Oxidation of the porphyrin ligand

The porphyrin Tr-system can donate a number of electrons to form the radical cation, dication, etc. Pulse radiolysis has been used to oxidize porphyrins to the TT-radical cations both in aqueous solutions and in organic solvents. In aqueous solutions, the - OH radical is not a good oxidant for porphyrins because it adds to the double bonds. However, oxidizing radicals such as Cl2 , Br2- N3-, CCI3O2 -, and CH2CHO are suitable for this purpose. For example, oxidation by Br2 takes place with rate constants in the range of 10 L mol  

The azide and dichloride radicals react more rapidly and the organic radicals shown above react more slowly. Many water-soluble metalloporphyrins were oxidized by these reactions, and radical cations were generated from Zn , Cd , Cu , Pd and Sn -porphyrins. Other metalloporphyrins were subsequently studied. 

The resulting porphyrin Tr-radical cations exhibit a fairly strong absorption band around 650-700 nm. The exact position of this band is dependent upon the nature of the porphyrin ligand.With a positively charged ligand (Zn TM4PyP), the spectrum of the radical cation was also slightly dependent 

In most cases, the radical cations were unstable and decayed within milliseconds. The rate of decay was greatly influenced by the nature of the substituents on the porphyrin ring, as outlined earlier for the various radical anions. Thus, the tetranegative Zn TSPP gave a relatively long-lived radical cation, with half-life —7 s at pH 7. In contrast, the tetrapositive Zrf TMPyP complexes gave very short-lived radical cations. They decay via disproportionation to form a dication. 

The influence of the central metal ion upon the reactivity of porphyrin radical cations in water was also studied. As described for the porphyrin radical anions, the electronegativity of the central metal ion was found to exert a pronounced effect on the rate of decay of the cations. For a given type of porphyrin ring, the stability of the porphyrin radical cation decreased in the order Pb, Mg, Cd, Zn, Pd, V 0, Fe, and again there was a good correlation between stability and oxidation potential. For example, Mg TSPP, because of the electropositive Mg ion, formed a particularly stable radical cation. In neutral aqueous solution the first half-life for decay of (Mg TSPP) was about 37 s, although it decreased at higher pH due to deprotonation of the axially-bound water molecule. 

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Substituents in the porphyrin ring systems may confer stability and selectivity toward oxidation these attributes are fundamental to the development of efficient model systems mimicking hemoprotein activities. Metalloporphyrins are very vulnerable to destructive oxidation of the porphyrin ligand under strongly oxidizing conditions. Thus, Groves et al. 

The addition of 0.5 equivalent of a two-electron reducing agent like veratryl alcohol generates LiP-Cpd II (with a Soret band at 420 nm), which corresponds to the iron(IV) 0x0 without oxidation of the porphyrin ligand. Site-directed mutations have demonstrated a key role for Trp-171 in veratryl alcohol oxidation, this amino acid being probably the location of an intermediate radical cation in the oxidation cascade. 

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