Speaker
Prof.
Olle Edholm
(Royal Institute of Technology KTH, Sweden)
Description
The free energy cost for creating water filled pores of
different sizes in model membranes is discussed and
calculated from molecular dynamics simulations. Clearly the
free energy cost for creating a hydrophilic pore with the
lipid head groups turned inwards towards that water is less
than that for creating a hydrophobic pore with hydrocarbon
towards the water. This depends, however, on the ratio
between bending modulus and surface tension and one might
think of systems in which the hydrophobic pore is more
stable. There are also systems for which a vacuum pore would
be more stable than the water filled one.
The free energy was calculated from atomistic molecular
dynamics simulations as a function of a reaction coordinate
using a constraining potential. The free energy profile that
came out of the simulations is quadratic for a radii less
than about 0.3 nm, and linear shape for larger radii. In the
outer region, a line tension can be calculated that is
consistent with experimentally measured values. Further,
this line tension can be rationalized and understood in
terms of the bending energy to deform the bilayer. The
region with small radii can be described and understood in
terms of statistical mechanics of density fluctuations. In
the region of cross over between a quadratic and linear free
energy there was some hysteresis associated with filling and
evacuation of the pore with water. The meta-stable pre-pore
state hypothesized to interpret experiments was not observed
in this region.
Reference: Free energy of a trans-membrane pore calculated
from atomistic molecular dynamics simulations, J. Wohlert,
W.K. den Otter, O. Edholm and W. J Briels, J. Chem. Phys.
124(2006) 154905.
Primary author
Prof.
Olle Edholm
(Royal Institute of Technology KTH, Sweden)
