Abstract
Solutions of liposomes composed of binary mixtures of anionic dioleoylphosphatidylserine (DOPS) and zwitterionic dioleoylphosphatidylcholine (DOPC) are investigated with label-free angle-resolved (AR) second harmonic scattering (SHS) and electrophoretic mobility measurements. The membrane surface potential is extracted from the AR-SHS response. The surface potential changes from -10 to -145 mV with varying DOPS content ( from 0% to 100%) and levels off already at ∼ 10 % DOPS content. The ζ-potential shows the same trend but with a drastically lower saturation value (-44 mV). This difference is explained by the formation of a condensed layer of Na+ counterions around the outer leaflet of the liposome as predicted by charge condensation theories for polyelectrolyte systems.
Highlights
Lipid bilayer membranes are primary building blocks of organisms
Solutions of liposomes composed of binary mixtures of anionic dioleoylphosphatidylserine (DOPS) and zwitterionic dioleoylphosphatidylcholine (DOPC) are investigated with label-free angle-resolved (AR) second harmonic scattering (SHS) and electrophoretic mobility measurements
Cells that undergo apoptosis increase the concentration of anionic phosphatidylserine (PS) in the outer leaflet of the plasma membrane to signal phagocytes to approach and digest them, whereas in healthy cells, PS lipids are only present in the inner leaflet
Summary
Lipid bilayer membranes are primary building blocks of organisms. These membranes exhibit a diverse composition in order to separate functional compartments and to control signalling processes. Cells that undergo apoptosis increase the concentration of anionic phosphatidylserine (PS) in the outer leaflet of the plasma membrane to signal phagocytes to approach and digest them, whereas in healthy cells, PS lipids are only present in the inner leaflet.. Combined AR-SHS and sum-frequency scattering (SFS) studies were recently used to probe the transmembrane asymmetry in single component anionic DOPS and zwitterionic DOPC liposomes, as well as liposomes composed of a 1:1 mixture of DOPC and DOPS. Transmembrane asymmetry in the form of a different number of lipid molecules in the inner and outer leaflets was absent for liposomes composed of DOPC, DOPS, or a 1:1 mixture of DOPC:DOPS.
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