Shapes and coiling of mixed phospholipid vesicles.

We have studied some physical properties of mixed phosphatidylcholine (SOPC)-phosphatidylserine (SOPS) vesicles. In a previous work (Paredes et al. in J Biol Phys 32:177-181, 2006) it was shown that the shape of the vesicles depends on the SOPC:SOPS composition, and that coiled cylindrical vesicles exist in samples with low SOPS contents. In this work, we further studied the same system of mixed vesicles. Differential scanning calorimetry (DSC) experiments displayed peaks characteristic of lipid mixing in the liquid state, ruling out a possible phase transition as an explanation of vesicle coiling. In addition, small-angle X-ray scattering (SAXS) experiments allowed us to estimate the periodicity distance inside the vesicles. This distance is d approximately equal 60 A, as revealed by the Bragg peaks observed in the experiments. Finally, the coiling transition of a cylindrical vesicle was observed under solvent flow. This observation indicates that the vesicle coiling reported previously for this system (Paredes et al. in J Biol Phys 32:177-181, 2006) does not depend on the SOPC:SOPS composition alone, but also on mechanical perturbations during the preparation steps.