Abstract
The reactions of antioxidants with superoxide radical were studied by cyclic voltammetry (CV)—and hydrodynamic voltammetry at a rotating ring-disk electrode (RRDE). In both methods, the superoxide is generated in solution from dissolved oxygen and then measured after being allowed to react with the antioxidant being studied. Both methods detected and measured the radical scavenging but the RRDE was able to give detailed insight into the antioxidant behavior. Three flavonoids, chrysin, quercetin and eriodictyol, were studied, their scavenging activity of superoxide was assessed and the molecular structure of each flavonoid was related to its scavenging capability. From our improved and novel RRDE method, we determine the ability of these 3 antioxidants to react with superoxide radical in a more quantitative manner than the classical CV. Density Functional Theory (DFT) and single crystal X-ray diffraction data provide structural information that assists in clarifying the scavenging molecular mechanism. Hydroxyls associated with the A ring, as found in chrysin, scavenge superoxide in a different manner than those found in the B ring of flavonoids, as those in quercetin and eriodictyol.
Highlights
Antioxidants are capable of preventing the oxidation of other molecules through the termination of detrimental free radical chain reactions by stabilizing the unpaired free electron characteristic of radical species
We use the forward sweep of cyclic voltammetry (CV) to generate the superoxide radical ion and subsequently, in the reverse sweep, we measure the amount of the superoxide radical ion remaining (Figure 1)
The superoxide radical ion is stable over the time of the experiment in an anhydrous aprotic solvent containing no reactive species so the loss of superoxide radical ion is attributed only to the added antioxidant
Summary
Antioxidants are capable of preventing the oxidation of other molecules through the termination of detrimental free radical chain reactions by stabilizing the unpaired free electron characteristic of radical species. Biological systems are equipped with endogenous defenses against superoxide’s damaging effects, such as the enzyme superoxide dismutase [2] and catalase [3] These defense mechanisms are assisted by exogenous antioxidants from dietary sources, mainly found in fruit and vegetables. There are several sources which are used to generate the superoxide radical experimentally for its study [4], including an enzymatic reaction by xanthine dehydrogenase [5], or a non enzymatic option using phenazine methosulphate, NADH and molecular oxygen [6]. Once generated it is followed using spectrophotometric, colorimetric, chemiluminescence, and fluorescence detecting techniques
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