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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.
Christian Heinis, Alexander Lund Nielsen, Manuel Leonardo Merz, Xinjian Ji, Ganesh Kumar Mothukuri, Anne Sofie Luise Zarda, Zsolt Bognár