Comparison of the effects of surface tension and osmotic pressure on the interfacial hydration of a fluid phospholipid bilayer

Abstract

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 degrees C was estimated to be approximately 34 mN/m.

FIGURE 1

Effects of increasing concentrations of betaine (▪), choline chloride (▴), and sucrose (•) on the GP of Laurdan (X = 0.01) in POPC LUVs. Solutes were dissolved in buffer (5 mM HEPES and 0.1 mM EDTA, at pH 7.4) at concentrations corresponding to given increasing values of osmolarity Π, namely, 0.5, 1, and 2 osm/kg. Changes in fluorophore emission spectra were measured at 25°C. Total lipid concentration was 25 mM. Each data point represents the average of at least three separate measurements with the error bars indicating standard deviation.

FIGURE 2

The GP values for each Π (0.5, 1, and 2 osm/kg) shown as a function of Δγ. The fitted curve starts from zero osmotic pressure (buffer). Data were taken from Fig. 1 and Table 1. The symbols for betaine, choline chloride, and sucrose are as in Fig. 1.

FIGURE 3

The ΔGP values for POPC/Laurdan (99:1) LUVs as a function of Δγ varying due to temperature (○, open symbols) and due to the kosmotropes at 25°C (From Fig. 2, solid symbols). The temperatures were (from left to right) 50, 40, 30, 25, 20, 18, 15, 10, and 5°C. See text for details. The values for γ as a function of temperature were obtained from the literature (Weast, 1979).

FIGURE 4

Intermolecular distance (ID) for POPC as a function of the increment in surface tension (Δγ) due to increasing concentration of the kosmotropic solutes. The symbols are as in Fig. 1.

FIGURE 5

Schematic representation of the geometries used in the calculation of molecular areas. See text for details.