Spontaneous formation of small and ultrasmall unilamellar vesicles in mixtures of drug surfactant and phospholipid: Effect of chemical structure of phospholipid tails on vesicle size

We have investigated the effect of length and chemical structure of phospholipid tails on the spontaneous formation of unilamellar liposomal vesicles in binary solute mixtures of cationic drug surfactant and zwitterionic phosphatidylcholine phospholipids. Binary drug surfactant-phospholipid mixtures with four different phospholipids with identical headgroups (two saturated phospholipids 1,2-dimyristoyl-sn-glycero-3-phosphocholine (DMPC, 14:0) and 1,2-Dipalmitoyl-sn-glycero-3-phosphocholine (DPPC, 16:0), and two unsaturated lipids 1,2-dioleoyl-sn-glycero-3-phosphocholine (DOPC, 18:1) and 1,2-Dierucoyl-sn-Glycero-3-Phosphatidylcholine (DEPC, 22:1)) combined with two different tricyclic antidepressant drugs (amitriptyline hydrochloride (AMT) and doxepin hydrochloride (DXP)) have been investigated with small-angle neutron scattering (SANS) and cryo-transmission electron microscopy (cryo-TEM). We observe a conspicuous impact of phospholipid tail structure on both micelle-to-vesicle transition point and vesicle size. In particular, ultrasmall unilamellar vesicles, i.e. with a diameter less than 20 nm, were observed in several samples with the two unsaturated phospholipids DOPC and DEPC, but not in any samples with the saturated phospholipids DMPC and DPPC. The smallest vesicles observed in DOPC and DEPC mixtures were smaller than 18 nm in diameter. In contrast, the smallest vesicles observed in DMPC mixtures were about 30 nm in diameter and always larger than 100 nm in DPPC mixtures. The ultrasmall vesicles showed exceptional colloidal stability. Moreover, bilayer vesicles predominated over micelles in a much wider range of concentrations for DOPC and DEPC mixtures as a result of having a smaller phospholipid mole fraction in the aggregates at the micelle-to-vesicle transition. Our results have been theoretically rationalized by combining solution thermodynamics with bending elasticity theory.