A laser-induced europium (III) ion luminescence study of the interaction of this ion with phospholipid bilayer vesicles above and below the gel to liquid-crystalline phase transition temperature.

Laser-induced europium(III) luminescence spectroscopy was used to investigate the formation and integrity of phospholipid bilayer vesicles produced by ultrasonication and detergent dialysis. Eu(III) ion interactions with these model biological membrane systems were explored. 7F0----5D0 spectral and excited-state lifetime data reveal two distinct, temperature-dependent binding sites, one involving a weak, 'superficial' interaction with the phosphate moiety of the phosphatidylcholine head group, the other involving a more tightly bound ion in a relatively dehydrated region of the head group. This latter 'sequestered' species appeared only at temperatures equal to or below that of the gel----liquid-crystalline phase transition. Systems containing various amounts of cholesterol showed a decrease in the formation of the sequestered species, indicative of a decrease in ion permeability. The results of this study demonstrate that the Eu(III) luminescence technique is useful for detecting major phase alterations in phospholipid bilayer vesicles.