Research involving three important interactions of synthetic cationic porphyrins with nucleic acids: DNA binding, oxidative-reductive strand scission and photosensitized strand scission, is examine...

Porphyrin—Nucleic Acid Interactions: A Review

The slow kinetics of the electrochemical oxygen reduction reaction (ORR) is a crucial bottleneck in the development of microbial fuel cells (MFCs). This article firstly gives an overview of the particular constraints imposed on ORR by MFC operating conditions: neutral pH, slow oxygen mass transfer, sensitivity to reactive oxygen species, fouling and biofouling. A review of the literature is then proposed to assess how microbial catalysis could afford suitable solutions. Actually, microbial catalysis of ORR occurs spontaneously on the surface of metallic materials and is an effective motor of microbial corrosion. In this framework, several mechanisms have been proposed, which are reviewed in the second part of the article. The last part describes the efforts made in the domain of MFCs to determine the microbial ecology of electroactive biofilms and define efficient protocols for the formation of microbial oxygen-reducing cathodes. Although no clear mechanism has been established yet, several promising solutions have been recently proposed.

/pdf/microbial-catalysis-of-the-oxygen-reduction-reaction-for-i356rhvbw1.pdf

Microbial Catalysis of the Oxygen Reduction Reaction for Microbial Fuel Cells: A Review

Hydrogen sulfide evolved from an acidified sample is pre-concentrated by permeation in a stationary alkaline acceptor solution enclosed in a silicone rubber sample loop. Depending on the sample volume pre-concentrated, the applicable analytical range spans low micrograms/L to tens of mg/L for both methods. The methylene blue method is more sensitive by a factor of approximately 30 and actually permits practical determinations in the sub-micrograms/L levels. The limit of detection (LOD) for the nitroprusside method ranges from 20 micrograms/L for a 20 microL sample by conventional flow-injection determination (no membrane, throughput 30 samples/h) to less than 2 micrograms/L for 12 mL sample pre-concentrated in the membrane system (throughput 5 samples/h). The membrane is highly resistant to fouling and permits analysis of untreated wastewater samples bearing suspended solids, oil, grease, etc. without any pretreatment. No significant interference is observed with either chemistry. Although the nitroprusside chemistry is less sensitive, it does not involve the use of concentrated aggressive reagents and is recommended unless ultratrace determinations are essential. Viable reaction mechanisms are proposed for both of these chemistries.

Nitroprusside and methylene blue methods for silicone membrane differentiated flow injection determination of sulfide in water and wastewater.

The mechanism of the electrochemical reduction of nitric oxide (NO) by hemin adsorbed at pyrolitic graphite was investigated. The selectivity of NO reduction was probed by combining the rotating ring disk electrode (RRDE) technique with a newly developed technique called on-line electrochemical mass spectroscopy (OLEMS). These techniques show that NO reduction by adsorbed heme groups results in production of hydroxylamine (NH2OH) with almost 100% selectivity at low potentials. Small amounts of nitrous oxide (N2O) were only observed at higher potentials. The rate-determining step in NO reduction most likely consists of an electrochemical equilibrium involving a proton transfer, as can be derived from the Tafel slope value of 62 mV/dec and the pH dependence of −42 mV/pH. The almost 100% selectivity toward NH2OH distinguishes this system both from NO reduction on bare metal electrodes, which often yields NH3, and from biological NO reduction in cytochrome P450nor, which yields N2O exclusively.

Electrochemical reduction of NO by hemin adsorbed at pyrolitic graphite

Carbon-supported hemin (hemin/C) was investigated as a nonprecious metal electrocatalyst mediating the oxygen reduction reaction (ORR). The effect of the heat treatment and metal content on the electrocatalytic activity was extensively studied. We found that the electrocatalytic activity was significantly improved after the heat treatment, and the best performance was achieved at the treatment temperature of 600 °C. Physico-chemical characterizations indicated that the heat treatment temperature yielded a coincident effect on the composition and microstructure of the catalyst. X-ray photoelectron spectroscopy (XPS) revealed that the atomic ratio of Fe to N was 1:4.7 for pristine hemin/C, which amounted to the highest value of 1:1.9 for hemin/C heat-treated at 600 °C. Nitrogen ad/desorption analysis showed that the pyrolysis at 600 °C resulted in the largest microporous surface area. On the basis of the above findings, we suggest that FeN2 be the active center in hemin/C and the micropore play an important...

Hemin: A Highly Effective Electrocatalyst Mediating the Oxygen Reduction Reaction

The equilibria among the species of [tetrakis(1-methylpyridinium-4-yl)porphine]iron(III) ion (Fe(III)–TMPyP) in aqueous solution and the redox reactions of their central metals were investigated by means of spectrophotometric, electrochemical and spectroelectrochemical methods. Two acid-dissociation equilibria involving three monomeric species of Fe(III)–TMPyP were found: one with a pKa of 5.5 and the other with a pKa of ca. 12. After electrochemical reduction, an equilibrium between two monomeric species of Fe(II)–TMPyP with a pKa of 11 was observed. In the Fe(III)–TMPyP solution at relatively high concentration, two additional different species, probably dimeric, were found in the pH regions of 5–8 and 9–13, respectively. The consistency of specification of the species with the scheme of redox reactions is discussed.

Spectrophotometric and Electrochemical Investigations of [Tetrakis(1-methylpyridinium-4-yl)porphine]iron(III) Ion in Aqueous Solution

The catalytic electroreduction of dioxygen at iron(III) protoporphyrin IX (hemin) modified glassy carbon electrodes was investigated in an aqueous solution within the pH range 1–13. The reduction potential of dioxygen on a hemin chemically modified electrode was more anodic than that on a hemin adsorbed electrode by a few tens of mV. The results of rotating disk and ring-disk voltammetric measurements indicated that the oxygen molecule is reduced via a one-step reduction accompanying four-electron and four-proton transfer at pH 12. The quantity of peroxide formation at hemin-adsorbed electrodes decreased with increasing pH, up to 10, and then increased with increasing pH beyond 12. The possible overall mechanisms for the catalytic reduction of dioxygen are described.

The Catalytic Electroreduction of Dioxygen on Iron Protoporphyrin IX Modified Glassy Carbon Electrodes

The axial-ligand replacement of the [Tetrakis(1-methyl-4-pyridinio)porphine]iron(III) ion by the cyanide ion was investigated in a highly alkaline aqueous solution by means of the spectrophotometric method at 25 °C. In the course of the replacement, a protolytic equilibrium between monocyanomonoaqua- and monocyanomonohydroxo complex ions was observed; the p(Ka/mol dm−3 value of the monocyanomonoaqua complex ion was found to be 12.2±1.1. In the formation of the dicyano complex ion, the rate-determining step was the second replacement of axial ligands, i.e., from the monocyano complex ion to the dicyanocomplex ion; according as the pH is 11 or 13, the equilibrium constant was found to be 38.0 mol−1 dm3 or 11.0, while the rate constant was 1.07×10−3 or 4.23×10−3 mol−3 dm3 s−1.

The Axial-Ligand Replacement of the [Tetrakis(1-methyl-4-pyridinio)porphine]iron(III) Ion by the Cyanide Ion in a Highly Alkaline Aqueous Solution

The catalytic electroreduction of an oxygen molecule on a glassy carbon electrode covalently modified with [5,10,15,20-tetrakis(4-carboxyphenyl)porphinato]iron(III) (abbreviated as GC-O-Fe(III)TCPP) was investigated in an aqueous solution. The reduction of oxygen on the GC-O-Fe(III)TCPP electrode occurs at a more anodic potential than on the glassy carbon electrode adsorbing Fe(III)TCPP. This anodic shift of the oxygen reduction potential may be closely related to the number of bonds between a metalloporphyrin molecule and the electrode material. The oxygen molecule is reduced via the one-step reduction accompanying four-electron and four-proton transfer at pH 7–11 and via two successive reductions accompanying two-electron transfer and subsequently two-electron and four-proton transfer at pH>11.5. At pH<6, the number of transferred electron could not be reproducibly determined because of the cleavage of several ester linkages.

The Catalytic Electroreduction of Oxygen in an Aqueous Solution on Glassy Carbon Electrodes Covalently Modified with [5,10,15,20-Tetrakis(4-carboxyphenyl)porphinato]iron(III)

Fumihiro Arifuku

Papers

Spectrophotometric and Electrochemical Investigations of [Tetrakis(1-methylpyridinium-4-yl)porphine]iron(III) Ion in Aqueous Solution

The Catalytic Electroreduction of Dioxygen on Iron Protoporphyrin IX Modified Glassy Carbon Electrodes

The Axial-Ligand Replacement of the [Tetrakis(1-methyl-4-pyridinio)porphine]iron(III) Ion by the Cyanide Ion in a Highly Alkaline Aqueous Solution

The Catalytic Electroreduction of Oxygen in an Aqueous Solution on Glassy Carbon Electrodes Covalently Modified with [5,10,15,20-Tetrakis(4-carboxyphenyl)porphinato]iron(III)

Anodic Oxidation of N , N -Dimethyl- p -phenylenediamine with Sodium Sulfite