Mechanism of unassisted ion transport across membrane bilayers
M. A. Wilson and A. Pohorille, J. Am. Chem. Soc., 118, 6580-6587
(1996).
To establish how charged species move from water to the nonpolar membrane interior and
to determine the energetic and structural effects accompanying this process, we performed
molecular dynamics simulations of the transport of Na and Cl across a lipid bilayer located between two
water lamellae. The total length of molecular dynamics trajectories generated for each ion
was 10 ns. Our simulations demonstrate that permeation of ions into the membrane is
accompanied by the formation of deep, asymmetric thinning defects in the bilayer, whereby
polar lipid head groups and water penetrate the nonpolar membrane interior. Once the ion
crosses the mid-plane of the bilayer the deformation ``switches sides''; the initial
defect slowly relaxes and a defect forms in the outgoing side of the bilayer. As a result,
the ion remains well solvated during the process; the total number of oxygen atoms from
water and lipid head groups in the first solvation shell remains constant. A similar
membrane deformation is formed when the ion is instantaneously inserted into the interior
of the bilayer. The formation of defects considerably lowers the free energy barrier to
transfer of the ion across the bilayer and, consequently, increases the permeabilities of
the membrane to ions, compared to the rigid, planar structure, by approximately 14 orders
of magnitude. Our results have implications for drug delivery using liposomes and peptide
insertion into membranes.
and Cl
across a lipid bilayer located between two
water lamellae. The total length of molecular dynamics trajectories generated for each ion
was 10 ns. Our simulations demonstrate that permeation of ions into the membrane is
accompanied by the formation of deep, asymmetric thinning defects in the bilayer, whereby
polar lipid head groups and water penetrate the nonpolar membrane interior. Once the ion
crosses the mid-plane of the bilayer the deformation ``switches sides''; the initial
defect slowly relaxes and a defect forms in the outgoing side of the bilayer. As a result,
the ion remains well solvated during the process; the total number of oxygen atoms from
water and lipid head groups in the first solvation shell remains constant. A similar
membrane deformation is formed when the ion is instantaneously inserted into the interior
of the bilayer. The formation of defects considerably lowers the free energy barrier to
transfer of the ion across the bilayer and, consequently, increases the permeabilities of
the membrane to ions, compared to the rigid, planar structure, by approximately 14 orders
of magnitude. Our results have implications for drug delivery using liposomes and peptide
insertion into membranes.