J. Phys. II France
Volume 7, Numéro 8, August 1997
Page(s) 1141 - 1157
DOI: 10.1051/jp2:1997177
J. Phys. II France 7 (1997) 1141-1157

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

W. Häckl1, U. Seifert2 and E. Sackmann1

1  Physik Department E22, Biophysics Laboratory, Technische Universität München, James-Franck-Strasse, 85747 Garching, Germany
2  Max-Planck-Institut für Kolloid- und Grenzflächenforschung, Kantstrasse 55, 14513 Teltow-Seehof, Germany

(Received 30 June 1997, revised 8 November 1996, accepted 22 April 1997)

We report the modification of the bending elastic modulus $k_{\rm c}$ of lipid bilayers (here DMPC) by small amounts ( $c \leq 5$ 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 $k_{\rm c}$ by a factor of two. The effect is rationalised in terms of local thinning of the bilayer. The strong effect of solutes on $k_{\rm c}$ 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 $k_{\rm c}$ 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 $\beta = 10.4 \times 10^{-3}$ 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.

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