Kc. Tu et al., CONSTANT-PRESSURE MOLECULAR-DYNAMICS INVESTIGATION OF CHOLESTEROL EFFECTS IN A DIPALMITOYLPHOSPHATIDYLCHOLINE BILAYER, Biophysical journal, 75(5), 1998, pp. 2147-2156
We report a 1.4-ns constant-pressure molecular dynamics simulation of
cholesterol at 12.5 mol% in a dipalmitoylphosphatidylcholine (DPPC) bi
layer at 50 degrees C and compare the results to our previous simulati
on of a pure DPPC bilayer. The interlamellar spacing was increased by
2.5 Angstrom in the cholesterol-containing bilayer, consistent with x-
ray diffraction results, whereas the bilayer thickness was increased b
y only 1 Angstrom. The bilayer/water interface was more abrupt because
the lipid headgroups lie flatter to fill spaces left by the cholester
ol molecules. This leads to less compensation by the lipid headgroups
of the oriented water contribution to the membrane dipole potential an
d could explain the experimentally observed increase in the magnitude
of the dipole potential by cholesterol, Our calculations suggested tha
t 12.5 mol% cholesterol does not significantly affect the conformation
s and packing of the hydrocarbon chains and produces only a slight red
uction in the empty free volume. However, cholesterol has a significan
t: influence on the subnanosecond time scale lipid dynamics: the diffu
sion constant for the center-of-mass ''rattling'' motion was reduced b
y a factor of 3, and the reorientational motion of the methylene group
s was slowed along the entire length of the hydrocarbon chains.