Reparameterized United Atom Model for Molecular Dynamics
Simulations of Gel and Fluid Phosphatidylcholine Bilayers
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Abstract
A new united atom
parametrization of diacyl lipids like dipalmitoylphosphatidylcholine
(DPPC) and the dimyristoylphosphatidylcholine (DMPC) has been constructed
based on <i>ab initio</i> calculations to obtain fractional
charges and the dihedral potential of the hydrocarbon chains, while
the Lennard-Jones parameters of the acyl chains were fitted to reproduce
the properties of liquid hydrocarbons. The results have been validated
against published experimental X-ray and neutron scattering data for
fluid and gel phase DPPC. The derived charges of the lipid phosphatidylcholine
(PC) headgroup are shown to yield dipole components in the range suggested
by experiments. The aim has been to construct a new force field that
retains and improves the good agreement for the fluid phase and at
the same time produces a gel phase at low temperatures, with properties
coherent with experimental findings. The gel phase of diacyl-PC lipids
forms a regular triangular lattice in the hydrocarbon region. The
global bilayer tilt obtains an azimuthal value of 31° and is
aligned between lattice vectors in the bilayer plane. We also show
that the model yields a correct heat of melting as well as decent
heat capacities in the fluid and gel phase of DPPC