|M.Sc Student||Meital Eckshtain|
|Subject||Corrole Metal Complexes as Catalysts for Decomposition of|
Superoxide Anion Radical
|Department||Department of Chemistry||Supervisor||Full Professor Gross Zeev|
|Full Thesis text - in Hebrew|
Research goals of this study were to examine the catalytic activity of superoxide dismutation for different metallocorroles, in order to elucidate the main factors that could influence this activity and identify plausible reaction mechanism for the iron(ІІІ) complex.
Hydrophobic corrole manganese complexes with electron-withdrawing groups such as C6F5 exhibited a catalytic rate constant of ~105 M-1s-1 and a higher redox potential for their Mn3+/Mn4+ process. On the other hand, analogous complexes with electron-donating groups such as anisyl exhibited catalytic constant of ~106 M-1s-1 and redox potential closer to the midpoint of oxidation and reduction processes of superoxide.
Redox potentials for negatively-charged-metallocorroles with C6F4OMe rings were lower than the analogous metallocorrole with C6F5 rings. Moreover, both the cytochrome c assay and the attenuation effects regarding the oxidation of benzyl to benzoic acid by superoxide indicate that the former complex is more efficient in dismutation of superoxide, compared to the metallocorrole with C6F5 rings.
Cyclic voltammetry of the positively-charged-metallocorroles indicates that positive charges proximal to metal ion increases its redox potential. The catalytic constants from the cytochrome c assay show that positive charges increase the catalytic activity of the metallocorrole, as the bis-ortho-pyridinium corrole is a more efficient catalyst than the bis-para-pyridinium derivative.
Mechanistic investigations for the reaction of Fe(tpfc)(SO3H)2 with superoxide were performed by following changes in the absorbance at 450 nm. This revealed the formation of a short lived intermediate (2 ms) that decayed back to the starting material within 40 ms. The kinetic were analyzed in terms of a fast formed [(SO3H)2(tpfc)Fe-O2.-] intermediate that decomposed in a rate limiting process to hydrogen peroxide and FeIV, followed by fast oxidation of superoxide to provide molecular oxygen and FeІІІ(tpfc)(SO3H)2. The high redox potential of Fe(tpfc)(SO3H)2 is also consistent with oxidation (by FeIV) of superoxide being more facile than its reduction (by FeIII).
In summary, this research revealed that manganese(ІІІ) and iron(ІІІ) complexes of β-pyrrole-sulfonated corroles and manganese(ІІІ) complex of 10,15-bis-pyridinium substituted corroles can catalyze the dismutation of superoxide. In contrast with all other synthetic and natural SODs, the iron and manganese corroles shuttle between Mn3+/Mn4+ rather than between Mn2+/Mn3+ oxidation state. We have uncovered two structure-activity relationships for the corroles: low redox potentials and close proximity of positive charges to the metal center lead to enhanced catalytic rates. This kind of information is crucial for the design of an optimal catalyst for the dismutation of superoxide.