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 less than or equal to 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/C18:1-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, beta, for which a value of beta = 10.4 x 10(-3) K-1 is obtained. A much stronger tendency for budding on mu m-scale (micro budding) during thermal area expansion of POPC and SOPC compared to DMPC was observed.