A METASTABLE STATE OF HIGH SURFACE-ACTIVITY PRODUCED BY SONICATION OFPHOSPHOLIPIDS

Sonication of phosphatidylcholine dispersions generates a metastable h igh energy assembly of molecules, the existence of which is revealed b y its conspicuous surface activity. Freshly sonicated liposome dispers ions release molecules to the air/water interface at rates sufficient to produce a close-packed monolayer within minutes. In contrast, monol ayers at the surface of multilamellar and extruded vesicles take hours to form. The highly surface active species appears within the first f ew minutes of sonication, long before a major reduction in turbidity o ccurs, and accumulates over the course of a few hours of sonication. I t disappears upon exhaustive sonication, extrusion, addition of extrud ed vesicles, or, more slowly, simply on standing. Tests for extraneous substances in the lipids before as well as after sonication revealed amounts of degradation products too small to represent the observed su rfactant. Direct evidence that the metastable aggregate releases intac t phospholipids was provided by a novel procedure to characterize mono layer composition by comparing surface tension with surface potential, both as a function of surface density. Centrifugation and gel filtrat ion chromatography indicate that the surface activity is associated wi th a particle of diameter larger than a lysophosphatidylcholine micell e but not larger than limit sonicated vesicles. The metastable materia l appears to be lipid molecules in other than the normal stable vesicu lar state, perhaps an incompletely closed vesicle, one in which the in ner and outer monolayers have not equilibrated, or possibly a micellar form.