Kinetics of Amphiphile Association with Two-Phase Lipid Bilayer Vesicles

AbstractWe examined the consequences of membrane heterogeneity for the association of a simple amphiphilic molecule with phospholipid vesicles with solid-liquid and liquid-liquid phase coexistence. To address this problem we studied the association of a single-chain, fluorescent amphiphile with dimyristoylphosphatidylcholine (DMPC) vesicles containing varying amounts of cholesterol. DMPC bilayers containing 15 mol% cholesterol show a region of solid–liquid-ordered (s-ℓo) coexistence below the Tm of pure DMPC (23.9°C) and a region of liquid-disordered–liquid-ordered coexistence (ℓd-ℓo) above the Tm. We first examined equilibrium binding and kinetics of amphiphile insertion into single-phase vesicles (s, ℓd, and ℓo phase). The data 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 obtained from a two-phase system. We found that association of the amphiphile with lipid vesicles is not influenced by the existence of ℓd-ℓo phase boundaries but occurs much more slowly in the s-ℓo phase coexistence region than expected on the basis of phase composition