(225e) Stability and Aggregation of Dipalmitoylphosphatidylcholine (DPPC) Vesicles and DPPC- Fibrinogen Interactions

The effect of sonication and freezing-thawing on the aggregate size and dynamic surface tension of aqueous dipalmitoylphosphatidylcholine (DPPC) dispersions and the effects of DPPC particles on the stability of aqueous fibrinogen (FB) solutions were studied by cryogenic-transmission electron microscopy (cryo-TEM), dynamic light scattering (DLS), UV-VIS spectroturbidimetry, and tensiometry. When 1000 ppm (0.1 wt%) DPPC dispersions were prepared with a certain protocol, including extensive sonication, they contained mostly frozen vesicles and were quite clear, transparent, and stable for at least 30 days. The average dispersed vesicles diameter was 80 nm in water and 90 nm in standard phosphate saline buffer. After a freeze-thaw cycle, such dispersions became turbid, and precipitates were observed with large particles of average size of 1.5 microns. These dispersions had much higher equilibrium and dynamic surface tensions than those before freezing. When freeze-thawed dispersions were subjected to a resonication at 55 °C, smaller vesicles sizes of ca. 70 nm were produced, and a lower surface tension behavior was restored as before freezing. Similar behavior was observed at 30 ppm DPPC. These results indicate that the freeze-thaw cycle causes substantial aggregation and precipitation of the vesicles.

FB-water mixtures were quite unstable and biphasic. They formed large aggregates which eventually precipitated. The addition of DPPC vesicles into these unstable FB dispersions reversed FB aggregation and precipitation, and produced stable translucent microdispersions. The inferred lipid/protein aggregates were limited in size, with average diameters ranging from 200 to 300 nm. In buffer, FB molecules formed aggregates with average aggregation number of 2, and were stable. The average hydrodynamic diameter of FB in solutions, determined from dynamic light scattering, was 30 nm. Mixing a stable FB solution in buffer and a stable DPPC dispersion in buffer produced highly unstable mixtures, in which large aggregates precipitated. These results have implications for designing efficient protocols of lipid dispersion preparation and lung surfactant replacement formulations in treating respiratory disease and for effective administration, and understanding the interactions of lipids and proteins in many biological and food processing applications.