Influence of toluene on the bending elastic properties of giant phosphatidylcholine vesicles

In the present study the influence of toluene on the bending elastic modulu s k(c), of dimyristoylphosphatidylcholine (DMPC) vesicles in the L-alpha ph ase is investigated using videomicroscopy. For giant unilamellar DMPC vesic les, a mean value of k(c) = (1.42 +/- 0.21) x 10(-19) J is determined at 30 degrees C. Addition of toluene, which is predominantly incorporated in the lipophilic core of the bilayer. induces a lowering of the bending elastic modulus with increasing concentration up to a plateau value of k(c) = (0.36 +/- 0.10) x 10(-19) J. This effect seems to be dominated by the mobility o f the nonpolar molecules in the lipophilic part of the bilayer. Vesicles ne ar the maximum solubilization capacity exhibit an anomalous increase of the bending elastic constant because of the formation of small hydrophobic dus ters of phase-separated toluene. In the case of complete membrane saturatio n, these clusters can be clearly visualized in the vesicle membrane region. Thermally induced undulations vanish as a consequence of the reduction of the excess area a. Microscopy investigations of the temperature dependence of the bending elastic modulus were performed for both pure DMPC and toluen e containing DMPC vesicles. Further analysis of the isobaric thermal area e xpansion coefficient beta indicates a reduced thermal expansivity for vesic les containing toluene. The values of beta vary from (6.9 +/- 0.8) x 10(-3) K-1 for DMPC vesicles to (1.6 +/- 0.3) x 10(-3) K-1 in the presence of tol uene.