Kaempferol and quercetin interactions with model lipid membranes

• Kaempferol and quercetin interactions with model lipid membranes (without cholesterol) were investigated.
• Two fluorophores and two spin probes were used to monitor the membrane characteristics.
• Kaempferol and quercetin interacted mainly with the polar headgroup regions of the bilayer.
• A difference of one –OH group promotes different membrane effects.

In the present study, the membrane interactions of the flavonoids kaempferol and quercetin at different temperatures with two different model lipid membranes (without cholesterol) were investigated. A combination of fluorescence spectroscopy, electron paramagnetic resonance spectroscopy, and differential scanning calorimetry were used to detect changes in membrane fluidity. Two fluorophores and two spin probes were used to monitor the membrane characteristics at different depths in the membrane. The dependence of the lipid order on the molar ratio of flavonoid to membranes of phosphatidylcholine:sphingomyelin (2.4:1) and of 1,2-dipalmitoyl-sn-glycero-3-phosphocholine showed that flavonoids increase the lipid order. As with fluorescence spectroscopy, for phosphatidylcholine:sphingomyelin liposomes, the fluidity decrease detected with the spin probes of the electron paramagnetic resonance spectroscopy was less with kaempferol than quercetin, and was higher in the middle of the membrane than close to the water–lipid interface. Below the phase-transition temperature, the polarity surrounding the nitroxide group of the spin probes in the most-ordered membrane domains increases in the presence of kaempferol and quercetin, which indicates that these flavonoids facilitate the penetration of water molecules into the membranes. Kaempferol and quercetin caused a small thermal destabilisation of DPPC transition which shows that the molecules interacted mainly with the polar headgroup regions of the bilayer. With the kaempferol and quercetin, which differ only in their number of –OH groups, their effect on the lipid membranes was more pronounced in the absence of cholesterol and confirmed that even a difference of one –OH group is enough to promote different membrane effects.