Line Tension Of Membrane Domains Calculated From Chemical Interactions Betweem Lipids And Elastic Splay And Tilt

Abstract

Model is developed to calculate the line tension of a domain in bilayer lipid membrane from both the chemical interactions between lipid molecules and the elastic deformations of the membrane. Two-component lipid bilayer is used to display the essential physics that determines line tension, without being obscured by complexities of the multi-component bilayer. Means to expand the approach to multi-component system has been formulated. The domain is assumed to be in thermodynamic equilibrium with the surround. Chemical interactions are incorporated by using regular solution theory, mean field approximation. Whenever a height mismatch exists at the boundary between the domain and the surround, the membrane deforms so as to prevent exposure of hydrophobic surfaces to water. The deformation energy is calculated by assuming that deformations occur through splay and tilt. The calculated line tension can be written as a sum of “mechanical” and “chemical” terms; each term is implicitly dependent on the other. For height mismatch of only a few Angstroms, line tension is accurately determined from the chemical interactions between lipids alone. For greater height mismatch, both chemical interactions and elastic deformations contribute. The calculated line tension is a function of temperature. Differences in spontaneous curvatures of the membrane lipid components lowers the effective critical temperature for domain formation. Below the critical temperature, the characteristic thickness of the transitional zone between the phases is several nanometers; it rapidly increases as the critical point is approached. If line tension and compositions of domains and surround are known for one temperature, they can be calculated over the entire temperature range. The model therefore allows values of line tension and domain composition that is experimentally measured at one temperature to be theoretically extended to a large range.