## 4 Conclusions  
Fluorescence measurements showed differences in the effects of the analyzed substances on the phosphatidylcholine lipid membrane. Oleanolic acid located in the hydrophobic core of the bilayer caused changes in the membrane structure, leading to increased exposure of its surface to the aqueous environment. In the PC membrane enriched with oleanolic acid (20 \(\upmu\)mol), apigenin and rutin were accumulated at the bilayer surface as well as in the hydrophobic core. In turn, RS was located in the hydrophobic membrane region, and at concentrations above 20 \(\upmu\)mol, changes in the bilayer structure occurred under the influence of this compound and an increased exposure of the hydrophilic membrane region to the water environment was observed. The weakest interactions with the bilayer were found in the case of ferulic acid. It was located at the membrane surface and is not present in the hydrophobic core. Oleanolic acid causes rearrangement of phospholipid packing in the bilayer, which leads to a change in the membrane curvature and formation of liposomes of smaller size. This phenomenon probably influences changes of the liposomes' hydrodynamic diameter in the presence of apigenin and rutin. For apigenin, a decrease of the liposomes' hydrodynamic diameter from about 275 nm to about 140 nm at 100 \(\upmu\)mol of flavonoid was found. In the case of rutin (100 \(\upmu\)mol), a size decrease from about 220 nm to about 155 nm was recorded. In turn, zeta potential measurements showed that, at least in some part, all the substances analyzed interact with the membrane surface. This led to potential decreases, which in the case of rutin was reduced to \(-\)29 mV. The results of structural studies were compared with the antioxidant activity of the investigated substances in the lipid membrane. In liposomes loaded with oleanolic acid, the pro-oxidant activity of resveratrol observed in the present study is likely not due to its ability to scavenge free radicals. Instead, it arises from its impact on the membrane structure, leading to an augmented exposure of the hydrophilic region to the surrounding aqueous environment. Thus, the availability of peroxidation-sensitive parts of the bilayer for free radicals generated from AAPH in water is increased, resulting in a pro-oxidant effect. A similar mechanism may be involved in the pro-oxidant action induced by the presence of oleanolic acid in the membrane. On the contrary, apigenin, rutin and ferulic acid are present at the membrane surface, as shown by fluorescence measurements. Their presence in this region protects the bilayer from radicals generated from AAPH in the aqueous phase. Lower antioxidant activity observed in the case of ferulic acid is probably related to weaker interaction of this compound with the membrane, compared to the flavonoids tested. This is illustrated by the Fo/F value, which at a concentration of 100 \(\upmu\)mol is equal to 1.44 for ferulic acid, compared to 1.31 and 1.23 for apigenin and rutin, respectively. The data presented above suggest that the antioxidant/pro-oxidative properties of the investigated phenolic compounds in the lipid membrane containing oleanolic acid result mainly from