Takayuki Ikeuchi

Formation and structure of stably dispersed small particles composed of phosphatidylcholine and ubiquinone-10: Coexistence of emulsion particles with bilayer vesicles

Stable aqueous dispersions of ubiquinone-10 (Q 10 ) were produced by cosonication with egg yolk phosphatidylcholine (PC) in a Q 10 mole fraction range of lipid mixture of 0.1–0.7. Electron microscopic and dynamic light scattering measurements showed the diameter of the dispersed particles to be 65–75 nm. The trapped aqueous volume inside the particles was determined fluorometrically with an aqueous space marker, calcein. Trapped volume decreased remarkably with the addition of Q 10 into small unilamellar vesicles of PC. The decline in the fraction of vesicular particles was also confirmed by 1 H-NMR measurements of the choline methyl of PC in the presence of a paramagnetic cation, Pr 31 . These results indicate that the excessive amount of Q 10 separated from PC bilayers is stabilized as emulsion particles by the PC monolayer on the surface. Monolayer-bilayer equilibrium of the PC-Q 10 mixture was estimated by measurements of spreading and collapse pressures. Results show that the coexistence of the emulsion particles (surface monolayer of PC + core of Q 10 ) with the vesicular particles (PC bilayers) is responsible for the maximum surface pressure of the monolayer. The coexistence is probably critically important to the production of stably dispersed particles of the lipid mixture.