Saponins induce lipid-dependent structural heterogeneity and modulate membrane organization in phospholipid bilayers

Abstract Saponins are amphiphilic natural compounds whose membrane activity is commonly attributed to sterol complexation, whereas their effects on sterol-free membranes remain poorly understood. Here, we investigated how lipid packing governs the interaction of a Saponaria officinalis saponin extract with cholesterol-free phosphatidylcholine (PC) and phosphatidylethanolamine (PE) membranes using differential scanning calorimetry, Fourier transform infrared spectroscopy, small-angle X-ray scattering, dynamic light scattering, atomic force microscopy, and calcein-release assays. Saponins reduced the cooperativity of lipid phase transitions and disrupted collective lipid rearrangements in all systems, but the extent of membrane reorganization was strongly lipid dependent. PC membranes showed pronounced disruption of lamellar organization, reduced assembly size, and increased structural heterogeneity, whereas PE membranes were more resistant, with short-chain PE displaying heterogeneous lamellar organization and long-chain PE showing only minor structural changes. These differences were accompanied by lipid-dependent membrane permeabilization, with substantially greater calcein release from PC than from PE liposomes. Overall, saponins can reorganize sterol-free membranes through mechanisms governed by lipid packing and interfacial interactions. Saponins therefore affect membrane barrier function even without cholesterol, with phospholipid composition determining the magnitude and nature of the response.