## 3 Results and Discussion
### Antioxidant Properties of Extracts Obtained from Walnut Husks
In methods where the SET mechanism dominates (DPPH, ABTS, CUPRAC and FRAP), neutralization of reactive species takes place as a result of ionization/dissociation of the antioxidant, which is promoted by a solvent with a high dielectric constant (\(\varepsilon\)). Therefore, it is not surprising that in the mentioned methods, the highest antioxidant properties are exhibited by methanol and acetone extracts (\(\varepsilon=32.6\) and \(\varepsilon=20.7\), respectively) [33; 34]. In turn, the low antioxidant properties of chloroform extracts can be explained, on the one hand, by the low content of polyphenols (see Table 2), and, on the other hand, by the low dielectric constant and difficult dissociation/ionization of the antioxidant (\(\varepsilon=4.8\)). In the \(\beta\)-carotene method, the dominant mechanism is the HAT mechanism, or more precisely, its variant PCET (Proton Couple Electron Transfer), in which the neutralization of peroxyl radicals (LOO*) occurs as a result of the formation of a transition complex between the radical and the antioxidant, and then the separation of hydrogen from the antioxidant as a result of the homolytic dissociation of the molecular bond. Hydrogen transport is favored in a solvent characterized by a high value \(\beta\), which expresses the hydrogen acceptor capacity of the solvent. Among the solvents used, chloroform has the lowest value of this parameter (\(\beta=0.02\)), while for the other solvents, it ranges from 0.41-0.5 [35], hence the better antioxidant properties of the extracts obtained in these solvents. The Influence of Juglone on the Antioxidant Properties of Selected Extracts Obtained from Walnut Husks  
As mentioned in the Introduction, juglone is one of the characteristic compounds of walnut found, among others, in husks. The content of this compound is typically determined using chromatographic methods. However, to our surprise, chromatographic measurements performed on the obtained extracts showed the absence of juglone (see Figure S1 in the Supplementary Materials). While looking for the reasons for this situation, we found literature data revealing that this compound is not only thermally unstable, but also susceptible to the negative influence of air, water or bacteria [32; 36]. Thus, the reasons for the absence of this compound in extracts can be attributed to the late harvest of walnut husks (late September and October). However, as mentioned, juglone has documented antibacterial, antiviral and antifungal properties. Taking into account the above and the fact that juglone has an -OH group in its structure with potential antioxidant properties (see Figure 3A), it was decided to check whether juglone also has antioxidant properties and whether its addition to the extracts changes their antioxidant properties. Figure 3: Structural formula of juglone (**A**). Spectra of the cation radical solution (green curve, A\({}_{0}=0.7\)) and the solution containing the cation radical ABTS (2900 \(\mu\)L, A\({}_{0}=0.7\)) and juglone (100 \(\mu\)L, c = 0.2 mg/mL); red curve—spectrum obtained at 0 min of reaction (immediately after mixing the ingredients) and blue curve—spectrum obtained after 60 min of reaction (**B**). In this series of studies, the antioxidant properties of juglone, methanol extracts (obtained in the Soxhlet apparatus and by maceration) and their binary mixtures were assessed using the ABTS method. Experiments were performed for two concentrations of juglone(c = 0.06 mg/mL and 0.2 mg/mL), which were mixed in different volume ratios with methanol extracts obtained from walnut husks. The concentrations of juglone selected for the experiment were based on literature data [37]. Since juglone solutions are characterized by intense color, it was first decided to check whether their addition would not affect the spectrum of the radical and disturb the measurement.