Tuning of HNO trapping by Mn(III) porphyrins: effect of the redox potential

SEBASTIÁN A. SUÁREZ; LUCIA ALVAREZ; DAMIÁN E. BIKIEL; MARCELO A. MARTÍ; FABIO DOCTOROVICH

Huatulco

Congreso; SILQOM 2013; 2013

In this work, we sought to understand the relationship between the redox potential of metalloporphyrins and its mechanism and rate of reaction with HNO and HNO donors. The metalloporphyrins that we are studying have oxidation potentials from 200mV to -200 mV. A direct relation between the oxidation potential and the reaction mechanism was found. On the other hand, sulfohydroxamic acids, known as Piloty acids (PA), are commonly used HNO donors. Their decomposition, between pH 9-13,(1) takes place by following first order kinetics with a rate constant ~10-4 s-1. In the presence of these compounds, our Mn(III) porphyrins [Mn(III)P] react to produce [MnPNO] as product. As Mn(III)P do not react with NO, the product formation is attributed to the reaction of the porphyrin with HNO/NO?.(2) However, for some Mn(III)P, nitroxylation is faster than the decomposition of the corresponding donor.(2) This fact strongly indicates that a direct reaction between them might be taking place. Even though both processes go through separate pathways, they can be complementary, being the direct (second order) mechanism more important at high concentrations of reagents. The fact that product formation happens even at values of pH lower than the pKa of the donor is an important proof of our mechanism. As it is known from literature, HNO donation happens from the deprotonated donor. This observation would indicate a decrease of the donor pKa due to coordination to the porphyrin metal center. Electronic structure calculations (DFT) were performed to model the effects of deprotonation in the presence and absence of porphyrin.