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.