Permeability investigations of phospholipid liposomes by adding cholesterol

The sublytic changes caused by sodium dodecyl sulfate (SDS) in egg phosphatidylcholine (PC) liposomes with rising concentrations of cholesterol (CH) (PC:CH mole ratios from 9:1 to 7:3) were investigated. The effective molar ratios Re of membrane-bound surfactant to lipid and the bilayer/aqueous phase partition coefficients (K) were determined. A linear increase and decrease in Re and K, respectively, take place as the CH proportion increase. Given that the surfactant capacity to interact with liposomes is inversely related to the Re value, the rise of CH reduces linearly both the SDS activity against vesicles (increase in Re) and its affinity with these structures (decrease in K). These linear variations may be due to the linear increase of hydrophobicity in the interfacial region of bilayers and to their reduced translational fluidity by changes in the bilayer packing and ordering. The increase in Re always results in two opposite effects on K. At low Re, K firstly increases because only the outer vesicle leaflet is available for interaction with SDS. The following abrupt fall in K is due to the fact that the binding of additional SDS molecules to bilayer is hampered. The K peaks obtained are correlated in all cases with the saturation of the outer vesicle leaflet by SDS. Increasing Re values, lead to an increased rate of flip-flop of the surfactant molecules (or permeabilization of the bilayers to SIDS), thus also making the inner monolayer available for interaction with added SDS. The free SIDS concentrations are always lower than its critical micelle concentration indicating that the SDS-liposome interaction is mainly ruled by the action of surfactant monomers regardless of the CH concentration in bilayers. (C) 2003 Elsevier Science B.V. All rights reserved.