Mesoscopic models of lipid bilayers and bilayers with embedded ...

36 Surface tension in lipid bilayers
3.5 Conclusion
In this Chapter we have introduced the definition of surface tension, and we have
described the basic formalism and computational technique to compute pressure
profiles (i.e. the local values of the pressure) in molecular simulations.
We have discussed the necessity of performing simulations of lipid bilayers in an
ensemble where the surface tension is one of the thermodynamic variables that can
be controlled. To this purpose, we have developed an efficient MC based scheme to
impose a given value of the surface tension, of which the zero value, that corresponds
to the natural state of tension of unconstrained bilayers, is a particular case.
We have then applied this MC scheme to investigate the dependence of the area
per lipid on the value of the applied surface tension and on the system size. We
observed that out-of-plane undulations are suppressed in small patches of a compressed
bilayer (i.e. negative surface tension), while they are not in larger patches. As
a consequence, finite-size effects on the projected area per lipid in compressed bilayers
are significant. However, for tensionless or stretched (positive surface tension)
bilayers we do not observe any finite-size effect.
We have also shown that the area per lipid has a linear dependence on the surface
tension for values of the latter close to the free energy minimum, i.e. zero surface tension.
From this linear dependence the bilayer area compressibility was derived, and
compared with experimental and MD simulation values, as well as with the value
of area compressibility derived from the spontaneous area fluctuations at equilibrium
in a tensionless bilayer. Good quantitative agreement between these values
was found.