## 3 Results and Discussion
### Antioxidant Properties of the Examined Compounds and Their Mixtures  
Figure 6 shows the antioxidant activity, expressed as the percentage of scavenging of the colored cation radical ABTS, which was determined for the individual substances: caffeic acid (CFA), protocatechuic acid (PCA), nicotinamide (NA) and isonicotinamide (INA). As can be seen in the presented data, the greatest antioxidant properties were shown by caffeic acid, and the weakest were demonstrated by nicotinamide and isonicotinamide, whose properties were comparable and extremely weak (after 60 min of the neutralization reaction, they neutralized the colored radical by only about 10%). Figure 6: Antioxidant activity, expressed as percentage of inhibition of the ABTS cation radical, obtained for individual CFA, PCA, NA and INA substances. The protocatechuic acid had slightly worse antioxidant properties than caffe acid. This is not surprising because according to the literature, the presence of the CH=CH-COOH group in cinnamic acids provides greater activity than the COOH group in benzoic acids. The studies conducted in [35] showed that caffe, sinapic, ferulic and p-coumaric acids show greater activity than benzoic acids containing the same number of hydroxyl groups (i.e., protocatechuic, syringic, vanillic and p-hydroxybenzoic acids, respectively). It is stated that the double bond in the aliphatic chain in cinnamic acids participates in stabilization of the radical formed after neutralization by the antioxidant. Moreover, the presence of a single COOH group in protocatechuic acid may negatively affect its antioxidant properties. This was confirmed by studies performed on protocatechuic acid and pyrocatechol. Gallic acid and pyrogallol showed similar antioxidant behavior. Regarding NA and INA, the literature reports their strong anti-inflammatory properties, but it is emphasized that they have a weak capacity to neutralize reactive oxygen species [36]. What is more, studies conducted on the artificial DPPH radical using isonicotinamide and nicotinamide showed that both compounds exhibit extremely weak antiradical activity compared with other niacin-related compounds [37]. Taking the above into account, one question arises: Which antioxidant properties are exhibited by binary mixtures of the tested polyphenic acids and pyridinecarboxamide? Figure 7 shows the antioxidant activity obtained for binary mixtures: caffe acid with nicotinamide (designated as CFA-NA), caffe acid with isonicotinamide (designated as CFA-INA), protocatechuic acid with nicotinamide (designated as PCA-NA) as well as protocatechuic acid with isonicotinamide (designated as PCA-INA). The blue bars show the antioxidant activity for the binary mixtures (marked with the letter \"m\"), which were prepared immediately before measurement by mixing equal amounts of the parent solutions. The orange bars correspond to the antioxidant activity of the solutions (marked with the letter \"s\"), which were prepared by dissolving the corresponding complexes (**1**-**4**) in such amounts that the concentrations of the resulting solutions were identical to the proper mixtures marked with the letter \"m\". The gray bars show the so-called summary data, which were constructed based on the sum of activities obtained for individual antioxidants in single-component systems containing the same amount of component as in the two-component mixture. Comparing the values of the inhibition percentages obtained for the binary mixtures (blue bars) with the expected data (gray bars) shows that the experimental data were lower than what would result from the sum of the antioxidant activity values obtained for the individual components. The same trend was visible for all tested mixtures. As shown in Figure 7, the binary mixtures of the  
Figure 7: Antioxidant activity, expressed as percentage of ABTS radical inhibition, obtained for binary mixture of caffe acid or protocatechuic acid with nicotinamide or isonicotinamide.