Antioxidant reactivity evaluated by competitive kinetics: Influence of the target molecule concentration

In the present work, a competitive kinetic method using c-phycocyanin (from Arthospira maxima species) as target molecule was employed to estimate the reactivity of several phenols and flavonoids present in the human diet towards peroxyl radicals. The results obtained indicate that the protection afforded by a given compound strongly depends upon the experimental conditions employed and, in particular, on the concentration of the target molecule. This dependence is related to increased role of secondary reactions of the phenol-derived radicals initially formed. Secondary reactions can explain the strong downward curvature observed in R0/R (where R0 is the rate of the process in the absence of additive, and R is the rate of the process in the presence of additive) vs. additive concentration plots, for compounds such as kaempferol and protocatechuic acid, particularly at high c-phycocyanin concentration. Also, it can explain the prooxidant role played by phenolic compounds of low reactivity, such as 3-hydroxyflavone at low c-phycocyanin concentrations.