pH Dependence Of Redox Potential Research Articles

Cytochromes as electron shuttles from FAD-dependent glucose dehydrogenase to electrodes

Cytochromes are a large and well-known group of electron transfer proteins, but only very few are used in bioelectrochemistry, e.g. to serve as electron shuttles between biocatalysts and electrodes. There, cytochromes generate an artificial electron transfer pathway if their structural and electrochemical properties are fitting. Cytochromes considered suitable are small and feature an exposed heme cofactor, a suitable redox potential, an electrochemically reversible redox process, sufficient protein production, and a fast electron transfer between other enzymes and hemoproteins. Out of 850 screened hemoproteins, 58 cytochromes met the requirements to function as an electron shuttle. They can be classified into five distinguished protein folds. Four representative b-type and three c-type cytochromes were selected for further analyses. The proteins were obtained in a yield of 0.075 to 375 mg L-1 in Escherichia coli or Komagataella phaffii. The electrochemical reversibility on 1-thioglycerol functionalized gold electrodes of all cytochromes was confirmed. The pH-dependent redox potentials range from -50 mV to +298 mV vs. SHE at pH 7.0. The electronic wiring ability was tested using the biocatalyst FAD-dependent glucose dehydrogenase (GDH). Cytochrome reduction by the GDH was monitored by spectrophotometry and amperometry. c-Type cytochromes showed a faster electron uptake from GDH than b-type cytochromes. Complementary surface charges at the cytochrome/GDH interface were not a predictor for a fast electron transfer but a higher difference in the redox potential of the cofactors was. This study supports the design of bioelectrocatalysts with high electron transfer rates, which are of great interest in 3rd generation biosensors and for direct bioelectrocatalysis.

Deactivation Mechanism of Pyridinic Nitrogen-Doped Carbon for ORR in Acidic Media

Nitrogen doped (N-doped) carbons are attracting attention as Pt-free catalysts for oxygen reduction reaction (ORR) in fuel cells. N-doped carbon catalysts exhibit high activity comparable to that of platinum in basic media whereas the activity is significantly reduced in acidic media. To solve the deactivation mechanism based on kinetics, we have investigated the reaction pathway of N-doped carbon catalysts. The active site of pyridinic nitrogen (pyri-N) is protonated to be pyridinium (pyri-NH+) in acidic media due to acid-base equilibrium ofpyri-N + H+ ⇄ pyriNH+ (1)Following to Eq. (1), we recently exhibited thatpyriNH+ + e- + O2⇄ O2a + pyri-NH(2)proceeds using model catalysts with homogenous active sites. In the step, the electrochemical reduction from pyri-NH+ to pyri-NH and thermal O2 adsorption simultaneously occurs wherein the adsorption of O2 promotes the electron transfer into π* orbital of pyri-NH. We proposed that the hydration of pyri-NH+ lowers the redox potential of Eq. (2) decreasing the ORR activity in acidic media. In the present study, we further considered the acid-base equilibrium of pyri-N in Eq. (1) to comprehensively answer why the ORR activity of N-doped carbon catalyst is high in alkaline media and is decreased in acidic media.To investigate the relationship between acid-base equilibrium and the activity, the electrochemical measurements were performed in electrolyte solutions of various pH. Figure 1 shows the onset potential (voltage at -100 μA/cm2) as a function of pH measure with a model catalyst of 1,10-phenanthroline/(carbon black). In the range from pH 5 to 13, the onset potential decreased with a slope of ~59 mV/pH, while at below pH 5, the onset potentials were almost constant. This indicates that the reaction mechanism changes due to the shift of the acid-base equilibrium. In low pH, the reactions determining the onset potential is assumed to be Eq. (2). On the other hand, at high pH, the reaction should start from pyri-N but not from pyri-NH+.We have figured out that pyri-N + H2O +e- + O2⇄ O2a + pyri-NH + OH- (3) takes place in alkaline madia. Figure 2 shows the N1s peak of the model catalyst applied voltage at pH 13 in N2-(left) and O2-(right)saturated electrolyte. A peak at 400.5 eV appeared at higher applied voltage in O2-saturated electrolyte. This means that pyri-N (399.2 eV) is reduced at a higher potential in the presence of O2 to form pyri-NH. In other words, oxygen adsorption and reduction of pyri-N simultaneously occurred in the presence of O2.The redox potentials of Eq. (2) and (3) explain the pH dependences of the onset potentials. The Nernst equations of Eq. (2) and Eq. (3) show pH-independent redox potential vs. RHE and pH-dependent redox potential vs. RHE with the slope of 59 mV/pH, respectively. This also means that the activity depends on the acid-base equilibrium of pyri-N.We also calculated the redox potentials based on DFT as shown in Figure 3. The additional important point is that the redox potentials of Eq. (2) is strongly influenced by the hydration whereas the redox potentials of Eq. (3) is not influenced by the hydration because the stability of pyri-N is not changed by the hydration. Therefore, the ORR activity in alkaline media is high because the redox potential of Eq. (2) vs. RHE is higher for the higher pH and is not influenced by the hydration. On the other hand, the activity in acidic media is lowered by the hydration. This is that the suppression of the hydration by the introduction of the hydrophobicity at the vicinity of the active site can significantly promote the activity in acidic media. Figure 1

Calculations of pH-Dependent Redox Potentials for Proton-Coupled Electron Transfer Reactions

We will present a computational protocol that combines density functional theory (DFT) and cheminformatics approaches to predict redox potentials of organic molecules as a function of the pH of the aqueous solution.[1] The methodology is based on an improved version of the Alberty-Legendre transform to account for varying protonation states of the redox reaction products at different pH values. Our approach circumvents the known problems with the use of DFT calculations and implicit solvation models for highly charged anions. To validate the protocol, we use a consistent set of quinones which can undergo up to two-electron two-proton redox reaction. As the reference we use the experimental redox potentials measured at pH 0, 7, and 13.[2] We demonstrate that with the proposed methodology we can achieve the MAE=0.144 V and MSE=-0.016 V at pH=13.[1] Fornari, R.P.; de Silva, P. A Computational Protocol Combining DFT and Cheminformatics for Prediction of pH-Dependent Redox Potentials. Molecules 2021, 26, 3978.[2] Wedege, K.; Dražević, E.; Konya, D.; Bentien, A. Organic Redox Species in Aqueous Flow Batteries: Redox Potentials, Chemical Stability and Solubility. Sci. Rep. 2016, 6, 39101.

Laccase-catalyzed polymerization of diaminobenzenesulfonic acid for pH-responsive color-changing and conductive wool fabrics

In recent years, there has been extensive interest in the research of smart fabrics and functional textiles. The present work has successfully developed the enzymatic approach for dyed wool fabrics in possession of special pH-responsive color-changing and conductive properties, via in-situ polymerization of 2,5-diaminobenzenesulfonic acid (DABSA) by laccase from Trametes versicolor. The enzymatically synthesized product, poly(2,5-diaminobenzenesulfonic acid) (PDABSA), was characterized by Fourier transform infrared spectroscopy, ultraviolet-visible (UV-Vis) spectrophotometry and matrix-assisted laser desorption ionization time-of-flight mass spectrometry. The redox property and thermal stability of the polymer products were studied by cyclic voltammetry and thermogravimetric analysis, respectively. The results proved that the resulting polymer reached 2 KDa and showed strong pH-dependence of UV-Vis absorption, electrochemical activity and high thermal stability. Utilizing the doping/dedoping process of PDABSA, the dyed wool fabrics were endowed with a pH-dependent redox potential. Furthermore, the dyed wool fabrics exhibited reversible color change from dark purple at pH 1.8 to yellowish-brown at pH 10.0, indicating that the PDABSA showed unusual pH-dependent color-changing properties on dyed wool fabrics.

Organic Redox Species in Aqueous Flow Batteries: Redox Potentials, Chemical Stability and Solubility

Organic molecules are currently investigated as redox species for aqueous low-cost redox flow batteries (RFBs). The envisioned features of using organic redox species are low cost and increased flexibility with respect to tailoring redox potential and solubility from molecular engineering of side groups on the organic redox-active species. In this paper 33, mainly quinone-based, compounds are studied experimentially in terms of pH dependent redox potential, solubility and stability, combined with single cell battery RFB tests on selected redox pairs. Data shows that both the solubility and redox potential are determined by the position of the side groups and only to a small extent by the number of side groups. Additionally, the chemical stability and possible degradation mechanisms leading to capacity loss over time are discussed. The main challenge for the development of all-organic RFBs is to identify a redox pair for the positive side with sufficiently high stability and redox potential that enables battery cell potentials above 1 V.

Monitoring the Transmembrane Proton Gradient Generated by Cytochrome bo3 in Tethered Bilayer Lipid Membranes Using SEIRA Spectroscopy.

Membrane proteins act as biocatalysts or ion/proton pumps to convert and store energy from ubiquitous environmental sources. Interfacing these proteins to electrodes allows utilizing the energy for enzymatic biofuel cells or other auspicious biotechnological applications. To optimize the efficiency of these devices, appropriate membrane models are required that ensure structural and functional integrity of the embedded enzymes and provide structural insight. We present a spectroelectrochemical surface-enhanced infrared absorption (SEIRA) and electrical impedance spectroscopy (EIS) study of the bacterial respiratory ubiquinol/cytochrome bo3 (cyt bo3) couple incorporated into a tethered bilayer lipid membrane (tBLM). Here, we employed a new lipid tether (WK3SH, dihydrocholesteryl (2-(2-(2-ethoxy)ethoxy)ethanethiol), which was synthesized using a three-step procedure with very good yield and allowed measuring IR spectra without significant spectral interference of the tBLM. The functional integrity of the incorporated cyt bo3 was demonstrated by monitoring the enzymatic O2 reduction current and the formation of the transmembrane proton gradient. Based on a SEIRA-spectroscopic redox titration, a shift of the pH-dependent redox potential of the ubiquinones under turnover conditions was correlated with an alkalinization of the submembrane reservoir by +0.8 pH units. This study demonstrates the high potential of tBLMs and the SEIRA spectroscopic approach to study bioenergetic processes.

Determination of the pH dependent redox potential of glucose oxidase by spectroelectrochemistry.

The pH dependent redox potential of the oxidoreductase glucose oxidase (GOx) from Aspergillus niger, which is the most frequently applied enzyme in electrochemical glucose biosensors and biofuel cells, was measured between pH 4.5 and 8.5 using UV/vis spectroelectrochemistry. In the entire pH range under investigation, the flavin adenine dinucleotide cofactor of GOx changed directly from the oxidized quinone to the doubly reduced hydroquinone. No stable semiquinoid species could be detected if electrochemical equilibrium was reached. From the pH dependency of the GOx redox potential, a pK(a) of 7.2 has been determined for the GOx flavohydroquinone. At pH values ≤6.0, a dependency of the reduction mechanism and the GOx redox potential on the presence of halides, especially on Cl(-), was observed. For the development of glucose biosensors and glucose biofuel cell anodes working at physiological or neutral pH, the GOx redox potentials at pH 7.4 and pH 7.0 are of main interest. Here values of E(1/2 pH 7.4) = -97 ± 3 mV and E(1/2 pH 7.0) = -80 ± 4 mV have been determined.

The sole tryptophan of amicyanin enhances its thermal stability but does not influence the electronic properties of the type 1 copper site

The cupredoxin amicyanin possesses a single tryptophan residue, Trp45. Its fluorescence is quenched when copper is bound even though it is separated by 10.1Å. Mutation of Trp45 to Ala, Phe, Leu and Lys resulted in undetectable protein expression. A W45Y amicyanin variant was isolated. The W45Y mutation did not alter the spectroscopic properties or intrinsic redox potential of amicyanin, but increased the pKa value for the pH-dependent redox potential by 0.5units. This is due to a hydrogen-bond involving the His95 copper ligand which is present in reduced W45Y amicyanin but not in native amicyanin. The W45Y mutation significantly decreased the thermal stability of amicyanin, as determined by changes in the visible absorbance of oxidized amicyanin and in the circular dichroism spectra for oxidized, reduced and apo forms of amicyanin. Comparison of the crystal structures suggests that the decreased stability of W45Y amicyanin may be attributed to the loss of a strong interior hydrogen bond between Trp45 and Tyr90 in native amicyanin which links two of the β-sheets that comprise the overall structure of amicyanin. Thus, Trp45 is critical for stabilizing the structure of amicyanin but it does not influence the electronic properties of the copper which quenches its fluorescence.

Determination of the Redox Potential of Immobilized Oxo-Bridged Metals in Porous Supports. The Ti–O–Mn–SBA System

The present work describes for the first time the determination of a redox potential of an oxo-bridged metal unit anchored to a mesoporous surface, which is not accessible to electrode reaction. The redox pair Ti(IV)OMnII/MnIII-SBA has been chosen because of the importance of the oxo-bridged TiOMn MMCT unit for photochemical energy storage research. The only feasible technique for determination of the standard redox potential, E0(MnII/MnIII), segments in the porous Ti(IV)O(MnII/MnIII)-SBA is from equilibrium constants with redox couples of known electrode potential. The present work describes the determination of E0(TiOMnII/MnIII) with the aid of 2,2′-azino-bis(3-ethylbenz-thiazoline-6-sulfonic acid (ABTS2-) and the 2,9-dimethyl-1,10-phenanthroline (Neocuproine or NC) complex with Cu(II). The results show that TiOMnIII is a moderate oxidant with pH dependent redox potential. The ABTS2-/ABTS•- reference redox pair yields E0(TiOMnII/MnIII) = 0.57 V at pH 7, increasing to 0.62 V at pH 4. The Cu(NC)2+/Cu(NC)22+ reference system yields E0(TiOMnII/MnIII) = 0.62 and 0.69 at pH 7 and 6, respectively. The difference of ∼0.1 V is interpreted in terms of the local charge effect of the positive Cu(NC)22+ ions. The moderate redox potential of Mn(III) produced by slow heating of Mn(II) and measured in aqueous suspensions is in contrast with the high oxidative reactivity of Mn(III) produced by photolysis of solid Ti(IV)OMnII-SBA under gas atmosphere. This apparent discrepancy is attributed to the formation of excited Mn(III) in contrast to ground state Mn(III) produced by heating at 305 °C and to partial hydration of the manganese in the TiOMn-SBA suspensions. The titanium in TiOMn-SBA remained T(IV) under all conditions of the present work.

Expanding the Range of Redox Potentials of the 2Fe-2S Clusters of the Outer Mitochondrial Membrane Protein MitoNEET

MitoNEET is a recently discovered outer mitochondrial membrane protein that harbors a 2Fe-2S cluster bound to a unique 3Cys-1His coordination (1). We measured a pH-dependent redox potential with a value near 0 mV at pH 7.0 (Em,7). This value lies intermediate between most low potential 4Cys-coordinated ferredoxin-like (Fd) centers (∼-300 mV) and most high potential 2Cys-2His-coordinated Rieske centers (∼+300 mV) (2). Upon replacing the single His87 ligand with Cys, we obtained an Em,7 near −300 mV closer to that of Fd clusters (Figure). Upon replacing Lys55 located near His87 with Met, Em,7 increases to near +250 mV, closer to that of high potential Rieske clusters (Figure). This shows that there is a large interaction between Lys55 and His87 and that mitoNEET is robust to large changes in the Em,7 of its 2Fe-2S clusters. Thus, we have engineered stable mutant mitoNEET with Em,7 values over a range of almost 600 mV.(1) Paddock et al. (2007) Proc Natl. Acad. Sci USA 104, 14342-14347(2) Meyer (2008) J Biol Inorg Chem 13, 157-170Supported by NIH (GM41637, GM54038 and DK54441).View Large Image | View Hi-Res Image | Download PowerPoint Slide

Redox Potential of the Outer-Mitochondrial Membrane 2Fe-2S Protein MitoNEET

MitoNEET was recently discovered as a binding target for the anti-diabetes drug pioglitazone (1). It harbors a pH-labile 2Fe-2S cluster coordinated by three cysteines and one histidine (His87) (2). We measured a pH-dependent redox potential of +35 mV (pH 7.5) that lies intermediate between most low potential 4Cys-coordinated ferredoxin-like centers (∼-300 mV) and most high potential 2Cys-2His-coordinated Rieske centers (∼+300 mV) (3). In addition, its redox potential was ∼40mV lower in the presence of phosphate ions. This can be explained by binding of a phosphate ion near the cluster as reported elsewhere (Homer, poster). The H87C mutant, which becomes 4 Cys coordinated, has a more negative reduction potential similar to a ferredoxin (∼ -200mV). Our results show that the redox potential is sensitive to the coordination of the cluster and that mitoNEET's unique coordination geometry is likely essential for its unknown redox function. (1) Colca et al. (2004) Am J Physiol Endocrinol Metab 286 E252-E260. (2) Paddock et al. (2007) Proc Natl. Acad. Sci USA 104, 14342-14347. (3) Meyer (2008) J Biol Inorg Chem 13, 157-170 Supported by NIH (GM41637, GM54038 and DK54441).View Large Image Figure ViewerDownload Hi-res image Download (PPT) View Large Image Figure ViewerDownload Hi-res image Download (PPT)

Simulate methylation reaction of arsenic(III) with methyl iodide in an aquatic system

AbstractThe methylation reaction of inorganic arsenic occurring in aquatic systems was studied by HPLC‐HG‐AFS as a method to separate and detect soluble methylarsenic species. Transformation from inorganic arsenic to methylarsenic was essential for major changes in toxicity to organisms. Monomethylarsenic [AsOCH3(OH)2] was the only product in the methylation reaction of inorganic arsenic(III) with methyl iodide (MeI). This process can be described as an oxidative carbonium‐ion transfer, with MeI acting as a methyl donor. From a thermodynamic point of view, the activity of the carbonium ion and pH were the two major influencing factors. The pH dependence of redox potential of As(III) was the reason for the effect of pH on methylation of arsenic. The influences of salinity and concentration of the methyl donor may be explained by their effects on the activity of carbonium. Moreover, kinetics experiments demonstrated that the methylation reaction was first‐order for both As(III) and methyl iodide. First‐order reaction rates were also calculated at different pH, salinity and MeI, and were found to be in the range 0.0026–0.0123 h−1. The methylation rate varied largely under different reaction conditions. Copyright © 2005 John Wiley & Sons, Ltd.

PH‐Dependent Redox Potential: How to Use It Correctly in the Activation Energy Analysis

AbstractFor Abstract see ChemInform Abstract in Full Text.

PH-dependent redox potential: how to use it correctly in the activation energy analysis

The activation barrier (the activation free energy) for the reaction's elementary act proper does not depend on the presence of reactants outside the reaction complex. The barrier is determined directly by the concentration-independent configurational free energy. In the case of redox reactants with pH-dependent redox potential, only the pH-independent quantity, the configurational redox potential enters immediately into expression for activation energy. Some typical cases of such reactions have been discussed (e.g., simultaneous proton and electron detachment, acid dissociation followed by oxidation, dissociation after oxidation, and others). For these mechanisms, the algorithms for calculation of the configurational redox potential from the experimentally determined redox potentials have been described both for the data related to a dissolved reactant or to a prosthetic group of an enzyme. Some examples of pH-dependent enzymatic redox reactions, in particular for the Rieske iron–sulfur protein, have been discussed.

Homo- and heterobimetallic complexes of ruthenium(II) and osmium(II) with pyrazole-3,5-dicarboxylic acid and 2,2′-bipyridine as co-ligands. Synthesis, structure, stereochemistry, spectroscopy and redox activities

Mononuclear, homodinuclear and heterodinuclear ruthenium(II) and osmium(II) complexes of composition [(bpy)2Ru(H2pzdc)](ClO4)·3H2O (1), [(bpy)2Os(H2pzdc)](ClO4)·3H2O (2), [(bpy)2Ru(pzdc)Ru(bpy)2](ClO4)·H2O (3), [(bpy)2Os(pzdc)Os(bpy)2](ClO4)·H2O (4) and [(bpy)2Ru(pzdc)Os(bpy)2](ClO4)·H2O (5) have been synthesized with pyrazole-3,5-dicarboxylic acid (H3pzdc) and 2,2′-bipyridine (bpy). Complexes 3–5 isolated solely in the homochiral forms have been characterized by their 1H and {1H1H} COSY NMR spectra. The crystal structures determined for 3 and 5 have revealed that the two compounds are isostructural and the diruthenium compound 3 is present in the rac(ΛΛ/ΔΔ) form, while the osmiumruthenium compound 5 is present as the enantiomeric pair (ΛΛ and ΔΔ). The metal-centered oxidation and bipyridine-centred reduction in 1–5 take place reversibly. As compared with the diruthenium complex 3, the stepwise oxidation of the metal centers in the diosmium complex 4 take place at less positive potentials by 0.45 V. The equilibrium constants for the comproportionation reaction MIIMII+MIIIMIII⇌2MIIMIII are found to be almost equal (ca. 6×102) for the diruthenium and diosmium species. From pH-dependent redox potential measurements the acid dissociation constants for the mononuclear complexes 1 and 2 have been determined. The luminescence spectral behavior of the complexes have been investigated in methanol–ethanol (1:4) solution (at 300 K) and in glassy state (77 K) and the quantum yields for the ruthenium(II) complexes are reported.

Electrochemical detection of thiols with a coenzyme pyrroloquinoline quinone modified electrode.

A modified electrode sensor for the detection of thiols is described. The sensor was constructed by incorporation of the coenzyme pyrroloquinoline quinone (PQQ) into a polypyrrole (PPy) film on a glassy carbon electrode substrate by the electropolymerization of pyrrole in the presence of PQQ. The electrochemical properties of entrapped PQQ in the PPy film were influenced by the applied potential during electropolymerization and by film thickness, both of which were optimized to yield a stable and reproducible response for entrapped PQQ. The PQQ/ PPy sensor was utilized for the amperometric detection of cysteine, homocysteine, penicillamine, N-acetylcysteine, and glutathione. The response for each thiol in pH 8.42 borate buffer was found to be linear with detection limits (S/N = 3) ranging from 13.2 microM for glutathione to 63.7 nM for cysteine with sensitivities of 0.023 nA/microM and 4.71 nA/microM, respectively. The response and detection limits were found to be sensitive to the nature of the thiol and the solution pH. Furthermore, in the presence of dopamine, ascorbic acid, or uric acid, the pH-dependent redox potential of the PQQ catalyst allows tuning of the detection potential to enhance the selectivity for thiols over these potential electroactive interferences.

Effect of serinate ligation at each of the iron sites of the [Fe4S4] cluster of Pyrococcus furiosus ferredoxin on the redox, spectroscopic, and biological properties.

Pyrococcus furiosus ferredoxin (Fd) contains a single [Fe(4)S(4)] cluster coordinated by three cysteine (at positions 11, 17, and 56) and one aspartate ligand (at position 14). In this study, the spectroscopic, redox, and functional consequences of D14C, D14C/C11S, D14S, D14C/C17S, and D14C/C56S mutations have been investigated. The four serine variants each contain a potential cluster coordination sphere of one serine and three cysteine residues, with serine ligation at each of the four Fe sites of the [Fe(4)S(4)] cluster. All five variants were expressed in Escherichia coli, and each contained a [Fe(4)S(4)](2+,+) cluster as shown by UV-visible absorption and resonance Raman studies of the oxidized protein and EPR and variable-temperature magnetic circular dichroism (VTMCD) studies of the as-prepared, dithionite-reduced protein. Changes in both the absorption and resonance Raman spectra are consistent with changing from complete cysteinyl cluster ligation in the D14C variant to three cysteines and one oxygenic ligand in each of the four serine variants. EPR and VTMCD studies show distinctive ground and excited state properties for the paramagnetic [Fe(4)S(4)](+) centers in each of these variant proteins, with the D14C and D14C/C11S variants having homogeneous S = (1)/(2) ground states and the D14S, D14C/C17S, and D14C/C56S variants having mixed-spin, S = (1)/(2) and (3)/(2) ground states. The midpoint potentials (pH 7.0, 23 degrees C) of the D14C/C11S and D14C/C17S variants were unchanged compared to that of the D14C variant (E(m) = -427 mV) within experimental error, but the potentials of D14C/C56S and D14S variants were more negative by 49 and 78 mV, respectively. Since the VTMCD spectra indicate the presence of a valence-delocalized Fe(2. 5+)Fe(2.5+) pair in all five variants, the midpoint potentials are interpreted in terms of Cys11 and Cys17 ligating the nonreducible valence-delocalized pair in D14C. Only the D14S variant exhibited a pH-dependent redox potential over the range of 3.5-10, and this is attributed to protonation of the serinate ligand to the reduced cluster (pK(a) = 4.75). All five variants had similar K(m) and V(m) values in a coupled assay in which Fd was reduced by pyruvate ferredoxin oxidoreductase (POR) and oxidized by ferredoxin NADP oxidoreductase (FNOR), both purified from P. furiosus. Hence, the mode of ligation at each Fe atom in the [Fe(4)S(4)] cluster appears to have little effect on the interaction and the electron transfer between Fd and FNOR.

Determination of Basicity of Core X and Terminal Y Ligands (X, Y = S and Se) of Reduced, Oxidized, and Super-Oxidized Forms of [Fe4X4(YAd)4]2− (Ad = 1-Adamantyl) in Aqueous Solutions

Abstract A series of [Fe4X4(YAd)4]2− (X, Y = S, Se; Ad = 1-adamantyl) were prepared as a model of high potential iron–sulfur proteins. Hydrolysis of the clusters was effectively depressed in aqueous poly[2-(dimethylamino)hexanamide] (PDAH) solutions due to the enbedding effect in hydrophobic environment and/or inhibition of dissociation of the terminal ligand into the aqueous media. Cyclic voltammetry of those clusters in aqueous PDAH solutions showed pH-dependent redox potentials of not only the [Fe4X4]+/2+ but also the [Fe4X4]2+/3+ (X = S and Se) couples, resulting from redox-linked protonation reactions of three oxidation states of [Fe4X4(YAd)4]n− (n = 1—3). pKavalues of the reduced, oxidized, and super-oxidized forms of [Fe4X4(YAd)4]2− were determined by computer simulation of the pH dependent redox potentials. The basicity of the core X (X = S and Se) of three oxidation states of [Fe4X4(YAd)4]n− (n = 1, 2, 3) was stronger than the terminal YAd (Y = S and Se) ligands: in the case of the mono-protonated [Fe4X4(YAd)4]3−(H+) and [Fe4X4(YAd)4]2−(H+), basicity of the terminal YAd ligand of [Fe4X4(YAd)4]2−(H+) becomes stronger than that of core X, although the core X of [Fe4X4(YAd)4]3−(H+) still showed a stronger basicity than those of terminal YAd ligands.

Modulating the pH-dependent redox potential of a flavin analog via incorporation into a self-assembled monolayer on gold

We report the first example of modifying the pH-dependent electrochemical properties of a flavin analog on gold by changing the pKa value of its N1 proton.

PH-Dependent metal-based redox couples as models for proton-coupled electron transfer reactions

An overview of proton-coupled electron transfer in transition metal–ligand systems known to exhibit pH-dependent metal-based redox potentials is presented. The systems of interest can be thought of as MLH m complexes, where M is a redox-active metal and LH m is a ligand in the inner-coordination sphere containing at least one ionizable hydrogen. The use of pH-dependent redox potentials to determine proton-coupled electron transfer reactions is described, along with some important factors for designing and studying complexes that exhibit this behavior. This chemistry is illustrated by a survey of the common transition metal–ligand systems known to give pH-dependent redox potentials indicative of proton-coupled electron transfer.