Search

The development of new electrocatalysts for the oxygen reduction reaction (ORR) and hydrogen evolution reaction (HER) at physiological pH is critical for several fields, including fuel cells and biological applications. Herein, the assembly of an electrode based on carboxyl-functionalised hydrophilic multiwalled carbon nanotubes (MWCNTs) filled with Fe phases and their excellent performance as electrocatalysts for ORR and HER at physiological pH are reported. The encapsulated Fe dramatically enhances the catalytic activity, and the graphitic shells play a double role of efficiently mediating the electron transfer to O2 and H2O reactants and providing a cocoon that prevents uncontrolled Fe oxidation or leaching.

The electrochemical behaviour of 4,6-dimethyl-2-thiopyrimidine (LH) and 4,6-dimethyl-1-phenyl-2-thiopyrimidine (LΦ) and their protonated forms was investigated in dimethylformamide on Hg electrodes. Adsorption processes affect the oxidation mechanism of both the compounds and the formation of Hg(I) complexes was observed. The electrochemical reduction of LH provides an example of the so-called “father—son reaction” but this kind of mechanism cannot occur with LΦ. Only in the species LH is a proton present which can be abstracted by the product of the first electron transfer LH · − while LΦ can be protonated by the solvent. However, in the presence of strong acids, for both molecules LH and LΦ the formation of the dimer from the radical anion obtained in the first electron transfer is kinetically preferred to the father—son reaction.

Unfolding turns immobilized cytochrome c into a His-His ligated form endowed with catalytic activity towards O(2), which is absent in the native protein. Dioxygen could be used by naturally occurring unfolded cytochrome c as a substrate for the production of partially reduced oxygen species (PROS) contributing to the cell oxidative stress.

The electron transfer (ET) process of progressively unfolded bovine cytochrome c immobilized on different self-assembled monolayers (SAMs) was investigated. Insight is gained on the role of the SAM surface on the functionality of the partially unfolded and non-native forms of the adsorbed protein. Direct electrochemical measurements were performed on cytochrome c adsorbed on mercaptopyridine (MP) and mixed 11-mercapto-1-undecanoic acid/11-mercapto-1-undecanol (MUA/MU) at varying temperature, in the presence of urea as unfolding agent. Under strongly unfolding conditions, a non-native form of cytochrome c , in which the methionine ligand is replaced by a histidine, was observed on both MP and MUA/MU SAMs. The E °′ of the native form, in which the haem is axially coordinated by methionine and histidine, slightly shifts to negative values upon increasing urea concentration. However, the non-native bis-histidinate species shows a much lower E °′ value (by approximately 0.4 V) which is by far enthalpic in origin and largely determined by axial ligand swapping. Analysis of the reduction enthalpies and entropies and of the ET rate constants indicate that the nature of the SAM (hydrophilic or anionic) results in changes in the conformational rearrangement of the cytochrome c under unfolding conditions.

We have studied the effect of urea-induced unfolding on the electron transfer process of yeast iso-1-cytochrome c and its mutant K72AK73AK79A adsorbed on electrodes coated by mixed 11-mercapto-1-undecanoic acid/11-mercapto-1-undecanol self-assembled monolayers. Electrochemical measurements, complemented by surface enhanced resonance Raman studies, indicate two distinct states of the adsorbed proteins that mainly differ with respect to the ligation pattern of the haem. The native state, in which the haem is axially coordinated by Met80 and His18, displays a reduction potential that slightly shifts to negative values with increasing urea concentration. At urea concentrations higher than 6 M, a second state prevails in which the Met80 ligand is replaced by an additional histidine residue. This structural change in the haem pocket is associated with an approximately 0.4 V shift of the reduction potential to negative values. These two states were found for both the wild-type protein and the mutant in which lysine residues 72, 73 and 79 had been substituted by alanines. The analysis of the reduction potentials, the reaction enthalpies and entropies as well as the rate constants indicates that these three lysine residues have an important effect on stabilising the protein structure in the adsorbed state and facilitating the electron transfer dynamics. © 2010 SBIC.

The thermodynamics of the electron transfer (ET) process for beef heart and yeast cytochromes c and the Lys72Ala/Lys73Ala/Lys79Ala mutant of the latter species subjected to progressive urea-induced unfolding was determined electrochemically. The results indicate the presence of at least three protein forms which were assigned to a low-temperature and a high-temperature His-Met intermediate species and a bis-histidinate form (although the presence of a His-Lys form cannot be excluded). The much lower E°' value of the bis-histidinate conformer as compared to His-Met ligated species is largely determined by the enthalpic contribution induced by axial ligand substitution. The biphasic E°' versus T profile for the His-Met species is due to a difference in reduction entropy between the conformers at low and high temperatures. Enthalpy-entropy compensation phenomena for the reduction reaction at varying urea concentration for all the forms of the investigated cytochromes c were addressed and discussed. © 2009 Springer Science+Business Media B.V.

The electrochemical reduction of a series of thio- and oxopyrimidine derivatives has been investigated in organic solvents on mercury electrodes. In all cases the electrochemical process gave a dimeric species as the major product. The overall reduction mechanism is the same for oxo and thio derivatives, and is found to be dependent only on the nature of the ring nitrogen substituent. A ‘father–son reaction’ is observed when hydrogen is bound to the ring nitrogen atom: the radical anion obtained from the first electron transfer draws out the nitrogen proton of a non-reduced molecule and this then dimerizes. In the presence of a protonating agent as well as for the N-substituted derivatives, the ‘father–son reaction’ is not observed. Theoretical calculations have been performed to gain insight into the proposed mechanisms: the LUMO energy and the vertical electron affinity show a linear correlation with the reduction potentials. Analysis of the theoretical parameters has allowed step-by-step determination of the electrochemical reduction process. The manner in which solvent properties influence electrochemical behaviour has been examined, and the role of the acceptor number (AN) has been discussed.

Heterogeneous electron transfer rate constants were determined as a function of electrode potential for one-electron oxidation in acetonitrile (AN) at O °C of a series of organocobaloximes [R-Co(DH)2L] bearing widely different organic groups. Reaction entropies were determined by voltammetric half-wave potential (Er 1 2 ) measurements in a non-isothermal cell. The electron transfer coefficients and reorganization parameters were calculated following the Marcus theory. The reaction free energies relative to a reference couple ΔG∘ are linearly correlated with the polar Taft constant of the organic substituent R. The steric effects on ΔG∘ are shown by the correlation of Er sol1 2 with the CoC bond distance. Assuming constancy of double layer effects along the series in the given solution composition, the trends of the apparent rate constants kapp were considered in order to evaluate the effects of the nature of the organic ligand on the activation energy ΔG‡ of the electron transfer. The steric effects on ΔG‡ are pointed out i.a. by consideration of the relationship between ΔG‡ and ΔG∘.

The electrochemical reduction of alkylcobaloximes at the dropping mercury electrode involves an initial one electron transfer process to give a radical anion with the cobalt in the CoII formal oxidation state. The CoC bond cleavage in the radical anion is an homolytic process yielding the CoI cobaloxime and competing with a second electron transfer. The nature of the organic group and of the solvent coordinated in the trans position to the Co atom influences the thermodynamics of the electron transfer and the stability of the radical anion. Electronic and steric effects on the thermodynamics of the first electron transfer can be distinguished. These effects have only a small influence on the kinetics of the first electron transfer.

The electrochemistry of a series of indium chelates with phenanthroline or substituted phenanthroline and cyclooctadiene on Hg, Pt, Au and glassy carbon electrodes is reported. Two quasi-reversible one-electron reductions between −0.8 and −1.1 V and between −1.25 and −1.55 V vs. a saturated calomel electrode were studied. Adsorption and a slow chemical reaction following the electron transfer were observed. The relationship between the reduction potentials of the chelates and those of the corresponding free phenanthroline ligands, as well as the shifts of the reduction potential caused by substitution, shows that the redox orbital is a π* ligand orbital. This explains the lack of correlation between the redox potentials and the molecular properties determined by the central metal atom such as the catalytic efficiency.

Diaquocobaloxime and three other chelating ligands obtained by substitution of the O ⋯ H ⋯ O bridge with (CH2)3 or BF2 groups have been examined in order to compare the effects of changes in the in-plane ligand on the relative stability of different oxidation states of the cobalt atom and on the cis-effect of the equatorial structure with those observed in the corresponding organometallic derivatives. The cis-effect of the in-plane ligand was evaluated from the electrochemistry of the CoIII/CoII electron transfer which is dependent upon the Co-solvent bond strength changes in the axial positions. The axial interaction depends mainly on the nature of the axial ligand. The introduction of either one (CH2)3 group or two BF2 groups or both leads to a displacement of the stability range of the CoII oxidation state towards more anodic redox potentials. The (CH2)3 group appears to be more effective both in the displacement of the redox potential and in teh bathochromic shift of the visible band at 450–500 nm of the CoII chelates.