## 3 Results and Discussion
The incorporation of RS into the hydrophobic core of the membrane likely \"loosens\" its structure and increases exposure to the surrounding water, which quenches NBD-PE emission and creates a less favorable environment for DPH fluorescence. Figure 2: Fluorescence intensity of NBD-PE probe in lipid membrane in the presence of oleanolic acid (**a**), and oleanolic acid mixed with apigenin (**b**), rutin (**c**), resveratrol (**d**) and ferulic acid (**e**). Figure 3: Fluorescence intensity of DPPH probe in lipid membrane in the presence of oleanolic acid. (**a**), and oleanolic acid mixed with apigenin (**b**), rutin (**c**), resveratrol (**d**) and ferulic acid (**e**). A previous study reported that flavonoids have the ability to quench DPH emission [27]. In the case of apigenin and rutin (Figure 3b,c) a strong decrease of DPH fluorescence in the presence of these two flavonoids was recorded. This phenomenon is due to the incorporation of AP and RT into the hydrophobic core of the membrane, where the probe is present. No statistically significant changes in DPH fluorescence were found in the presence of fertile acid, indicating its absence in the hydrophobic membrane region (Figure 3e). Fluorescent probes' emission measurements revealed significant differences in the effects of the analyzed substances on the phosphatidylcholine lipid membrane. OA is located in the hydrophobic core of the bilayer, and at higher concentration (100 \(\upmu\)mol), causes changes in the membrane structure leading to increased exposure of its surface to the aqueous environment. In the phosphatidylcholine membrane enriched with oleanolic acid (20 \(\upmu\)mol), AP and RT are accumulated at the bilayer surface as well as in the hydrophobic core. In turn, RS is located in the hydrophobic membrane region, and at concentrations above 20 \(\upmu\)mol, changes in the bilayer structure under the influence of this compound increase exposure of the hydrophilic membrane region to the water environment. The weakest interactions with the bilayer were found in the case of FA. It is located at the membrane surface and is not present in the hydrophobic core, as indicated by the DPH probe measurements. Influence of Oleanolic Acid and Polyphenols on Lipid Membranes Structure--Fluorescence Anisotropy Measurements  
NBD-PE and DPH fluorescence anisotropy measurements revealed changes in the lipid membranes' structure in the presence of the analyzed substances. DPH is a probe situated in the hydrophobic bilayer region parallel to the acyl chains [28]. Its fluorescence anisotropy measurements are used to monitor acyl chains' rotational diffusion. Reduction of the probe's rotational mobility results in the increase of the anisotropy value [29]. In turn, the moiety responsible for NBD-PE fluorescence is located in hydrophilic part of bilayer. A slight increase of NBD-PE fluorescence anisotropy was recorded in the case of a bilayer containing 20 \(\upmu\)mol of OA and 100 \(\upmu\)mol of resveratrol simultaneously (Figure 4d). This confirms the structural changes observed in the hydrophilic part of the membrane during steady-state fluorescence measurements. In the other cases, there was no statistically significant effect of OA and polyphenols on the NBD-PE emission anisotropy (Figure 4a-c,e). A slight increase in DPH fluorescence anisotropy was observed in the samples containing 100 \(\upmu\)mol OA as well as OA (20 \(\upmu\)mol) and apigenin (20-100 \(\upmu\)mol) simultaneously (Figure 5a,b). This indicates a decrease in membrane fluidity in the presence of the substances used. No statistically significant changes in DPH fluorescence anisotropy were observed in the other samples (Figure 5c-e). [PERSON] and co-authors [5] discovered that OA reduces the flexing motion of fatty acyl chain regions in DPPC (dipalmitoyl phosphatidylcholine), being in liquid-crystalline phase. This phenomenon leads to a decrease of bilayer fluidity [5]. A similar effect was observed for cholesterol, whereby the modulating influence of OA is weaker.