evidence. These data provide a novel view of the electronic structure of CuA centers and support previously neglected electron transfer pathways.

Fine Tuning of Functional Features of the CuA Site by Loop-Directed Mutagenesis

Here we report the spectroscopic and electrochemical characterization of three novel chimeric Cu_A proteins in which either one or the three loops surrounding the metal ions in the Thermus thermophilus protein have been replaced by homologous human and plant sequences while preserving the set of coordinating amino acids. These conservative modifications mimic basic differences between Cu_A sites from different organisms and allow for fine tuning the energy gap between alternative electronic ground states of Cu_A.. This results in a systematic modulation of thermodynamic and kinetic electron transfer (ET) parameters and in the selection of one of two possible redox-active molecular orbitals, which differ in the ET reorganization energy by a factor of 2. Moreover, the ET mechanism is found to be frictionally controlled, and the modifications introduced into the different chimeras do not affect the frictional activation parameter.

Dramatic Electronic Perturbations of CuA Centers via Subtle Geometric Changes

Cu_A is a binuclear copper site acting as electron entry port in terminal heme-copper oxidases. In the oxidized form, Cu_A is a mixed valence pair whose electronic structure can be described using a potential energy surface with two minima, σ_u* and π_u, that are variably populated at room temperature. We report that mutations in the first and second coordination spheres of the binuclear metallocofactor can be combined in an additive manner to tune the energy gap and, thus, the relative populations of the two lowest-lying states. A series of designed mutants span σ_u*/π_u energy gaps ranging from 900 to 13 cm^-1. The smallest gap corresponds to a variant with an effectively degenerate ground state. All engineered sites preserve the mixed-valence character of this metal center and the electron transfer functionality. An increase of the Cu-Cu distance less than 0.06 Å modifies the σ_u*/π_u energy gap by almost 2 orders of magnitude, with longer distances eliciting a larger population of the π_u state. This scenario offers a stark contrast to synthetic systems, as model compounds require a lengthening of 0.5 Å in the Cu-Cu distance to stabilize the π_u state. These findings show that the tight control of the protein environment allows drastic perturbations in the electronic structure of Cu_A sites with minor geometric changes.

Manganese porphyrin redox state in endothelial cells: Resonance Raman studies and implications for antioxidant protection towards peroxynitrite

Cationic manganese(III) ortho N-substituted pyridylporphyrins (MnP) act as efficient antioxidants catalyzing superoxide dismutation and accelerating peroxynitrite reduction. Importantly, MnP can reach mitochondria offering protection against reactive species in different animal models of disease. Although an LC-MS/MS-based method for MnP quantitation and subcellular distribution has been reported, a direct method capable of evaluating both the uptake and the redox state of MnP in living cells has not yet been developed. In the present work we applied resonance Raman (RR) spectroscopy to analyze the intracellular accumulation of two potent MnP-based lipophilic SOD mimics, MnTnBuOE-2-PyP⁵⁺ and MnTnHex-2-PyP⁵⁺ within endothelial cells. RR experiments with isolated mitochondria revealed that the reduction of Mn(III)P was affected by inhibitors of the electron transport chain, supporting the action of MnP as efficient redox active compounds in mitochondria. Indeed, RR spectra confirmed that MnP added in the Mn(III) state can be incorporated into the cells, readily reduced by intracellular components to the Mn(II) state and oxidized by peroxynitrite. To assess the combined impact of reactivity and bioavailability, we studied the kinetics of Mn(III)TnBuOE-2-PyP⁵⁺ with peroxynitrite and evaluated the cytoprotective capacity of MnP by exposing the endothelial cells to nitro-oxidative stress induced by peroxynitrite. We observed a preservation of normal mitochondrial function, attenuation of cell damage and prevention of apoptotic cell death. These data introduce a novel application of RR spectroscopy for the direct detection of MnP and their redox states inside living cells, and helps to rationalize their antioxidant capacity in biological systems.

Engineering a bifunctional copper site in the cupredoxin fold by loop-directed mutagenesis

Copper sites in proteins are designed to perform either electron transfer or redox catalysis. Type 1 and CuA sites are electron transfer hubs bound to a rigid protein fold that prevents binding of exogenous ligands and side reactions. Here we report the engineering of two Type 1 sites by loop-directed mutagenesis within a CuA scaffold with unique electronic structures and functional features. A copper-thioether axial bond shorter than the copper-thiolate bond is responsible for the electronic structure features, in contrast to all other natural or chimeric sites where the copper thiolate bond is short. These sites display highly unusual features, such as: (1) a high reduction potential despite a strong interaction with the axial ligand, which we attribute to changes in the hydrogen bond network and (2) the ability to bind exogenous ligands such as imidazole and azide. This strategy widens the possibility of using natural protein scaffolds with functional features not present in nature.

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.

Tuning of Enthalpic/Entropic Parameters of a Protein Redox Center through Manipulation of the Electronic Partition Function

Manipulation of the partition function (Q) of the redox center Cu_A from cytochrome c oxidase is attained by tuning the accessibility of a low lying alternative electronic ground state and by perturbation of the electrostatic potential through point mutations, loop engineering and pH variation. We report clear correlations of the entropic and enthalpic contributions to redox potentials with Q and with the identity and hydrophobicity of the weak axial ligand, respectively.

Active Site Structure and Peroxidase Activity of Oxidatively Modified Cytochrome c Species in Complexes with Cardiolipin

We report a resonance Raman and UV-vis characterization of the active site structure of oxidatively modified forms of cytochrome c (Cyt-c) free in solution and in complexes with cardiolipin (CL). The studied post-translational modifications of Cyt-c include methionine sulfoxidation and tyrosine nitration, which lead to altered heme axial ligation and increased peroxidase activity with respect to those of the wild-type protein. In spite of the structural and activity differences between the protein variants free in solution, binding to CL liposomes induces in all cases the formation of a spectroscopically identical bis-His axial coordination conformer that more efficiently promotes lipid peroxidation. The spectroscopic results indicate that the bis-His form is in equilibrium with small amounts of high-spin species, thus suggesting a labile distal His ligand as the basis for the CL-induced increase in enzymatic activity observed for all protein variants. For Cyt-c nitrated at Tyr74 and sulfoxidized at Met80, the measured apparent binding affinities for CL are ∼4 times larger than for wild-type Cyt-c. On the basis of these results, we propose that these post-translational modifications may amplify the pro-apoptotic signal of Cyt-c under oxidative stress conditions at CL concentrations lower than for the unmodified protein.

Reversible Switching of Redox-Active Molecular Orbitals and Electron Transfer Pathways in Cu(A) Sites of Cytochrome c Oxidase

The Cu(A) site of cytochrome c oxidase is a redox hub that participates in rapid electron transfer at low driving forces with two redox cofactors in nearly perpendicular orientations. Spectroscopic and electrochemical characterizations performed on first and second-sphere mutants have allowed us to experimentally detect the reversible switching between two alternative electronic states that confer different directionalities to the redox reaction. Specifically, the M160H variant of a native Cu(A) shows a reversible pH transition that allows to functionally probe both states in the same protein species. Alternation between states exerts a dramatic impact on the kinetic redox parameters, thereby suggesting this effect as the mechanism underlying the efficiency and directionality of Cu(A) electron transfer in vivo. These findings may also prove useful for the development of molecular electronics.

Coupling of tyrosine deprotonation and axial ligand exchange in nitrocytochrome c

Here we report a spectroscopic, electrochemical and computational study of cytochrome c showing that nitration of Tyr74 induces Tyr deprotonation, which is coupled to Met/Lys axial ligand exchange, and results in concomitant gain of peroxidatic activity at physiological pH.

Specific methionine oxidation of cytochrome c in complexes with zwitterionic lipids by hydrogen peroxide: potential implications for apoptosis

The rise of H₂O₂ concentration that characterizes the initiation of apoptosis can specifically oxidize Met80 in cytochrome c bound to zwitterionic phospholipids, yielding a stable peroxidase.

Cytochrome c (Cyt-c) has been previously shown to participate in cardiolipin (CL) oxidation and, therefore, in mitochondrial membrane permeabilization during the early events of apoptosis. The gain in this function has been ascribed to specific CL/Cyt-c interactions. Here we report that the cationic protein Cyt-c is also able to interact electrostatically with the main lipid components of the mitochondrial membranes, the zwitterionic lipids phosphatidylcholine (PC) and phosphatidylethanolamine (PE), through the mediation of phosphate anions that bind specifically to amino groups in the surfaces of protein and model membranes. In these complexes, Cyt-c reacts efficiently with H₂O₂ at submillimolar levels, which oxidizes the sulfur atom of the axial ligand Met80. The modified protein is stable and presents significantly enhanced peroxidatic activity. Based on these results, we postulate that the rise of H₂O₂ concentrations to the submillimolar levels registered during initiation of the apoptotic program may represent one signaling event that triggers the gain in peroxidatic function of the Cyt-c molecules bound to the abundant PE and PC membrane components. As the activated protein is a chemically stable species, it can potentially bind and oxidize important targets, such as CL.

Effect of gold nanoparticles on the structure and electron-transfer characteristics of glucose oxidase redox polyelectrolyte-surfactant complexes

Efficient electrical communication between redox proteins and electrodes is a critical issue in the operation and development of amperometric biosensors. The present study explores the advantages of a nanostructured redox-active polyelectrolyte-surfactant complex containing [Os(bpy)2Clpy](2+) (bpy=2,2'-bipyridine, py= pyridine) as the redox centers and gold nanoparticles (AuNPs) as nanodomains for boosting the electron-transfer propagation throughout the assembled film in the presence of glucose oxidase (GOx). Film structure was characterized by grazing-incidence small-angle X-ray scattering (GISAXS) and atomic force microscopy (AFM), GOx incorporation was followed by surface plasmon resonance (SPR) and quartz-crystal microbalance with dissipation (QCM-D), whereas Raman spectroelectrochemistry and electrochemical studies confirmed the ability of the entrapped gold nanoparticles to enhance the electron-transfer processes between the enzyme and the electrode surface. Our results show that nanocomposite films exhibit five-fold increase in current response to glucose compared with analogous supramolecular AuNP-free films. The introduction of colloidal gold promotes drastic mesostructural changes in the film, which in turn leads to a rigid, amorphous interfacial architecture where nanoparticles, redox centers, and GOx remain in close proximity, thus improving the electron-transfer process.

The role of protein dynamics and thermal fluctuations in regulating cytochrome c/cytochrome c oxidase electron transfer

In this overview we present recent combined electrochemical, spectroelectrochemical, spectroscopic and computational studies from our group on the electron transfer reactions of cytochrome c and of the primary electron acceptor of cytochrome c oxidase, the CuA site, in biomimetic complexes. Based on these results, we discuss how protein dynamics and thermal fluctuations may impact on protein ET reactions, comment on the possible physiological relevance of these results, and finally propose a regulatory mechanism that may operate in the Cyt/CcO electron transfer reaction in vivo. This article is part of a Special Issue entitled: 18th European Bioenergetic Conference.

Native Cu(A) redox sites are largely resilient to pH variations within a physiological range

Previous studies on engineered CuA centres have shown that one of the histidine ligands is protonated and dissociated from the metal site at physiological pH values, thus suggesting a role in regulating proton-coupled electron transfer of cytochrome c oxidases in vivo. Here we report that for native CuA such protonation does not take place at physiologically relevant pH values and, furthermore, no significant changes in the spectroscopic and redox properties of the metal site occur at low pH.

Phosphate mediated adsorption and electron transfer of cytochrome c. A time-resolved SERR spectroelectrochemical study

The study of proteins immobilized on biomimetic or biocompatible electrodes represents an active field of research as it pursues both fundamental and technological interests. In this context, adsorption and redox properties of cytochrome c (Cyt) on different electrode surfaces have been extensively reported, although in some cases with contradictory results. Here we report a SERR spectroelectrochemical study of the adsorption and electron transfer behaviour of the basic protein Cyt on electrodes coated with amino-terminated monolayers. The obtained results show that inorganic phosphate (Pi) and ATP anions are able to mediate high affinity binding of the protein with preservation of the native structure and rendering an average orientation that guarantees efficient pathways for direct electron transfer. These findings aid the design of Cyt-based bioelectronic devices and understanding the modulation by Pi and ATP of physiological functions of Cyt.

Self-assembled monolayers of NH2-terminated thiolates: order, pKa, and specific adsorption

Self-assembled monolayers (SAMs) of amino-terminated alkanethiols on Au were characterized by a combination of electrochemical (LSV, CV, and EIS) and spectroscopic (XPS and SER) techniques. Clear correlations were obtained between the apparent surface pKa values determined by impedimetric titrations and order parameters such as the content of trans conformers in the SAMs. These results contrast with previous studies that exhibit dispersions of up to 6 pH units in the reported pKa values. In addition, we determined that inorganic and organic phosphate species bind specifically to these SAMs mediating adsorption and heterogeneous electron transfer of positively charged macromolecules such as cytochrome c.

Surface-enhanced vibrational spectroscopy for probing transient interactions of proteins with biomimetic interfaces: electric field effects on structure, dynamics and function of cytochrome c

Most of the biochemical and biophysical processes of proteins take place at membranes, and are thus under the influence of strong local electric fields, which are likely to affect the structure as well as the reaction mechanism and dynamics. To analyse such electric field effects, biomimetic interfaces may be employed that consist of membrane models deposited on nanostructured metal electrodes. For such devices, surface-enhanced resonance Raman and IR absorption spectroscopy are powerful techniques to disentangle the complex interfacial processes of proteins in terms of rotational diffusion, electron transfer, and protein and cofactor structural changes. The present article reviews the results obtained for the haem protein cytochrome c, which is widely used as a model protein for studying the various reaction steps of interfacial redox processes in general. In addition, it is shown that electric field effects may be functional for the natural redox processes of cytochrome c in the respiratory chain, as well as for the switch from the redox to the peroxidase function, one of the key events preceding apoptosis.

Distance-dependent electron transfer rate of immobilized redox proteins: a statistical physics approach

The electron transfer kinetics of redox proteins adsorbed on metal electrodes coated with self-assembled monolayers (SAMs) of mercaptanes shows an unusual distance-dependence. For thick SAMs, the experimentally measured electron transfer rate constant k{exp} obeys the behavior predicted by Marcus theory [R. A. Marcus and N. Sutin, Biochim. Biophys. Acta 811, 265 (1985)], whereas for thin SAMs, k{exp} remains virtually constant [Z. Q. Feng, J. Chem. Soc., Faraday Trans. 93, 1367 (1997)]. In this work, we present a simple theoretical model system for the redox protein cytochrome c electrostatically bound to a SAM-coated electrode. A statistical average of the electron tunneling rate is calculated by accounting for all possible orientations of the model protein. This approach, which takes into account the electric field dependent orientational distribution, allows for a satisfactory description of the "saturation" regime in the high electric field limit. It further predicts a nonexponential behavior of the average electron transfer processes that may be experimentally checked by extending kinetic experiments to shorter sampling times, i.e., 1/k{exp}. For a comprehensive description of the overall kinetics in the saturation regime at sampling times of the order of <<1/k{exp}, it is essential to consider the dynamics of protein reorientation, which is not implemented in the present model.