Line tension at a boundary of two lipid monolayers of different thickness
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The line tension (excessive energy per unit length) of the contact of two planar lipid monolayers of different equilibrium thickness is calculated. Since the exposure of hydrophobic lipid tails to water is energetically unfavorable their packing in the boundary vicinity is different from that for homogeneous monolayer. In order to diminish an area of acyl chains-water contact lipid molecules deform - compress or stretch, accumulating energy. Calculations are carried out under assumption that monolayers could be considered as a continuous elastic media characterized by two moduli of elasticity - area compression/stretching modulus K-a and tilt modulus K-t Spatial distribution of deformations and line tension are obtained by minimization of total (elastic and hydrophobic) energy. For reasonable values of the parameters it is shown that hydrophobic core-water contact is absolutely eliminated, so the energy of the system is exclusively determined by the elastic deformations of monolayers in the narrow region of similar to2-4 nm width in the vicinity of the boundary. The value of the line tension obtained is similar to0.2-0.5 kT/nm for the monolayer heights difference similar to0.3-0.5 nm. This value is sufficient to support circular shape of cholesterol-sphingomyelin domains and is in a good agreement with available experimental data.