The coulometric approach to the superoxide scavenging activity determination: The case of porphyrin derivatives influence on oxygen electroreduction

Abstract

The electrochemical and antioxidant properties of 5,10,15,20-tetrakis(3′-hydroxyphenyl)porphyrin ( H 2 T (m- OHPh ) P ) and 5,10,15,20-tetrakis(4′-hydroxyphenyl)porphyrin ( H 2 T (p- OHPh ) P ) were tested by the cyclic voltammetry (CV) method. It is shown that in dimethylsulfoxide (DMSO) the electroreduction processes of oxygen and porphyrins are under diffusion control. The electroreduction of H 2 T (m- OHPh ) P and H 2 T (p- OHPh ) P with potentials about -1 V are irreversible due to the chemical step (EC process) which leads to products with oxidation potentials about -0.5 V. Additionally in case of oxygen and porphyrin coreduction, both H 2 T (m- OHPh ) P and H 2 T (p- OHPh ) P influence the O 2•- electrosynthesis. The nonlinear dependence of the O 2•- peak current vs. porphyrin concentrations makes the available amperometric approach unsuitable for antioxidant activity estimation. To solve this problem, the coulometric parameters were calculated. The excellent linearity of the coulometric response of superoxide ion vs. porphyrins concentration was demonstrated for a wide concentration range. On the basis of coulometric responses, we constructed a parameter which separated the radical scavenging activity and variation of oxygen electroreduction. The superoxide scavenging activities of H 2 T (m- OHPh ) P and H 2 T (p- OHPh ) P were determined using the developed approach. The effect of OH group position on the superoxide scavenge activity is shown: H 2 T (p- OHPh ) P has a higher activity (slope = 0.75 L.mmol-1) than H 2 T (m- OHPh ) P (slope = 0.60 L.mmol-1).