ALTERATIONS IN PHOSPHOLIPID-BILAYERS CAUSED BY OXYETHYLENATED NONYLPHENOL SURFACTANTS

The interactions of oxyethylenated nonylphenols [ethylene oxide units (EO) averaging between 5 and 30] with phosphatidylcholine liposomes we re investigated, Three parameters were regarded as corresponding to th e effective surfactant/lipid molar ratios at which the surfactant syst em (a) resulted in the transient liposome reorganization for the relea se of 50% of 5(6)-carboxyfluorescein (CF) trapped in the liposomes, R( 50%CF); (b) saturated the liposomes, R(SAT); and (c) led to a complete liposome solubilization, R(SOL). From these parameters the surfactant partition coefficients R(50%CF), K-SAT, and K-SOL were determined, Pe rmeability alterations were determined as a change in CF released from the interior of vesicles and solubilization as a decrease in the stat ic light scattering of liposome suspensions, R(SAT) and R(SOL) showed the lowest values in the critical micellar concentration (CMC) range b etween 0.040 and 0.087 mM and 0.050 and 0.075 mM, respectively [hydrop hilic-lipophilic balance (HLB) number ranging between 13 and 16 and 14 and 15], which corresponded approximately to the nonylphenols with 10 -20 and 15 EO units, respectively, However, K-50%CF, K-SAT, and K-SOL drastically decreased as the number of EO units increased, especially in the EO range between 5 and 15. Thus, the highest surfactant affinit y with bilayers (maximum R values) and its maximum ability to alter th ese structures (minimum R values) were governed by the CMC surfactants and their HLB numbers and, consequently, by the EO units present in t he surfactant structure, The free surfactant concentrations were lower than their CMCs at sublytic level, whereas they remained similar to t hese values during bilayer saturation and solubilization, The nonylphe nols investigated showed higher activity versus liposomes than that re ported for Triton X-100, the oxyethylenated nonylphenol with an averag e of 10 ethylene oxide units showing low toxicity in biological tests. (C) 1996 Academic Press, Inc.