  Much of our understanding of the physical properties of raft domains in
biological membranes, and some insight into the mechanisms underlying their
formation stem from atomistic simulations of simple model systems, especially
ternary mixtures consisting of saturated and unsaturated lipids, and
cholesterol (Chol). To explore the properties of such systems at large spatial
scales, we here present a simple ternary mixture lattice model, involving a
small number of nearest neighbor interaction terms. Monte Carlo simulations of
mixtures with different compositions show an excellent agreement with
experimental and atomistic simulation observations across multiple scale,
ranging from the local distributions of lipids to the phase diagram of the
system. The simplicity of the model allows us to identify the roles played by
the different interactions between components, and the interplay between them.
Importantly, by changing the value of one of the model parameters, we can tune
the size of the liquid-ordered domains, thereby to simulate both Type II
mixtures exhibiting macroscopic phase separation and Type I mixtures with
nanoscopic domains. The Type II mixture simulation results fit well to the
experimentally-determined phase diagram of mixtures containing saturated
DPPC/unsaturated DOPC/Chol. When the tunable parameter is changed, we obtain
the Type I version of DPPC/DOPC/Chol, i.e., a mixture not showing thermodynamic
phase transitions but one that may be fitted to the same phase diagram if local
measures are used to distinguish between the different states. Our model
results suggest that short range packing is likely to be a key regulator of the
stability and size distribution of biological rafts.
