Effect of lipid composition on hydrophobic properties of liposomes.

Partition behavior of liposomes of different composition in the aqueous Ficoll-Dextran biphasic system was examined. The relative hydrophobicity of the liposomes was estimated. The effect of cholesterol on the solvent interactions of lecithin and sphingomyelin liposomes was studied. It is found that the effect of phosphatidylethanolamine on the hydrophobic character of lecithin liposomes exceeds that produced by cholesterol. The relative hydrophobicity of all the liposomes examined was found to be independent of the ionic composition of the system containing varied amounts of NaCl and Na-phosphate buffer, pH 7.4. The hydrophobic character of the liposomes under study is shown to increase in the following order: phosphatidylserine less than phosphatidylethanolamine less than phosphatidylcholine less than sphingomyelin. Hydrophobic properties of the liposomes are compared to those of human red cells. It is suggested that the dependence of the hydrophobic character of membrane particles on the ionic composition of an aqueous environment can be of fundamental importance for the selective distribution of the particles in vivo.

Action of surfactants on egg lecithin liposomes.

The lytic action of several homologous series of surfactants including N-acyl derivatives of the Na-salt of amino acids on the egg lecithin multilamellar liposomes was examined. The affinity for the lipid membrane and the solubilising capacity of the agents were estimated. The contribution of a CH2 group and that of the polar head group of surfactants to the free energy of the agent's binding to the membrane were evaluated. The results obtained indicate that the contribution of a CH2 group to the free binding energy depends on the nature of the surfactants' head group. This dependence is attributed to either various localisation of the agent's molecules in the lipid bilayer or to different properties of the agent's hydrocarbon tails. The contributions of the head groups of the surfactants are assumed to reflect the affinity of these head groups for the lecithin polar head group at the membrane interface. The results obtained indicate some degree of specificity involved in the interactions of the head groups.

A new approach to the study of lipid-protein interactions in the structure of biological membranes.

The lytic action of a number of N-acyl amino acids on lecithin liposomes was examined. The agents' affinity for lecithin liposome membrane was measured and the results obtained were treated to estimate the interactions of the amino acid residues with the lecithin polar head group at the surface of the liposome membrane. The data were considered in relation to the study of the surfactant effects on the erythrocyte volume. The ability of the suggested approach to obtain information on protein-lipid interactions inaccessible by other techniques is briefly commented on.