Electrical phenomena at the surfaces of phospholipid membranes caused by the binding of ionic compounds

Copper-phospholipid interaction at cell membrane model hydrophobic surfaces

Detailed investigation of Cu (II) binding with natural lipid phosphatidylglycerol (PG) in aqueous solution was carried out by voltammetric measurements at the mercury drop electrode, complemented by monolayer studies in a Langmuir trough and electrophoretic measurements, all used as models for hydrophobic cell membranes. Penetration of copper ions into the PG layer was facilitated by the formation of hydrophilic Cu-Phenanthroline (Phen) complex in the subphase, followed by the mixed ligand Cu-Phen-PG complex formation at the hydrophobic interface. Electrophoretic measurements indicated a comparatively low abundance of the formed mixed ligand complex within the PG vesicles, resulting it the zeta potential change of +0.83mV, while monolayer studies confirmed their co-existence at the interface. The Cu-Phen-PG complex was identified in the pH range from 6 to 9. The stoichiometry of the complex ([PhenCuOHPG]), as well as its stability and kinetics of formation, were determined at the mercury drop electrode. Cu-Phen-PG reduces quasireversibly at about -0.7V vs. Ag/AgCl including reactant adsorption, followed by irreversible mixed complex dissociation, indicating a two-electron transfer - chemical reaction (EC mechanism). Consequently, the surface concentration (γ) of the adsorbed [PhenCuOHPG] complex at the hydrophobic electrode surface was calculated to be (3.35±0.67)×10^-11molcm^-2. Information on the mechanism of Cu (II) - lipid complex formation is a significant contribution to the understanding of complex processes at natural cell membranes.

Physical and oxidation stability of self-emulsifying krill oil-in-water emulsions

Krill oil is a unique source of omega-3 fatty acids since it is a mixture of phospholipids and triacylglycerols. Due to the presence of phospholipids, it can form oil-in-water emulsions without additional food additives. In this work, the physical stability of krill oil-in-water emulsions was determined at various pH values (3-7) and NaCl concentrations (50-1000 mM). The initial particle size ranged from 150 to 165 nm. The emulsions were the most stable at pH ≥ 5.0 and salt concentrations below 100 mM. Lipid oxidation was accelerated by iron and inhibited by Trolox and α-tocopherol. Trolox was a more effective antioxidant than α-tocopherol. α-Tocopherol had a better inhibitory effect when it was added after homogenization than when added to the lipid prior to homogenization. These results indicate that krill oil emulsions could represent a self-emulsifying, oxidatively stable source of omega-3 fatty acids that may be used in functional foods.

Size Dependence of Protein-Induced Flocculation of Phosphatidylcholine Liposomes

The adsorption of lysozyme and cytochrome C on phosphatidylcholine liposomes essentially changes the physical properties of the phospholipid membranes and under certain circumstances greatly affects the stability of the colloid dispersion by inducing bridging liposome flocculation. This study was designed to examine experimentally the influence of liposome size on two kinetic parameters of the flocculation, its rate constant and activation energy. As the liposome radius increased in the range 50-500 nm, the activation energy tended to decrease, resulting in an increased flocculation rate, except for the flocculation of 400-nm liposomes, which was greatly impeded. The pronounced influence of the liposome size on the flocculation rate constant was evident, since a well-defined minimum in the kinetic rate of flocculation of 400-nm liposomes was detected experimentally. The obtained nonlinear radius dependencies of the flocculation rates and activation energies are interpreted in terms of the bridging mechanism of the protein-induced liposome flocculation and the supplementary concept of the stability of thin liquid films formed between approaching protein-adsorbed liposomes. Copyright 2000 Academic Press.