  We study the effect of cholesterol on the structure of
dipalmitoylphosphatidylcholine (DPPC) phospholipid bilayers. Using extensive
molecular dynamics computer simulations at atomistic resolution we observe and
quantify several structural changes upon increasing cholesterol content that
are collectively known as the condensing effect: a thickening of the bilayer,
an increase in lipid tail order, and a decrease in lateral area. We also
observe a change in leaflet interdigitation and a lack thereof in the
distributions of DPPC head group orientations. These results, obtained over a
wide range of cholesterol mole fractions, are then used to calibrate the
analysis of phospholipid properties in bilayers containing a single cholesterol
molecule per leaflet, which we perform in a spatially resolved way. We find
that a single cholesterol molecule affects phospholipids in its first and
second solvation shells, which puts the range of this interaction to be on the
order of one to two nanometers. We also observe a tendency of phospholipids to
orient their polar head groups toward the cholesterol, which provides
additional support for the umbrella model of bilayer organization.
