Metal phthalocyanines and porphyrins as photosensitizers for reduction of water to hydrogen

Oxidative stress has become widely viewed as an underlying condition in a number of diseases, such as ischemia–reperfusion disorders, central nervous system disorders, cardiovascular conditions, cancer, and diabetes. Thus, natural and synthetic antioxidants have been actively sought. Superoxide dismutase is a first line of defense against oxidative stress under physiological and pathological conditions. Therefore, the development of therapeutics aimed at mimicking superoxide dismutase was a natural maneuver. Metalloporphyrins, as well as Mn cyclic polyamines, Mn salen derivatives and nitroxides were all originally developed as SOD mimics. The same thermodynamic and electrostatic properties that make them potent SOD mimics may allow them to reduce other reactive species such as peroxynitrite, peroxynitrite-derived CO3·−, peroxyl radical, and less efficiently H2O2. By doing so SOD mimics can decrease both primary and secondary oxidative events, the latter arising from the inhibition of cellular tra...

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Superoxide Dismutase Mimics: Chemistry, Pharmacology, and Therapeutic Potential

The log kcat values for the dismutation of O2•- by a series of monohydroxoiron(III) and aquamanganese(III) porphyrins, including ortho, meta, and para isomers of 5,10,15,20-tetrakis(N-alkylpyridiniumyl)porphyrins, were found to vary linearly with the metal-centered redox potentials (E1/2) for the M(III)/M(II) couple. Each 120 mV increase in E1/2 imparted a 10-fold increase in kcat. The observed behavior is in accord with the Marcus equation for outer-sphere electron-transfer reactions, suggesting that the same mechanism is operative for iron and manganese porphyrins. The Marcus plot enabled us to estimate the self-exchange rate constants of monohydroxoiron porphyrins to be ∼1 order of magnitude higher than those of aquamanganese porphyrins. Furthermore, E1/2 values for all of the metalloporhyrins investigated were linearly related to the acid dissociation constants (pKa3) of the pyrrolic nitrogen of the metal-free porphyrins, indicating that either E1/2, or the more readily measured pKa3, may be useful in...

Relationship among Redox Potentials, Proton Dissociation Constants of Pyrrolic Nitrogens, and in Vivo and in Vitro Superoxide Dismutating Activities of Manganese(III) and Iron(III) Water-Soluble Porphyrins

Reactions of manganese porphyrins with peroxynitrite and carbonate radical anion.

Diverse functions of cationic Mn(III) N-substituted pyridylporphyrins, recognized as SOD mimics.

Oxidation of metal tetraphenylporphyrins

Water-soluble manganese(III) porphyrins are oxidised in alkaline aqueous solution to the corresponding manganese(IV) porphyrins which, from magnetic moment measurements, appear to exist in solution as µ-oxo-dimers. Midpoint potentials and rate constants for oxidation of the manganese(III) porphyrins have been measured for a series of oxidants and throughout the range 9 < pH < 14, but the overall electronic charge on the metalloporphyrin had little effect upon either parameter. The midpoint potentials for the MnIII/IV couple are strongly dependent upon pH and increase with decreasing pH. Although manganese(IV) porphyrins are mild oxidants at pH 14, they should be capable of oxidising water to molecular oxygen in neutral solution. With hypochlorite as oxidant, a second oxidation step is possible and the final product is believed to be a manganese(V) oxo-porphyrin.

Photochemistry of manganese porphyrins. Part 6. Oxidation–reduction equilibria of manganese(III) porphyrins in aqueous solution

Studies with aqueous and organic solvents and with micelles, membranes, and microemulsions showed that the environment had little effect upon the spectroscopic and chemical properties of MnII and MnIII porphyrins. However, for oxidation of a MnIII porphyrin, the environment makes important contributions towards the ease of oxidation and the nature of the oxidation process. In alkaline aqueous solution, oxidation was facile and resulted in formation of MnIV and MnV porphyrins. Identical, but less efficient, oxidation processes were observed in membranes and positively charged micelles but in both negatively charged micelles and microemulsions oxidation was extremely inefficient and the MnV porphyrin was not observed. In CH2Cl2 solution, reaction with phenoxathiinylium hexachloroantimonate gave the MnIII porphyrin π-radical cation whilst reaction with iodosylbenzene resulted in formation of the MnIV porphyrin µ-oxo-dimer.

Nilton Carnieri

Papers

Oxidation of metal tetraphenylporphyrins

Photochemistry of manganese porphyrins. Part 6. Oxidation–reduction equilibria of manganese(III) porphyrins in aqueous solution

Photochemistry of manganese porphyrins. Part 7. Characterisation of manganese porphyrins in organic and aqueous/organic microheterogeneous systems

Photochemistry of manganese porphyrins V: The photoreduction of methyl viologen