Monolayer Spontaneous Curvature of Raft-Forming Membrane Lipids

We have derived the monolayer spontaneous curvatures J0 for a series of phosphatidylcholines, phosphatidylethanolamines, sphingomyelin, and cholesterol as a function of temperature. For this purpose lipids were transferred into inverted hexagonal phases using dipalmitoyl phosphatidylethanolamine according to established protocols (see, e.g. [1-3]) and investigated by small angle x-ray scattering. The neutral plane for each lipid, which defines J0, was determined from the corresponding electron density maps. Spontaneous curvatures of phosphatidylethanolamines and cholesterol were found to be at least a factor of two more negative than those of phosphatidylcholines, whose J0 is closer to zero. Interestingly, a significant positive J0 = +0.1 nm−1 was retrieved for dipalmitoylphosphatidylcholine at 25°C. Spontaneous curvatures in general were found to become more negative with increasing temperature with thermal expansion coefficients that ranged between −1·10−3 to −3·10−3 (°C nm)−1 for most investigated lipids. The data allowed us to estimate the monolayer spontaneous curvatures of ternary lipid mixtures showing liquid ordered / liquid disordered phase coexistence. We report selected spontaneous curvature phase diagrams and discuss effects on protein insertion and line tension.