The mechanisms governing the solubilization by Triton X-100, octyl glucoside, and sodium cholate of large unilamellar liposomes prepared by reverse-phase evaporation were investigated. The solubilization process is described by the three-stage model previously proposed for these detergents [Lichtenberg, D., Robson, R.J., & Dennis, E.A.(1983) Biochim. Biophys. Acta 737, 285-304]. In stage I, detergent monomers are incorporated into the phospholipid bilayers until they saturate the liposomes. At that point, i.e., stage II, mixed phospholipid-detergent micelles begin to form. By stage III, the lamellar to micellar transition is complete and all the phospholipids are present as mixed micelles. The turbidity of liposome preparations was systematically measured as a function of the amount of detergent added for a wide range of phospholipid concentrations (from 0.25 to 20 mM phospholipid). The results allowed a quantitative determination of RSat, the effective detergent to lipid molar ratios in the saturated liposomes, which were 0.64, 1.3, and 0.30 for Triton X-100, octyl glucoside, and sodium cholate, respectively. The corresponding ratios in the mixed micelles, RSol, were 2.5, 3.8, and 0.9 mol of detergent/mol of phospholipid. The monomer concentrations of the three detergents in the aqueous phase were also determined at the lamellar to micellar transitions (0.18, 17, and 2.8 mM, respectively). These transitions were also investigated by 31P NMR spectroscopy, and complete agreement was found with turbidity measurements. Freeze-fracture electron microscopy and permeability studies in the sublytic range of detergent concentrations indicated that during stage I of solubilization detergent partitioning between the aqueous phase and the lipid bilayer greatly affects the basic permeability of the liposomes without significantly changing the morphology of the preparations. A rough approximation of the partition coefficients was derived from the turbidity and permeability data (K = 3.5, 0.09, and 0.11 mM-1 for Triton X-100, octyl glucoside, and sodium cholate, respectively). It is concluded that when performed systematically, turbidity measurements constitute a very convenient and powerful technique for the quantitative study of the liposome solubilization process by detergents.

No abstract

The binding of calcium, magnesium, lithium, potassium, and sodium to membrane bilayers of 5 to 1 (M/M) 1-palmitoyl-2-oleoylphosphatidylcholine (POPC) and 1-palmitoyl- 2-oleoylphosphatidylserine (POPS) was investigated by using deuterium nuclear magnetic resonance (2H NMR). Both lipids were deuteriated on their polar headgroups, and spectra were obtained at 25 degrees C in the liquid-crystalline phase as a function of salt concentration. The spectra obtained with calcium were correlated with 45CaCl2 binding studies to determine the effective membrane-bound calcium at low calcium binding, up to 0.78 calcium per POPS. Deuterium quadrupolar splittings of both POPC and POPS headgroups were shown to be very sensitive to calcium binding. The behavior of these two headgroups over a wide range of CaCl2 concentrations suggests that Ca2+ binding occurs in at least two steps, the first step being achieved with 0.5 M CaCl2, with a stoichiometry of 0.5 Ca2+ per POPS. Correlations of the deuterium Ca2+ binding data with related data obtained after incorporation of a cationic integral peptide showed that the effects of these two cationic molecules of the POPS headgroup are qualitatively similar, and provided further support for two-step Ca2+ binding to the POPC/POPS 5:1 membranes. The corresponding data obtained with magnesium, lithium, and potassium indicate that these cations interact with both the choline and serine headgroups. The amplitudes of headgroup perturbations could be partly correlated to the relative affinities of the metallic cations for the lipid membrane. The two-step binding described with Ca2+ appears to be relevant to the Mg2+ data, and in certain limits to the Li+ data. The data were interpreted in terms of conformational changes of the lipid headgroups induced by an electric field due to the charges of the membrane-bound metallic cations. A conformational change of the serine headgroup induced by the membrane-bound charges is proposed. We propose that the metallic cations can be differentiated on the basis of their respective spatial distribution functions relative to the choline and serine headgroups. According to this interpretation, the divalent cations Ca2+ and Mg2+ are more deeply buried in the membrane than monovalent Na+ and K+, the case of Li+ being intermediate of the latter two. This conclusion is discussed in relation to fundamental theories of the spatial distribution of ions near the interface between water and smooth charged solid surfaces.

Deuterium nuclear magnetic resonance (2H NMR) was used to study the interaction of a cationic amphiphilic peptide with pure DMPC membranes and with mixed bilayers of dimyristoylphosphatidylcholine (DMPC) and dimyristoylphosphatidylserine (DMPS). The choline and serine headgroups were selectively deuteriated at the alpha and beta positions. The amphiphilic peptide, with 20 leucine residues in the hydrophobic core and two cationic hydrophilic lysine residues at each end, spanned the lipid bilayer. Although 2H NMR experiments using DMPC with perdeuteriated fatty acyl chains showed that the average order parameter of the hydrophobic region was not significantly modified by the incorporation of the amphiphilic peptide, for either DMPC or DMPC/DMPS (5:1) bilayers, large perturbations of the quadrupolar splittings of the choline and serine headgroups were observed. The results obtained with the DMPC headgroup suggest that the incorporation of the cationic peptide in both DMPC and DMPC/DMPS (5:1) bilayers leads to a structural perturbation directly related to the net charge on the membrane surface. The magnitude of the observed effect seems to be similar to those observed previously with other cationic molecules [Seelig, J., MacDonald, P.M., & Scherer, P.G. (1987) Biochemistry 26, 7535-7541]. Two of the three quadrupolar splittings of the PS headgroup exhibited large variations in the presence of the amphiphilic peptide, while the third one remained unchanged. Our data have led us to propose a model describing the influence of membrane surface charges on headgroup conformation. In this model, the surface charge is represented as a uniform charge distribution. The electric field due to the charges produces a torque which rotates the polar headgroups.(ABSTRACT TRUNCATED AT 250 WORDS)