Lipid bilayer membranes are important as fundamental structures in biology and possess characteristic water-permeability, stability, and mechanical properties. Water permeation through a lipid bilayer membrane occurs readily, and more readily at higher temperature, which is largely due to an enthalpy cost of the liquid-to-gas phase transition of water. A fullerene bilayer membrane formed by dissolution of a water-soluble fullerene, Ph 5C60K, has now been shown to possess properties entirely different from those of the lipid membranes. The fullerene membrane is several orders of magnitude less permeable to water than a lipid membrane, and the permeability decreases at higher temperature. Water permeation is burdened by a very large entropy loss and may be favored slightly by an enthalpy gain, which is contrary to the energetics observed for the lipid membrane. We ascribe this energetics to favorable interactions of water molecules to the surface of the fullerene molecules as they pass through the clefts of the rigid fullerene bilayer. The findings provide possibilities of membrane design in science and technology.bilayer membrane ͉ vesicle ͉ entropy barrier V esicles made of a lipid bilayer membrane such as a cell membrane are fundamental structures in biology. The membrane maintains stability and mechanical properties to sustain the integrity of the biological machinery while allowing water to permeate through the liquid crystalline film of the hydrocarbon chains (1). The low energy barrier for water permeation is largely due to the small enthalpy cost of the liquid-to-gas phase transition of water (2). The lipid membrane and its water permeation mechanism have long served as important models for design and analysis of membrane structures. We have reported that, upon dissolution in water, water-soluble fullerene (Ph 5 C 60 K) ( Fig. 1) (3) produces spherical bilayer vesicles (Ϸ15-nm radius) [see supporting information (SI) Fig. 7] as an exclusive self-assembly object (4, 5). We report here that the vesicles made from water-soluble fullerene molecules show water-permeability, stability, and mechanical characteristics entirely different from those of the conventional vesicles. The fullerene membrane is several orders of magnitude less permeable to water than a lipid membrane (2, 6, 7), and the permeability decreases at higher temperature. The high energy barrier for permeation is due to a very large negative activation entropy. We suggest that water molecules lose their mobility in the clefts of the rigid fullerene bilayer, just as in hydration of crystalline proteins or metal salts.
Results and DiscussionA solution study of the thermodynamics of water permeation through the fullerene membrane revealed that the mechanism of permeation is very different from that so far known for lipid bilayers. The vesicles were found to be quite tough on a mica surface, as examined by atomic force microscopy (4). In contrast to lipid vesicles, the fullerene vesicles show neither 1 H nor 13 C NMR signals in water (at 10 -80°C), an...