Spontaneous vesiculation of aqueous lipid dispersions

The swelling properties of lipid mixtures consisting of phosphatidylcholine and a charged single-chain detergent have been studied. The work presented here is confined to lipid mixtures forming smectic lamellar phases in H2O. These mixtures exhibit continuous swelling with increasing water content, provided the surface charge density exceeds a threshold value of about 1-2 microC/cm2. In excess H2O, such mixtures undergo spontaneous vesiculation: unilamellar vesicles form spontaneously when excess H2O or salt solutions of moderate ionic strength (I less than 0.2) are added to the dried film of such lipid mixtures. The resulting dispersion of unilamellar vesicles is usually polydisperse. Its average size depends on the detergent/phospholipid mole ratio, decreasing with increasing detergent content. It is shown that in the phase diagram of three-component systems consisting of phosphatidylcholine, a charged single-chain detergent, and excess H2O there is a compositional range, though narrow, within which the small unilamellar vesicle (diameter less than 100 nm) is the thermodynamically most stable structure. This behavior is characteristic of charged, single-chain detergents of 14 and more C atoms. Many pharmacologically active compounds are amphiphilic and surface-active, and as such, they will orient at phospholipid-water interfaces, imparting a net surface charge to neutral lipid surfaces. It is shown that such drugs exhibit detergent-like action. Mixed films of phosphatidylcholine and a pharmacologically active compound behave similarly to phosphatidylcholine-detergent mixtures: they undergo spontaneous vesiculation when excess H2O or salt solutions of moderate ionic strength are added. In this case, the drug itself induces vesiculation; possible pharmacological implications of this finding are discussed.