SOLUTE-INDUCED SHIFT OF PHASE-TRANSITION TEMPERATURE IN DI-SATURATED PC LIPOSOMES - ADOPTION OF RIPPLE PHASE CREATES OSMOTIC-STRESS

We have examined the calorimetric behavior of large liposomes consisti ng of symmetric saturated chain phosphatidylcholines. Most notably, fo r systems made in solutions containing solute (e.g., NaCl, glucose, et c.) there was an additional major endotherm just below the main phase transition temperature. The new endotherm was found to represent a pop ulation of lipid whose main phase transition was shifted to lower temp erature due to an induced osmotic stress across the membrane. Absent f or isoosmotic systems, the osmotic stress was created when the liposom e internal volume decreased, a consequence of the L-beta, (gel) to P-b eta' (rippled) phase transition. That is, rippling of the membrane cau sed vesicle volume to decrease (greater than or equal to 28%) and beca use the free flow of water outward was restricted by solute, an osmoti c gradient was created where none had existed before. The distribution of enthalpy between the new shifted T-m and the expected T-m correlat ed with the percent of lipid in the outer bilayer and it was concluded that only the outer bilayer sensed the induced stress. Internalized l iposome structures were shielded, thus explaining the persistence of t he expected T-m in preparations made in solute. The shift in T-m (Delt a T-m) was discrete and linearly dependent upon lipid chain length for the PC series di-17:0 (Delta T-m approximate to 1.4 degrees C) throug h di-20:0 (Delta T-m approximate to 0.6 degrees C), suggesting a struc tural change (i.e., lipid packing/orientation) was involved. Although freeze-fracture electron microscopy of stressed and unstressed bilayer s revealed no differences in ripple periodicity there were differences in surface features and in vesicle shape. The fact that this phenomen on has gone unnoticed for MLVs is probably due to the fact that these systems are known to exclude solute and thus exist under osmotic compr ession. (C) 1997 Elsevier Science B.V.