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Phospholipids when dispersed in excess water generally form vesicular membrane structures. Cryo-transmission and freeze-fracture electron microscopy are combined here with calorimetry and viscometry to demonstrate the reversible conversion of phosphatidylglycerol aqueous vesicle suspensions to a three-dimensional structure that consists of extended bilayer networks. Thermodynamic analysis indicates that the structural transitions arise from two effects: (i) the enhanced membrane elasticity accompanying the lipid state fluctuations on chain melting and (ii) solventassociated interactions (including electrostatics) that favor a change in membrane curvature. The material properties of the hydrogels and their reversible formation offer the possibility of future applications, for example in drug delivery, the design of structural switches, or for understanding vesicle fusion or fission processes. Lipids may undergo a cooperative melting reaction linked to the loss in conformational order of the lipid chains (1). This transition is accompanied by a large change of the internal energy (or enthalpy) that can be studied by calorimetry. It has long been known that the melting profiles are influenced by secondary components, e.g., proteins (2), cholesterol, or anesthetics (3, 4). Phase diagrams of lipid mixtures usually are constructed as a function of the chemical potential of the components and the temperature (5). Another kind of possible transition involves changes in vesicular shape. These transitions have been studied in some detail by theoretical means, resulting in phase diagrams for vesicle structure as a function of the elastic constants (6, 7). Changes in these constants can explain theoretically the wealth of shapes of erythrocytes (8). There are therefore two kinds of phase diagrams found for lipid systems: one describing the calorimetric features of chain melting, mainly dependent on short-range cooperative events within the membrane plane, and the other describing transitions in shape and long-range order influenced by changes in the elastic constants.Although seemingly different features of a lipid system, the elastic constants and the heat capacity of the lipid membrane are related because they are coupled to thermodynamic fluctuations in the membrane properties (9). Maximum elasticity is achieved in the regime of ordered (gel)-fluid phase coexistence (10), which implies that close to the heat capacity maximum the elastic constants change markedly and as a consequence the membranes become more flexible. From this it is expected that changes in structural equilibria may be found close to the chain-melting transition.We demonstrate here thermodynamically how the structural changes couple to the chain melting. Large viscosity changes of dimyristoyl phosphatidylglycerol (DMPG) dispersions close to the calorimetric melting transition have been reported that are accompanied by marked changes in heat capacity, light scattering, and electrical conductance (11,12). The changes in viscosity suggest a change in the l...

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