The Disordering Effect of Plant Metabolites on Model Lipid Membranes of Various Thickness

Abstract: This study was focused on the effect of plant metabolites (phloretin, capsaicin, digitonin, diosgenin and betulin) on the model lipid membranes. The methods of assessing the permeability of lipid bilayers based on measuring the leakage of a fluorescent marker (calcein) from liposomes and differential scanning microcalorimetry of vesicle suspension were used. It was found that the release rate of calcein from 1-palmitoyl-2-oleyl-sn-glycero-3-phosphocholine (POPC) liposomes with test compounds added to the suspension at a ratio with lipid 1 : 50 decreased in the order capsaicin > phloretin >> betulin ≈ diosgenin ≈ digitonin. In the case of cholesterol- and ergosterol-containing POPC liposomes, the activity decreased as diosgenin ≈ digitonin > betulin > capsaicin > phloretin. It was demonstrated that phloretin and capsaicin significantly reduce the melting temperature (Tm) and to increase the half-width of the main peak on the endotherm (T1/2) of dipalmitoylphosphocholine (DPPC), distearoylphosphocholine (DSPC) and diarachidoylphosphocholine (DAPC). These results show that the incorporation of these small molecules between the polar “heads” of phosphocholine. It was found that the increase in the length of saturated chains of membrane-forming lipids (from 16 to 20 hydrocarbon units), the absolute values of ΔTm and ΔT1/2 decreased in the presence of phloretin and increased with capsaicin. This may be the result of differences in the localization of phloretin and capsaicin in the membrane. Steroid saponins exhibited a weak effect on the thermotropic behavior of phosphocholines. The absolute values of ΔTm and ΔT1/2 decreased in the order DPPC, DSPC, and DAPC and increased in the order betulin, diosgenin, and digitonin. Steroid saponins are characterized by a more pronounced effect on the thermotropic behavior of the sterol–phospholipid mixture. The findings are consistent with the assumption of a high affinity of the tested saponins for sterol-containing membranes. © 2020, Pleiades Publishing, Ltd.