  Cholesterol (CHOL) is one of the most important components of plasma
membranes of higher cells and one of the main factors for the formation of
(nano)domains. In this work, molecular dynamics simulations of mixtures of CHOL
with DPPC (saturated lipid) and DLiPC (unsaturated lipid) as standard
phospholipids (PLs) are presented in a wide range of CHOL concentrations. The
key idea is to systematically extract all structural and enthalpic properties
relevant to the formulation of a lattice model of these systems and express
them in dependence of the acyl chain order parameters. Detailed interpretation
is simplified by the observation that, to a good approximation, the interaction
effects do not depend on the total CHOL concentration, but only on the local
CHOL arrangement. The resulting information can be used to motivate the
agglomeration of CHOL molecules, the relevance of entropic rather than
enthalpic effects for understanding the stronger influence of CHOL on DPPC
compared to DLiPC, or the thermodynamic background of raft formation. It is
verified that the interaction functions hardly change during the transition
from binary to ternary mixtures, suggesting the applicability of the concepts
to more complex mixtures.
