From discoid micelles to spherical vesicles. The concept of edge activity
The edge energy of a lipid bilayer is considered to be the crucial parameter controlling the formation of closed vesicles. It is considered to be modulated by amphiphiles accumulating at the edge as described by a Gibbs isotherm. The approach is tested for the system egg-lecithin – taurochenodesoxycholate. The parameters entered into the Gibbs' isotherm are determined by dynamic light scattering in the regime of mixed micelles (vanishing edge tension). Adjustment of an appropriate finite edge tension then leads to the discovery of metastable discs after sonication which close to vesicles spontaneously. This disc–vesicle transition is observed by electron microscopy. It is described in terms of a phenomenological potential profile. Estimates of the intrinsic edge tension and of the elastic modulus of an open lipid bilayer are obtained.