Lateral Pressure Profile In Membrane With Lipids Interdigitation: Analytical Derivation

Abstract

We derive analytically thermodynamic characteristics of a lipid bilayer membrane with interdigitation: lipid tails of the opposite monolayers interpenetrate. To allow for interdigitation, our microscopic model of bilayer treats lipids as semi-flexible chains with tails linked across the mid-plane of the membrane. We found striking difference between lateral pressure profiles for linked and not linked chains in the vicinity of the monolayers interface, see figure. Lateral pressure mid-plane peak disappears in the linked-tails case, while the free energy per chain increases by amount ΔFint∼6kBT (per chain) . This is purely entropic contribution to the free energy due to linking of the opposite chains. From this we deduced critical pressure capable of forcing interdigitation to a depth of a single lipid-chain CH2-CH2 segment of a volume Δv∼70A3 : Pint= ΔFint / Δv ∼3.5MPa, in good agreement with experiment in DPPC bilayer (Chemistry Letters. Vol. 37 (2008) , p.604, Nobutake Tamai et al.). We also studied geometric constraints imposed by the balance between the interdigitation free energy increase ΔFint and energy of the hydrophobic mismatch in case of a liquid-condensed domain embedded in the liquid-expanded surrounding during e.g. the liquid-gel phase transition.View Large Image | View Hi-Res Image | Download PowerPoint Slide