Kinetics of amphiphile association with two-phase lipid bilayer vesicles

We examined the consequences of membrane heterogeneity for the association of a simple amphiphilic molecule with phospholipid vesicles with solid-liqu id and liquid-liquid phase coexistence. To address this problem we studied the association of a single-chain, fluorescent amphiphile with dimyristoylp hosphatidylcholine (DMPC) vesicles containing varying amounts of cholestero l. DMPC bilayers containing 15 mol% cholesterol show a region of solid-liqu id-ordered (s-l(o)) coexistence below the T-m of pure DMPC (23.9 degrees C) and a region of liquid-disordered-liquid-ordered coexistence (l(d)-l(o)) a bove the T-m. We first examined equilibrium binding and kinetics of amphiph ile insertion into single-phase vesicles (s, l(d), and l(o) phase). The dat a obtained were then used to predict the behavior of the equivalent process in a two-phase system, taking into account the fractions of phases present . Next, the predicted kinetics were compared to experimental kinetics obtai ned from a two-phase system. We found that association of the amphiphile wi th lipid vesicles is not influenced by the existence of l(d)-l(o) phase bou ndaries but occurs much more slowly in the s-l(o) phase coexistence region than expected on the basis of phase composition.