Water hydroxide, redox activation

The redox-active natural product (+)-methanophenazine (MP) is the first phenazine to be isolated from archea. This compound is able to mediate the electron transport between membrane-bound enzymes and was characterized as the first phenazine derivative involved in the electron transport of biological systems. The research team of U. Beifuss prepared this natural product by using the Williamson ether synthesis in the last step of the synthetic sequence. The etherification was conducted under phase-transfer conditions in a THF/water system in the presence of methyltrioctyl-ammonium chloride and using potassium hydroxide as a base. 

The rate of electron transfer that occurs to/from the metal center is high. Structure based modeling of the active site of human MnSOD [40], which includes calculating the energies of both the oxidized and reduced states with either water or hydroxide bound to the metal, suggests the rate of this internal electron transfer is enhanced by electron-relaxation effects. In addition, a 0.17 V redox potential is calculated, a value that is low compared with the experimental values of 0.31 V fori . coli and 0.26 V for B. stearothermophilus, respectively. A potential of —0.30 V seems to be optimal as it lies midway between the redox potentials of the two half reactions of the dismutation process [41],... 

In the substrate-free situation, Fe is in a low-spin state. The crystal structure indicates that several water molecules or hydroxide ions are present in the active site. One of them occupies the sixth coordination site at Fe. The hexacoordinated low-spin state is presumably achieved by coordinating the strong-field hydroxide ligand, OH , rather than the weak-field ligand, H2O [137]. The low redox potential is due to stabilization of the ferric state by an environment of high dielectric constant, i.e. by the water molecules in the active site [137]. When the camphor substrate diffuses into the enzyme, the change towards a high-spin complex can be followed spectroscopically. A transition to square pyramidal coordination at Fe occurs, and ordered... 

Babu et al. investigated the case of the commercially available iron-benzenetricarboxylate-based MOF Basolite F300 and its electrocatalytic activity towards the oxidation of hydroxide to O2 (anodic water splitting). In aqueous media well-defined Fe(III/II) redox processes of the solid microcrystalline powder attached to platinum electrodes were observed, but only in the presence of hydrochloric acid. The HCl concentration effect as well as the Fe(III/II) reversible potential for this process clearly indicated a CE-type reaction with framework dissolution prior to reduction of Fe (aq)... 

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