Effects of Fully and Partially Solubilized Amphiphiles on Bilayer Bending Stiffness and Temperature Dependence of the Effective Tension of Giant Vesicles

We report the modification of the bending elastic modulus k_c of lipid bilayers (here DMPC) by small amounts (c ≤ 5 mol %) of (i) small amphiphiles which exchange between the bilayer and the aqueous phase (e.g. the ion carrier valinomycin and the Ca⁺⁺ carrier A23187) and (ii) amphiphiles solubilized in the membrane (cholanic acid). Large reductions of the bending stiffness may be induced by a few percent of the solutes, e.g. 1 mol % of valinomycin reduce k_c by a factor of two. The effect is rationalised in terms of local thinning of the bilayer. The strong effect of solutes on k_c contrasts with its weak dependence on the lipid structure since the C18:0/C18:l-lipid stearoyl-oleoyl-phosphatidyl-choline (SOPC) exhibits only a 15% higher value of k_c than DMPC. The effect of temperature on the flicker behaviour was analysed in order to establish correlations between the effective tension and the excess area of the quasi-spherical vesicles. The temperature dependence of the bilayer excess area for a DMPC vesicle leads to the thermal expansion coefficient, β, for which a value of β= 10.4 × 10^-3 K^-1 is obtained. A much stronger tendency for budding on μm-scale (micro budding) during thermal area expansion of POPC and SOPC compared to DMPC was observed.