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Comparison of the Effects of Surface Tension and Osmotic Pressure on the Interfacial Hydration of a Fluid Phospholipid Bilayer

Tim Söderlund ^*, Juha-Matti I. Alakoskela ^*, Antti L. Pakkanen ^* and Paavo K. J. Kinnunen ^* 

^* Helsinki Biophysics & Biomembrane Group, Institute of Biomedicine/Biochemistry, University of Helsinki, Helsinki, Finland; and MEMPHYS-Center for Biomembrane Physics

Correspondence: Address reprint requests to Paavo K. J. Kinnunen, Helsinki Biophysics & Biomembrane Group, Institute of Biomedicine/Biochemistry, Biomedicum, PO Box 63 (Haartmaninkatu 8), FIN-00014, University of Helsinki, Helsinki, Finland. Tel.: 358-9-191-2542; Fax: 358-9-191-2-5444; E-mail: paavo.kinnunen{at}helsinki.fi.

The effects of three so-called kosmotropic solutes, namely, betaine, sucrose, and choline chloride on 1-palmitoyl-2-oleoyl-sn-glycero-3-phosphocholine large unilamellar vesicles, were studied by measuring the generalized polarization (GP) for the fluorescence emission of the membrane partitioning probe Laurdan. The latter has been shown to be sensitive to the depth of water penetration into phospholipid bilayers. At equal osmotic pressures the three solutes produced different increments in GP, with a qualitative positive correlation. However, the increments in GP correlated also quantitatively with the increase of air-water surface tension caused by the three kosmotropes. Our findings suggest surface tension to determine the impact of these solutes on the lateral packing of the lipid bilayer. Based on the changes in area/lipid at different surface tensions, the equilibrium lateral pressure for a 1-palmitoyl-2-oleoyl-sn-glycero-3-phosphocholine bilayer at 25°C was estimated to be 34 mN/m.

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