Theory of cation-phospholipid-induced shape changes in a lipid bilayer couple |
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Authors: | Gregory S B Lin |
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Institution: | (1) Department of Physiology-Anatomy, and Group in Biophysics and Medical Physics, University of California, 94720 Berkeley, CA, USA |
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Abstract: | A quantitative model of ion binding and molecular interactions in the lipid bilayer membrane is proposed and found to be useful
in examining the factors underlying such membrane characteristics as shape, sidedness, stability and vesicle size at various
cation concentrations. The lipid membrane behaves as a bilayer couple whose preferential radius of curvature depends on the
expansion or contraction of one monolayer relative to the other. It is proposed that molecular packing may be altered by electrostatic
repulsion of adjacent like-charged phospholipid headgroups, or by bringing two headgroups closer together by divalent cation
crossbridging. The surface concentrations of each type of cation-phospholipid complex can be described by simple binding equilibria
and the Gouy-Chapman-Stern formulation for the surface potential in a diffuse double layer. The asymmetric distribution of
acidic phospholipids in most biological membranes can account for the differential effects of identical ionic environments
on either side of the bilayer. The fraction of vesicle material which tends to have a right-side-out orientation may be approximated
by a normal distribution about the mean curvature. The theory generates vesicle sidedness distributions that, when fitted
to experimental results from human erythrocyte membranes, provide an alternative method of estimating intrinsic cationphospholipid
dissociation constants and other molecular parameters of the bilayer. The results also corroborate earlier suggestions that
the Gouy-Chapman theory tends to overestimate free counter-ion concentrations at the surface under large surface potentials. |
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