## 3 Results and Discussion
### Antioxidant Activity Assays  
The EEP was evaluated for its ability to quench the DPPH, which is one of the few stable organic nitrogen radicals and bears a purple colour. This assay is based on the measurement of the loss of DPPH after reaction with samples. It is considered as the prior mechanism involved in the electron transfer. The IC\({}_{50}\) value is a parameter widely used to measure the antioxidant activity of test samples. It is calculated as the concentration of antioxidants needed to decrease the initial DPPH concentration by 50% [23]. Thus, the lower IC\({}_{50}\) value the higher antioxidant activity. The EEP GUA-4 showed an antioxidant activity (IC\({}_{50}\) = 67.9 \(\upmu\)g/mL) comparable to the reference ascorbic acid (IC\({}_{50}\) = 43.2 \(\upmu\)g/mL) and tenfold lower than Trolox (IC\({}_{50}\) = 6.3 \(\upmu\)g/mL) and seven-fold lower than quercetin (IC\({}_{50}\) = 9.9 \(\upmu\)g/mL). While caffe acid (12) showed the highest activity (IC\({}_{50}\) = 5.9 \(\upmu\)g/mL) along with ferulic acid (10) (IC\({}_{50}\) = 9.9 \(\upmu\)g/mL) and syringic acid (11) (IC\({}_{50}\) = 9.8 \(\upmu\)g/mL). The lowest antioxidant activity corresponds to 5-methylchrysin ether (9) (IC\({}_{50}\) = 112.9 \(\upmu\)g/mL) and 5-methyl-pinobanksin ether (5) (IC\({}_{50}\) = 98.4 \(\upmu\)g/mL). There are reports of the DPPH scavenging capacity, in terms of IC\({}_{50}\), for pinochemrin (**1**), chrysin (**2**), galangin (**3**), isorhamnetin (**7**), ferulic acid (**10**), syringic acid, (**11**) and caffe acid (**12**) [23; 24; 25; 26]. For alpinetin (**4**) only the percentage of scavenging activity of DPPH is reported [27]. Nevertheless, the information available for dillentin (**6**), 5-methyl-pinobanksin ether (**5**), 5-methylchrysin ether (**9**), and 5-methylgalangin ether (**8**) is scarce. Thus, the present work stands for the first report of the IC\({}_{50}\) as a measurement of their antioxidant activity (Table 2). In the ABTS assay, the antioxidant activity is measured as the ability of test compounds to decrease the color reacting directly with the radical ABTS*[28]. Ferulic acid (**10**), syringic acid (**11**), caffeic acid (**12**), and chrysin (**2**) showed the lowest IC\({}_{50}\) (Table 2). The EEP, 5-methylchrysin ether (**9**), and 5-methyl-pinobanksin ether (**5**) showed weak antioxidant activity (Table 2). The analysis of their chemical structures can explain the weak antioxidant activity of the methylated flavonoids. According to [PERSON] et al. [29] the catechol structure in the B-ring, 2,3-double bond in conjugation with a 4-oxo function in the C-ring and the hydroxyl groups in meta position in ring-A. On the other hand, the activity of phenolic acids lies on the presence of two \(o\)-hydroxyl groups in the aromatic ring. \begin{table}
\begin{tabular}{c c c} \hline \hline \multirow{2}{*}{**Compounds**} & \multicolumn{2}{c}{**IC\({}_{50}\) (\(\upmu\)g/mL)**} \\ \cline{2-3}  & **DPPH** & **ABTS** \\ \hline EEP GUA-4 & 67.9 \(\pm\) 0.1 & 98.7 \(\pm\) 0.5 \\ pinochemrin (**1**) & 23.5 \(\pm\) 1.8 & 44.8 \(\pm\) 2.1 \\ chrysin (**2**) & 10.7 \(\pm\) 0.2 & 16.3 \(\pm\) 0.8 \\ galangin (**3**) & 15.3 \(\pm\) 0.5 & 26.8 \(\pm\) 0.1 \\ alpinetin (**4**) & 47.3 \(\pm\) 1.9 & 69.5 \(\pm\) 0.7 \\
5-methylpinobanksin ether (**5**) & 98.4 \(\pm\) 2.3 & 126.9 \(\pm\) 4.5 \\ dillenetin (**6**) & 35.7 \(\pm\) 3.5 & 48.9 \(\pm\) 2.9 \\ isorhamnetin (**7**) & 21.4 \(\pm\) 3.1 & 36.7 \(\pm\) 4.6 \\
5-methylglangin ether (**8**) & 63.2 \(\pm\) 2.6 & 94.2 \(\pm\) 6.2 \\
5-methylchrysin ether (9) & 112.9 \(\pm\) 1.9 & 158.4 \(\pm\) 4.9 \\ ferulic acid (**10**) & 9.9 \(\pm\) 0.7 & 16.7 \(\pm\) 0.2 \\ syringe acid (**11**) & 9.8 \(\pm\) 0.4 & 13.0 \(\pm\) 0.9 \\ caffeic acid (**12**) & 5.9 \(\pm\) 0.4 & 9.7 \(\pm\) 0.5 \\ Quercetin & 9.9 \(\pm\) 2.5 & 16.1 \(\pm\) 2.1 \\ Trolox & 6.3 \(\pm\) 1.4 & 3.8 \(\pm\) 1.2 \\ ascorbic acid & 43.2 \(\pm\) 10.3 & 36.8 \(\pm\) 2.5 \\ \hline \hline \end{tabular}
\end{table} 
Table 2: Scavenging ability of ethanol extract of propolis (EEP) GUA-4 and compounds **1–12**.